hep-ph/0603175
LU TP 06–13
FERMILAB-PUB-06-052-CD-T
March 2006

PYTHIA 6.4
Physics and Manual
Torbjörn Sjöstrand
Department of Theoretical Physics, Lund University,Sölvegatan 14A, S-223 62 Lund, Sweden
E-mail: torbjorn@thep.lu.se

Stephen Mrenna
Computing Division, Simulations Group, Fermi National Accelerator Laboratory,
MS 234, Batavia, IL 60510, USA
E-mail: mrenna@fnal.gov

Peter Skands
Theoretical Physics Department, Fermi National Accelerator Laboratory,
MS 106, Batavia, IL 60510, USA
E-mail: skands@fnal.gov

Abstract
The Pythia program can be used to generate high-energy-physics ‘events’,i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a
representation of event properties in a wide range of reactions, within andbeyond the Standard Model, with emphasis on those where strong interactions play a rôle, directly or indirectly, and therefore multihadronic final
states are produced. The physics is then not understood well enough togive an exact description; instead the program has to be based on a combination of analytical results and various QCD-based models. This physics
input is summarized here, for areas such as hard subprocesses, initial- andfinal-state parton showers, underlying events and beam remnants, fragmentation and decays, and much more. Furthermore, extensive information isprovided on all program elements: subroutines and functions, switches andparameters, and particle and process data. This should allow the user totailor the generation task to the topics of interest.
The code and further information may be found on the Pythia web page:
http://www.thep.lu.se/∼torbjorn/Pythia.html.

Dedicated to
the Memory of

Bo Andersson
1937 – 2002
originator, inspirator

Preface
The Pythia program is frequently used for event generation in high-energy physics. The
emphasis is on multiparticle production in collisions between elementary particles. Thisin particular means hard interactions in e+ e− , pp and ep colliders, although also otherapplications are envisaged. The program is intended to generate complete events, in asmuch detail as experimentally observable ones, within the bounds of our current understanding of the underlying physics. Many of the components of the program represent
original research, in the sense that models have been developed and implemented for anumber of aspects not covered by standard theory.
Event generators often have a reputation for being ‘black boxes’; if nothing else, thisreport should provide you with a glimpse of what goes on inside the program. Some such
understanding may be of special interest for new users, who have no background in thefield. An attempt has been made to structure the report sufficiently well so that many of
the sections can be read independently of each other, so you can pick the sections thatinterest you. We have tried to keep together the physics and the manual sections on
specific topics, where practicable.
A large number of persons should be thanked for their contributions. Bo Andersson
and Gösta Gustafson are the originators of the Lund model, and strongly influenced
the early development of related programs. (Begun with Jetset in 1978, now fusedwith Pythia.) Hans-Uno Bengtsson is the originator of the Pythia program. Mats
Bengtsson is the main author of the old final-state parton-shower algorithm. Patrik Edén
has contributed an improved popcorn scenario for baryon production. Maria van Zijl has
helped develop the original multiple-interactions scenarios, Christer Friberg the expanded
photon physics machinery, Emanuel Norrbin the new matrix-element matching of the
final-state parton shower algorithm and the handling of low-mass strings, Leif Lönnblad
the Bose–Einstein models, and Gabriela Miu the matching of initial-state showers. Stefan
Wolf provided an implementation of onium production in NRQCD.
Further bug reports, smaller pieces of code and general comments on the program havebeen obtained from users too numerous to be mentioned here, but who are all gratefully
acknowledged. To write programs of this size and complexity would be impossible withouta strong support and user feedback. So, if you find errors, please let us know.
The moral responsibility for any remaining errors clearly rests with the authors. However, kindly note that this is a ‘University World’ product, distributed ‘as is’, free ofcharge, without any binding guarantees. And always remember that the program does
not represent a dead collection of established truths, but rather one of many possible
approaches to the problem of multiparticle production in high-energy physics, at thefrontline of current research. Be critical!

Contents
1 Introduction
1.1 The Complexity of High-Energy Processes . . . . . . . . . . . . . . . . . .
1.2 Event Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 The Origins of the Current Program . . . . . . . . . . . . . . . . . . . . .
1.4 About this Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.6 Appendix: The Historical Pythia . . . . . . . . . . . . . . . . . . . . . . .

1
1
2
3
5
7
7

2 Physics Overview
2.1 Hard Processes and Parton Distributions . . . . . . . . . . . . . . . . . . .
2.1.1 Hard Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Resonance Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 Parton Distributions . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Initial- and Final-State Radiation . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Matrix elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Parton showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Beam Remnants and Multiple Interactions . . . . . . . . . . . . . . . . . .
2.4 Hadronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1 String Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2 Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10
10
11
12
13
14
15
16
17
19
19
22

3 Program Overview
3.1 Update History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Program Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Program Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Manual Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Getting Started with the Simple Routines . . . . . . . . . . . . . . . . . .
3.6 Getting Started with the Event Generation Machinery . . . . . . . . . . .

23
23
27
29
31
33
37

4 Monte Carlo Techniques
4.1 Selection From a Distribution . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 The Veto Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 The Random Number Generator . . . . . . . . . . . . . . . . . . . . . . . .

43
43
45
47

5 The Event Record
5.1 Particle Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 The Event Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 How The Event Record Works . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 A simple example . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2 Complete PYTHIA events . . . . . . . . . . . . . . . . . . . . . . .
5.4 The HEPEVT Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51
51
58
62
63
63
66

6 The Old Electron–Positron Annihilation Routines
6.1 Annihilation Events in the Continuum . . . . . . . . . . . . . . . . . . . .
6.1.1 Electroweak cross sections . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 First-order QCD matrix elements . . . . . . . . . . . . . . . . . . .
6.1.3 Four-jet matrix elements . . . . . . . . . . . . . . . . . . . . . . . .
6.1.4 Second-order three-jet matrix elements . . . . . . . . . . . . . . . .
6.1.5 The matrix-element event generator scheme . . . . . . . . . . . . .
6.1.6 Optimized perturbation theory . . . . . . . . . . . . . . . . . . . .
6.1.7 Angular orientation . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.8 Initial-state radiation . . . . . . . . . . . . . . . . . . . . . . . . . .

70
70
70
72
73
74
75
76
77
78

6.2
6.3

6.4

6.1.9 Alternative matrix elements . . . . . . . . . . . . . . . . . . . . . .
Decays of Onia Resonances . . . . . . . . . . . . . . . . . . . . . . . . . . .
Routines and Common-Block Variables . . . . . . . . . . . . . . . . . . . .
6.3.1 e+ e− continuum event generation . . . . . . . . . . . . . . . . . . .
6.3.2 A routine for onium decay . . . . . . . . . . . . . . . . . . . . . . .
6.3.3 Common-block variables . . . . . . . . . . . . . . . . . . . . . . . .
Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

79
79
80
80
81
82
87

7 Process Generation
89
7.1 Parton Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.1.1 Baryons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.1.2 Mesons and photons . . . . . . . . . . . . . . . . . . . . . . . . . . 90
7.1.3 Leptons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
7.1.4 Equivalent photon flux in leptons . . . . . . . . . . . . . . . . . . . 94
7.2 Kinematics and Cross Section for a Two-body Process . . . . . . . . . . . 96
7.3 Resonance Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
7.4 Cross-section Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7.4.1 The simple two-body processes . . . . . . . . . . . . . . . . . . . . 102
7.4.2 Resonance production . . . . . . . . . . . . . . . . . . . . . . . . . 105
7.4.3 Lepton beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
7.4.4 Mixing processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.5 Three- and Four-body Processes . . . . . . . . . . . . . . . . . . . . . . . . 108
7.6 Resonance Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.6.1 The decay scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
7.6.2 Cross-section considerations . . . . . . . . . . . . . . . . . . . . . . 110
7.7 Nonperturbative Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7.7.1 Hadron–hadron interactions . . . . . . . . . . . . . . . . . . . . . . 112
7.7.2 Photoproduction and γγ physics . . . . . . . . . . . . . . . . . . . . 115
8 Physics Processes
130
8.1 The Process Classification Scheme . . . . . . . . . . . . . . . . . . . . . . . 130
8.2 QCD Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
8.2.1 QCD jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
8.2.2 Heavy flavours . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
8.2.3 J/ψ and other Hidden Heavy Flavours . . . . . . . . . . . . . . . . 136
8.2.4 Minimum bias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
8.3 Physics with Incoming Photons . . . . . . . . . . . . . . . . . . . . . . . . 138
8.3.1 Photoproduction and γγ physics . . . . . . . . . . . . . . . . . . . . 138
8.3.2 Deeply Inelastic Scattering and γ ∗ γ ∗ physics . . . . . . . . . . . . . 141
8.3.3 Photon physics at all virtualities . . . . . . . . . . . . . . . . . . . . 143
8.4 Electroweak Gauge Bosons . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
8.4.1 Prompt photon production . . . . . . . . . . . . . . . . . . . . . . . 144
8.4.2 Single W/Z production . . . . . . . . . . . . . . . . . . . . . . . . . 145
8.4.3 W/Z pair production . . . . . . . . . . . . . . . . . . . . . . . . . . 147
8.5 Higgs Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
8.5.1 Light Standard Model Higgs . . . . . . . . . . . . . . . . . . . . . . 148
8.5.2 Heavy Standard Model Higgs . . . . . . . . . . . . . . . . . . . . . 150
8.5.3 Extended neutral Higgs sector . . . . . . . . . . . . . . . . . . . . . 151
8.5.4 Charged Higgs sector . . . . . . . . . . . . . . . . . . . . . . . . . . 153
8.5.5 Higgs pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
8.6 Non-Standard Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
8.6.1 Fourth-generation fermions . . . . . . . . . . . . . . . . . . . . . . . 154
8.6.2 New gauge bosons . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

8.7

8.8
8.9

8.6.3 Left–Right Symmetry and Doubly Charged Higgs Bosons . . . . . . 155
8.6.4 Leptoquarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
8.6.5 Compositeness and anomalous couplings . . . . . . . . . . . . . . . 157
8.6.6 Excited fermions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
8.6.7 Technicolor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
8.6.8 Extra Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
Supersymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
8.7.1 General Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 165
8.7.2 Extended Higgs Sector . . . . . . . . . . . . . . . . . . . . . . . . . 166
8.7.3 Superpartners of Gauge and Higgs Bosons . . . . . . . . . . . . . . 168
8.7.4 Superpartners of Standard Model Fermions . . . . . . . . . . . . . . 169
8.7.5 Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
8.7.6 SUSY examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
8.7.7 R-Parity Violation . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
8.7.8 NMSSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
8.7.9 Long-lived coloured sparticles . . . . . . . . . . . . . . . . . . . . . 177
Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
Main Processes by Machine . . . . . . . . . . . . . . . . . . . . . . . . . . 178
8.9.1 e+ e− collisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
8.9.2 Lepton–hadron collisions . . . . . . . . . . . . . . . . . . . . . . . . 179
8.9.3 Hadron–hadron collisions . . . . . . . . . . . . . . . . . . . . . . . . 180

9 The Process Generation Program Elements
181
9.1 The Main Subroutines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
9.2 Switches for Event Type and Kinematics Selection . . . . . . . . . . . . . . 185
9.3 The General Switches and Parameters . . . . . . . . . . . . . . . . . . . . 193
9.4 Further Couplings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
9.5 Supersymmetry Common-Blocks and Routines . . . . . . . . . . . . . . . . 223
9.6 General Event Information . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
9.7 How to Generate Weighted Events . . . . . . . . . . . . . . . . . . . . . . 234
9.8 How to Run with Varying Energies . . . . . . . . . . . . . . . . . . . . . . 240
9.9 How to Include External Processes . . . . . . . . . . . . . . . . . . . . . . 242
9.9.1 Run information . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
9.9.2 Event information . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
9.9.3 An example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
9.9.4 PYTHIA as a generator of external processes . . . . . . . . . . . . 260
9.9.5 Further comments . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
9.10 Interfaces to Other Generators . . . . . . . . . . . . . . . . . . . . . . . . . 264
9.11 Other Routines and Common Blocks . . . . . . . . . . . . . . . . . . . . . 268
10 Initial- and Final-State Radiation286
10.1 Shower Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
10.1.1 The evolution equations . . . . . . . . . . . . . . . . . . . . . . . . 287
10.1.2 The Sudakov form factor . . . . . . . . . . . . . . . . . . . . . . . . 288
10.1.3 Matching to the hard scattering . . . . . . . . . . . . . . . . . . . . 289
10.2 Final-State Showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
10.2.1 The choice of evolution variable . . . . . . . . . . . . . . . . . . . . 291
10.2.2 The choice of energy splitting variable . . . . . . . . . . . . . . . . 291
10.2.3 First branchings and matrix-element matching . . . . . . . . . . . . 292
10.2.4 Subsequent branches and angular ordering . . . . . . . . . . . . . . 294
10.2.5 Other final-state shower aspects . . . . . . . . . . . . . . . . . . . . 296
10.2.6 Merging with massive matrix elements . . . . . . . . . . . . . . . . 297
10.2.7 Matching to four-parton events . . . . . . . . . . . . . . . . . . . . 299

10.2.8 A new p⊥ -ordered final-state shower . . . . . . . . . . . . . . . . . . 300
10.3 Initial-State Showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
10.3.1 The shower structure . . . . . . . . . . . . . . . . . . . . . . . . . . 302
10.3.2 Longitudinal evolution . . . . . . . . . . . . . . . . . . . . . . . . . 304
10.3.3 Transverse evolution . . . . . . . . . . . . . . . . . . . . . . . . . . 306
10.3.4 Other initial-state shower aspects . . . . . . . . . . . . . . . . . . . 308
10.3.5 Matrix-element matching . . . . . . . . . . . . . . . . . . . . . . . . 310
10.3.6 A new p⊥ -ordered initial-state shower . . . . . . . . . . . . . . . . . 311
10.4 Routines and Common-Block Variables . . . . . . . . . . . . . . . . . . . . 313
11 Beam Remnants and Underlying Events
325
11.1 Beam Remnants — Old Model . . . . . . . . . . . . . . . . . . . . . . . . . 326
11.1.1 Hadron Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326
11.1.2 Photon Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327
11.1.3 Lepton Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327
11.1.4 Primordial k⊥ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
11.1.5 Remnant Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . 328
11.2 Multiple Interactions — Old Model . . . . . . . . . . . . . . . . . . . . . . 329
11.2.1 The basic cross sections . . . . . . . . . . . . . . . . . . . . . . . . 330
11.2.2 The simple model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332
11.2.3 A model with varying impact parameters . . . . . . . . . . . . . . . 334
11.3 Beam Remnants (and Multiple Interactions) — Intermediate Model . . . . 337
11.3.1 Flavour and x Correlations . . . . . . . . . . . . . . . . . . . . . . . 338
11.3.2 Colour Topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339
11.3.3 Primordial k⊥ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
11.3.4 Beam-Remnant Kinematics . . . . . . . . . . . . . . . . . . . . . . 342
11.4 Multiple Interactions (and Beam Remnants) – New Model . . . . . . . . . 342
11.4.1 Joined Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . 345
11.5 Pile-up Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345
11.6 Common-Block Variables and Routines . . . . . . . . . . . . . . . . . . . . 346
12 Fragmentation359
12.1 Flavour Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359
12.1.1 Quark flavours and transverse momenta . . . . . . . . . . . . . . . 359
12.1.2 Meson production . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
12.1.3 Baryon production . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
12.2 String Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
12.2.1 Fragmentation functions . . . . . . . . . . . . . . . . . . . . . . . . 365
12.2.2 Joining the jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
12.2.3 String motion and infrared stability . . . . . . . . . . . . . . . . . . 369
12.2.4 Fragmentation of multiparton systems . . . . . . . . . . . . . . . . 370
12.2.5 Junction topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . 373
12.3 Independent Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . 374
12.3.1 Fragmentation of a single jet . . . . . . . . . . . . . . . . . . . . . . 374
12.3.2 Fragmentation of a jet system . . . . . . . . . . . . . . . . . . . . . 375
12.4 Other Fragmentation Aspects . . . . . . . . . . . . . . . . . . . . . . . . . 376
12.4.1 Small-mass systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 376
12.4.2 Interconnection Effects . . . . . . . . . . . . . . . . . . . . . . . . . 378
12.4.3 Bose–Einstein effects . . . . . . . . . . . . . . . . . . . . . . . . . . 380

13 Particles and Their Decays384
13.1 The Particle Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384
13.2 Masses, Widths and Lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . 385
13.2.1 Masses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
13.2.2 Widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386
13.2.3 Lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
13.3 Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387
13.3.1 Strong and electromagnetic decays . . . . . . . . . . . . . . . . . . 387
13.3.2 Weak decays of the τ lepton . . . . . . . . . . . . . . . . . . . . . . 388
13.3.3 Weak decays of charm hadrons . . . . . . . . . . . . . . . . . . . . 388
13.3.4 Weak decays of bottom hadrons . . . . . . . . . . . . . . . . . . . . 390
13.3.5 Other decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391
14 The Fragmentation and Decay Program Elements
392
14.1 Definition of Initial Configuration or Variables . . . . . . . . . . . . . . . . 392
14.2 The Physics Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395
14.3 The General Switches and Parameters . . . . . . . . . . . . . . . . . . . . 397
14.3.1 The advanced popcorn code for baryon production . . . . . . . . . 409
14.4 Further Parameters and Particle Data . . . . . . . . . . . . . . . . . . . . 413
14.5 Miscellaneous Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420
14.5.1 Interfacing to detector simulation . . . . . . . . . . . . . . . . . . . 420
14.5.2 Parameter values . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421
14.6 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422
15 Event Study and Analysis Routines
426
15.1 Event Study Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
15.2 Event Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432
15.2.1 Sphericity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432
15.2.2 Thrust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433
15.2.3 Fox-Wolfram moments . . . . . . . . . . . . . . . . . . . . . . . . . 434
15.2.4 Jet masses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
15.3 Cluster Finding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
15.3.1 Cluster finding in an e+ e− type of environment . . . . . . . . . . . 436
15.3.2 Cluster finding in a pp type of environment . . . . . . . . . . . . . 439
15.4 Event Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440
15.4.1 Multiplicities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 440
15.4.2 Energy-Energy Correlation . . . . . . . . . . . . . . . . . . . . . . . 441
15.4.3 Factorial moments . . . . . . . . . . . . . . . . . . . . . . . . . . . 441
15.5 Routines and Common-Block Variables . . . . . . . . . . . . . . . . . . . . 442
15.6 Histograms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452
16 Summary and Outlook

455

References

456

Appendix A: Subprocess Summary Table

474

Appendix B: Index of Subprograms and Common-Block Variables

478

1

Introduction

Multiparticle production is the most characteristic feature of current high-energy physics.
Today, observed particle multiplicities are typically between ten and a hundred, and with
future machines this range will be extended upward. The bulk of the multiplicity is foundin jets, i.e. in collimated bunches of hadrons (or decay products of hadrons) producedby the hadronization of partons, i.e. quarks and gluons. (For some applications it will
be convenient to extend the parton concept also to some non-coloured but showering
particles, such as electrons and photons.)

1.1

The Complexity of High-Energy Processes

To first approximation, all processes have a simple structure at the level of interactions
between the fundamental objects of nature, i.e. quarks, leptons and gauge bosons. For
instance, a lot can be understood about the structure of hadronic events at LEP just fromthe ‘skeleton’ process e+ e− → Z0 → qq. Corrections to this picture can be subdivided,
arbitrarily but conveniently, into three main classes.
Firstly, there are bremsstrahlung-type modifications, i.e. the emission of additional
final-state particles by branchings such as e → eγ or q → qg. Because of the largeness of the strong coupling constant αs , and because of the presence of the triple gluon
vertex, QCD emission off quarks and gluons is especially prolific. We therefore speak
about ‘parton showers’, wherein a single initial parton may give rise to a whole bunch ofpartons in the final state. Also photon emission may give sizable effects in e+ e− and ep
processes. The bulk of the bremsstrahlung corrections are universal, i.e. do not dependon the details of the process studied, but only on one or a few key numbers, such as themomentum transfer scale of the process. Such universal corrections may be included toarbitrarily high orders, using a probabilistic language. Alternatively, exact calculationsof bremsstrahlung corrections may be carried out order by order in perturbation theory, but rapidly the calculations then become prohibitively complicated and the answers
correspondingly lengthy.
Secondly, we have ‘true’ higher-order corrections, which involve a combination of loop
graphs and the soft parts of the bremsstrahlung graphs above, a combination needed tocancel some divergences. In a complete description it is therefore not possible to consider
bremsstrahlung separately, as assumed here. The necessary perturbative calculations areusually very difficult; only rarely have results been presented that include more than one
non-‘trivial’ order, i.e. more than one loop. As above, answers are usually very lengthy,
but some results are sufficiently simple to be generally known and used, such as therunning of αs , or the correction factor 1 + αs /π + · · · in the partial widths of Z0 → qq
decay channels. For high-precision studies it is imperative to take into account the resultsof loop calculations, but usually effects are minor for the qualitative aspects of high-energy
processes.
Thirdly, quarks and gluons are confined. In the two points above, we have used a
perturbative language to describe the short-distance interactions of quarks, leptons andgauge bosons. For leptons and colourless bosons this language is sufficient. However, forquarks and gluons it must be complemented with the structure of incoming hadrons, anda picture for the hadronization process, wherein the coloured partons are transformed
into jets of colourless hadrons, photons and leptons. The hadronization can be further
subdivided into fragmentation and decays, where the former describes the way the creationof new quark-antiquark pairs can break up a high-mass system into lower-mass ones,
ultimately hadrons. (The word ‘fragmentation’ is also sometimes used in a broader sense,
but we will here use it with this specific meaning.) This process is still not yet understoodfrom first principles, but has to be based on models. In one sense, hadronization effects
are overwhelmingly large, since this is where the bulk of the multiplicity comes from. In
1

another sense, the overall energy flow of a high-energy event is mainly determined by the
perturbative processes, with only a minor additional smearing caused by the hadronization
step. One may therefore pick different levels of ambition, but in general detailed studies
require a detailed modelling of the hadronization process.
The simple structure that we started out with has now become considerably more
complex — instead of maybe two final-state partons we have a hundred final particles.
The original physics is not gone, but the skeleton process has been dressed up and is nolonger directly visible. A direct comparison between theory and experiment is therefore
complicated at best, and impossible at worst.

1.2

Event Generators

It is here that event generators come to the rescue. In an event generator, the objective
striven for is to use computers to generate events as detailed as could be observed by aperfect detector. This is not done in one step, but rather by ‘factorizing’ the full problem into a number of components, each of which can be handled reasonably accurately.
Basically, this means that the hard process is used as input to generate bremsstrahlung
corrections, and that the result of this exercise is thereafter left to hadronize. This sounds
a bit easier than it really is — else this report would be a lot thinner. However, the basic
idea is there: if the full problem is too complicated to be solved in one go, it may bepossible to subdivide it into smaller tasks of more manageable proportions. In the actual
generation procedure, most steps therefore involve the branching of one object into two,or at least into a very small number, with the daughters free to branch in their turn. A
lot of book-keeping is involved, but much is of a repetitive nature, and can therefore beleft for the computer to handle.
As the name indicates, the output of an event generator should be in the form of‘events’, with the same average behaviour and the same fluctuations as real data. In
the data, fluctuations arise from the quantum mechanics of the underlying theory. In
generators, Monte Carlo techniques are used to select all relevant variables according to
the desired probability distributions, and thereby ensure (quasi-)randomness in the final
events. Clearly some loss of information is entailed: quantum mechanics is based on amplitudes, not probabilities. However, only very rarely do (known) interference phenomena
appear that cannot be cast in a probabilistic language. This is therefore not a more
restraining approximation than many others.
Once there, an event generator can be used in many different ways. The five main
applications are probably the following:• To give physicists a feeling for the kind of events one may expect/hope to find, andat what rates.
• As a help in the planning of a new detector, so that detector performance is optimized, within other constraints, for the study of interesting physics scenarios.
• As a tool for devising the analysis strategies that should be used on real data, sothat signal-to-background conditions are optimized.
• As a method for estimating detector acceptance corrections that have to be applied
to raw data, in order to extract the ‘true’ physics signal.
• As a convenient framework within which to interpret the observed phenomena interms of a more fundamental underlying theory (usually the Standard Model).
Where does a generator fit into the overall analysis chain of an experiment? In ‘real
life’, the machine produces interactions. These events are observed by detectors, and theinteresting ones are written to tape by the data acquisition system. Afterward the events
may be reconstructed, i.e. the electronics signals (from wire chambers, calorimeters, andall the rest) may be translated into a deduced setup of charged tracks or neutral energy
depositions, in the best of worlds with full knowledge of momenta and particle species.
Based on this cleaned-up information, one may proceed with the physics analysis. In the2Monte Carlo world, the rôle of the machine, namely to produce events, is taken by theevent generators described in this report. The behaviour of the detectors — how particles
produced by the event generator traverse the detector, spiral in magnetic fields, shower incalorimeters, or sneak out through cracks, etc. — is simulated in programs such as Geant[Bru89]. Be warned that this latter activity is sometimes called event simulation, which issomewhat unfortunate since the same words could equally well be applied to what, here,
we call event generation. A more appropriate term is detector simulation. Ideally, theoutput of this simulation has exactly the same format as the real data recorded by thedetector, and can therefore be put through the same event reconstruction and physics
analysis chain, except that here we know what the ‘right answer’ should be, and so cansee how well we are doing.
Since the full chain of detector simulation and event reconstruction is very timeconsuming, one often does ‘quick and dirty’ studies in which these steps are skipped
entirely, or at least replaced by very simplified procedures which only take into accountthe geometric acceptance of the detector and other trivial effects. One may then use theoutput of the event generator directly in the physics studies.
There are still many holes in our understanding of the full event structure, despite
an impressive amount of work and detailed calculations. To put together a generator
therefore involves making a choice on what to include, and how to include it. At best,
the spread between generators can be used to give some impression of the uncertainties
involved. A multitude of approximations will be discussed in the main part of this report,
but already here is should be noted that many major approximations are related to thealmost complete neglect of non-‘trivial’ higher-order effects, as already mentioned. It
can therefore only be hoped that the ‘trivial’ higher order parts give the bulk of theexperimental behaviour. By and large, this seems to be the case; for e+ e− annihilation it
even turns out to be a very good approximation.
The necessity to make compromises has one major implication: to write a good event
generator is an art, not an exact science. It is therefore essential not to blindly trust
the results of any single event generator, but always to make several cross-checks. In
addition, with computer programs of tens of thousands of lines, the question is not whether
bugs exist, but how many there are, and how critical their positions. Further, an event
generator cannot be thought of as all-powerful, or able to give intelligent answers to illposed questions; sound judgement and some understanding of a generator are necessary
prerequisites for successful use. In spite of these limitations, the event-generator approachis the most powerful tool at our disposal if we wish to gain a detailed and realistic
understanding of physics at current or future high-energy colliders.

1.3

The Origins of the Current Program

Over the years, many event generators have appeared. A recent comprehensive overview
is the Les Houches guidebook to Monte Carlo event generators [Dob04]. Surveys ofgenerators for e+ e− physics in general and LEP in particular may be found in [Kle89,
Sjö89, Kno96, Lön96, Bam00], for high-energy hadron–hadron (pp) physics in [Ans90,
Sjö92, Kno93, LHC00], and for ep physics in [HER92, HER99]. We refer the reader
to those for additional details and references. In this particular report, the two closely
connected programs Jetset and Pythia, now merged under the Pythia label, will be
described.
Jetset has its roots in the efforts of the Lund group to understand the hadronization process, starting in the late seventies [And83]. The so-called string fragmentation
model was developed as an explicit and detailed framework, within which the long-range
confinement forces are allowed to distribute the energies and flavours of a parton configuration among a collection of primary hadrons, which subsequently may decay further.
This model, known as the Lund string model, or ‘Lund’ for short, contained a number3

of specific predictions, which were confirmed by data from e+ e− annihilation around 30
GeV at PETRA and PEP, whence the model gained a widespread acceptance. The Lund
string model is still today the most elaborate and widely used fragmentation model atour disposal. It remains at the heart of the Pythia program.
In order to predict the shape of events at PETRA/PEP, and to study the fragmentation
process in detail, it was necessary to start out from the partonic configurations thatwere to fragment. The generation of complete e+ e− hadronic events was therefore added,
originally based on simple γ exchange and first-order QCD matrix elements, later extended
to full γ ∗ /Z0 exchange with first-order initial-state QED radiation and second-order QCD
matrix elements. A number of utility routines were also provided early on, for everythingfrom event listing to jet finding.
By the mid-eighties it was clear that the pure matrix-element approach had reachedthe limit of its usefulness, in the sense that it could not fully describe the exclusive multijet topologies of the data. (It is still useful for inclusive descriptions, like the optimized
perturbation theory discussed in section 6.1.6, and in combination with renormalon contributions [Dok97].) Therefore a parton-shower description was developed [Ben87a] as analternative to the higher-order matrix-element one. (Or rather as a complement, since
the trend over the years has been towards the development of methods to marry the two
approaches.) The combination of parton showers and string fragmentation has been very
successful, and has formed the main approach to the description of hadronic Z0 events.
This way, the Jetset code came to cover the four main areas of fragmentation, finalstate parton showers, e+ e− event generation and general utilities.
The successes of string fragmentation in e+ e− made it interesting to try to extend thisframework to other processes, and explore possible physics consequences. Therefore anumber of other programs were written, which combined a process-specific description of
the hard interactions with the general fragmentation framework of Jetset. The Pythia
program evolved out of early studies on fixed-target proton–proton processes, addressed
mainly at issues related to string drawing.
With time, the interest shifted towards hadron collisions at higher energies, first to the
SPS pp collider, and later to the Tevatron, SSC and LHC, in the context of a number ofworkshops in the USA and Europe. Parton showers were added, for final-state radiation
by making use of the Jetset routine, for initial-state one by the development of theconcept of ‘backwards evolution’, specifically for Pythia [Sjö85]. Also a framework wasdeveloped for minimum-bias and underlying events [Sjö87a].
Another main change was the introduction of an increasing number of hard processes,
within the Standard Model and beyond. A special emphasis was put on the search for
the Standard Model Higgs, in different mass ranges and in different channels, with due
respect to possible background processes.
The bulk of the machinery developed for hard processes actually depended little on the
choice of initial state, as long as the appropriate parton distributions were there for theincoming partons and particles. It therefore made sense to extend the program from being
only a pp generator to working also for e+ e− and ep. This process was completed in 1991,
again spurred on by physics workshop activities. Currently Pythia should therefore work
well for a selection of different possible incoming beam particles.
An effort independent of the Lund group activities got going to include supersymmetric
event simulation in Pythia. This resulted in the SPythia program [Mre97].
While Jetset was independent of Pythia until 1996, their ties had grown muchstronger over the years, and the border-line between the two programs had become more
and more artificial. It was therefore decided to merge the two, and also include theSPythia extensions, starting from Pythia 6.1. The different origins in part still arereflected in this manual, but the striving is towards a seamless merger.
Among the most recent developments, primarily intended for Tevatron and LHC
physics studies, is the introduction of ‘interleaved evolution’ in Pythia 6.3, with new4p⊥ -ordered parton showers and a more sophisticated framework for minimum-bias andunderlying events [Sjö04, Sjö04a]. The possibilities for studying physics beyond the Standard Model have also been extended significantly, to include supersymmetric models with
R-parity violation, Technicolor models, Z0 /W0 models, as well as models with (Randall–
Sundrum) extra dimensions. This still only includes the models available internally in
Pythia. Versatility is further enhanced by the addition of an interface to external user
processes, according to the Les Houches Accord (LHA) standard [Boo01], and by interfaces to SUSY RGE and decay packages via the SUSY Les Houches Accord (SLHA)[Ska03].
The tasks of including new processes, and of improving the simulation of parton showers and other aspects of already present processes, are never-ending. Work therefore
continues apace.

1.4

About this Report

As we see, Jetset and Pythia started out as very ideologically motivated programs, developed to study specific physics questions in enough detail that explicit predictions could
be made for experimental quantities. As it was recognized that experimental imperfections could distort the basic predictions, the programs were made available for general use
by experimentalists. It thus became feasible to explore the models in more detail thanwould otherwise have been possible. As time went by, the emphasis came to shift somewhat, away from the original strong coupling to a specific fragmentation model, towards a
description of high-energy multiparticle production processes in general. Correspondingly,
the use expanded from being one of just comparing data with specific model predictions,to one of extensive use for the understanding of detector performance, for the derivation of acceptance correction factors, for the prediction of physics at future high-energy
accelerators, and for the design of related detectors.
While the ideology may be less apparent, it is still there, however. This is not something unique to the programs discussed here, but inherent in any event generator, orat least any generator that attempts to go beyond the simple parton level skeleton description of a hard process. Do not accept the myth that everything available in Monte
Carlo form represents ages-old common knowledge, tested and true. Ideology is present
by commissions or omissions in any number of details. A program like Pythia represents
a major amount of original physics research, often on complicated topics where no simple
answers are available. As a (potential) program user you must be aware of this, so thatyou can form your own opinion, not just about what to trust and what not to trust, butalso how much to trust a given prediction, i.e. how uncertain it is likely to be. Pythia
is particularly well endowed in this respect, since a number of publications exist wheremost of the relevant physics is explained in considerable detail. In fact, the problem mayrather be the opposite, to find the relevant information among all the possible places.
One main objective of the current report is therefore to collect much of this informationin one single place. Not all the material found in specialized papers is reproduced, by awide margin, but at least enough should be found here to understand the general picture
and to know where to go for details.
The official reference for Pythia is therefore the current report. It is intended to
update and extend the previous round of published physics descriptions and program
manuals [Sjö01, Sjö01a, Sjö03a]. Further specification could include a statement of thetype ‘We use Pythia version X.xxx’. (If you are a LATEX fan, you may want to know thatthe program name in this report has been generated by the command \textsc{Pythia}.)
Kindly do not refer to Pythia as ‘unpublished’, ‘private communication’ or ‘in preparation’: such phrases are incorrect and only create unnecessary confusion.
In addition, remember that many of the individual physics components are documented in separate publications. If some of these contain ideas that are useful to you,5

there is every reason to cite them. A reasonable selection would vary as a function of thephysics you are studying. The criterion for which to pick should be simple: imagine thata Monte Carlo implementation had not been available. Would you then have cited a given
paper on the grounds of its physics contents alone? If so, do not punish the extra effort
of turning these ideas into publicly available software. (Monte Carlo manuals are goodfor nothing in the eyes of many theorists, so often only the acceptance of ‘mainstream’
publications counts.) Here follows a list of some main areas where the Pythia programs
contain original research:
Fragmentation/Hadronization:• The string fragmentation model [And83, And98].
• The string effect [And80].
• Baryon production (diquark/popcorn) [And82, And85, Edé97].
• Fragmentation of systems with string junctions [Sjö03].
• Small-mass string fragmentation [Nor98].
• Fragmentation of multiparton systems [Sjö84].
• Colour rearrangement [Sjö94a] and Bose-Einstein effects [Lön95].
• Fragmentation effects on αs determinations [Sjö84a].
Parton Showers:• Initial-state parton showers (Q2 -ordering) [Sjö85, Miu99].
• Final-state parton showers (Q2 -ordering) [Ben87a, Nor01].
• Initial-state parton showers (p2⊥ -ordering) [Sjö04a].
• Final-state parton showers (p2⊥ -ordering) [Sjö04a].
• Photon radiation from quarks [Sjö92c]
DIS and photon physics:• Deeply Inelastic Scattering [And81a, Ben88].
• Photoproduction [Sch93a], γγ [Sch94a] and γ ∗ p/γ ∗ γ/γ ∗ γ ∗ [Fri00] physics.
• Parton distributions of the photon [Sch95, Sch96].
Beyond the Standard Model physics:• Supersymmetry [Amb96, Mre99a], with R-parity violation [Ska01, Sjö03].
• Technicolor [Lan02a].
• Extra dimensions [Bij01]• Z0 models [Lyn00]
Other topics:• Colour flow in hard scatterings [Ben84].
• Elastic and diffractive cross sections [Sch94].
• Minijets, underlying event, and minimum-bias (multiple parton–parton interactions)[Sjö87a, Sjö04, Sjö04a].
• Rapidity gaps [Dok92].
• Jet clustering in k⊥ [Sjö83].
In addition to a physics survey, the current report also contains a complete manual
for the program. Such manuals have always been updated and distributed jointly withthe programs, but have grown in size with time. A word of warning may therefore be inplace. The program description is fairly lengthy, and certainly could not be absorbed in
one sitting. This is not even necessary, since all switches and parameters are provided
with sensible default values, based on our best understanding (of the physics, and of what6you expect to happen if you do not specify any options). As a new user, you can therefore
disregard all the fancy options, and just run the program with a minimum ado. Lateron, as you gain experience, the options that seem useful can be tried out. No single user
is ever likely to find need for more than a fraction of the total number of possibilities
available, yet many of them have been added to meet specific user requests.
In some instances, not even this report will provide you with all the information you
desire. You may wish to find out about recent versions of the program, know about related
software, pick up a few sample main programs to get going, or get hold of related physics
papers. Some such material can be found on the Pythia web page:
http://www.thep.lu.se/∼torbjorn/Pythia.html.

1.5Disclaimer

At all times it should be remembered that this is not a commercial product, developedand supported by professionals. Instead it is a ‘University World’ product, developed bya very few physicists (mainly the current first author) originally for their own needs, andsupplied to other physicists on an ‘as-is’ basis, free of charge. (It is protected by copyright,
however, so is not ‘free software’ in the nowadays common meaning. This is not intended
to stifle research, but to make people respect some common-sense ‘intellectual property’rights: code should not be ‘borrowed’ and redistributed in such a form that credit would
not go to the people who did the work.)
No guarantees are therefore given for the proper functioning of the program, nor for thevalidity of physics results. In the end, it is always up to you to decide for yourself whether
to trust a given result or not. Usually this requires comparison either with analyticalresults or with results of other programs, or with both. Even this is not necessarily
foolproof: for instance, if an error is made in the calculation of a matrix element for agiven process, this error will be propagated both into the analytical results based on theoriginal calculation and into all the event generators which subsequently make use of the
published formulae. In the end, there is no substitute for a sound physics judgement.
This does not mean that you are all on your own, with a program nobody feels responsible for. Attempts are made to check processes as carefully as possible, to write
programs that do not invite unnecessary errors, and to provide a detailed and accurate
documentation. All of this while maintaining the full power and flexibility, of course,
since the physics must always take precedence in any conflict of interests. If neverthelessany errors or unclear statements are found, please do communicate them to one of theauthors. Every attempt will be made to solve problems as soon as is reasonably possible,
given that this support is by a few persons, who mainly have other responsibilities.
However, in order to make debugging at all possible, we request that any sample
code you want to submit as evidence be completely self-contained, and peeled off from allirrelevant aspects. Use simple write statements or the Pythia histogramming routines tomake your point. Chances are that, if the error cannot be reproduced by fifty lines of code,in a main program linked only to Pythia, the problem is sitting elsewhere. Numerous
errors have been caused by linking to other (flawed) libraries, e.g. collaboration-specificframeworks for running Pythia. Then you should put the blame elsewhere.

1.6

Appendix: The Historical Pythia

The ‘Pythia’ label may need some explanation.
The myth tells how Apollon, the God of Wisdom, killed the powerful dragon-likemonster Python, close to the village of Delphi in Greece. To commemorate this victory,
Apollon founded the Pythic Oracle in Delphi, on the slopes of Mount Parnassos. Heremen could come to learn the will of the Gods and the course of the future. The oracle

7

plays an important rôle in many of the other Greek myths, such as those of Heracles andof King Oedipus.
Questions were to be put to the Pythia, the ‘Priestess’ or ‘Prophetess’ of the Oracle. In
fact, she was a local woman, usually a young maiden, of no particular religious schooling.
Seated on a tripod, she inhaled the obnoxious vapours that seeped up through a crevice inthe ground. This brought her to a trance-like state, in which she would scream seemingly
random words and sounds. It was the task of the professional priests in Delphi to record
those utterings and edit them into the official Oracle prophecies, which often took theform of poems in perfect hexameter. In fact, even these edited replies were often less thaneasy to interpret. The Pythic oracle acquired a reputation for ambiguous answers.
The Oracle existed already at the beginning of the historical era in Greece, and wasuniversally recognized as the foremost religious seat. Individuals and city states came toconsult, on everything from cures for childlessness to matters of war. Lavish gifts allowed
the temple area to be built and decorated. Many states supplied their own treasury halls,
where especially beautiful gifts were on display. Sideshows included the Omphalos, astone reputedly marking the centre of the Earth, and the Pythic games, second only to
the Olympic ones in importance.
Strife inside Greece eventually led to a decline in the power of the Oracle. A serious
blow was dealt when the Oracle of Zeus Ammon (see below) declared Alexander the Greatto be the son of Zeus. The Pythic Oracle lived on, however, and was only closed by a
Roman Imperial decree in 390 ad, at a time when Christianity was ruthlessly destroying
any religious opposition. Pythia then had been at the service of man and Gods for amillennium and a half.
The rôle of the Pythic Oracle prophecies on the course of history is nowhere better
described than in ‘The Histories’ by Herodotus [HerBC], the classical and captivating
description of the Ancient World at the time of the Great War between Greeks andPersians. Especially famous is the episode with King Croisus of Lydia. Contemplating a
war against the upstart Persian Empire, he resolves to ask an oracle what the outcomeof a potential battle would be. However, to have some guarantee for the veracity of any
prophecy, he decides to send embassies to all the renowned oracles of the known World.
The messengers are instructed to inquire the various divinities, on the hundredth day
after their departure, what King Croisus is doing at that very moment. From the Pythia
the messengers bring back the reply
I know the number of grains of sand as well as the expanse of the sea,
And I comprehend the dumb and hear him who does not speak,
There came to my mind the smell of the hard-shelled turtle,
Boiled in copper together with the lamb,With copper below and copper above.
The veracity of the Pythia is thus established by the crafty ruler, who had waited until
the appointed day, slaughtered a turtle and a lamb, and boiled them together in a copper
cauldron with a copper lid. Also the Oracle of Zeus Ammon in the Libyan desert is able
to give a correct reply (lost to posterity), while all others fail. King Croisus now sends asecond embassy to Delphi, inquiring after the outcome of a battle against the Persians.
The Pythia answers
If Croisus passes over the Halys he will dissolve a great Empire.
Taking this to mean he would win, the King collects his army and crosses the border river,
only to suffer a crushing defeat and see his Kingdom conquered. When the victorious King
Cyrus allows Croisus to send an embassy to upbraid the Oracle, the God Apollon answers
through his Prophetess that he has correctly predicted the destruction of a great empire— Croisus’ own — and that he cannot be held responsible if people choose to interpretthe Oracle answers to their own liking.
8The history of the Pythia program is neither as long nor as dignified as that ofits eponym. However, some points of contact exist. You must be very careful when
you formulate the questions: any ambiguities will corrupt the reply you get. And you
must be even more careful not to misinterpret the answers; in particular not to pick theinterpretation that suits you before considering the alternatives. Finally, even a perfect
God has servants that are only human: a priest might mishear the screams of the Pythia
and therefore produce an erroneous oracle reply; the current authors might unwittingly
let a bug free in the program Pythia.

9

2

Physics Overview

In this section we will try to give an overview of the main physics features of Pythia, andalso to introduce some terminology. The details will be discussed in subsequent sections.
For the description of a typical high-energy event, an event generator should containa simulation of several physics aspects. If we try to follow the evolution of an event insome semblance of a time order, one may arrange these aspects as follows:
1. Initially two beam particles are coming in towards each other. Normally each particle is characterized by a set of parton distributions, which defines the partonic
substructure in terms of flavour composition and energy sharing.
2. One shower initiator parton from each beam starts off a sequence of branchings,
such as q → qg, which build up an initial-state shower.
3. One incoming parton from each of the two showers enters the hard process, wherethen a number of outgoing partons are produced, usually two. It is the nature ofthis process that determines the main characteristics of the event.
4. The hard process may produce a set of short-lived resonances, like the Z0 /W± gaugebosons, whose decay to normal partons has to be considered in close association withthe hard process itself.
5. The outgoing partons may branch, just like the incoming did, to build up final-state
showers.
6. In addition to the hard process considered above, further semihard interactions may
occur between the other partons of two incoming hadrons.
7. When a shower initiator is taken out of a beam particle, a beam remnant is left
behind. This remnant may have an internal structure, and a net colour charge that
relates it to the rest of the final state.
8. The QCD confinement mechanism ensures that the outgoing quarks and gluons are
not observable, but instead fragment to colour neutral hadrons.
9. Normally the fragmentation mechanism can be seen as occurring in a set of separate
colour singlet subsystems, but interconnection effects such as colour rearrangement
or Bose–Einstein may complicate the picture.
10. Many of the produced hadrons are unstable and decay further.
Conventionally, only quarks and gluons are counted as partons, while leptons andphotons are not. If pushed ad absurdum this may lead to some unwieldy terminology. We
will therefore, where it does not matter, speak of an electron or a photon in the ‘partonic’
substructure of an electron, lump branchings e → eγ together with other ‘parton shower’branchings such as q → qg, and so on. With this notation, the division into the above
ten points applies equally well to an interaction between two leptons, between a lepton
and a hadron, and between two hadrons.
In the following sections, we will survey the above ten aspects, not in the same orderas given here, but rather in the order in which they appear in the program execution, i.e.
starting with the hard process.

2.1

Hard Processes and Parton Distributions

In the original Jetset code, only two hard processes were available. The first and main
one is e+ e− → γ ∗ /Z0 → qq. Here the ‘∗’ of γ ∗ is used to denote that the photon must be
off the mass shell. The distinction is of some importance, since a photon on the mass shellcannot decay. Of course also the Z0 can be off the mass shell, but here the distinction isless relevant (strictly speaking, a Z0 is always off the mass shell). In the following we may
not always use ‘∗’ consistently, but the rule of thumb is to use a ‘∗’ only when a process is
not kinematically possible for a particle of nominal mass. The quark q in the final stateof e+ e− → γ ∗ /Z0 → qq may be u, d, s, c, b or t; the flavour in each event is picked at
10

random, according to the relative couplings, evaluated at the hadronic c.m. energy. Also
the angular distribution of the final qq pair is included. No parton-distribution functions
are needed.
The other original Jetset process is a routine to generate ggg and γgg final states,
as expected in onium 1−− decays such as Υ. Given the large top mass, toponium decays weakly much too fast for these processes to be of any interest, so therefore no new
applications are expected.
2.1.1

Hard Processes

The current Pythia contains a much richer selection, with around 300 different hard
processes. These may be classified in many different ways.
One is according to the number of final-state objects: we speak of ‘2 → 1’ processes,‘2 → 2’ ones, ‘2 → 3’ ones, etc. This aspect is very relevant from a programming pointof view: the more particles in the final state, the more complicated the phase space andtherefore the whole generation procedure. In fact, Pythia is optimized for 2 → 1 and
2 → 2 processes. There is currently no generic treatment of processes with three or moreparticles in the final state, but rather a few different machineries, each tailored to thepole structure of a specific class of graphs.
Another classification is according to the physics scenario. This will be the main themeof section 8. The following major groups may be distinguished:• Hard QCD processes, e.g. qg → qg.
• Soft QCD processes, such as diffractive and elastic scattering, and minimum-bias
events. Hidden in this class is also process 96, which is used internally for themerging of soft and hard physics, and for the generation of multiple interactions.
• Heavy-flavour production, both open and hidden, e.g. gg → tt and gg → J/ψg.
• Prompt-photon production, e.g. qg → qγ.
• Photon-induced processes, e.g. γg → qq.
• Deeply Inelastic Scattering, e.g. q` → q`.
• W/Z production, such as the e+ e− → γ ∗ /Z0 or qq → W+ W− .
• Standard Model Higgs production, where the Higgs is reasonably light and narrow,and can therefore still be considered as a resonance.
• Gauge boson scattering processes, such as WW → WW, when the Standard Model
Higgs is so heavy and broad that resonant and non-resonant contributions have to
be considered together.
• Non-standard Higgs particle production, within the framework of a two-Higgsdoublet scenario with three neutral (h0 , H0 and A0 ) and two charged (H± ) Higgs
states. Normally associated with SUSY (see below), but does not have to be.
• Production of new gauge bosons, such as a Z0 , W0 and R (a horizontal boson,
coupling between generations).
• Technicolor production, as an alternative scenario to the standard picture of electroweak symmetry breaking by a fundamental Higgs.
• Compositeness is a possibility not only in the Higgs sector, but may also apply tofermions, e.g. giving d∗ and u∗ production. At energies below the threshold for newparticle production, contact interactions may still modify the standard behaviour.
• Left–right symmetric models give rise to doubly charged Higgs states, in fact one
set belonging to the left and one to the right SU(2) gauge group. Decays involve
right-handed W’s and neutrinos.
• Leptoquark (LQ ) production is encountered in some beyond-the-Standard-Model
scenarios.
• Supersymmetry (SUSY) is probably the favourite scenario for physics beyond theStandard Model. A rich set of processes are allowed, already if one obeys R-parity
11

conservation, and even more so if one does not. The main supersymmetric machinery and process selection is inherited from SPythia [Mre97], however with many
improvements in the event generation chain. Many different SUSY scenarios havebeen proposed, and the program is flexible enough to allow input from several ofthese, in addition to the ones provided internally.
• The possibility of extra dimensions at low energies has been a topic of much study inrecent years, but has still not settled down to some standard scenarios. Its inclusion
into Pythia is also only in a very first stage.
This is by no means a survey of all interesting physics. Also, within the scenarios studied,not all contributing graphs have always been included, but only the more important
and/or more interesting ones. In many cases, various approximations are involved in thematrix elements coded.
2.1.2

Resonance Decays

As we noted above, the bulk of the processes above are of the 2 → 2 kind, with very
few leading to the production of more than two final-state particles. This may be seen
as a major limitation, and indeed is so at times. However, often one can come quite far
with only one or two particles in the final state, since showers will add the required extra
activity. The classification may also be misleading at times, since an s-channel resonanceis considered as a single particle, even if it is assumed always to decay into two final-stateparticles. Thus the process e+ e− → W+ W− → q1 q01 q2 q02 is classified as 2 → 2, although
the decay treatment of the W pair includes the full 2 → 4 matrix elements (in the doubly
resonant approximation, i.e. excluding interference with non-WW four-fermion graphs).
Particles which admit this close connection between the hard process and the subsequent evolution are collectively called resonances in this manual. It includes all particles
in mass above the b quark system, such as t, Z0 , W± , h0 , supersymmetric particles, andmany more. Typically their decays are given by electroweak physics, or physics beyond
the Standard Model. What characterizes a (Pythia) resonance is that partial widths
and branching ratios can be calculated dynamically, as a function of the actual massof a particle. Therefore not only do branching ratios change between an h0 of nominal
mass 100 GeV and one of 200 GeV, but also for a Higgs of nominal mass 200 GeV, thebranching ratios would change between an actual mass of 190 GeV and 210 GeV, say.
This is particularly relevant for reasonably broad resonances, and in threshold regions.
For an approach like this to work, it is clearly necessary to have perturbative expressions
available for all partial widths.
Decay chains can become quite lengthy, e.g. for supersymmetric processes, but follow
a straight perturbative pattern. If the simulation is restricted to only some set of decays,
the corresponding cross section reduction can easily be calculated. (Except in some rarecases where a nontrivial threshold behaviour could complicate matters.) It is therefore
standard in Pythia to quote cross sections with such reductions already included. Notethat the branching ratios of a particle is affected also by restrictions made in the secondaryor subsequent decays. For instance, the branching ratio of h0 → W+ W− , relative to
h0 → Z0 Z0 and other channels, is changed if the allowed W decays are restricted.
The decay products of resonances are typically quarks, leptons, or other resonances,e.g. W → qq0 or h0 → W+ W− . Ordinary hadrons are not produced in these decays,
but only in subsequent hadronization steps. In decays to quarks, parton showers areautomatically added to give a more realistic multijet structure, and one may also allow
photon emission off leptons. If the decay products in turn are resonances, further decays
are necessary. Often spin information is available in resonance decay matrix elements.
This means that the angular orientations in the two decays of a W+ W− pair are properly
correlated. In other cases, the information is not available, and then resonances decay
isotropically.
12

Of course, the above ‘resonance’ terminology is arbitrary. A ρ, for instance, could
also be called a resonance, but not in the above sense. The width is not perturbatively
calculable, it decays to hadrons by strong interactions, and so on. From a practical pointof view, the main dividing line is that the values of — or a change in — branchingratios cannot affect the cross section of a process. For instance, if one wanted to consider
the decay Z0 → cc, with a D meson producing a lepton, not only would there thenbe the problem of different leptonic branching ratios for different D’s (which means thatfragmentation and decay treatments would no longer decouple), but also that of additional
cc pair production in parton-shower evolution, at a rate that is unknown beforehand. In
practice, it is therefore next to impossible to force D decay modes in a consistent manner.
2.1.3

Parton Distributions

The cross section for a process ij → k is given byZ

σij→k =

Z

dx1

dx2 fi1 (x1 ) fj2 (x2 ) σ̂ij→k .

(1)Here σ̂ is the cross section for the hard partonic process, as codified in the matrix elements
for each specific process. For processes with many particles in the final state it would
be replaced by an integral over the allowed final-state phase space. The fia (x) are theparton-distribution functions, which describe the probability to find a parton i insidebeam particle a, with parton i carrying a fraction x of the total a momentum. Actually,
parton distributions also depend on some momentum scale Q2 that characterizes the hard
process.
Parton distributions are most familiar for hadrons, such as the proton, which are
inherently composite objects, made up of quarks and gluons. Since we do not understand
QCD, a derivation from first principles of hadron parton distributions does not yet exist,
although some progress is being made in lattice QCD studies. It is therefore necessary
to rely on parameterizations, where experimental data are used in conjunction with theevolution equations for the Q2 dependence, to pin down the parton distributions. Several
different groups have therefore produced their own fits, based on slightly different sets ofdata, and with some variation in the theoretical assumptions.
Also for fundamental particles, such as the electron, is it convenient to introduce parton
distributions. The function fee (x) thus parameterizes the probability that the electron thattakes part in the hard process retains a fraction x of the original energy, the rest being
radiated (into photons) in the initial state. Of course, such radiation could equally well bemade part of the hard interaction, but the parton-distribution approach usually is much
more convenient. If need be, a description with fundamental electrons is recovered forthe choice fee (x, Q2 ) = δ(x − 1). Note that, contrary to the proton case, electron parton
distributions are calculable from first principles, and reduce to the δ function above forQ2 → 0.
The electron may also contain photons, and the photon may in its turn contain quarks
and gluons. The internal structure of the photon is a bit of a problem, since the photon
contains a point-like part, which is perturbatively calculable, and a resolved part (withfurther subdivisions), which is not. Normally, the photon parton distributions are therefore parameterized, just as the hadron ones. Since the electron ultimately contains quarks
and gluons, hard QCD processes like qg → qg therefore not only appear in pp collisions,but also in ep ones (‘resolved photoproduction’) and in e+ e− ones (‘doubly resolved 2γ
events’). The parton distribution function approach here makes it much easier to reuse
one and the same hard process in different contexts.
There is also another kind of possible generalization. The two processes qq → γ ∗ /Z0 ,studied in hadron colliders, and e+ e− → γ ∗ /Z0 , studied in e+ e− colliders, are really special
cases of a common process, ff → γ ∗ /Z0 , where f denotes a fundamental fermion, i.e. a
13quark, lepton or neutrino. The whole structure is therefore only coded once, and thenslightly different couplings and colour prefactors are used, depending on the initial state
considered. Usually the interesting cross section is a sum over several different initialstates, e.g. uu → γ ∗ /Z0 and dd → γ ∗ /Z0 in a hadron collider. This kind of summation isalways implicitly done, even when not explicitly mentioned in the text.
A final comment on parton distributions is that, in general, the composite structureof hadrons allow for multiple parton–parton scatterings to occur, in which case correllatedparton distributions should be used to describe the multi-parton structure of the incoming
beams. This will be discussed in section 2.3.

2.2

Initial- and Final-State Radiation

In every process that contains coloured and/or charged objects in the initial or final state,gluon and/or photon radiation may give large corrections to the overall topology of events.
Starting from a basic 2 → 2 process, this kind of corrections will generate 2 → 3, 2 → 4,
and so on, final-state topologies. As the available energies are increased, hard emissionof this kind is increasingly important, relative to fragmentation, in determining the event
structure.
Two traditional approaches exist to the modelling of perturbative corrections. One is
the matrix-element method, in which Feynman diagrams are calculated, order by order.
In principle, this is the correct approach, which takes into account exact kinematics,
and the full interference and helicity structure. The only problem is that calculations
become increasingly difficult in higher orders, in particular for the loop graphs. Only in
exceptional cases have therefore more than one loop been calculated in full, and often
we do not have any loop corrections at all at our disposal. On the other hand, we haveindirect but strong evidence that, in fact, the emission of multiple soft gluons plays asignificant rôle in building up the event structure, e.g. at LEP, and this sets a limit to
the applicability of matrix elements. Since the phase space available for gluon emission
increases with the available energy, the matrix-element approach becomes less relevant
for the full structure of events at higher energies. However, the perturbative expansion
is better behaved at higher energy scales, owing to the running of αs . As a consequence,
inclusive measurements, e.g. of the rate of well-separated jets, should yield more reliable
results at high energies.
The second possible approach is the parton-shower one. Here an arbitrary number ofbranchings of one parton into two (or more) may be combined, to yield a description ofmultijet events, with no explicit upper limit on the number of partons involved. This ispossible since the full matrix-element expressions are not used, but only approximations
derived by simplifying the kinematics, and the interference and helicity structure. Parton
showers are therefore expected to give a good description of the substructure of jets, but inprinciple the shower approach has limited predictive power for the rate of well-separated
jets (i.e. the 2/3/4/5-jet composition). In practice, shower programs may be matched to
first-order matrix elements to describe the hard-gluon emission region reasonably well, inparticular for the e+ e− annihilation process. Nevertheless, the shower description is not
optimal for absolute αs determinations.
Thus the two approaches are complementary in many respects, and both have founduse. Because of its simplicity and flexibility, the parton-shower option is often the first
choice, while the full higher-order matrix elements one (i.e. including loops) is mainly
used for αs determinations, angular distribution of jets, triple-gluon vertex studies, andother specialized studies. With improved calculational techniques and faster computers,
Born-level calculations have been pushed to higher orders, and have seen increasing use.
Obviously, the ultimate goal would be to have an approach where the best aspects of thetwo worlds are harmoniously married. This is currently a topic of quite some study, withseveral new approaches having emerged over the last few years.
142.2.1

Matrix elements

Matrix elements are especially made use of in the older Jetset-originated implementationof the process e+ e− → γ ∗ /Z0 → qq.
For initial-state QED radiation, a first-order (un-exponentiated) description has been
adopted. This means that events are subdivided into two classes, those where a photon
is radiated above some minimum energy, and those without such a photon. In the latterclass, the soft and virtual corrections have been lumped together to give a total event ratethat is correct up to one loop. This approach worked fine at PETRA/PEP energies, butdoes not do so well for the Z0 line shape, i.e. in regions where the cross section is rapidly
varying and high precision is strived for.
For final-state QCD radiation, several options are available. The default is the partonshower one (see below), but some matrix-elements options also exist. In the definition of
3- or 4-jet events, a cut is introduced whereby it is required that any two partons havean invariant mass bigger than some fraction of the c.m. energy. 3-jet events which do
not fulfil this requirement are lumped with the 2-jet ones. The first-order matrix-element
option, which only contains 3- and 2-jet events therefore involves no ambiguities. In
second order, where also 4-jets have to be considered, a main issue is what to do with
4-jet events that fail the cuts. Depending on the choice of recombination scheme, whereby
the two nearby partons are joined into one, different 3-jet events are produced. Therefore
the second-order differential 3-jet rate has been the subject of some controversy, and theprogram actually contains two different implementations.
By contrast, the normal Pythia event generation machinery does not contain any fullhigher-order matrix elements, with loop contributions included. There are several cases
where higher-order matrix elements are included at the Born level. Consider the case ofresonance production at a hadron collider, e.g. of a W, which is contained in the lowestorder process qq0 → W. In an inclusive description, additional jets recoiling against the W
may be generated by parton showers. Pythia also contains the two first-order processes
qg → Wq0 and qq0 → Wg. The cross sections for these processes are divergent when thep⊥ → 0. In this region a correct treatment would therefore have to take into account loop
corrections, which are not available in Pythia.
Even without having these accessible, we know approximately what the outcome
should be. The virtual corrections have to cancel the p⊥ → 0 singularities of the realemission. The total cross section of W production therefore receives finite O(αs ) corrections to the lowest-order answer. These corrections can often be neglected to first
approximation, except when high precision is required. As for the shape of the W p⊥
spectrum, the large cross section for low-p⊥ emission has to be interpreted as allowing
more than one emission to take place. A resummation procedure is therefore necessary
to have matrix element make sense at small p⊥ . The outcome is a cross section below thenaı̈ve one, with a finite behaviour in the p⊥ → 0 limit.
Depending on the physics application, one could then use Pythia in one of two
ways. In an inclusive description, which is dominated by the region of reasonably smallp⊥ , the preferred option is lowest-order matrix elements combined with parton showers,
which actually is one way of achieving the required resummation. For W production as
background to some other process, say, only the large-p⊥ tail might be of interest. Thenthe shower approach may be inefficient, since only few events will end up in the interestingregion, while the matrix-element alternative allows reasonable cuts to be inserted fromthe beginning of the generation procedure. (One would probably still want to add showers
to describe additional softer radiation, at the cost of some smearing of the original cuts.)
Furthermore, and not less importantly, the matrix elements should give a more precise
prediction of the high-p⊥ event rate than the approximate shower procedure.
In the particular case considered here, that of W production, and a few similar processes, actually the shower has been improved by a matching to first-order matrix ele-15

ments, thus giving a decent description over the whole p⊥ range. This does not provide
the first-order corrections to the total W production rate, however, nor the possibility toselect only a high-p⊥ tail of events.
2.2.2

Parton showers

The separation of radiation into initial- and final-state showers is arbitrary, but very
convenient. There are also situations where it is appropriate: for instance, the processe+ e− → Z0 → qq only contains final-state QCD radiation (QED radiation, however, ispossible both in the initial and final state), while qq → Z0 → e+ e− only contains initialstate QCD one. Similarly, the distinction of emission as coming either from the q or fromthe q is arbitrary. In general, the assignment of radiation to a given mother parton is agood approximation for an emission close to the direction of motion of that parton, but
not for the wide-angle emission in between two jets, where interference terms are expected
to be important.
In both initial- and final-state showers, the structure is given in terms of branchingsa → bc, specifically e → eγ, q → qg, q → qγ, g → gg, and g → qq. (Further branchings,
like γ → e+ e− and γ → qq, could also have been added, but have not yet been of interest.)
Each of these processes is characterized
by a splitting kernel Pa→bc (z). The branching rateR
is proportional to the integral Pa→bc (z) dz. The z value picked for a branching describes
the energy sharing, with daughter b taking a fraction z and daughter c the remaining 1−zof the mother energy. Once formed, the daughters b and c may in turn branch, and so on.
Each parton is characterized by some virtuality scale Q2 , which gives an approximate
sense of time ordering to the cascade. We stress here that somewhat different definitionof Q2 are possible, and that Pythia actually implements two distinct alternatives, as
you will see. In the initial-state shower, Q2 values are gradually increasing as the hard
scattering is approached, while Q2 is decreasing in the final-state showers. Shower evolution is cut off at some lower scale Q0 , typically around 1 GeV for QCD branchings. From
above, a maximum scale Qmax is introduced, where the showers are matched to the hardinteraction itself. The relation between Qmax and the kinematics of the hard scattering is
uncertain, and the choice made can strongly affect the amount of well-separated jets.
Despite a number of common traits, the initial- and final-state radiation machineries
are in fact quite different, and are described separately below.
Final-state showers are time-like, i.e. partons have m2 = E 2 − p2 ≥ 0. The evolution
variable Q2 of the cascade has therefore traditionally in Pythia been associated with them2 of the branching parton. As discussed above, this choice is not unique, and in more
recent versions of Pythia, a p⊥ -ordered shower algorithm, with Q2 = p2⊥ = z(1 − z)m2 ,is available in addition to the mass-ordered one. Regardless of the exact definition of
the ordering variable, the general strategy is the same: starting from some maximum
scale Q2max , an original parton is evolved downwards in Q2 until a branching occurs. The
selected Q2 value defines the mass of the branching parton, or the p⊥ of the branching,depending on whether the mass-ordering or the p⊥ -ordering is used. In both cases, thez value obtained from the splitting kernel represents the parton energy division between
the daughters. These daughters may now, in turn, evolve downwards, in this case withmaximum virtuality already defined by the previous branching, and so on down to theQ0 cut-off.
In QCD showers, corrections to the leading-log picture, so-called coherence effects,
lead to an ordering of subsequent emissions in terms of decreasing angles. For the massordering constraint, this does not follow automatically, but is implemented as an additional requirement on allowed emissions. The p⊥ -ordered shower leads to the correct
behaviour without such modifications [Gus86]. Photon emission is not affected by angularordering. It is also possible to obtain non-trivial correlations between azimuthal angles inthe various branchings, some of which are implemented as options. Finally, the theoretical
16

analysis strongly suggests the scale choice αs = αs (p2⊥ ) = αs (z(1 − z)m2 ), and this is thedefault in the program, for both shower algorithms.
The final-state radiation machinery is normally applied in the c.m. frame of the hard
scattering or a decaying resonance. The total energy and momentum of that subsystem ispreserved, as is the direction of the outgoing partons (in their common rest frame), whereapplicable.
In contrast to final-state showers, initial-state ones are space-like. This means that,in the sequence of branchings a → bc that lead up from the shower initiator to the hardinteraction, particles a and b have m2 = E 2 − p2 < 0. The ‘side branch’ particle c, which
does not participate in the hard scattering, may be on the mass shell, or have a time-likevirtuality. In the latter case a time-like shower will evolve off it, rather like the finalstate radiation described above. To first approximation, the evolution of the space-likemain branch is characterized by the evolution variable Q2 = −m2 , which is required to
be strictly increasing along the shower, i.e. Q2b > Q2a . Corrections to this picture havebeen calculated, but are basically absent in Pythia. Again, in more recent versions ofPythia, a p⊥ -ordered ISR algorithm is also available, with Q2 = p2⊥ = −(1 − z)m2 .
Initial-state radiation is handled within the backwards evolution scheme. In this approach, the choice of the hard scattering is based on the use of evolved parton distributions,which means that the inclusive effects of initial-state radiation are already included. Whatremains is therefore to construct the exclusive showers. This is done starting from thetwo incoming partons at the hard interaction, tracing the showers ‘backwards in time’,back to the two shower initiators. In other words, given a parton b, one tries to find theparton a that branched into b. The evolution in the Monte Carlo is therefore in terms ofa sequence of decreasing Q2 (space-like virtuality or transverse momentum, as applicable)
and increasing momentum fractions x. Branchings on the two sides are interleaved in acommon sequence of decreasing Q2 values.
In the above formalism, there is no real distinction between gluon and photon emission.
Some of the details actually do differ, as will be explained in the full description.
The initial- and final-state radiation shifts around the kinematics of the original hardinteraction. In Deeply Inelastic Scattering, this means that the x and Q2 values that canbe derived from the momentum of the scattered lepton do not automatically agree withthe values originally picked. In high-p⊥ processes, it means that one no longer has twojets with opposite and compensating p⊥ , but more complicated topologies. Effects of any
original kinematics selection cuts are therefore smeared out, an unfortunate side-effect ofthe parton-shower approach.

2.3

Beam Remnants and Multiple Interactions

To begin with, consider a hadron–hadron collision where only a single parton–parton
interaction occurs, i.e. we ignore the possibility of multiple interactions for the moment.
In that case, the initial-state radiation algorithm reconstructs one shower initiator ineach beam. This initiator only takes some fraction of the total beam energy, leaving
behind a beam remnant which takes the rest. For a proton beam, a u quark initiatorwould leave behind a ud diquark beam remnant, with an antitriplet colour charge. The
remnant is therefore colour-connected to the hard interaction, and forms part of the samefragmenting system. It is further customary to assign a primordial transverse momentum
to the shower initiator, to take into account the motion of quarks inside the original
hadron, at least as required by the uncertainty principle by the proton size, probably
augmented by unresolved (i.e. not simulated) soft shower activity. This primordial k⊥ isselected according to some suitable distribution, and the recoil is assumed to be taken up
by the beam remnant.
Often the remnant is more complicated, e.g. a gluon initiator would leave behind a uud
proton remnant system in a colour octet state, which can conveniently be subdivided into17

a colour triplet quark and a colour antitriplet diquark, each of which are colour-connectedto the hard interaction. The energy sharing between these two remnant objects, and theirrelative transverse momentum, introduces additional degrees of freedom, which are notunderstood from first principles.
Naı̈vely, one would expect an ep event to have only one beam remnant, and an e+ e−
event none. This is not always correct, e.g. a γγ → qq interaction in an e+ e− event would
leave behind the e+ and e− as beam remnants, and a qq → gg interaction in resolved
photoproduction in an e+ e− event would leave behind one e± and one q or q in eachremnant. Corresponding complications occur for photoproduction in ep events.
There is another source of beam remnants. If parton distributions are used to resolve
an electron inside an electron, some of the original energy is not used in the hard interaction, but is rather associated with initial-state photon radiation. The initial-state
shower is in principle intended to trace this evolution and reconstruct the original electron before any radiation at all took place. However, because of cut-off procedures, somesmall amount may be left unaccounted for. Alternatively, you may have chosen to switch
off initial-state radiation altogether, but still preserved the resolved electron parton distributions. In either case the remaining energy is given to a single photon of vanishingtransverse momentum, which is then considered in the same spirit as ‘true’ beam remnants.
So far we have assumed that each event only contains one hard interaction, i.e. thateach incoming particle has only one parton which takes part in hard processes, and that
all other constituents sail through unaffected. This is appropriate in e+ e− or ep events,
but not necessarily so in hadron–hadron collisions (where a resolved photon counts as a
hadron). Here each of the beam particles contains a multitude of partons, and so theprobability for several interactions in one and the same event need not be negligible.
In principle these additional interactions could arise because one single parton from one
beam scatters against several different partons from the other beam, or because several
partons from each beam take part in separate 2 → 2 scatterings. Both are expected, but
combinatorics should favour the latter, which is the mechanism considered in Pythia.
The dominant 2 → 2 QCD cross sections are divergent for p⊥ → 0, and drop rapidly
for larger p⊥ . Probably the lowest-order perturbative cross sections will be regularized
at small p⊥ by colour coherence effects: an exchanged gluon of small p⊥ has a largetransverse wave function and can therefore not resolve the individual colour charges of
the two incoming hadrons; it will only couple to an average colour charge that vanishes
in the limit p⊥ → 0. In the program, some effective p⊥min scale is therefore introduced,
below which the perturbative cross section is either assumed completely vanishing or atleast strongly damped. Phenomenologically, p⊥min comes out to be a number of the orderof 1.5–2.5 GeV, with some energy dependence.
In a typical ‘minimum-bias’ event one therefore expects to find one or a few scatterings
at scales around or a bit above p⊥min , while a high-p⊥ event also may have additional
scatterings at the p⊥min scale. The probability to have several high-p⊥ scatterings in thesame event is small, since the cross section drops so rapidly with p⊥ .
The understanding of multiple interaction is still very primitive. Pythia therefore
contains several different options. These differ e.g. on the issue of the ‘pedestal’ effect: isthere an increased probability or not for additional interactions in an event which is known
to contain a hard scattering, compared with one that contains no hard interactions? Other
differences concern the level of detail in the generation of scatterings after the first one,and the model that describes how the scatterings are intercorrelated in flavour, colour,and momentum space.
The default underlying-event scenario obtained in a call to PYEVNT corresponds to theso-called ’Tune A’ [Fie02] (although with a slightly different energy dependence), which
reproduces many aspects of Tevatron data correctly. Starting from Pythia version 6.3,
a more advanced model for the underlying event is also available. This model is obtained18

by calling PYEVNW instead of PYEVNT, but one should then not forget to also change therelevant parameter settings to an appropriate tune of the new model.

2.4

Hadronization

QCD perturbation theory, formulated in terms of quarks and gluons, is valid at shortdistances. At long distances, QCD becomes strongly interacting and perturbation theory
breaks down. In this confinement regime, the coloured partons are transformed intocolourless hadrons, a process called either hadronization or fragmentation. In this paper
we reserve the former term for the combination of fragmentation and the subsequent decayof unstable particles.
The fragmentation process has yet to be understood from first principles, starting fromthe QCD Lagrangian. This has left the way clear for the development of a number ofdifferent phenomenological models. Three main schools are usually distinguished, string
fragmentation (SF), independent fragmentation (IF) and cluster fragmentation (CF), butmany variants and hybrids exist. Being models, none of them can lay claims to being‘correct’, although some may be better founded than others. The best that can be aimed
for is internal consistency, a good representation of existing data, and a predictive powerfor properties not yet studied or results at higher energies.
2.4.1

String Fragmentation

The original Jetset program is intimately connected with string fragmentation, in theform of the time-honoured ‘Lund model’. This is the default for all Pythia applications,but independent fragmentation options also exist (although not actively maintained), forapplications where one wishes to study the importance of string effects.
All current models are of a probabilistic and iterative nature. This means that the
fragmentation process as a whole is described in terms of one or a few simple underlying
branchings, of the type jet → hadron + remainder-jet, string → hadron + remainderstring, and so on. At each branching, probabilistic rules are given for the production of
new flavours, and for the sharing of energy and momentum between the products.
To understand fragmentation models, it is useful to start with the simplest possible
system, a colour-singlet qq 2-jet event, as produced in e+ e− annihilation. Here lattice
QCD studies lend support to a linear confinement picture (in the absence of dynamicalquarks), i.e. the energy stored in the colour dipole field between a charge and an anticharge
increases linearly with the separation between the charges, if the short-distance Coulomb
term is neglected. This is quite different from the behaviour in QED, and is related tothe presence of a triple-gluon vertex in QCD. The details are not yet well understood,
however.
The assumption of linear confinement provides the starting point for the string model.
As the q and q partons move apart from their common production vertex, the physicalpicture is that of a colour flux tube (or maybe a colour vortex line) being stretched between
the q and the q. The transverse dimensions of the tube are of typical hadronic sizes,
roughly 1 fm. If the tube is assumed to be uniform along its length, this automaticallyleads to a confinement picture with a linearly rising potential. In order to obtain a Lorentz
covariant and causal description of the energy flow due to this linear confinement, the moststraightforward way is to use the dynamics of the massless relativistic string with notransverse degrees of freedom. The mathematical, one-dimensional string can be thoughtof as parameterizing the position of the axis of a cylindrically symmetric flux tube. From
hadron spectroscopy, the string constant, i.e. the amount of energy per unit length, isdeduced to be κ ≈ 1 GeV/fm. The expression ‘massless’ relativistic string is somewhat
of a misnomer: κ effectively corresponds to a ‘mass density’ along the string.

19Let us now turn to the fragmentation process. As the q and q move apart, the potential
energy stored in the string increases, and the string may break by the production of a
new q0 q0 pair, so that the system splits into two colour-singlet systems qq0 and q0 q. If theinvariant mass of either of these string pieces is large enough, further breaks may occur.
In the Lund string model, the string break-up process is assumed to proceed until only
on-mass-shell hadrons remain, each hadron corresponding to a small piece of string witha quark in one end and an antiquark in the other.
In order to generate the quark–antiquark pairs q0 q0 which lead to string break-ups, theLund model invokes the idea of quantum mechanical tunnelling. This leads to a flavourindependent Gaussian spectrum for the p⊥ of q0 q0 pairs. Since the string is assumed tohave no transverse excitations, this p⊥ is locally compensated between the quark and theantiquark of the pair. The total p⊥ of a hadron is made up out of the p⊥ contributions
from the quark and antiquark that together form the hadron. Some contribution of very
soft perturbative gluon emission may also effectively be included in this description.
The tunnelling picture also implies a suppression of heavy-quark production, u : d : s :c ≈ 1 : 1 : 0.3 : 10−11 . Charm and heavier quarks hence are not expected to be producedin the soft fragmentation, but only in perturbative parton-shower branchings g → qq.
When the quark and antiquark from two adjacent string breaks are combined to forma meson, it is necessary to invoke an algorithm to choose between the different allowed
possibilities, notably between pseudoscalar and vector mesons. Here the string model
is not particularly predictive. Qualitatively one expects a 1 : 3 ratio, from counting
the number of spin states, multiplied by some wave-function normalization factor, whichshould disfavour heavier states.
A tunnelling mechanism can also be used to explain the production of baryons. Thisis still a poorly understood area. In the simplest possible approach, a diquark in acolour antitriplet state is just treated like an ordinary antiquark, such that a string canbreak either by quark–antiquark or antidiquark–diquark pair production. A more complex
scenario is the ‘popcorn’ one, where diquarks as such do not exist, but rather quark–
antiquark pairs are produced one after the other. This latter picture gives a less strong
correlation in flavour and momentum space between the baryon and the antibaryon of apair.
In general, the different string breaks are causally disconnected. This means that it ispossible to describe the breaks in any convenient order, e.g. from the quark end inwards.
One therefore is led to write down an iterative scheme for the fragmentation, as follows.
Assume an initial quark q moving out along the +z axis, with the antiquark going out
in the opposite direction. By the production of a q1 q1 pair, a meson with flavour content
qq1 is produced, leaving behind an unpaired quark q1 . A second pair q2 q2 may now be
produced, to give a new meson with flavours q1 q2 , etc. At each step the produced hadrontakes some fraction of the available energy and momentum. This process may be iterated
until all energy is used up, with some modifications close to the q end of the string inorder to make total energy and momentum come out right.
The choice of starting the fragmentation from the quark end is arbitrary, however.
A fragmentation process described in terms of starting at the q end of the system andfragmenting towards the q end should be equivalent. This ‘left–right’ symmetry constrains
the allowed shape of the fragmentation function f (z), where z is the fraction of theremaining light-cone momentum E ± pz (+ for the q jet, − for the q one) taken byeach new particle. The resulting ‘Lund symmetric fragmentation function’ has two freeparameters, which are determined from data.
If several partons are moving apart from a common origin, the details of the stringdrawing become more complicated. For a qqg event, a string is stretched from the q
end via the g to the q end, i.e. the gluon is a kink on the string, carrying energy andmomentum. As a consequence, the gluon has two string pieces attached, and the ratio ofgluon to quark string force is 2, a number which can be compared with the ratio of colour
20charge Casimir operators, NC /CF = 2/(1 − 1/NC2 ) = 9/4. In this, as in other respects,
the string model can be viewed as a variant of QCD where the number of colours NC
is not 3 but infinite. Note that the factor 2 above does not depend on the kinematicalconfiguration: a smaller opening angle between two partons corresponds to a smaller
string length drawn out per unit time, but also to an increased transverse velocity of thestring piece, which gives an exactly compensating boost factor in the energy density perunit string length.
The qqg string will fragment along its length. To first approximation this meansthat there is one fragmenting string piece between q and g and a second one between gand q. One hadron is straddling both string pieces, i.e. sitting around the gluon corner.
The rest of the particles are produced as in two simple qq strings, but strings boosted
with respect to the overall c.m. frame. When considered in detail, the string motion
and fragmentation is more complicated, with the appearance of additional string regions
during the time evolution of the system. These corrections are especially important forsoft and collinear gluons, since they provide a smooth transition between events wheresuch radiation took place and events where it did not. Therefore the string fragmentation
scheme is ‘infrared safe’ with respect to soft or collinear gluon emission.
Another possible colour topology arises when considering baryon-number-violatingprocesses, or events where more than one valence quark has been knocked out of a beam
baryon (as can happen when multiple parton–parton interactions occur). In this case,there will be three (anti-)colour carriers connected antisymmetrically in colour, and ofwhich no two may naturally be considered to form a diquark system. The string topology
will thus not be of the simple qq type, but rather a ‘Y’ shaped topology is spanned
between the endpoints. The vertex of the ‘Y’ topology comes to be of special interestin the fragmentation, and will be referred to as a ‘string junction’. Each of the threestring pieces undergo a fragmentation process subject to exactly the same principles as
outlined above, only a baryon containing the junction will eventually be formed. The
picture is essentially that of three jets going out, with the junction baryon formed ‘in the
middle’, hence the junction baryon will tend to have a soft spectrum when the jets arewidely separated. Note that, in the limit that two of the endpoints of the ’Y’ come close
together, the diquark picture for beam remnants mentioned above is effectively recovered,with only minor differences remaining.
For events that involve many partons, there may be several possible topologies fortheir ordering along the string. An example would be a qqg1 g2 (the gluon indices are here
used to label two different gluon-momentum vectors), where the string can connect thepartons in either of the sequences q − g1 − g2 − q and q − g2 − g1 − q. The matrix elementsthat are calculable in perturbation theory contain interference terms between these two
possibilities, which means that the colour flow is not always well-defined. Fortunately, theinterference terms are down in magnitude by a factor 1/NC2 , where NC = 3 is the numberof colours, so approximate recipes can be found. In the leading log shower description,on the other hand, the rules for the colour flow are well-defined.
A final comment: in the argumentation for the importance of colour flows there isa tacit assumption that soft-gluon exchanges between partons will not normally messup the original colour assignment. Colour rearrangement models provide toy scenarios
wherein deviations from this rule could be studied. Of particular interest has been theprocess e+ e− → W+ W− → q1 q2 q3 q4 , where the original singlets q1 q2 and q3 q4 could
be rearranged to q1 q4 and q3 q2 . So far, there are no experimental evidence for dramaticeffects of this kind, but the more realistic models predict effects sufficiently small that thesehave not been ruled out. Another example of nontrivial effects is that of Bose–Einsteincorrelations between identical final-state particles, which reflect the true quantum natureof the hadronization process.

21

2.4.2

Decays

A large fraction of the particles produced by fragmentation are unstable and subsequently
decay into the observable stable (or almost stable) ones. It is therefore important to include all particles with their proper mass distributions and decay properties. Althoughinvolving little deep physics, this is less trivial than it may sound: while a lot of experimental information is available, there is also very much that is missing. For charmmesons, it is necessary to put together measured exclusive branching ratios with someinclusive multiplicity distributions to obtain a consistent and reasonably complete set of
decay channels, a rather delicate task. For bottom even less is known, and for some B
baryons only a rather simple phase-space type of generator has been used for hadronic
decays.
Normally it is assumed that decay products are distributed according to phase space,i.e. that there is no dynamics involved in their relative distribution. However, in manycases additional assumptions are necessary, e.g. for semileptonic decays of charm andbottom hadrons one needs to include the proper weak matrix elements. Particles may
also be produced polarized and impart a non-isotropic distribution to their decay products.
Many of these effects are not at all treated in the program. In fact, spin information is
not at all carried along, but has to be reconstructed explicitly when needed.
This normal decay treatment makes use of a set of tables where branching ratiosand decay modes are stored. It encompasses all hadrons made out of d, u, s, c and b
quarks, and also the leptons. The decay products are hadrons, leptons and photons.
Some bb states are sufficiently heavy that they are allowed to decay to partonic states,
like Υ → ggg, which subsequently fragment, but these are exceptions.
You may at will change the particle properties, decay channels or branching ratios of
the above particles. There is no censorship of what is allowed or not allowed, beyond
energy–momentum, spin, and (electrical and colour) charge conservation. There is also
no impact e.g. on the cross section of processes, since there is no way of knowing e.g. if therestriction to one specific decay of a particle is because that decay is of particular interest
to us, or because recent measurement have shown that this indeed is the only channel.
Furthermore, the number of particles produced of each species in the hadronization process
is not known beforehand, and so cannot be used to correctly bias the preceding steps ofthe generation chain. All of this contrasts with the class of ‘resonances’ described above,in section 2.1.2.

22

3

Program Overview

This section contains a diverse collection of information. The first part is an overview ofprevious Jetset and Pythia versions. The second gives instructions for installation of
the program and describes its philosophy: how it is constructed and how it is supposed
to be used. It also contains some information on how to read this manual. The third andfinal part contains several examples of pieces of code or short programs, to illustrate thegeneral style of program usage. This last part is mainly intended as an introduction forcompletely new users, and can be skipped by more experienced ones.
The combined Pythia package is completely self-contained. Interfaces to externallydefined subprocesses, parton-distribution function libraries, SUSY parameter calculators,
τ decay libraries, and a time routine are provided, however, plus a few other optional
interfaces.
Many programs written by other persons make use of Pythia, especially the string
fragmentation machinery. It is not the intention to give a complete list here. A majority ofthese programs are specific to given collaborations, and therefore not publicly distributed.
Below we give a list of a few public programs from the ‘Lund group’, which may have asomewhat wider application. None of them are supported by the Pythia author team,so any requests should be directed to the persons mentioned.
• Ariadne is a generator for dipole emission, written mainly by L. Lönnblad [Pet88].
• LDCMC is a related program for initial-state radiation according to the Linked
Dipole Chain model, also written mainly by L. Lönnblad [Kha99].
• Aroma is a generator for heavy-flavour processes in leptoproduction, written by
G. Ingelman, J. Rathsman and G. Schuler [Ing88].
• Fritiof is a generator for hadron–hadron, hadron–nucleus and nucleus–nucleus
collisions [Nil87].
• Lepto is a leptoproduction event generator, written mainly by G. Ingelman [Ing80].
It can generate parton configurations in Deeply Inelastic Scattering according to anumber of possibilities.
• PomPyt is a generator for pomeron interactions written by G. Ingelman and collaborators [Bru96].
One should also note that a version of Pythia has been modified to include the effectsof longitudinally polarized incoming protons. This is the work of St. Güllenstern et al.
[Gül93].

3.1

Update History

For the record, in Tables 1 and 2 we list the official main versions of Jetset and Pythia,
respectively, with some brief comments.
Versions before 6.1: All versions preceding Pythia 6.1 should now be consideredobsolete, and are no longer maintained. For stable applications, the earlier combination
Jetset 7.4 and Pythia 5.7 could still be used, however.
Changes in version 6.1: The move from Jetset 7.4 and Pythia 5.7 to Pythia 6.1
was a major one. For reasons of space, individual points are therefore not listed separately
below, but only the main ones. The Pythia web page contains complete update notes,
where all changes are documented by topic and subversion.
The main new features of Pythia 6.1, either present from the beginning or added
later on, include:• Pythia and Jetset have been merged.
• All real variables are declared in double precision.
23

Table 1: The main versions of Jetset, with their date of appearance, published manuals,
and main changes from previous versions.
No.
Date
1
Nov 78
2
May 793.1 Aug 79
3.2 Apr 80
3.3
4.1

Aug 80
Apr 81

4.2
4.3
5.1

Nov 81Mar 82
Jul 82
Apr 83

5.2

Nov 83

5.3
6.1
6.2
6.3
7.1

May 84
Jan 85
Oct 85
Oct 86
Feb 897.2

Nov 897.3
7.4

May 90Dec 93Publ.
[Sjö78][Sjö79]—[Sjö80]

Main new or improved features
single-quark jets
heavy-flavour jets
2-jets in e+ e− , preliminary 3-jets
3-jets in e+ e− with full matrix elements,
toponium → ggg decays—
softer fragmentation spectrum—
baryon production and diquark fragmentation,
fourth-generation quarks, larger jet systems—
low-p⊥ physics[Sjö82] 4-jets and QFD structure in e+ e− ,[Sjö83] event-analysis routines—
improved string fragmentation scheme, symmetricfragmentation, full 2nd order QCD for e+ e−—
momentum-conservation schemes for IF,
initial-state photon radiation in e+ e−—‘popcorn’ model for baryon production—common blocks restructured, parton showers[Sjö86] error detection[Sjö87] new parton-shower scheme—
new particle codes and common-block structure,
more mesons, improved decays, vertex information,
Abelian gluon model, Bose–Einstein effects—
interface to new standard common block,photon emission in showers[Sjö92d] expanded support for non-standard particles[Sjö94] updated particle data and defaults• The internal mapping of particle codes has changed.
• The supersymmetric process machinery of SPythia has been included and further
improved, with several new processes.
• Many new processes of beyond-the-Standard-Model physics, in areas such as technicolor and doubly-charged Higgs bosons.
• An expanded description of QCD processes in virtual-photon interactions, combinedwith a new machinery for the flux of virtual photons from leptons.
• Initial-state parton showers are matched to the next-to-leading order matrix elements for gauge boson production.
• Final-state parton showers are matched to a number of different first-order matrix
elements for gluon emission, including full mass dependence.
• The hadronization description of low-mass strings has been improved, with consequences especially for heavy-flavour production.
• An alternative baryon production model has been introduced.
• Colour rearrangement is included as a new option, and several alternative BoseEinstein descriptions are added.

24

Table 2: The main versions of Pythia, with their date of appearance, published manuals,
and main changes from previous versions.
No.
1

Date
Dec 82

2
3.1
3.2
3.3
3.4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
5.1
5.2
5.3

—
—
—
Feb 84
Sep 84
Dec 84
Jun 85
Aug 85
Nov 85
Jan 86
May 86
May 86
Jan 87
May 87
May 87
Jun 87
Oct 89

5.4

Jun 905.5
5.6

Jan 91
Sep 91

5.7
6.1

Dec 93
Mar 976.2
6.3

Aug 01
Aug 03

6.4

Mar 06

Publ.
[Ben84]Main new or improved features
synthesis of predecessors Compton, Highpt andKassandra[Ben84a] scale-breaking parton distributions[Ben85] more efficient kinematics selection
initial- and final-state parton showers, W and Z
multiple interactions
WW, WZ, ZZ and R processes
γW, γZ, γγ processes
H0 production, diffractive and elastic events
angular correlation in resonance pair decays
Z00 and H+ processes[Ben87] variable impact parameter in multiple interactions
gH+ process
massive matrix elements for heavy quarks
intermediate boson scattering
new particle and subprocess codes, new common-block
structure, new kinematics selection, some
lepton–hadron interactions, new subprocesses
s-dependent widths, resonances not on the mass shell,
new processes, new parton distributions
improved e+ e− and ep, several new processes[Sjö92d] reorganized parton distributions, new processes,
user-defined external processes[Sjö94] new total cross sections, photoproduction, top decay[Sjö01] merger with Jetset, double precision, supersymmetry,technicolor, extra dimensions, etc. new processes,
improved showers, virtual-photon processes[Sjö01a] Les Houches Accord user processes, R-parity violation[Sjö03a] improved multiple interactions, p⊥ –ordered showers,
SUSY Les Houches Accord interface
thisnone so far

Changes in version 6.2: By comparison, the move from Pythia 6.1 to Pythia 6.2
was rather less dramatic. Again update notes tell the full story. Some of the main
new features, present from the beginning or added later on, which may affect backwards
compatibility, are:
• A new machinery to handle user-defined external processes, according to the Les
Houches Accord standard in [Boo01]. The old machinery is no longer available.
Some of the alternatives for the FRAME argument in the PYINIT call have also beenrenamed to make way for a new ’USER’ option.
• The maximum size of the decay channel table has been increased from 4000 to 8000,
affecting the MDME, BRAT and KFDP arrays in the PYDAT3 common block.
• A number of internally used and passed arrays, such as WDTP, WDTE, WDTPP, WDTEP,
25

WDTPM, WDTEM, XLAM and IDLAM, have been expanded from dimension 300 to 400.
• Lepton- and baryon-number-violating decay channels have been included for supersymmetric particles [Ska01, Sjö03]. Thus the decay tables have grown considerably
longer.
• The string hadronization scheme has been improved and expanded better to handle
junction topologies, where three strings meet. This is relevant for baryon-numberviolating processes, and also for the handling of baryon beam remnants. Thus newroutines have been introduced, and also e.g. new K(I,1) status codes.
• A runtime interface to Isasusy has been added, for determining the SUSY mass
spectrum and mixing parameters more accurately than with the internal Pythia
routines.
• The Technicolor scenario is updated and extended. A new common block, PYTCSM,
is introduced for the parameters and switches in Technicolor and related scenarios,and variables are moved to it from a few other common blocks. New processes
381–388 are introduced for standard QCD 2 → 2 interactions with Technicolor (orother compositeness) extensions, while the processes 11, 12, 13, 28, 53, 68, 81 and
82 now revert back to being pure QCD.
• The PYSHOW time-like showering routine has been expanded to allow showering inside
systems consisting of up to 80 particles, which can be made use of in some resonancedecays and in user-defined processes.
• The PYSSPA space-like showering routine has been expanded with a q → qγ branching.
• The PYSIGH routine has been split into several, in order to make it more manageable. (It had increased to a size of over 7000 lines, which gave some compilers
problems.) All the phase-space and parton-density generic weights remain, whereas
the process-specific matrix elements have been grouped into new routines PYSGQC,
PYSGHF, PYSGWZ, PYSGHG, PYSGSU, PYSGTC and PYSGEX.
• Some exotic particles and QCD effective states have been moved from temporary
flavour codes to a PDG-consistent naming, and a few new codes have been introduced.
• The maximum number of documentation lines in the beginning of the event record
has been expanded from 50 to 100.
• The default parton distribution set for the proton is now CTEQ 5L.
• The default Standard Model Higgs mass has been changed to 115 GeV.
Changes in version 6.3: The major changes in 6.3 was the introduction of a newunderlying–event framework for hadron collisions, together with new transverse-momentum-ordered initial- and final-state parton showers. Other changes include an interface
to SUSY spectrum and decay calculators conformant to the SUSY Les Houches Accord.
Early releases of Pythia 6.3, up to 6.312, were still experimental, and did not include thenew parton showers. Details on these versions can be found in the update notes available
on the web. We here concentrate on the structure of the program after (and including)version 6.312.
Firstly, there are no incompatibilities between Pythia 6.2 and Pythia 6.3 at thelevel of commonblock sizes or subroutine arguments (except for the PYSUGI interface to
Isajet, see below). That is, any program that ran with Pythia 6.2 also ought to run
with Pythia 6.3, without any change required. Note, however, the following news:• The old (6.2) master event-generation routine PYEVNT is still available; it uses the oldunderlying–event machinery (PYMULT) and parton showers (PYSSPA and PYSHOW). A‘Tune A’–like set of parameters [Fie02] has been adopted as default. It reproduces
Tune A at the Tevatron, but implies a slower energy rescaling, to give a moreconservative estimate of the underlying activity at the LHC.
26

• A new master event-generation routine PYEVNW has been introduced, in parallel withPYEVNT. By calling PYEVNW instead of PYEVNT a completely new scenario for multiple
interactions [Sjö04] and parton showers [Sjö04a] is obtained. This framework has
still only been fully developed for hadron-hadron collisions (where a resolved photon
counts as a hadron). Note that many of the default parameters now in Pythia are
directly related to the PYEVNT model. These defaults are not appropriate to use with
the new framework, and so all parameters should be set explicitly whenever PYEVNW
is used. See e.g. examples available on the web.
• The current PYSUGI run-time interface to the Isasusy evolution package is based on
Isajet version 7.71 (from Pythia 6.319 onwards). The dimensions of the SSPAR,SUGMG, SUGPAS and SUGXIN common blocks found in the PYSUGI routine could haveto be modified if another Isajet version is linked.
• The new routine PYSLHA introduces a generic interface to SUSY spectrum and decay calculators in agreement with the SUSY Les Houches Accord [Ska03] (SLHA),
and thus offers a long-term solution to the incompatibility problems noted above.
Without SUSY switched on, PYSLHA can still be used stand-alone to read in SLHA
decay tables for any particle, see section 14.4.
• From version 6.321 a run-time interface to FeynHiggs [Hei99] is available, to correct
the Higgs sector of SUSY spectra obtained with either of the PYSUGI and PYSLHA
interfaces.
• From version 6.321 a new set of toy models for investigating final state colour reconnection effects are available, see [San05].
• From version 6.322 an option exists to extend the particle content recognized by
Pythia to that of the NMSSM, for use in the context of interfaces to other programs,see [Puk05].
• Some new processes added, notably J/ψ and Υ production in an NRQCD colouroctet-model framework.
The update notes tell exactly with what version a new feature or a bug fix is introduced.
Changes in version 6.4: No new features are introduced in Pythia 6.400 relative
to the preceding Pythia 6.327. The main reason for the new version number is tosynchronize with the documentation, i.e. with this updated manual. As changes are madeto this baseline version, they will be documented in the update notes.

3.2

Program Installation

The Pythia ‘master copy’ is the one found on the web page
http://www.thep.lu.se/∼torbjorn/Pythia.html
There you have, for several subversions xxx:
pythia6xxx.f
the Pythia 6.xxx code,pythia6xxx.tex
editions of this Pythia manual, andpythia6xxx.update plain text update notes to the manual.
In addition to these, one may also find sample main programs and other pieces of related
software, and some physics papers.
The program is written essentially entirely in standard Fortran 77, and should run on
any platform with such a compiler. To a first approximation, program compilation should
therefore be straightforward.

27

Unfortunately, experience with many different compilers has been uniform: the options available for obtaining optimized code may actually produce erroneous code (e.g.
operations inside DO loops are moved out before them, where some of the variables have
not yet been properly set). Therefore the general advice is to use a low optimization level.
Note that this is often not the default setting.
SAVE statements have been included in accordance with the Fortran standard.
All default settings and particle and process data are stored in BLOCK DATA PYDATA.
This subprogram must be linked for a proper functioning of the other routines. On some
platforms this is not done automatically but must be forced by you, e.g. by having a line
EXTERNAL PYDATA
at the beginning of your main program. This applies in particular if Pythia is maintained as a library from which routines are to be loaded only when they are needed. In
this connection we note that the library approach does not give any significant spaceadvantages over a loading of the packages as a whole, since a normal run will call on mostof the routines anyway, directly or indirectly.
With the move towards higher energies, e.g. for LHC applications, single-precision (32bit) real arithmetic has become inappropriate. Therefore a declaration IMPLICIT DOUBLEPRECISION(A-H,O-Z) at the beginning of each subprogram is inserted to ensure doubleprecision (64 bit) real arithmetic. Remember that this also means that all calls to Pythia
routines have to be done with real variables declared correspondingly in the user-written
calling program. An IMPLICIT INTEGER(I-N) is also included to avoid problems on somecompilers. Integer functions beginning with PY have to be declared explicitly. In total,
therefore all routines begin with
C...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
INTEGER PYK,PYCHGE,PYCOMP
and you are recommended to do the same in your main programs. Note that, in runningtext and in description of common-block default values, the more cumbersome doubleprecision notation is not always made explicit, but code examples should be correct.
On a machine where DOUBLE PRECISION would give 128 bits, it may make sense touse compiler options to revert to 64 bits, since the program is anyway not constructed tomake use of 128 bit precision.
Fortran 77 makes no provision for double-precision complex numbers. Therefore complex numbers have been used only sparingly. However, some matrix element expressions,mainly for supersymmetric and technicolor processes, simplify considerably when written
in terms of complex variables. In order to achieve a uniform precision, such variables
have been declared COMPLEX*16, and are manipulated with functions such as DCMPLX
and DCONJG. Affected are PYSIGH, PYWIDT and several of the supersymmetry routines.
Should the compiler not accept this deviation from the standard, or some simple equivalent thereof (like DOUBLE COMPLEX instead of COMPLEX*16) these code pieces could be
rewritten to ordinary COMPLEX, also converting the real numbers involved to and fromsingle precision, with some drop in accuracy for the affected processes. PYRESD alreadycontains some ordinary COMPLEX variables, and should not cause any problems.
Several compilers report problems when an odd number of integers precede a doubleprecision variable in a common block. Therefore an extra integer has been introduced as
padding in a few instances, e.g. NPAD, MSELPD and NGENPD.
Since Fortran 77 provides no date-and-time routine, PYTIME allows a system-specificroutine to be interfaced, with some commented-out examples given in the code. Thisroutine is only used for cosmetic improvements of the output, however, so can be left at
the default with time 0 given.
28

A test program, PYTEST, is included in the Pythia package. It is disguised as a
subroutine, so you have to run a main program
CALL PYTEST(1)ENDThis program will generate over a thousand events of different types, under a variety ofconditions. If Pythia has not been properly installed, this program is likely to crash, orat least generate a number of erroneous events. This will then clearly be marked in the
output, which otherwise will just contain a few sample event listings and a table of thenumber of different particles produced. To switch off the output of normal events andfinal table, use PYTEST(0) instead of PYTEST(1). The final tally of errors detected should
read 0.
For a program written to run Pythia 5 and Jetset 7, most of the conversion requiredfor Pythia 6 is fairly straightforward, and can be automatized. Both a simple Fortranroutine and a more sophisticated Perl [Gar98] script exist to this end, see the Pythia
web page. Some manual checks and interventions may still be required.

3.3

Program Philosophy

The Monte Carlo program is built as a slave system, i.e. you, the user, have to supply
the main program. From this the various subroutines are called on to execute specifictasks, after which control is returned to the main program. Some of these tasks may bevery trivial, whereas the ‘high-level’ routines by themselves may make a large number ofsubroutine calls. Many routines are not intended to be called directly by you, but only
from higher-level routines such as PYEXEC, PYEEVT, PYINIT, PYEVNT, or PYEVNW.
Basically, this means that there are three ways by which you communicate with theprograms. First, by setting common-block variables, you specify the details of how the
programs should perform specific tasks, e.g. which subprocesses should be generated,which particle masses should be assumed, which coupling constants used, which fragmentation scenarios, and so on with hundreds of options and parameters. Second, by calling
subroutines you tell the programs to generate events according to the rules establishedabove. Normally there are few subroutine arguments, and those are usually related todetails of the physical situation, such as what c.m. energy to assume for events. Third,
you can either look at the common block PYJETS to extract information on the generated
event, or you can call on various functions and subroutines to analyse the event further
for you.
It should be noted that, while the physics content is obviously at the centre of attention, the Pythia package also contains a very extensive setup of auxiliary service
routines. The hope is that this will provide a comfortable working environment, where
not only events are generated, but where you also linger on to perform a lot of the subsequent studies. Of course, for detailed studies, it may be necessary to interface the output
directly to a detector simulation program.
The general rule is that all routines have names that are six characters long, beginning with PY. Apart from dummy copies of routines from other libraries, there are three
exceptions the length rules: PYK, PYP and PYR. The former two functions are strongly
coupled to the K and P matrices in the PYJETS common block, while the latter is veryfrequently used. Also common-block names are six characters long and start with PY.
There are three integer functions, PYK, PYCHGE and PYCOMP. In all routines where they are
to be used, they have to be declared INTEGER.
An index to (almost) all subprograms and common-block variables is found in Appendix B.
On the issue of initialization, the routines of different origin and functionality behave
quite differently. Routines that are intended to be called from many different places, such29

as showers, fragmentation and decays, require no specific initialization (except for the
one implied by the presence of BLOCK DATA PYDATA, see above), i.e. each event and eachtask stands on its own. Current common-block values are used to perform the tasks in
specific ways, and those rules can be changed from one event to the next (or even within
the generation of one and the same event) without any penalty. The random-number
generator is initialized at the first call, but usually this is transparent.
In the core process generation machinery (e.g. selection of the hard process kinematics),
on the other hand, a sizable amount of initialization is performed in the PYINIT call, andthereafter the events generated by PYEVNT all obey the rules established at that point.
This improves the efficiency of the generation process, and also ties in with the Monte
Carlo integration of the process cross section over many events. Therefore common-block
variables that specify methods and constraints to be used have to be set before the PYINIT
call and then not be changed afterwards, with few exceptions. Of course, it is possible
to perform several PYINIT calls in the same run, but there is a significant time overhead
involved, so this is not something one would do for each new event. The two older separate
process generation routines PYEEVT (and some of the routines called by it) and PYONIA
also contain some elements of initialization, where there are a few advantages if events are
generated in a coherent fashion. The cross section is not as complicated here, however, so
the penalty for reinitialization is small, and also does not require any special user calls.
Apart from writing a title page, giving a brief initialization information, printing error
messages if need be, and responding to explicit requests for listings, all tasks of the
program are performed ‘silently’. All output is directed to unit MSTU(11), by default 6,
and it is up to you to set this unit open for write. The only exceptions are PYRGET, PYRSET
and PYUPDA where, for obvious reasons, the input/output file number is specified at eachcall. Here you again have to see to it that proper read/write access is set.
The programs are extremely versatile, but the price to be paid for this is having alarge number of adjustable parameters and switches for alternative modes of operation.
No single user is ever likely to need more than a fraction of the available options. Since
all these parameters and switches are assigned sensible default values, there is no reason
to worry about them until the need arises.
Unless explicitly stated (or obvious from the context) all switches and parameters canbe changed independently of each other. One should note, however, that if only a few
switches/parameters are changed, this may result in an artificially bad agreement withdata. Many disagreements can often be cured by a subsequent retuning of some otherparameters of the model, in particular those that were once determined by a comparison
with data in the context of the default scenario. For example, for e+ e− annihilation, sucha retuning could involve one QCD parameter (αs or Λ), the longitudinal fragmentation
function, and the average transverse momentum in fragmentation.
The program contains a number of checks that requested processes have been implemented, that flavours specified for jet systems make sense, that the energy is sufficient toallow hadronization, that the memory space in PYJETS is large enough, etc. If anything
goes wrong that the program can catch (obviously this may not always be possible), an
error message will be printed and the treatment of the corresponding event will be cut
short. In serious cases, the program will abort. As long as no error messages appear on
the output, it may not be worthwhile to look into the rules for error checking, but if but
one message appears, it should be enough cause for alarm to receive prompt attention.
Also warnings are sometimes printed. These are less serious, and the experienced usermight deliberately do operations which go against the rules, but still can be made to
make sense in their context. Only the first few warnings will be printed, thereafter the
program will be quiet. By default, the program is set to stop execution after ten errors,
after printing the last erroneous event.
It must be emphasized that not all errors will be caught. In particular, one tricky question is what happens if an integer-valued common-block switch or subroutine/function
30

argument is used with a value that is not defined. In some subroutine calls, a prompt
return will be expedited, but in most instances the subsequent action is entirely unpredictable, and often completely haywire. The same goes for real-valued variables that areassigned values outside the physically sensible range. One example will suffice here: if
PARJ(2) is defined as the s/u suppression factor, a value > 1 will not give more profuseproduction of s than of u, but actually a spillover into c production. Users, beware!

3.4

Manual Conventions

In the manual parts of this report, some conventions are used. All names of subprograms,
common blocks and variables are given in upper-case ‘typewriter’ style, e.g. MSTP(111)= 0. Also program examples are given in this style.
If a common-block variable must have a value set at the beginning of execution, thena default value is stored in the block data subprogram PYDATA. Such a default value isusually indicated by a ‘(D = . . . )’ immediately after the variable name, e.g.
MSTJ(1) : (D = 1) choice of fragmentation scheme.
All variables in the fragmentation-related common blocks (with very few exceptions,clearly marked) can be freely changed from one event to the next, or even within thetreatment of one single event; see discussion on initialization in the previous section. In
the process-generation machinery common blocks the situation is more complicated. The
values of many switches and parameters are used already in the PYINIT call, and cannot
be changed after that. The problem is mentioned in the preamble to the afflicted common
blocks, which in particular means PYPARS and PYSUBS. For the variables which may still
be changed from one event to the next, a ‘(C)’ is added after the ‘(D = . . . )’ statement.
Normally, variables internal to the program are kept in separate common blocks andarrays, but in a few cases such internal variables appear among arrays of switches andparameters, mainly for historical reasons. These are denoted by ‘(R)’ for variables you
may want to read, because they contain potentially interesting information, and by ‘(I)’for purely internal variables. In neither case may the variables be changed by you.
In the description of a switch, the alternatives that this switch may take are often
enumerated, e.g.
MSTJ(1) : (D = 1) choice of fragmentation scheme.
= 0 : no jet fragmentation at all.
= 1 : string fragmentation according to the Lund model.
= 2 : independent fragmentation, according to specification in MSTJ(2) andMSTJ(3).
If you then use any value other than 0, 1 or 2, results are undefined. The action could
even be different in different parts of the program, depending on the order in which thealternatives are identified.
It is also up to you to choose physically sensible values for parameters: there is nocheck on the allowed ranges of variables. We gave an example of this at the end of thepreceding section.
Subroutines you are expected to use are enclosed in a box at the point where they aredefined:CALL PYLIST(MLIST)
This is followed by a description of input or output parameters. The difference between
input and output is not explicitly marked, but should be obvious from the context. In
fact, the event-analysis routines of section 15.5 return values, while all the rest only have
input variables.

31

Routines that are only used internally are not boxed in. However, we use boxes for allcommon blocks, so as to enhance the readability.
In running text, often specific switches and parameters will be mentioned, without areference to the place where they are described further. The Index at the very end of
the document allows you to find this place. Often names for switches begin with MST
and parameters with PAR. No common-block variables begin with PY. There is thus no
possibility to confuse an array element with a function or subroutine call.
An almost complete list of common blocks, with brief comments on their main functions, is the following:
C...The event record.
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
C...Parameters.
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
C...Particle properties + some flavour parameters.
COMMON/PYDAT2KCHG(500,4),PMAS(500,4),PARF(2000),VCKM(4,4)
C...Decay information.
COMMON/PYDAT3/MDCY(500,3),MDME(8000,2),BRAT(8000),KFDP(8000,5)
C...Particle names
COMMON/PYDAT4/CHAF(500,2)
CHARACTER CHAF*16
C...Random number generator information.
COMMON/PYDATR/MRPY(6),RRPY(100)
C...Selection of hard scattering subprocesses.
COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)
C...Parameters.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
C...Internal variables.
COMMON/PYINT1/MINT(400),VINT(400)
C...Process information.
COMMON/PYINT2/ISET(500),KFPR(500,2),COEF(500,20),ICOL(40,4,2)
C...Parton distributions and cross sections.
COMMON/PYINT3/XSFX(2,-40:40),ISIG(1000,3),SIGH(1000)
C...Resonance width and secondary decay treatment.
COMMON/PYINT4/MWID(500),WIDS(500,5)
C...Generation and cross section statistics.
COMMON/PYINT5/NGENPD,NGEN(0:500,3),XSEC(0:500,3)
C...Process names.
COMMON/PYINT6/PROC(0:500)
CHARACTER PROC*28
C...Total cross sections.
COMMON/PYINT7/SIGT(0:6,0:6,0:5)
C...Photon parton distributions: total and valence only.
COMMON/PYINT8/XPVMD(-6:6),XPANL(-6:6),XPANH(-6:6),XPBEH(-6:6),&XPDIR(-6:6)
COMMON/PYINT9/VXPVMD(-6:6),VXPANL(-6:6),VXPANH(-6:6),VXPDGM(-6:6)
C...Colour tag information in the Les Houches Accord format.
COMMON/PYCTAG/NCT, MCT(4000,2)
C...Partons and their scales for the new pT-ordered final-state showers.
PARAMETER (MAXNUP=500)
COMMON/PYPART/NPART,NPARTD,IPART(MAXNUP),PTPART(MAXNUP)C...Multiple interactions in the new model.
COMMON/PYINTM/KFIVAL(2,3),NMI(2),IMI(2,800,2),NVC(2,-6:6),&XASSOC(2,-6:6,240),XPSVC(-6:6,-1:240),PVCTOT(2,-1:1),
32

&XMI(2,240),Q2MI(240),IMISEP(0:240)
C...Hardest initial-state radiation in the new model.
COMMON/PYISMX/MIMX,JSMX,KFLAMX,KFLCMX,KFBEAM(2),NISGEN(2,240),&PT2MX,PT2AMX,ZMX,RM2CMX,Q2BMX,PHIMX
C...Possible joined interactions in backwards evolution.
COMMON/PYISJN/MJN1MX,MJN2MX,MJOIND(2,240)
C...Supersymmetry parameters.
COMMON/PYMSSM/IMSS(0:99),RMSS(0:99)
C...Supersymmetry mixing matrices.
COMMON/PYSSMT/ZMIX(4,4),UMIX(2,2),VMIX(2,2),SMZ(4),SMW(2),&SFMIX(16,4),ZMIXI(4,4),UMIXI(2,2),VMIXI(2,2)
C...R-parity-violating couplings in supersymmetry.
COMMON/PYMSRV/RVLAM(3,3,3), RVLAMP(3,3,3), RVLAMB(3,3,3)
C...Internal parameters for R-parity-violating processes.
COMMON/PYRVNV/AB(2,16,2),RMS(0:3),RES(6,5),IDR,IDR2,DCMASS,KFR(3)COMMON/PYRVPM/RM(0:3),A(2),B(2),RESM(2),RESW(2),MFLAGLOGICAL MFLAG
C...Parameters for Gauss integration of supersymmetric widths.
COMMON/PYINTS/XXM(20)COMMON/PYG2DX/X1
C...Parameters of TechniColor Strawman Model and other compositeness.
COMMON/PYTCSM/ITCM(0:99),RTCM(0:99)
C...Histogram information.
COMMON/PYBINS/IHIST(4),INDX(1000),BIN(20000)
C...HEPEVT common block.
PARAMETER (NMXHEP=4000)
COMMON/HEPEVT/NEVHEP,NHEP,ISTHEP(NMXHEP),IDHEP(NMXHEP),&JMOHEP(2,NMXHEP),JDAHEP(2,NMXHEP),PHEP(5,NMXHEP),VHEP(4,NMXHEP)DOUBLE PRECISION PHEP,VHEP
C...User process initialization common block.
INTEGER MAXPUP
PARAMETER (MAXPUP=100)
INTEGER IDBMUP,PDFGUP,PDFSUP,IDWTUP,NPRUP,LPRUPDOUBLE PRECISION EBMUP,XSECUP,XERRUP,XMAXUP
COMMON/HEPRUP/IDBMUP(2),EBMUP(2),PDFGUP(2),PDFSUP(2),&IDWTUP,NPRUP,XSECUP(MAXPUP),XERRUP(MAXPUP),XMAXUP(MAXPUP),&LPRUP(MAXPUP)
C...User process event common block.
INTEGER MAXNUPPARAMETER (MAXNUP=500)
INTEGER NUP,IDPRUP,IDUP,ISTUP,MOTHUP,ICOLUP
DOUBLE PRECISION XWGTUP,SCALUP,AQEDUP,AQCDUP,PUP,VTIMUP,SPINUP
COMMON/HEPEUP/NUP,IDPRUP,XWGTUP,SCALUP,AQEDUP,AQCDUP,IDUP(MAXNUP),&ISTUP(MAXNUP),MOTHUP(2,MAXNUP),ICOLUP(2,MAXNUP),PUP(5,MAXNUP),&VTIMUP(MAXNUP),SPINUP(MAXNUP)

3.5

Getting Started with the Simple Routines

Normally Pythia is expected to take care of the full event generation process. At times,
however, one may want to access the more simple underlying routines, which allow a large
flexibility to ‘do it yourself’. We therefore start with a few cases of this kind, at the sametime introducing some of the more frequently used utility routines.
As a first example, assume that you want to study the production of uu 2-jet systems
33

at 20 GeV energy. To do this, write a main program
IMPLICIT DOUBLE PRECISION(A-H, O-Z)
CALL PY2ENT(0,2,-2,20D0)
CALL PYLIST(1)END
and run this program, linked together with Pythia. The routine PY2ENT is specificallyintended for storing two entries (partons or particles). The first argument (0) is a command to perform fragmentation and decay directly after the entries have been stored, thesecond and third that the two entries are u (2) and u (−2), and the last that the c.m.
energy of the pair is 20 GeV, in double precision. When this is run, the resulting event isstored in the PYJETS common block. This information can then be read out by you. No
output is produced by PY2ENT itself, except for a title page which appears once for every
Pythia run.
Instead the second command, to PYLIST, provides a simple visible summary of theinformation stored in PYJETS. The argument (1) indicates that the short version should
be used, which is suitable for viewing the listing directly on an 80-column terminal screen.
It might look as shown here.
Event listing (summary)I

particle/jet KS

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26

(u)
A
(ubar)
V(string)(rho+)(rho-)
pi+(Sigma0)(K*+)
p~pipi+(pi0)
pi(pi0)(Lambda0)
gamma
K+
(pi0)
gamma
gamma
gamma
gamma
p+
pigammagamma

KF orig

12
2
11
-2
11
92
11
213
11
-213
1
211
11
3212
11
323
1 -2212
1
-211
1
211
11
111
1
-211
11
111
11
3122
1
22
1
321
11
111
1
22
1
22
1
22
1
22
1
2212
1
-211
1
22
1
22
sum: 0.00

0
0
1
3
3
3
3
3
3
3
4
4
5
5
7
7
8
8
12
12
14
14
15
15
18
18

p_x

p_y

p_z

0.000
0.000
0.000
0.098
-0.2270.125-0.254-0.1240.395-0.0130.109-0.0110.089-0.316-0.208-0.046-0.084-0.0400.059-0.070-0.3220.006-0.178-0.030-0.006-0.0340.000

0.000 10.000
0.000 -10.000
0.000
0.000
-0.154
2.710
0.145
6.538
-0.2660.097
0.034 -1.397
0.709 -2.753-0.614 -3.806
0.146 -1.389-0.456
2.164
0.301
0.546
0.343
2.089
-0.1974.449
0.014 -1.403
0.020
0.006
0.299 -2.139
0.410 -0.614
0.146
0.224
0.155
0.322
-0.1624.027-0.0350.422
0.033 -1.343-0.018 -0.059
0.384 -0.585
0.026 -0.029
0.000
0.000

E

m

10.00010.00020.000
2.856
6.590
0.339
1.855
2.968
3.988
1.403
2.218
0.638
2.124
4.467
1.804
0.050
2.217
0.751
0.274
0.364
4.043
0.423
1.649
0.156
0.699
0.052
20.0000.006
0.006
20.000
0.885
0.781
0.140
1.193
0.846
0.938
0.140
0.140
0.135
0.140
0.135
1.116
0.000
0.494
0.135
0.000
0.000
0.000
0.000
0.938
0.140
0.000
0.000
20.000(A few blanks have been removed between the columns to make it fit into the formatof this text.) Look in the particle/jet column and note that the first two lines are the34original u and u. The parentheses enclosing the names, ‘(u)’ and ‘(ubar)’, are there asa reminder that these partons actually have been allowed to fragment. The partons arestill retained so that event histories can be studied. Also note that the KF (flavour code)
column contains 2 in the first line and −2 in the second. These are the codes actuallystored to denote the presence of a u and a u, cf. the PY2ENT call, while the names written
are just conveniences used when producing visible output. The A and V near the end of theparticle/jet column indicate the beginning and end of a string (or cluster, or independent
fragmentation) parton system; any intermediate entries belonging to the same system
would have had an I in that column. (This gives a poor man’s representation of an
up-down arrow, l.)In the orig (origin) column, the zeros indicate that u and u are two initial entries.
The subsequent line, number 3, denotes the fragmenting uu string system as a whole, andhas origin 1, since the first parton of this string system is entry number 1. The particles
in lines 4–10 have origin 3 to denote that they come directly from the fragmentation ofthis string. In string fragmentation it is not meaningful to say that a particle comes fromonly the u quark or only the u one. It is the string system as a whole that gives a ρ+ , a
ρ− , a π + , a Σ0 , a K∗+ , a p− , and a π − . Note that some of the particle names are again
enclosed in parentheses, indicating that these particles are not present in the final state
either, but have decayed further. Thus the π − in line 13 and the π 0 in line 14 have origin
5, as an indication that they come from the decay of the ρ− in line 5. Only the names
not enclosed in parentheses remain at the end of the fragmentation/decay chain, andare thus experimentally observable. The actual status code used to distinguish between
different classes of entries is given in the KS column; codes in the range 1–10 correspondto remaining entries, and those above 10 to those that have fragmented or decayed.
The columns with p x, p y, p z, E and m are quite self-explanatory. All momenta,
energies and masses are given in units of GeV, since the speed of light is taken to be c = 1.
Note that energy and momentum are conserved at each step of the fragmentation/decay
process (although there exist options where this is not true). Also note that the z axis
plays the rôle of preferred direction, along which the original partons are placed. The final
line is intended as a quick check that nothing funny happened. It contains the summed
charge, summed momentum, summed energy and invariant mass of the final entries at theend of the fragmentation/decay chain, and the values should agree with the input impliedby the PY2ENT arguments. (In fact, warnings would normally appear on the output if
anything untoward happened, but that is another story.)
The above example has illustrated roughly what information is to be had in the event
record, but not so much about how it is stored. This is better seen by using a 132-column
format for listing events. Try e.g. the following program
IMPLICIT DOUBLE PRECISION(A-H, O-Z)
CALL PY3ENT(0,1,21,-1,30D0,0.9D0,0.7D0)
CALL PYLIST(2)CALL PYEDIT(3)CALL PYLIST(2)END
where a 3-jet dgd event is generated in the first executable line and listed in the second.
This listing will contain the numbers as directly stored in the common block PYJETSCOMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
For particle I, K(I,1) thus gives information on whether or not a parton or particle has
fragmented or decayed, K(I,2) gives the particle code, K(I,3) its origin, K(I,4) andK(I,5) the position of fragmentation/decay products, and P(I,1)–P(I,5) momentum,
energy and mass. The number of lines in current use is given by N, i.e. 1 ≤ I ≤ N. The35

V matrix contains decay vertices; to view those PYLIST(3) has to be used. NPAD is adummy, needed to avoid some compiler troubles. It is important to learn the rules for
how information is stored in PYJETS.
The third executable line in the program illustrates another important point about
Pythia: a number of routines are available for manipulating and analyzing the event
record after the event has been generated. Thus PYEDIT(3) will remove everything except
stable charged particles, as shown by the result of the second PYLIST call. More advanced
possibilities include things like sphericity or clustering routines. Pythia also contains
some simple routines for histogramming, used to give self-contained examples of analysis
procedures.
Apart from the input arguments of subroutine calls, control on the doings of Pythia
may be imposed via many common blocks. Here sensible default values are always provided. A user might want to switch off all particle decays by putting MSTJ(21) = 0 orincrease the s/u ratio in fragmentation by putting PARJ(2) = 0.40D0, to give but two
examples. It is by exploring the possibilities offered here that Pythia can be turned into
an extremely versatile tool, even if all the nice physics is already present in the default
values.
As a final, semi-realistic example, assume that the p⊥ spectrum of π + particles is to
be studied in 91.2 GeV e+ e− annihilation events, where p⊥ is to be defined with respect
to the sphericity axis. Using the internal routines for simple histogramming, a complete
program might look like
C...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
INTEGER PYK,PYCHGE,PYCOMPC...Common blocks.
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
C...Book histograms.
CALL PYBOOK(1,’pT spectrum of pi+’,100,0D0,5D0)
C...Number of events to generate. Loop over events.
NEVT=100
DO 110 IEVT=1,NEVTC...Generate event. List first one.
CALL PYEEVT(0,91.2D0)
IF(IEVT.EQ.1) CALL PYLIST(1)
C...Find sphericity axis.
CALL PYSPHE(SPH,APL)
C...Rotate event so that sphericity axis is along z axis.
CALL PYEDIT(31)
C...Loop over all particles, but skip if not pi+.
DO 100 I=1,N
IF(K(I,2).NE.211) GOTO 100
C...Calculate pT and fill in histogram.
PT=SQRT(P(I,1)**2+P(I,2)**2)
CALL PYFILL(1,PT,1D0)
36

C...End of particle and event loops.
100
CONTINUE
110 CONTINUE
C...Normalize histogram properly and list it.
CALL PYFACT(1,20D0/NEVT)CALL PYHIST
END
Study this program, try to understand what happens at each step, and run it to checkthat it works. You should then be ready to look at the relevant sections of this reportand start writing your own programs.

3.6

Getting Started with the Event Generation Machinery

A run with the full Pythia event generation machinery has to be more strictly organizedthan the simple examples above, in that it is necessary to initialize the generation before
events can be generated, and in that it is not possible to change switches and parameters
freely during the course of the run. A fairly precise recipe for how a run should be
structured can therefore be given.
Thus, the nowadays normal usage of Pythia can be subdivided into three steps.
1. The initialization step. It is here that all the basic characteristics of the coming
generation are specified. The material in this section includes the following.
• Declarations for double precision and integer variables and integer functions:
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
INTEGER PYK,PYCHGE,PYCOMP• Common blocks, at least the following, and maybe some more:
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)• Selection of required processes. Some fixed ‘menus’ of subprocesses can beselected with different MSEL values, but with MSEL=0 it is possible to compose‘à la carte’, using the subprocess numbers. To generate processes 14, 18 and29, for instance, one needs
MSEL=0MSUB(14)=1
MSUB(18)=1
MSUB(29)=1• Selection of kinematics cuts in the CKIN array. To generate hard scatterings
with 50 GeV ≤ p⊥ ≤ 100 GeV, for instance, use
CKIN(3)=50D0
CKIN(4)=100D0
Unfortunately, initial- and final-state radiation will shift around the kinematicsof the hard scattering, making the effects of cuts less predictable. One therefore
always has to be very careful that no desired event configurations are cut out.
• Definition of underlying physics scenario, e.g. Higgs mass.
• Selection of parton-distribution sets, Q2 definitions, showering and multiple
interactions parameters, and all other details of the generation.
37

• Switching off of generator parts not needed for toy simulations, e.g. fragmentation for parton level studies.
• Initialization of the event generation procedure. Here kinematics is set up,
maxima of differential cross sections are found for future Monte Carlo generation, and a number of other preparatory tasks carried out. Initializationis performed by PYINIT, which should be called only after the switches andparameters above have been set to their desired values. The frame, the beam
particles and the energy have to be specified, e.g.
CALL PYINIT(’CMS’,’p’,’pbar’,1800D0)• Any other initial material required by you, e.g. histogram booking.
2. The generation loop. It is here that events are generated and studied. It includes
the following tasks:• Generation of the next event, withCALL PYEVNT
or, for the new multiple interactions and showering model,
CALL PYEVNW• Printing of a few events, to check that everything is working as planned, withCALL PYLIST(1)• An analysis of the event for properties of interest, either directly reading out information from the PYJETS common block or making use of the utility routines
in Pythia.
• Saving of events on disk or tape, or interfacing to detector simulation.
3. The finishing step. Here the tasks are:
• Printing a table of deduced cross sections, obtained as a by-product of theMonte Carlo generation activity, with the command
CALL PYSTAT(1)• Printing histograms and other user output.
To illustrate this structure, imagine a toy example, where one wants to simulate theproduction of a 300 GeV Higgs particle. In Pythia, a program for this might looksomething like the following.
C...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
INTEGER PYK,PYCHGE,PYCOMPC...Common blocks.
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)COMMON/PYDAT2/KCHG(500,4),PMAS(500,4),PARF(2000),VCKM(4,4)
COMMON/PYDAT3/MDCY(500,3),MDME(8000,2),BRAT(8000),KFDP(8000,5)
COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
C...Number of events to generate. Switch on proper processes.
NEV=1000
MSEL=0MSUB(102)=1
MSUB(123)=1
MSUB(124)=1
C...Select Higgs mass and kinematics cuts in mass.
38

PMAS(25,1)=300D0
CKIN(1)=290D0
CKIN(2)=310D0
C...For simulation of hard process only: cut out unnecessary tasks.
MSTP(61)=0
MSTP(71)=0
MSTP(81)=0
MSTP(111)=0
C...Initialize and list partial widths.
CALL PYINIT(’CMS’,’p’,’p’,14000D0)
CALL PYSTAT(2)C...Book histogram.
CALL PYBOOK(1,’Higgs mass’,50,275D0,325D0)
C...Generate events. Look at first few.
DO 200 IEV=1,NEV
CALL PYEVNTIF(IEV.LE.3) CALL PYLIST(1)
C...Loop over particles to find Higgs and histogram its mass.
DO 100 I=1,N
IF(K(I,1).LT.20.AND.K(I,2).EQ.25) HMASS=P(I,5)
100
CONTINUE
CALL PYFILL(1,HMASS,1D0)
200 CONTINUE
C...Print cross sections and histograms.
CALL PYSTAT(1)CALL PYHIST
END
Here 102, 123 and 124 are the three main Higgs production graphs gg → h, ZZ → h,and WW → h, and MSUB(ISUB) = 1 is the command to switch on process ISUB. Fullfreedom to combine subprocesses ‘à la carte’ is ensured by MSEL = 0; ready-made ‘menus’
can be ordered with other MSEL numbers. The PMAS command sets the mass of the Higgs,
and the CKIN variables the desired mass range of the Higgs — a Higgs with a 300 GeV
nominal mass actually has a fairly broad Breit–Wigner type mass distribution. The MSTP
switches that come next are there to modify the generation procedure, in this case toswitch off initial- and final-state radiation, multiple interactions among beam jets, andfragmentation, to give only the ‘parton skeleton’ of the hard process. The PYINIT callinitializes Pythia, by finding maxima of cross sections, recalculating the Higgs decay
properties (which depend on the Higgs mass), etc. The decay properties can be listedwith PYSTAT(2).
Inside the event loop, PYEVNT is called to generate an event, and PYLIST(1) to list
the event. The information used by PYLIST(1) is the event record, stored in the common
block PYJETS. Here one finds all produced particles, both final and intermediate ones, withinformation on particle species and event history (K array), particle momenta (P array)and production vertices (V array). In the loop over all particles produced, 1 through N,
the Higgs particle is found by its code, K(I,2) = 25, and its mass is stored in P(I,5).
After all events have been generated, PYSTAT(1) gives a summary of the number of39

events generated in the various allowed channels, and the inferred cross sections.
In the run above, a typical event listing might look like the following.
Event listing (summary)I

particle/jet

KF

p_x

p_y

p_z

E

m

1 !p+!
2212
0.000
0.000 8000.000 8000.000
0.938
2 !p+!
2212
0.000
0.000-8000.000 8000.000
0.938======================================================================
3 !g!
21-0.505-0.22928.55328.558
0.000
4 !g!
21
0.224
0.041 -788.073 788.073
0.000
5 !g!
21
-0.505-0.22928.55328.558
0.000
6 !g!
21
0.224
0.041 -788.073 788.073
0.000
7 !H0!
25-0.281-0.188 -759.520 816.631 300.027
8 !W+!
24 120.648
35.239 -397.843 424.829
80.023
9 !W-!
-24 -120.929 -35.426 -361.677 391.801
82.57910 !e+!
-11
12.922-4.760 -160.940 161.528
0.001
11 !nu_e!
12 107.726
39.999 -236.903 263.302
0.000
12 !s!
3 -62.423
7.195 -256.713 264.292
0.199
13 !cbar!
-4 -58.506 -42.621 -104.963 127.509
1.350======================================================================
14 (H0)25-0.281-0.188 -759.520 816.631 300.027
15 (W+)
24 120.648
35.239 -397.843 424.829
80.02316 (W-)-24 -120.929 -35.426 -361.677 391.801
82.57917 e+-1112.922-4.760 -160.940 161.528
0.001
18 nu_e
12 107.726
39.999 -236.903 263.302
0.000
19 s
A
3 -62.423
7.195 -256.713 264.292
0.199
20 cbarV-4 -58.506 -42.621 -104.963 127.509
1.350
21 ud_1
A
2103-0.1010.176 7971.328 7971.328
0.771
22 d
V
1
-0.3160.001 -87.390
87.390
0.010
23 u
A
2
0.606
0.052
-0.7510.967
0.006
24 uu_1
V
2203
0.092
-0.042-7123.668 7123.668
0.771======================================================================
sum: 2.00
0.00
0.00
0.00 15999.98 15999.98
The above event listing is abnormally short, in part because some columns of information
were removed to make it fit into this text, in part because all initial- and final-state QCD
radiation, all non-trivial beam jet structure, and all fragmentation was inhibited in thegeneration. Therefore only the skeleton of the process is visible. In lines 1 and 2 one
recognizes the two incoming protons. In lines 3 and 4 are incoming partons before initialstate radiation and in 5 and 6 after — since there is no such radiation they coincide here.
Line 7 shows the Higgs produced by gg fusion, 8 and 9 its decay products and 10–13 thesecond-step decay products. Up to this point lines give a summary of the event history,
indicated by the exclamation marks that surround particle names (and also reflected in
the K(I,1) code, not shown). From line 14 onwards come the particles actually producedin the final states, first in lines 14–16 particles that subsequently decayed, which havetheir names surrounded by brackets, and finally the particles and partons left in the end,
including beam remnants. Here this also includes a number of unfragmented partons,
since fragmentation was inhibited. Ordinarily, the listing would have gone on for a few
hundred more lines, with the particles produced in the fragmentation and their decay
products. The final line gives total charge and momentum, as a convenient check that
nothing unexpected happened. The first column of the listing is just a counter, the second
gives the particle name and information on status and string drawing (the A and V), the40

third the particle-flavour code (which is used to give the name), and the subsequentcolumns give the momentum components.
One of the main problems is to select kinematics efficiently. Imagine for instance that
one is interested in the production of a single Z with a transverse momentum in excess of
50 GeV. If one tries to generate the inclusive sample of Z events, by the basic production
graphs qq → Z, then most events will have low transverse momenta and will have to bediscarded. That any of the desired events are produced at all is due to the initial-state
generation machinery, which can build up transverse momenta for the incoming q and
q. However, the amount of initial-state radiation cannot be constrained beforehand. To
increase the efficiency, one may therefore turn to the higher-order processes qg → Zq
and qq → Zg, where already the hard subprocess gives a transverse momentum to theZ. This transverse momentum can be constrained as one wishes, but again initial- andfinal-state radiation will smear the picture. If one were to set a p⊥ cut at 50 GeV for
the hard-process generation, those events where the Z was given only 40 GeV in the hard
process but got the rest from initial-state radiation would be missed. Not only therefore
would cross sections come out wrong, but so might the typical event shapes. In the end,
it is therefore necessary to find some reasonable compromise, by starting the generation
at 30 GeV, say, if one knows that only rarely do events below this value fluctuate up to
50 GeV. Of course, most events will therefore not contain a Z above 50 GeV, and one will
have to live with some inefficiency. It is not uncommon that only one event out of ten
can be used, and occasionally it can be even worse.
If it is difficult to set kinematics, it is often easier to set the flavour content of a process.
In a Higgs study, one might wish, for example, to consider the decay h0 → Z0 Z0 , witheach Z0 → e+ e− or µ+ µ− . It is therefore necessary to inhibit all other h0 and Z0 decay
channels, and also to adjust cross sections to take into account this change, all of which
is fairly straightforward. The same cannot be said for decays of ordinary hadrons, where
the number produced in a process is not known beforehand, and therefore inconsistencies
easily can arise if one tries to force specific decay channels.
In the examples given above, all run-specific parameters are set in the code (in themain program; alternatively it could be in a subroutine called by the main program).
This approach is allowing maximum flexibility to change parameters during the courseof the run. However, in many experimental collaborations one does not want to allow
this freedom, but only one set of parameters, to be read in from an external file at the
beginning of a run and thereafter never changed. This in particular applies when Pythia
is to be linked with other libraries, such as Geant [Bru89] and detector-specific software.
While a linking of a normal-sized main program with Pythia is essentially instantaneouson current platforms (typically less than a second), this may not hold for other libraries.
For this purpose one then needs a parser of Pythia parameters, the core of which canbe provided by the PYGIVE routine.
As an example, consider a main program of the form
C...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
INTEGER PYK,PYCHGE,PYCOMPC...Input and output strings.
CHARACTER FRAME*12,BEAM*12,TARGET*12,PARAM*100
C...Read parameters for PYINIT call.
READ(*,*) FRAME,BEAM,TARGET,ENERGYC...Read number of events to generate, and to print.
READ(*,*) NEV,NPRT

41

C...Loop over reading and setting parameters/switches.
100 READ(*,’(A)’,END=200) PARAMCALL PYGIVE(PARAM)
GOTO 100
C...Initialize PYTHIA.
200 CALL PYINIT(FRAME,BEAM,TARGET,ENERGY)
C...Event generation loop
DO 300 IEV=1,NEV
CALL PYEVNTIF(IEV.LE.NPRT) CALL PYLIST(1)300 CONTINUE
C...Print cross sections.
CALL PYSTAT(1)END
and a file indata with the contents
CMS,p,p,14000.
1000,3! below follows commands sent to PYGIVEMSEL=0! Mix processes freely
MSUB(102)=1! g + g -> h0
MSUB(123)=1! Z0 + Z0 -> h0
MSUB(124)=1! W+ + W- -> h0
PMAS(25,1)=300. ! Higgs mass
CKIN(1)=290.
! lower cutoff on mass
CKIN(2)=310.
! upper cutoff on mass
MSTP(61)=0! no initial-state showers
MSTP(71)=0! no final-state showers
MSTP(81)=0! no multiple interactions
MSTP(111)=0! no hadronization
Here the text following the exclamation marks is interpreted as a comment by PYGIVE, andthus purely intended to allow better documentation of changes. The main program could
then be linked to Pythia, to an executable a.out, and run e.g. with a Unix commandline
a.out < indata > output
to produce results on the file output. Here the indata could be changed without requiringa recompilation. Of course, the main program would have to be more realistic, e.g. with
events saved to disk or tape, but the principle should be clear.
Further examples of how to use Pythia are available on the Pythia webpage.

42

4Monte Carlo Techniques

Quantum mechanics introduces a concept of randomness in the behaviour of physicalprocesses. The virtue of event generators is that this randomness can be simulated by
the use of Monte Carlo techniques. In the process, the program authors have to use someingenuity to find the most efficient way to simulate an assumed probability distribution.
A detailed description of possible techniques would carry us too far, but in this sectionsome of the most frequently used approaches are presented, since they will appear indiscussions in subsequent sections. Further examples may be found e.g. in [Jam80].
First of all one assumes the existence of a random number generator. This is a (Fortran) function which, each time it is called, returns a number R in the range between 0
and 1, such that the inclusive distribution of numbers R is flat in the range, and such thatdifferent numbers R are uncorrelated. The random number generator that comes withPythia is described at the end of this section, and we defer the discussion until then.

4.1

Selection From a Distribution

The situation that is probably most common is that we know a function f (x) which is
non-negative in the allowed x range xmin ≤ x ≤ xmax . We want to select an x ‘at random’so that the probability in a small interval dx around a given x is proportional to f (x) dx.
Here f (x) might be a fragmentation function, a differential cross section, or any of anumber of distributions.
One does not have to assume that the integral of f (x) is explicitly normalized to unity:by the Monte Carlo procedure of picking exactly one accepted x value, normalization is
implicit in the final result. Sometimes the integral of f (x) does carry a physics content
of its own, as part of an overall weight factor we want to keep track of. Consider, forinstance, the case when x represents one or several phase-space variables and f (x) adifferential cross section; here the integral has a meaning of total cross section for theprocess studied. The task of a Monte Carlo is then, on the one hand, to generate events
one at a time, and, on the other hand, to estimate the total cross section. The discussionof this important example is deferred to section 7.4.
If it is possible to find a primitive function F (x) which has a known inverse F −1 (x),an x can be found as follows (method 1):
Z x
xmin

f (x) dx = R

Z xmaxxmin

f (x) dx=⇒ x = F −1 (F (xmin ) + R(F (xmax ) − F (xmin ))) .

(2)

The statement of the first line is that a fraction R of the total area under f (x) should beto the left of x. However, seldom are functions of interest so nice that the method above
works. It is therefore necessary to use more complicated schemes.
Special tricks can sometimes be found. Consider e.g. the generation of a Gaussianf (x) = exp(−x2 ). This function is not integrable, but if we combine it with the same
Gaussian distribution of a second variable y, it is possible to transform to polar coordinatesf (x) dx f (y) dy = exp(−x2 − y 2 ) dx dy = r exp(−r2 ) dr dϕ ,(3)and now the r and ϕ distributions may be easily generated and recombined to yield x.
At the same time we get a second number y, which can also be used. For the generationof transverse momenta in fragmentation, this is very convenient, since in fact we want toassign two transverse degrees of freedom.
If the maximum of f (x) is known, f (x) ≤ fmax in the x range considered, a hit-or-missmethod will always yield the correct answer (method 2):43

1. select an x with even probability in the allowed range, i.e. x = xmin +R(xmax −xmin );2. compare a (new) R with the ratio f (x)/fmax ; if f (x)/fmax ≤ R, then reject the xvalue and return to point 1 for a new try;
3. otherwise the most recent x value is retained as final answer.
The probability that f (x)/fmax > R is proportional to f (x); hence the correct distributionof retained x values. The
efficiency of this method, i.e. the average probability that anR
x will be retained, is ( f (x) dx)/(fmax (xmax − xmin )). The method is acceptable if thisnumber is not too low, i.e. if f (x) does not fluctuate too wildly.
Very often f (x) does have narrow spikes, and it may not even be possible to define
an fmax . An example of the former phenomenon is a function with a singularity justoutside the allowed region, an example of the latter an integrable singularity just at thexmin and/or xmax borders. Variable transformations may then be used to make a functionsmoother. Thus a function f (x) which blows up as 1/x for x → 0, with an xmin close to0, would instead be roughly constant if transformed to the variable y = ln x.
The variable transformation strategy may be seen as a combination of methods 1 and
2, as follows. Assume the existence of a function g(x), with f (x) ≤ g(x) over the x range
of interest. Here g(x) is picked to be a ‘simple’ function, such that the primitive function
G(x) and its inverse G−1 (x) are known. Then (method 3):
1. select an x according to the distribution g(x), using method 1;
2. compare a (new) R with the ratio f (x)/g(x); if f (x)/g(x) ≤ R, then reject the xvalue and return to point 1 for a new try;
3. otherwise the most recent x value is retained as final answer.
This works, since the first step will select x with a probability g(x) dx = dG(x) and thesecond retain this choice with probability f (x)/g(x). The total probability to pick a value
x is then just the product of the two, i.e. f (x) dx.
If f (x) has several spikes, method 3 may work for each spike separately, but it may
not be possible to find a g(x) that covers all of them at the same time, and which still
has an invertible primitive function. However, assume that we can find a function g(x) =Pi gi (x), such that f (x) ≤ g(x) over the x range considered, and such that the functions
gi (x) each are non-negative and simple, in the sense that we can find primitive functions
and their inverses. In that case (method 4):
1. select an i at random, with relative probability given by the integrals
Z xmax
xmin

gi (x) dx = Gi (xmax ) − Gi (xmin ) ;(4)

2. for the i selected, use method 1 to find an x, i.e.
x = G−1i (Gi (xmin ) + R(Gi (xmax ) − Gi (xmin ))) ;(5)

3. compare a (new) R with the ratio f (x)/g(x); if f (x)/g(x) ≤ R, then reject the xvalue and return to point 1 for a new try;
4. otherwise the most recent x value is retained as final answer.
This is just a trivial extension of method 3, where steps 1 and 2 ensure that, on the
average, each x value picked
there is distributed according to g(x): the first stepR
R picks iwith relative probability gi (x) dx, the second x with absolute probability gi (x)/ gi (x) dx(this is one place where one must remember to do normalization correctly); the productof the two is therefore gi (x) and the sum over all i gives back g(x).
We have now arrived at an approach that is sufficiently powerful for a large selectionof problems. In general, for a function f (x) which is known to have sharp peaks in a few
different places, the generic behaviour at each peak separately may be covered by one
or a few simple functions gi (x), to which one adds a few more gi (x) to cover the basic
behaviour away from the peaks. By a suitable selection of the relative strengths of the44different gi ’s, it is possible to find a function g(x) that matches well the general behaviourof f (x), and thus achieve a reasonable Monte Carlo efficiency.
The major additional complication is when x is a multidimensional variable. Usually
the problem is not so much f (x) itself, but rather that the phase-space boundaries may
be very complicated. If the boundaries factorize it is possible to pick phase-space points
restricted to the desired region. Otherwise the region may have to be inscribed in a hyperrectangle, with points picked within the whole hyper-rectangle but only retained if they
are inside the allowed region. This may lead to a significant loss in efficiency. Variabletransformations may often make the allowed region easier to handle.
There are two main methods to handle several dimensions, each with its set of variations. The first method is based on a factorized ansatz, i.e. one attempts to find afunction g(x) which is everywhere larger than f (x), and which can be factorized intog(x) = g (1) (x1 ) g (2) (x2 ) · · · g (n) (xn ), where x = (x1 , x2 , . . . , xn ). Here each g (j) (xj ) may(j)in its turn be a sum of functions gi , as in method 4 above. First, each xj is selectedindependently, and afterwards the ratio f (x)/g(x) is used to determine whether to retain
the point.
The second method is useful if the boundaries of the allowed region can be written in
a form where the maximum range of x1 is known, the allowed range of x2 only depends
on x1 , that of x3 only on x1 and x2 , and so on until xn , whose range may depend on all
the preceding variables. In that case it may be possible to find a function g(x) that canbe integrated over x2 through xn to yield a simple function of x1 , according to which x1 isselected. Having done that, x2 is selected according to a distribution which now depends
on x1 , but with x3 through xn integrated over. In particular, the allowed range for x2 is
known. The procedure is continued until xn is reached, where now the function depends
on all the preceding xj values. In the end, the ratio f (x)/g(x) is again used to determine
whether to retain the point.

4.2

The Veto Algorithm

The ‘radioactive decay’ type of problems is very common, in particular in parton showers,
but it is also used, e.g. in the multiple interactions description in Pythia. In this kindof problems there is one variable t, which may be thought of as giving a kind of time axis
along which different events are ordered. The probability that ‘something will happen’(a nucleus decay, a parton branch) at time t is described by a function f (t), which is
non-negative in the range of t values to be studied. However, this naı̈ve probability is
modified by the additional requirement that something can only happen at time t if it
did not happen at earlier times t0 < t. (The original nucleus cannot decay once again
if it already did decay; possibly the decay products may decay in their turn, but that is
another question.)
The probability that nothing has happened by time t is expressed by the function
N (t) and the differential probability that something happens at time t by P(t). The
basic equation then isdN= f (t) N (t) .
P(t) = −(6)dt
For simplicity, we shall assume that the process starts at time t = 0, with N (0) = 1.
The above equation can be solved easily if one notes that dN /N = d ln N :½

N (t) = N (0) exp −and thus

Z t
0

¾
0

0

f (t ) dt½

P(t) = f (t) exp −45½= exp −
Z t
0

Z t
0

¾
0

f (t ) dt

0

,

(7)

¾
0

0

f (t ) dt.

(8)

With f (t) = c this is nothing but the textbook formulae for radioactive decay. In particular, at small times the correct decay probability, P(t), agrees well with the input one,f (t), since the exponential factor is close to unity there. At larger t, the exponential
gives a dampening which ensures that the integral of P(t) never can exceed unity, even
if the integral of f (t) does. The exponential can be seen as the probability that nothing
happens between the original time 0 and the final time t. In the parton-shower language,
this corresponds to the so-called Sudakov form factor.
If f (t) has a primitive function with a known inverse, it is easy to select t values
correctly:
Z t
0

½

Z t

P(t0 ) dt0 = N (0) − N (t) = 1 − exp −0¾

f (t0 ) dt0 = 1 − R ,(9)which has the solution
F (0) − F (t) = ln R=⇒

t = F −1 (F (0) − ln R) .

(10)

If f (t) is not sufficiently nice, one may again try to find a better function g(t), withf (t) ≤ g(t) for all t ≥ 0. However to use method 3 with this g(t) would not work, since
the method would not correctly take into account the effects of the exponential term inP(t). Instead one may use the so-called veto algorithm:
1. start with i = 0 and t0 = 0;
2. add 1 to i and select ti = G−1 (G(ti−1 ) − ln R), i.e. according to g(t), but with theconstraint that ti > ti−1 ,3. compare a (new) R with the ratio f (ti )/g(ti ); if f (ti )/g(ti ) ≤ R, then return topoint 2 for a new try;
4. otherwise ti is retained as final answer.
It may not be apparent why this works. Consider, however, the various ways in which
one can select a specific time t. The probability that the first try works, t = t1 , i.e. thatno intermediate t values need be rejected, is given by½

P0 (t) = exp −

Z t
0

¾

g(t0 ) dt0

½

g(t)

Z t
f (t)
= f (t) exp − g(t0 ) dt0
g(t)
0

¾

,

(11)

where the exponential times g(t) comes from eq. (8) applied to g, and the ratio f (t)/g(t)is the probability that t is accepted. Now consider the case where one intermediate time
t1 is rejected and t = t2 is only accepted in the second step. This gives
Z t

½

Z t1

"

¾

#

½

¾

Z t
f (t1 )f (t)
P1 (t) =dt1 exp −g(t ) dt g(t1 ) 1 −exp − g(t0 ) dt0 g(t),g(t1 )
g(t)
0
0
t1
(12)
where the first exponential times g(t1 ) gives the probability that t1 is first selected, thesquare brackets the probability that t1 is subsequently rejected, the following piece theprobability that t = t2 is selected when starting from t1 , and the final factor that t isretained. The whole is to be integrated over all possible intermediate times t1 . The
exponentials together give an integral over the range from 0 to t, just as in P0 , and thefactor for the final step being accepted is also the same, so therefore one finds that00

P1 (t) = P0 (t)Z t
0dt1 [g(t1 ) − f (t1 )] .

(13)

This generalizes. In P2 one has to consider two intermediate times, 0 ≤ t1 ≤ t2 ≤ t3 = t,and soP2 (t) = P0 (t)Z t1= P0 (t)20dt1 [g(t1 ) − f (t1 )]

µZ t
0

0

0

Z t

[g(t ) − f (t )] dt
46

t1
0

dt2 [g(t2 ) − f (t2 )]

¶2

.

(14)

The last equality is most easily seen if one also considers the alternative region 0 ≤ t2 ≤t1 ≤ t, where the rôles of t1 and t2 have just been interchanged, and the integral therefore
has the same value as in the region considered. Adding the two regions, however, theintegrals over t1 and t2 decouple, and become equal. In general, for Pi , the i intermediatetimes can be ordered in i! different ways. Therefore the total probability to accept t, in
any step, isP(t) =∞X
i=0

Pi (t) = P0 (t)½= f (t) exp −½= f (t) exp −

Z t
0

Z t
0

µZ t
∞
X
1
i=0 i!

¾

0[g(t0 ) − f (t0 )] dt0

g(t0 ) dt0 exp

½Z t

¾
0

0

f (t ) dt

,

0

¶i
¾

[g(t0 ) − f (t0 )] dt0(15)which is the desired answer.
If the process is to be stopped at some scale tmax , i.e. if one would like to remain
with a fraction N (tmax ) of events where nothing happens at all, this is easy to include in
the veto algorithm: just iterate upwards in t at usual, but stop the process if no allowed
branching is found before tmax .
Usually f (t) is a function also of additional variables x. The methods of the preceding
section are easy to generalize if one can find a suitable function g(t, x) with f (t, x) ≤g(t, x). The g(t) used in the veto algorithm is the integral of g(t, x) over x. Each timea ti has been selected also an xi is picked, according to g(ti , x) dx, and the (t, x) point is
accepted with probability f (ti , xi )/g(ti , xi ).

4.3

The Random Number Generator

In recent years, progress has been made in constructing portable generators with large
periods and other good properties; see the review [Jam90]. Therefore the current version
contains a random number generator based on the algorithm proposed by Marsaglia,
Zaman and Tsang [Mar90]. This routine should work on any machine with a mantissaof at least 48 digits, i.e. on computers with a 64-bit (or more) representation of double
precision real numbers. Given the same initial state, the sequence will also be identicalon different platforms. This need not mean that the same sequence of events will be
generated, since the different treatments of roundoff errors in numerical operations willlead to slightly different real numbers being tested against these random numbers in IF
statements. Also code optimization may lead to a divergence of the event sequence.
Apart from nomenclature issues, the coding of PYR as a function rather than a subroutine, and the extension to double precision, the only difference between our code andthe code given in [Jam90] is that slightly different algorithms are used to ensure that the
random number is not equal to 0 or 1 within the machine precision. Further developmentsof the algorithm has been proposed [Lüs94] to remove residual possibilities of small longrange correlations, at the price of a slower generation procedure. However, given that
Pythia is using random numbers for so many different tasks, without any fixed cycle,
this has been deemed unnecessary.
The generator has a period of over 1043 , and the possibility to obtain almost 109
different and disjoint subsequences, selected by giving an initial integer number. The
price to be paid for the long period is that the state of the generator at a given moment
cannot be described by a single integer, but requires about 100 words. Some of these
are real numbers, and are thus not correctly represented in decimal form. The old-styleprocedure, which made it possible to restart the generation from a seed value written to
the run output, is therefore not convenient. The CERN library implementation keeps
track of the number of random numbers generated since the start. With this value saved,47

in a subsequent run the random generator can be asked to skip ahead the corresponding
number of random numbers. Pythia is a heavy user of random numbers, however:typically 30% of the full run time is spent on random number generation. Of this, half isoverhead coming from the function call administration, but the other half is truly related
to the speed of the algorithm. Therefore a skipping ahead would take place with 15% of
the time cost of the original run, i.e. an uncomfortably high figure.
Instead a different solution is chosen here. Two special routines are provided forwriting and reading the state of the random number generator (plus some initialization
information) on a sequential file, in a platform-dependent internal representation. The fileused for this purpose has to be specified by you, and opened for read and write. A stateis written as a single record, in free format. It is possible to write an arbitrary number ofstates on a file, and a record can be overwritten, if so desired. The event generation loop
might then look something like:
1. save the state of the generator on file (using flag set in point 3 below),
2. generate an event,
3. study the event for errors or other reasons why to regenerate it later; set flag tooverwrite previous generator state if no errors, otherwise set flag to create newrecord;4. loop back to point 1.
With this procedure, the file will contain the state before each of the problematical events.
These events can therefore be generated in a shorter run, where further information canbe printed. (Inside Pythia, some initialization may take place in connection with thevery first event generated in a run, so it may be necessary to generate one ordinary
event before reading in a saved state to generate the interesting events.) An alternative
approach might be to save the state every 100 events or so. If the events are subsequently
processed through a detector simulation, you may have to save also other sets of seeds,
naturally.
Unfortunately, the procedure is not always going to work. For instance, if cross section
maximum violations have occured before the interesting event in the original run, there isa possibility that another event is picked in the re-started one, where the maximum weight
estimate has not been updated. Another problem is the multiple interaction machinery,
where some of the options contain an element of learning, which again means that theevent sequence may be broken.
In addition to the service routines, the common block which contains the state of thegenerator is available for manipulation, if you so desire. In particular, the initial seedvalue is by default 19780503, i.e. different from the Marsaglia/CERN default 54217137.
It is possible to change this value before any random numbers have been generated, or toforce re-initialization in mid-run with any desired new seed.
It should be noted that, of course, the appearance of a random number generator
package inside Pythia does in no way preclude the use of other routines. You can easily
revert to having PYR as nothing but an interface to an arbitrary external random number
generator; e.g. to call a routine RNDM all you need to have is
FUNCTION PYR(IDUMMY)IMPLICIT DOUBLE PRECISION(A-H, O-Z)
100 PYR=RNDM(IDUMMY)
IF(PYR.LE.0D0.OR.PYR.GE.1D0) GOTO 100
RETURNEND
The random generator subpackage consists of the following components.
R = PYR(IDUMMY)
48

Purpose: to generate a (pseudo)random number R uniformly in the range 0 < R < 1, i.e.
excluding the endpoints.
IDUMMY : dummy input argument; normally 0.
CALL PYRGET(LFN,MOVE)
Purpose: to dump the current state of the random number generator on a separate file,using internal representation for real and integer numbers. To be precise, the fullcontents of the PYDATR common block are written on the file, with the exceptionof MRPY(6).
LFN : (logical file number) the file number to which the state is dumped. You must
associate this number with a true file (with a platform-dependent name), andsee to it that this file is open for write.
MOVE : choice of adding a new record to the file or overwriting old record(s). Normally
only options 0 or −1 should be used.
= 0 (or > 0) : add a new record to the end of the file.
= -1 : overwrite the last record with a new one (i.e. do one BACKSPACE before the
new write).
= −n : back up n records before writing the new record. The records following after
the new one are lost, i.e. the last n old records are lost and one new added.
CALL PYRSET(LFN,MOVE)
Purpose: to read in a state for the random number generator, from which the subsequent
generation can proceed. The state must previously have been saved by a PYRGETcall. Again the full contents of the PYDATR common block are read, with the
exception of MRPY(6).
LFN : (logical file number) the file number from which the state is read. You must
associate this number with a true file previously written with a PYRGET call, andsee to it that this file is open for read.
MOVE : positioning in file before a record is read. With zero value, records are read one
after the other for each new call, while non-zero values may be used to navigate
back and forth, and e.g. return to the same initial state several times.
= 0 : read the next record.
= +n : skip ahead n records before reading the record that sets the state of therandom number generator.
= −n : back up n records before reading the record that sets the state of the random
number generator.
COMMON/PYDATR/MRPY(6),RRPY(100)Purpose: to contain the state of the random number generator at any moment (forcommunication between PYR, PYRGET and PYRSET), and also to provide you withthe possibility to initialize different random number sequences, and to know how
many numbers have been generated.
MRPY(1) : (D = 19780503) the integer number that specifies which of the possible subsequences will be initialized in the next PYR call for which MRPY(2) = 0. Allowed
values are 0 ≤ MRPY(1) ≤ 900 000 000, the original Marsaglia (and CERN library)
seed is 54217137. The MRPY(1) value is not changed by any of the Pythia routines.
MRPY(2) : (D = 0) initialization flag, put to 1 in the first PYR call of run. A re49

initialization of the random number generator can be made in mid-run by resetting MRPY(2) to 0 by hand. In addition, any time the counter MRPY(3) reaches
1000000000, it is reset to 0 and MRPY(2) is increased by 1.
MRPY(3) : (R) counter for the number of random numbers generated from the beginningof the run. To avoid overflow when very many numbers are generated, MRPY(2)is used as described above.
MRPY(4), MRPY(5) : I97 and J97 of the CERN library implementation; part of the stateof the generator.
MRPY(6) : (R) current position, i.e. how many records after beginning, in the file; usedby PYRGET and PYRSET.
RRPY(1) - RRPY(97) : the U array of the CERN library implementation; part of thestate of the generator.
RRPY(98) - RRPY(100) : C, CD and CM of the CERN library implementation; the firstpart of the state of the generator, the latter two constants calculated at initialization.

50

5

The Event Record

The event record is the central repository for information about the particles produced in
the current event: flavours, momenta, event history, and production vertices. It plays avery central rôle: without a proper understanding of what the record is and how information is stored, it is meaningless to try to use Pythia. The record is stored in the common
block PYJETS. Almost all the routines that the user calls can be viewed as performingsome action on the record: fill a new event, let partons fragment or particles decay, boost
it, list it, find clusters, etc.
In this section we will first describe the KF flavour code, subsequently the PYJETS
common block, and then give a few comments about the rôle of the event record in the
programs.
To ease the interfacing of different event generators, a HEPEVT standard common-block
structure for the event record has been agreed on. For historical reasons the standard
common blocks are not directly used in Pythia, but a conversion routine comes with theprogram, and is described at the end of this section.

5.1

Particle Codes

The Particle Data Group particle code [PDG88, PDG92, PDG00] is used consistently
throughout the program. Almost all known discrepancies between earlier versions of the
PDG standard and the Pythia usage have now been resolved. The one known exception isthe (very uncertain) classification of f0 (980), with f0 (1370) also affected as a consequence.
There is also a possible point of confusion in the technicolor sector between π 0 0tc andηtc . The latter is retained for historical reasons, whereas the former was introduced forconsistency in models of top-color-assisted technicolor. The PDG standard, with thelocal Pythia extensions, is referred to as the KF particle code. This code you have to bethoroughly familiar with. It is described below.
The KF code is not convenient for a direct storing of masses, decay data, or otherparticle properties, since the KF codes are so spread out. Instead a compressed code KC
between 1 and 500 is used here. A particle and its antiparticle are mapped to the same
KC code, but else the mapping is unique. Normally this code is only used at very specificplaces in the program, not visible to the user. If need be, the correspondence can always
be obtained by using the function PYCOMP, i.e. KC = PYCOMP(KF). This mapping is nothardcoded, but can be changed by user intervention, e.g. by introducing new particles
with the PYUPDA facility. It is therefore not intended that you should ever want or needto know any KC codes at all. It may be useful to know, however, that for codes smaller
than 80, KF and KC agree. Normally a user would never do the inverse mapping, but wenote that this is stored as KF = KCHG(KC,4), making use of the KCHG array in the PYDAT2
common block. Of course, the sign of a particle could never be recovered by this inverse
operation.
The particle names printed in the tables in this section correspond to the ones obtainedwith the routine PYNAME, which is used extensively, e.g. in PYLIST. Greek characters
are spelt out in full, with a capital first letter to correspond to a capital Greek letter.
Generically the name of a particle is made up of the following pieces:
1. The basic root name. This includes a * for most spin 1 (L = 0) mesons and spin
3/2 baryons, and a 0 for some spin 1/2 baryons (where there are two states to be
distinguished, cf. Λ–Σ0 ). The rules for heavy baryon naming are in accordance withthe 1986 Particle Data Group conventions [PDG86]. For mesons with one unit oforbital angular momentum, K (D, B, . . . ) is used for quark-spin 0 and K* (D*, B*,. . . ) for quark-spin 1 mesons; the convention for ‘*’ may here deviate slightly fromthe one used by the PDG.

51

KF
1
2
3
4
5
6
7
8
9
10

Table 3: Quark and lepton codes.
Name Printed KF Name Printed
d
d
11
e−
eu
u
12
νe
nu e
s
s
13
µ−
muc
c
14
νµ
nu mu
b
b
15
τ−
taut
t
16
ντ
nu tau
b0
b’
17
τ0
tau’
t0
t’
18
ντ0
nu’ tau
19
20

2. Any lower indices, separated from the root by a . For heavy hadrons, this is the
additional heavy-flavour content not inherent in the root itself. For a diquark, it is
the spin.
3. The characters ‘bar’ for an antiparticle, wherever the distinction between particle
and antiparticle is not inherent in the charge information.
4. Charge information: ++, +, 0, −, or −−. Charge is not given for quarks or diquarks.
Some neutral particles which are customarily given without a 0 also here lack it,such as neutrinos, g, γ, and flavour-diagonal mesons other than π 0 and ρ0 . Notethat charge is included both for the proton and the neutron. While non-standard,
it is helpful in avoiding misunderstandings when looking at an event listing.
Below follows a list of KF particle codes. The list is not complete; a more extensive one
may be obtained with CALL PYLIST(11). Particles are grouped together, and the basic
rules are described for each group. Whenever a distinct antiparticle exists, it is given thesame KF code with a minus sign (whereas KC codes are always positive).
1. Quarks and leptons, Table 3.
This group contains the basic building blocks of matter, arranged according tofamily, with the lower member of weak isodoublets also having the smaller code
(thus d precedes u). A fourth generation is included as part of the scenarios forexotic physics. The quark codes are used as building blocks for the diquark, mesonand baryon codes below.
2. Gauge bosons and other fundamental bosons, Table 4.
This group includes all the gauge and Higgs bosons of the Standard Model, as well
as some of the bosons appearing in various extensions of it. They correspond to
one extra U(1) and one extra SU(2) group, a further Higgs doublet, a graviton,
a horizontal gauge boson R (coupling between families), and a (scalar) leptoquark
LQ .
3. Exotic particle codes.
The positions 43–80 are used as temporary sites for exotic particles that eventually
may be shifted to a separate code sequence. Currently this list only consists of theparticle codes 45 and 46, described among the Supersymmetric codes below. The
ones not in use are at your disposal, but with no guarantees that they will remainso.
4. Various special codes, Table 5.
In a Monte Carlo, it is always necessary to have codes that do not correspond to
any specific particle, but are used to lump together groups of similar particles fordecay treatment (nowadays largely obsolete), to specify generic decay products (alsoobsolete), or generic intermediate states in external processes, or additional event
record information from jet searches. These codes, which again are non-standard,
52

Table 4: Gauge boson and other fundamental boson codes.
KF Name Printed KF Name Printed
21
g
g
31
22
γ
gamma
32
Z00
Z’0
0
23
Z
Z0
33
Z000
Z"0
24 W+
W+
34 W0+
W’+
0
0
25
h
h0
35
H
H0
26
36
A0
A0
27
37
H+
H+
28
38
29
39
G
Graviton
30
40
41
R0
R0
42
LQ
LQ
are found between numbers 81 and 100.
The junction, code 88, is not a physical particle but marks the place in the event
record where three string pieces come together in a point, e.g. a Y-shaped topology with a quark at each end. No distinction is made between a junction and anantijunction, i.e. whether a baryon or an antibaryon is going to be produced in theneighbourhood of the junction.
5. Diquark codes, Table 6.
A diquark made up of a quark with code i and another with code j, where i ≥ j,and with total spin s, is given the code
KF = 1000i + 100j + 2s + 1 ,(16)

i.e. the tens position is left empty (cf. the baryon code below). Some of the most

KF
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100

Printedspecflav
rndmflavphasespa
c-hadronb-hadronjunction
systemclusterstringindep.
CMshower
SPHEaxis
THRUaxis
CLUSjetCELLjettable

Table 5: Various special codes.
Meaning
Spectator flavour; used in decay-product listings
A random u, d, or s flavour; possible decay productSimple isotropic phase-space decay
Information on decay of generic charm hadron
Information on decay of generic bottom hadron
A junction of three string pieces(internal use for unspecified resonance data)
Intermediate pseudoparticle in external process
Parton system in cluster fragmentation
Parton system in string fragmentation
Parton system in independent fragmentation
Four-momentum of time-like showering system
Event axis found with PYSPHEEvent axis found with PYTHRU
Jet (cluster) found with PYCLUS
Jet (cluster) found with PYCELL
Tabular output from PYTABU

53

Table 6: Diquark codes. For brevity, diquarks containing c or b quarks are not listed, but
are defined analogously.
KF Name Printed
KF Name Printed
1103 dd1
dd 12101 ud0
ud 02103 ud1
ud 12203 uu1
uu 1
3101
sd0
sd 0
3103
sd1
sd 1
3201
su0
su 0
3203
su1
su 1
3303
ss1
ss 1
frequently used codes are listed in the table. All the lowest-lying spin 0 and 1
diquarks are included in the program.
6. Meson codes, Tables 7 and 8.
A meson made up of a quark with code i and an antiquark with code −j, j 6= i,and with total spin s, is given the code
KF = {100 max(i, j) + 10 min(i, j) + 2s + 1} sign(i − j) (−1)max(i,j) ,(17)

assuming it is not orbitally or radially excited. Note the presence of an extra − sign
if the heaviest quark is a down-type one. This is in accordance with the particle–
antiparticle distinction adopted in the 1986 Review of Particle Properties [PDG86].
It means for example that a B meson contains a b antiquark rather than a b quark.
The flavour-diagonal states are arranged in order of ascending mass. Thus theobvious generalization of eq. (17) to KF = 110i + 2s + 1 is only valid for charm andbottom. The lighter quark states can appear mixed, e.g. the π 0 (111) is an equal
mixture of dd (naı̈vely code 111) and uu (naı̈vely code 221).
The standard rule of having the last digit of the form 2s + 1 is broken for the K0S –K0L
system, where it is 0, and this convention should carry over to mixed states in the
B meson system, should one choose to define such. For higher multiplets with thesame spin, ±10000, ±20000, etc., are added to provide the extra distinction needed.
Some of the most frequently used codes are given below.
The full lowest-lying pseudoscalar and vector multiplets are included in the program,
Table 7.
Also the lowest-lying orbital angular momentum L = 1 mesons are included, Table 8:
one pseudovector multiplet obtained for total quark-spin 0 (L = 1, S = 0 ⇒ J = 1)
and one scalar, one pseudovector and one tensor multiplet obtained for total quarkspin 1 (L = 1, S = 1 ⇒ J = 0, 1 or 2), where J is what is conventionally called thespin s of the meson. Any mixing between the two pseudovector multiplets is nottaken into account. Please note that some members of these multiplets have still
not been found, and are included here only based on guesswork. Even for known
ones, the information on particles (mass, width, decay modes) is highly incomplete.
Only two radial excitations are included, the ψ 0 = ψ(2S) and Υ0 = Υ(2S).
7. Baryon codes, Table 9.
A baryon made up of quarks i, j and k, with i ≥ j ≥ k, and total spin s, is given
the code
KF = 1000i + 100j + 10k + 2s + 1 .
(18)
An exception is provided by spin 1/2 baryons made up of three different types ofquarks, where the two lightest quarks form a spin-0 diquark (Λ-like baryons). Herethe order of the j and k quarks is reversed, so as to provide a simple means ofdistinction to baryons with the lightest quarks in a spin-1 diquark (Σ-like baryons).

54

KF
211
311
321
411
421
431
511
521
531
541
111
221
331
441
551
130
310

Table 7: Meson codes, part 1.
Name Printed KF Name Printed
π+
pi+
213
ρ+
rho+
0
K
K0
313 K∗0
K*0
+
∗+
K
K+
323 K
K*+
D+
D+
413 D∗+D*+
D0
D0
423 D∗0
D*0
+
∗+
Ds
D s+
433 Ds
D* s+
B0
B0
513
B∗0
B*0
B+
B+
523 B∗+
B*+
0
∗0
Bs
B s0
533
Bs
B* s0
∗+
B+
B
c+
543
B
B* c+
c
c
π0
pi0
113
ρ0
rho0
η
eta
223
ω
omega
η0
eta’
333
φ
phi
ηc
eta c
443 J/ψ
J/psi
ηb
eta b
553
Υ
Upsilon
0
KL
K L0
K0S
K S0

Table 8: Meson codes, part 2. For brevity, states with b quark are omitted from thislisting, but are defined in the program.
KF
Name Printed
KF
Name Printed
10213
b1
b 1+
10211
a+
a 0+
0
0
10313
K1
K 10
10311 K∗0
K*
00
0
+
∗+
10323
K1
K 1+
10321 K0
K* 0+
∗+
10413
D+
D
1+
10411
D
D* 0+
1
0
10423
D01
D 10
10421 D∗0
D* 00
0
+
∗+
10433
D1s
D 1s+
10431 D0s
D* 0s+
10113
b01
b 10
10111
a00
a 00
10223
h01
h 10
10221
f00
f 00
00
00
10333
h1
h’ 10
10331
f0
f’ 00
10443
h01c
h 1c0
10441
χ00c
chi 0c0
20213
a+
a 1+
215
a+
a 2+
1
2
∗0
20313
K∗0
K*
10
315
K
K*
20
1
2
∗+
20323
K∗+
K*
1+
325
K
K*
2+
1
2
∗+
∗+
20413
D1
D* 1+
415
D2
D* 2+
∗0
20423
D∗0
D*
10
425
D
D* 20
1
2
∗+
∗+
20433
D1s
D* 1s+435
D2s
D* 2s+
20113
a01
a 10
115
a02
a 20
20223
f10
f 10
225
f20
f 20
00
00
20333
f1
f’ 10
335
f2
f’ 20
20443
χ01c
chi 1c0445
χ02cchi 2c0100443ψ0psi’
0100553
Υ
Upsilon’55

Table 9: Baryon codes. For brevity, some states with b quarks or multiple c ones areomitted from this listing, but are defined in the program.
KF NamePrintedKF NamePrinted−1114∆
Delta2112
n
n0
2114
∆0
Delta0
2212
p
p+
2214
∆+
Delta+++
2224 ∆
Delta++
3112
Σ−Sigma3114 Σ∗−
Sigma*3122
Λ0Lambda03212
Σ0Sigma03214
Σ∗0Sigma*0+∗+3222
ΣSigma+
3224 Σ
Sigma*+
3312
Ξ−Xi3314 Ξ∗−
Xi*3322
Ξ0
Xi0
3324
Ξ∗0
Xi*0
−
3334
Ω
Omega4112
Σ0c
Sigma c0 4114
Σ∗0Sigma*c0c4122
Λ+Lambdac+c4212
Σ+Sigma c+ 4214 Σ∗+
Sigma* c+
c
c
++
∗++
4222 Σc
Sigma c++ 4224 Σc
Sigma* c++
4132
Ξ0c
Xi c0
4312
Ξ00
Xi’ c04314
Ξ∗0
Xi* c0
c
c
+
4232
Ξc
Xi c+
4322
Ξ0+
Xi’
c+
4324 Ξ∗+Xi* c+
c
c
0
4332
Ωc
Omega c0 4334Ω∗0Omega*
c0
c
−
∗−
5112
Σb
Sigma b- 5114 Σb
Sigma* b5122
Λ0b
Lambda b05212
Σ0b
Sigma b0 5214
Σ∗0Sigma* b0
b
+
∗+
5222
Σb
Sigma b+ 5224 Σb
Sigma* b+
For hadrons with heavy flavours, the root names are Lambda or Sigma for hadrons
with two u or d quarks, Xi for those with one, and Omega for those without u or dquarks.
Some of the most frequently used codes are given in Table 9. The full lowest-lying
spin 1/2 and 3/2 multiplets are included in the program.
8. QCD effective states, Table 10.
We here include the pomeron IP and reggeon IR ‘particles’, which are important e.g.
in the description of diffractive scattering, but do not have a simple correspondencewith other particles in the classification scheme.
Also included are codes to be used for denoting diffractive states in Pythia, as partof the event history. The first two digits here are 99 to denote the non-standard
character. The second, third and fourth last digits give flavour content, while thevery last one is 0, to denote the somewhat unusual character of the code. Only a
few codes have been introduced with names; depending on circumstances these also
have to double up for other diffractive states. Other diffractive codes for strange
mesons and baryon beams are also accepted by the program, but do not give niceprintouts.
9. Supersymmetric codes, Table 11.
SUSY doubles the number of states of the Standard Model (at least). Fermions
have separate superpartners to the left- and right-handed components. In the third
generation these are assumed to mix to nontrivial mass eigenstates, while mixing
is not included in the first two. Note that all sparticle names begin with a tilde.
Default masses are arbitrary and branching ratios not set at all. This is taken
56

Table 10: QCD effective states.
KF
PrintedMeaning110
reggeonreggeon IR
990
pomeronpomeron IP
9900110 rho diff0 Diffractive π 0 /ρ0 /γ state9900210 pi diffr+
Diffractive π + state9900220 omega di0
Diffractive ω state
9900330 phi diff0
Diffractive φ state9900440 J/psi di0
Diffractive J/ψ state
9902110n diffr
Diffractive n state9902210 p diffr+
Diffractive p statecare of at initialization if IMSS(1) is positive. The H03 , A02 and χ̃05 states at thebottom of the table only appear in the Next-to-Minimal Supersymmetric Standard
Model (NMSSM), do not have standardized codes and are not fully implemented in
Pythia, but can optionally (see IMSS(13)) be used in the context of interfaces toother programs.
10. Technicolor codes, Table 12.
A set of colourless and coloured technihadrons have been included. The colourless technivector mesons and most of the colourless technipions are associatedwith the original strawman model of technicolor. The coloured technirho mesons(KF= 3100113, 3200113, 3300113 and 3100113), a Coloron (or V8 ), and additional
colour singlet (KF= 3100111) and colour octet (KF=3100111) technipions arise in
the extended model of Topcolor assisted Technicolor (TC2). Additional indices onthese technihadrons refer to two strongly interacting groups SU(3)1 ×SU(3)2 , one
for the first two generations and a second for the third generation, which is broken
down to ordinary SU(3)C .
The ηtc belongs to an older iteration of Technicolor modelling than the rest. It was

KF
1000001
1000002
1000003
1000004
1000005
1000006
1000011
1000012
1000013
1000014
1000015
1000016
1000021
1000022
1000023
1000024
45
46

Table 11: Supersymmetric codes.
Name
Printed
KF
Name
d̃L
d̃R
∼d L
2000001
ũL∼u L
2000002
ũR
s̃L
∼s L
2000003
s̃R
c̃L
∼c L
2000004
c̃R
b̃1
b̃2
∼b 1
2000005t̃1∼t 1
2000006t̃2
ẽL∼e L2000011
ẽRν̃eL∼nu eL
2000012ν̃eRµ̃L∼mu L2000013µ̃Rν̃µL∼nu muL 2000014 ν̃µRτ̃1∼tau L- 2000015
τ̃2
ν̃τ L
∼nu tauL 2000016
ν̃τ R
g̃
∼g
1000025
χ̃03
χ̃01∼chi 10 1000035
χ̃04
0
χ̃2
∼chi 20 1000037
χ̃+
2
χ̃+
∼chi 1+ 1000039
G̃
1
H03
H 30
1000045
χ̃05
0
A2
A 20

57

Printed
∼d R
∼u R
∼s R
∼c R
∼b 2
∼t 2
∼e R∼nu eR∼mu R∼nu muR∼tau R∼nu tauR∼chi 30∼chi 40∼chi 2+∼Gravitino∼chi 50

KF
3000111
3000211
3000221
300011330002133000223
3000331

Table 12: Technicolor codes.
Name
Printed
KF
Name
0
πtc
pi tc03100021 V8,tc+
0πtc
pi tc+
3100111 π22,1,tc0
π 0 0tc
pi’ tc03200111 π22,8,tc
ρ0tc
rho tc0
3100113 ρ011,tc+
ρtc
rho tc+3200113 ρ012,tc
0ωtc
omega tc0 3300113 ρ021,tc
ηtc
eta tc0
3400113 ρ022,tc

Printed
V8 tc
pi 22 1 tc
pi 22 8 tc
rho 11 tc
rho 12 tc
rho 21 tc
rho 22 tc

originally given the 3000221 code, and thereby now comes to clash with the π 0 0tc of
the current main scenario. Since the ηtc is one-of-a-kind, it was deemed better tomove it to make way for the π 0 0tc . This leads to a slight inconsistency with the PDG
codes.
11. Excited fermion codes, Table 13.
A first generation of excited fermions are included.
12. Exotic particle codes, Table 14.
This section includes the excited graviton, as the first (but probably not last) manifestation of the possibility of large extra dimensions. Although it is not yet in the
PDG standard, we assume that such states will go in a new series of numbers.
Included is also a set of particles associated with an extra SU(2) gauge group forright-handed states, as required in order to obtain a left–right symmetric theory
at high energies. This includes right-handed (Majorana) neutrinos, right-handed±
Z0R and WR
gauge bosons, and both left- and right-handed doubly charged Higgs
bosons. Such a scenario would also contain other Higgs states, but these do notbring anything new relative to the ones already introduced, from an observational
point of view. Here the first two digits are 99 to denote the non-standard character.
13. Colour octet state codes, Table 15.
Within the colour octet approach to charmonium and bottomonium production, intermediate colour octet states can be produced and subsequently ‘decay’ to the normal singlet states by soft-gluon emission. The codes have been chosen 9900000 bigger than the respective colour-singlet state, so that they occur among the generatorspecific codes. The names are based on spectroscopic notation, with additional
upper index (8) to reflect the colour octet nature.
A hint on large particle numbers: if you want to avoid mistyping the number of zeros,
it may pay off to define a statement like
PARAMETER (KSUSY1=1000000,KSUSY2=2000000,KTECHN=3000000,&KEXCIT=4000000,KDIMEN=5000000)
at the beginning of your program and then refer to particles as KSUSY1+1 = d̃L and soon. This then also agrees with the internal notation (where feasible).

5.2

The Event Record

Each new event generated is in its entirety stored in the common block PYJETS, which
thus forms the event record. Here each parton or particle that appears at some stage

KF
4000001
4000002

Table 13: Excited fermion codes.
Name Printed
KF
Name
u∗
d*
4000011e∗∗du*
4000012νe∗58

Printede*nu* e0

KF
5000039
9900012
9900014
9900016

Table 14: Exotic particle codes.
Name
PrintedKF
Name
G∗
Graviton*νRe
nu Re
9900023Z0R+
νRµnu Rmu9900024 WR++
νRτ
nu Rtau
9900041 HL
9900042 H++
R

Printed
Z R0
W R+
H L++
H R++

of the fragmentation or decay chain will occupy one line in the matrices. The different
components of this line will tell which parton/particle it is, from where it originates, itspresent status (fragmented/decayed or not), its momentum, energy and mass, and thespace–time position of its production vertex. Note that K(I,3)–K(I,5) and the P and V
vectors may take special meaning for some specific applications (e.g. sphericity or cluster
analysis), as described in those connections.
The event history information stored in K(I,3)–K(I,5) should not be taken too literally. In the particle decay chains, the meaning of a mother is well-defined, but the
fragmentation description is more complicated. The primary hadrons produced in string
fragmentation come from the string as a whole, rather than from an individual parton.
Even when the string is not included in the history (see MSTU(16)), the pointer fromhadron to parton is deceptive. For instance, in a qgq event, those hadrons are pointing
towards the q (q) parton that were produced by fragmentation from that end of the string,according to the random procedure used in the fragmentation routine. No particles point
to the g. This assignment seldom agrees with the visual impression, and is not intendedto.
The common block PYJETS has expanded with time, and can now house 4000 entries.
This figure may seem ridiculously large, but actually the previous limit of 2000 wasoften reached in studies of high-p⊥ processes at the LHC (and SSC). This is because
the event record contains not only the final particles, but also all intermediate partonsand hadrons, which subsequently showered, fragmented or decayed. Included are also a
wealth of photons coming from π 0 decays; the simplest way of reducing the size of theevent record is actually to switch off π 0 decays by MDCY(PYCOMP(111),1) = 0. Also notethat some routines, such as PYCLUS and PYCELL, use memory after the event record proper
as a working area. Still, to change the size of the common block, upwards or downwards,
is easy: just do a global substitute in the common block and change the MSTU(4) value tothe new number. If more than 10000 lines are to be used, the packing of colour information
should also be changed, see MSTU(5).
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
Purpose: to contain the event record, i.e. the complete list of all partons and particles(initial, intermediate and final) in the current event. (By parton we here mean
the subclass of particles that carry colour, for which extra colour flow information

KF
9900443
9900441
9910443

Table 15: Colour octet state codes.
Name
PrintedKF
NamePrinted3 (8)3 (8)cc[ S1 ] cc∼[3S18] 9900553 bb[ S1 ] bb∼[3S18](8)(8)
cc[1 S0 ] cc∼[1S08] 9900551 bb[1 S0 ] bb∼[1S08](8)(8)
cc[3 P0 ] cc∼[3P08] 9910553 bb[3 P0 ] bb∼[3P08]59

is then required. Normally this means quarks and gluons, which can fragment
to hadrons, but also squarks and other exotic particles fall in this category.)N :number of lines in the K, P and V matrices occupied by the current event. N
is continuously updated as the definition of the original configuration and thetreatment of fragmentation and decay proceed. In the following, the individual
parton/particle number, running between 1 and N, is called I.
NPAD : dummy to ensure an even number of integers before the double precision reals,
as required by some compilers.
K(I,1) : status code KS, which gives the current status of the parton/particle stored
in the line. The ground rule is that codes 1–10 correspond to currently existing
partons/particles, while larger codes contain partons/particles which no longer
exist, or other kinds of event information.
= 0 : empty line.
= 1 : an undecayed particle or an unfragmented parton, the latter being either asingle parton or the last one of a parton system.
= 2 : an unfragmented parton, which is followed by more partons in the same
colour-singlet parton system.
= 3 : an unfragmented parton with special colour flow information stored inK(I,4) and K(I,5), such that adjacent partons along the string need not
follow each other in the event record.
= 4 : a particle which could have decayed, but did not within the allowed volume
around the original vertex.
= 5 : a particle which is to be forced to decay in the next PYEXEC call, in thevertex position given (this code is only set by user intervention).
= 11 : a decayed particle or a fragmented parton, the latter being either a single
parton or the last one of a parton system, cf. = 1.
= 12 : a fragmented parton, which is followed by more partons in the same coloursinglet parton system, cf. = 2. Further, a B meson which decayed as a B
one, or vice versa, because of B–B mixing, is marked with this code rather
than = 11.
= 13 : a parton which has been removed when special colour flow information has
been used to rearrange a parton system, cf. = 3.
= 14 : a parton which has branched into further partons, with special colour-flow
information provided, cf. = 3.
= 15 : a particle which has been forced to decay (by user intervention), cf. = 5.
= 21 : documentation lines used to give a compressed story of the event at the
beginning of the event record.
= 31 : lines with information on sphericity, thrust or cluster search.
= 32 : tabular output, as generated by PYTABU.
= 41 : a junction, with partons arranged in colour, except that two quark lines
may precede or follow a junction. For instance, a configuration like
q1 g1 q2 g2 (junction) g3 q3 corresponds to having three strings q1 g1 , q2 g2 andq3 g3 meeting in the junction. The occurence of non-matching colours easily reveal the q2 as not being a continuation of the q1 g1 string. Hereeach g above is shorthand for an arbitrary number of gluons, including
none. The most general topology allows two junctions in a system, i.e.
q1 g1 q2 g2 (junction) g0 (junction) g3 q3 g4 q4 . The final q/q would have status
code 1, the other partons 2. Thus code = 41 occurs where = 2 would normally have been used, had the junction been an ordinary parton.
= 42 : a junction, with special colour flow information stored in K(I,4) and K(I,5),
such that adjacent partons along the string need not follow each other inthe event record. Thus this code matches the = 3 of ordinary partons.
= 51 : a junction of strings which have been fragmented, cf. = 41. Thus this code60

matches the = 12 of ordinary partons.
= 52 : a junction of strings which have been rearranged in colour, cf. = 42. Thus
this code matches the = 13 of ordinary partons.
< 0 : these codes are never used by the program, and are therefore usually notaffected by operations on the record, such as PYROBO, PYLIST and eventanalysis routines (the exception is some PYEDIT calls, where lines are moved
but not deleted). Such codes may therefore be useful in some connections.
K(I,2) : particle KF code, as described in section 5.1.
K(I,3) : line number of parent particle, where known, otherwise 0. Note that the assignment of a particle to a given parton in a parton system is unphysical, andwhat is given there is only related to the way the fragmentation was generated.
K(I,4) : normally the line number of the first daughter; it is 0 for an undecayed particle
or unfragmented parton.
For K(I,1) = 3, 13 or 14, instead, it contains special colour-flow information(for internal use only) of the form
K(I,4) = 200000000*MCFR + 100000000*MCTO + 10000*ICFR + ICTO,
where ICFR and ICTO give the line numbers of the partons from which thecolour comes and to where it goes, respectively; MCFR and MCTO originallyare 0 and are set to 1 when the corresponding colour connection has been traced
in the PYPREP rearrangement procedure. (The packing may be changed withMSTU(5).) The ‘from’ colour position may indicate a parton which branchedto produce the current parton, or a parton created together with the current
parton but with matched anticolour, while the ‘to’ normally indicates a partonthat the current parton branches into. Thus, for setting up an initial colour
configuration, it is normally only the ‘from’ part that is used, while the ‘to’ partis added by the program in a subsequent call to parton-shower evolution (forfinal-state radiation; it is the other way around for initial-state radiation). Note
that, when using PYEVNW to generate events, a complementary way of storing
the colour flow information is also used, so-called Les Houches style colour tags
[Boo01], cf. the /PYCTAG/ common block.
For K(I,1) = 42 or 52, see below.
Note: normally most users never have to worry about the exact rules for colourflow storage, since this is used mainly for internal purposes. However, when it isnecessary to define this flow, it is recommended to use the PYJOIN routine, since
it is likely that this would reduce the chances of making a mistake.
K(I,5) : normally the line number of the last daughter; it is 0 for an undecayed particle
or unfragmented parton.
For K(I,1) = 3, 13 or 14, instead, it contains special colour-flow information(for internal use only) of the form
K(I,5) = 200000000*MCFR + 100000000*MCTO + 10000*ICFR + ICTO,
where ICFR and ICTO give the line numbers of the partons from which theanticolour comes and to where it goes, respectively; MCFR and MCTO originallyare 0 and are set to 1 when the corresponding colour connection has been traced
in the PYPREP rearrangement procedure. For further discussion, see K(I,4).
For K(I,1) = 42 or 52, see below.
K(I,4), K(I,5) : For junctions with K(I,1) = 42 or 52 the colour flow information
scheme presented above has to be modified, since now three colour or anticolourlines meet. Thus the form isK(I,4) = 100000000*MC1 + 10000*ITP + IC1,
K(I,5) = 200000000*MC2 + 100000000*MC3 + 10000*IC2 + IC3.
The colour flow possibilities are
ITP = 1 : junction of three colours in the final state, with positions as stored in
IC1, IC2 and IC3. A typical example would be neutralino decay to three61

quarks. Note that the positions need not be filled by the line numbers of
the final quark themselves, but more likely by the immediate neutralino
decay products that thereafter initiate showers and branch further.
ITP = 2 : junction of three anticolours in the final state, with positions as
stored in IC1, IC2 and IC3.
ITP = 3 : junction of one incoming anticolour to two outgoing colours, withthe anticolour position stored in IC1 and the two colour ones in IC2
and IC3. A typical example would be an antisquark decaying to twoquarks.
ITP = 4 : junction of one incoming colour to two outgoing anticolours, withthe colour position stored in IC1 and the two anticolour ones in IC2
and IC3.
ITP = 5 : junction of a colour octet into three colours. The incoming colour is
supposed to pass through unchanged, and so is bookkept as usual forthe particle itself. IC1 is the position of the incoming anticolour, while
IC2 and IC3 are the positions of the new colours associated with thevanishing of this anticolour. A typical example would be gluino decay
to three quarks.
ITP = 6 : junction of a colour octet into three anticolours. The incoming anticolour is supposed to pass through unchanged, and so is bookkept as
usual for the particle itself. IC1 is the position of the incoming colour,
while IC2 and IC3 are the positions of the new anticolours associatedwith the vanishing of this colour.
Thus odd (even) ITP code corresponds to a +1 (−1) change in baryon number
across the junction.
The MC1, MC2 and MC3 mark which colour connections have been traced in a
PYPREP rearrangement procedure, as above.
P(I,1) : px , momentum in the x direction, in GeV/c.
P(I,2) : py , momentum in the y direction, in GeV/c.
P(I,3) : pz , momentum in the z direction, in GeV/c.
P(I,4) : E, energy, in GeV.
P(I,5) : m, mass, in GeV/c2 . In parton showers, with space-like virtualities, i.e. where
Q2 = −m2 > 0, one puts P(I,5)= −Q.
V(I,1) : x position of production vertex, in mm.
V(I,2) : y position of production vertex, in mm.
V(I,3) : z position of production vertex, in mm.
V(I,4) : time of production, in mm/c (≈ 3.33 × 10−12 s).
V(I,5) : proper lifetime of particle, in mm/c (≈ 3.33 × 10−12 s). If the particle is notexpected to decay, V(I,5) = 0. A line with K(I,1) = 4, i.e. a particle that
could have decayed, but did not within the allowed region, has the proper nonzero V(I,5).
In the absence of electric or magnetic fields, or other disturbances, the decay
vertex VP of an unstable particle may be calculated as
VP(j) = V(I,j) + V(I,5)*P(I,j)/P(I,5), j = 1–4.

5.3

How The Event Record Works

The event record is the main repository for information about an event. In the generation
chain, it is used as a ‘scoreboard’ for what has already been done and what remains to
do. This information can be studied by you, to access information not only about the
final state, but also about what came before.

62

5.3.1

A simple example

The first example of section 3.5 may help to clarify what is going on. When PY2ENT iscalled to generate a qq pair, the quarks are stored in lines 1 and 2 of the event record,
respectively. Colour information is set to show that they belong together as a colour
singlet. The counter N is also updated to the value of 2. At no stage is a previously
generated event removed. Lines 1 and 2 are overwritten, but lines 3 onwards still containwhatever may have been there before. This does not matter, since N indicates where the‘real’ record ends.
As PYEXEC is called, explicitly by you or indirectly by PY2ENT, the first entry is considered and found to be the first parton of a system. Therefore the second entry is also
found, and these two together form a colour singlet parton system, which may be allowed
to fragment. The ‘string’ that fragments is put in line 3 and the fragmentation products
in lines 4 through 10 (in this particular case). At the same time, the q and q in the firsttwo lines are marked as having fragmented, and the same for the string. At this stage, N
is 10. Internally in PYEXEC there is another counter with the value 2, which indicates how
far down in the record the event has been studied.
This second counter is gradually increased by one. If the entry in the corresponding
line can fragment or decay, then fragmentation or decay is performed. The fragmentation/decay products are added at the end of the event record, and N is updated accordingly.
The entry is then also marked as having been treated. For instance, when line 3 is considered, the ‘string’ entry of this line is seen to have been fragmented, and no action istaken. Line 4, a ρ+ , is allowed to decay to π + π 0 ; the decay products are stored in lines 11
and 12, and line 4 is marked as having decayed. Next, entry 5 is allowed to decay. The
entry in line 6, π + , is a stable particle (by default) and is therefore passed by without any
action being taken.
In the beginning of the process, entries are usually unstable, and N grows faster than
the second counter of treated entries. Later on, an increasing fraction of the entries arestable end products, and the rôles are now reversed, with the second counter growing
faster. When the two coincide, the end of the record has been reached, and the process
can be stopped. All unstable objects have now been allowed to fragment or decay. They
are still present in the record, so as to simplify the tracing of the history.
Notice that PYEXEC could well be called a second time. The second counter would thenstart all over from the beginning, but slide through until the end without causing any
action, since all objects that can be treated already have been. Unless some of the relevant
switches were changed meanwhile, that is. For instance, if π 0 decays were switched off
the first time around but on the second, all the π 0 ’s found in the record would be allowed
to decay in the second call. A particle once decayed is not ‘undecayed’, however, so if theπ 0 is put back stable and PYEXEC is called a third time, nothing will happen.
5.3.2

Complete PYTHIA events

In a full-blown event generated with Pythia, the usage of PYJETS is more complicated,
although the general principles survive. PYJETS is used extensively by many of the generation routines; indeed it provides the bridge between many of them. The Pythia event
listing begins (optionally) with a few lines of event summary, specific to the hard processsimulated and thus not described in the overview above. These specific parts are coveredin the following.
In most instances, only the particles actually produced are of interest. For MSTP(125)= 0, the event record starts off with the parton configuration existing after hard interaction, initial- and final-state radiation, multiple interactions and beam remnants have been
considered. The partons are arranged in colour singlet clusters, ordered as required forstring fragmentation. Also photons and leptons produced as part of the hard interaction(e.g. from qq → gγ or uu → Z0 → e+ e− ) appear in this part of the event record. These63

original entries appear with pointer K(I,3) = 0, whereas the products of the subsequentfragmentation and decay have K(I,3) numbers pointing back to the line of the parent.
The standard documentation, obtained with MSTP(125) = 1, includes a few lines atthe beginning of the event record, which contain a brief summary of the process thathas taken place. The number of lines used depends on the nature of the hard process
and is stored in MSTI(4) for the current event. These lines all have K(I,1) = 21. For
all processes, lines 1 and 2 give the two incoming particles. When listed with PYLIST,
these two lines will be separated from subsequent ones by a sequence of ‘======’ signs, toimprove readability. For diffractive and elastic events, the two outgoing states in lines 3
and 4 complete the list. Otherwise, lines 3 and 4 contain the two partons that initiate thetwo initial-state parton showers, and 5 and 6 the end products of these showers, i.e. thepartons that enter the hard interaction. With initial-state radiation switched off, lines 3
and 5 and lines 4 and 6 are identical. For a simple 2 → 2 hard scattering, lines 7 and 8give the two outgoing partons/particles from the hard interaction, before any final-stateradiation. For 2 → 2 processes proceeding via an intermediate resonance such as γ ∗ /Z0 ,W± or h0 , the resonance is found in line 7 and the two outgoing partons/particles in 8 and
9. In some cases one of these may be a resonance in its own right, or both of them, so thatfurther pairs of lines are added for subsequent decays. If the decay of a given resonancehas been switched off, then no decay products are listed either in this initial summary
or in the subsequent ordinary listing. Whenever partons are listed, they are assumed to
be on the mass shell for simplicity. The fact that effective masses may be generated byinitial- and final-state radiation is taken into account in the actual parton configurationthat is allowed to fragment, however. The listing of the event documentation closes withanother line made up of ‘======’ signs.
A few examples may help clarify the picture. For a single diffractive event pp → pdiffr p,
the event record will start withI K(I,1)
K(I,2) K(I,3) comment
1
21
2212
0 incoming p
2
21
-2212
0 incoming p========================= not part of record; appears in listings3
21 9902210
1 outgoing pdiffr
4
21
-2212
2 outgoing p========================= again not part of record
The typical QCD 2 → 2 process would be
I K(I,1)
K(I,2) K(I,3) comment
1
21
2212
0 incoming p
2
21
-2212
0 incoming p=========================
321
2
1 u picked from incoming p
421-1
2 d picked from incoming p
521
21
3 u evolved to g at hard scattering
621-1
4 still d at hard scattering
721
21
0 outgoing g from hard scattering
821-1
0 outgoing d from hard scattering=========================
Note that, where well defined, the K(I,3) code does contain information as to whichside the different partons come from, e.g. above the gluon in line 5 points back to the u
in line 3, which points back to the proton in line 1. In the example above, it would havebeen possible to associate the scattered g in line 7 with the incoming one in line 5, but
this is not possible in the general case, consider e.g. gg → gg.
A special case is provided by W+ W− or Z0 Z0 fusion to an h0 . Then the virtual W’s or
Z’s are shown in lines 7 and 8, the h0 in line 9, and the two recoiling quarks (that emitted
the bosons) in 10 and 11, followed by the Higgs decay products. Since the W’s and Z’s are64√
space-like, what is actually listed as the mass for them is − −m2 . Thus W+ W− fusion
to an h0 in process 8 (not process 124, which is lengthier) might look like
I K(I,1)
K(I,2) K(I,3) comment
1
21
2212
0 first incoming p
2
21
2212
0 second incoming p=========================
321
2
1 u picked from first p
4
21
21
2 g picked from second p
521
2
3 still u after initial-state radiation
621-4
4 g evolved to c
721
24
5 space-like W+ emitted by u quark
8
21-24
6 space-like W− emitted by c quark
921
25
0 Higgs produced by W+ W− fusion
1021
1
5 u turned into d by emission of W+
1121-3
6 c turned into s by emission of W−
1221
23
9 first Z0 coming from decay of h0
1321
23
9 second Z0 coming from decay of h0142112
12 νe from first Z0 decay
15
21
-12
12 ν e from first Z0 decay
1621
5
13 b quark from second Z0 decay
1721-5
13 b antiquark from second Z0 decay=========================
Another special case is when a spectrum of virtual photons are generated inside a
lepton beam, i.e. when PYINIT is called with one or two ’gamma/lepton’ arguments.
(Where lepton could be either of e-, e+, mu-, mu+, tau- or tau+.) Then the documentation
section is expanded to reflect the new layer of administration. Positions 1 and 2 containthe original beam particles, e.g. e and p (or e+ and e− ). In position 3 (and 4 for e+ e− ) is(are) the scattered outgoing lepton(s). Thereafter comes the normal documentation, but
starting from the photon rather than a lepton. For ep, this means 4 and 5 are the γ ∗ andp, 6 and 7 the shower initiators, 8 and 9 the incoming partons to the hard interaction,and 10 and 11 the outgoing ones. Thus the documentation is 3 lines longer (4 for e+ e− )than normally.
The documentation lines are often helpful to understand in broad outline what happened in a given event. However, they only provide the main points of the process, withmany intermediate layers of parton showers omitted. The documentation can therefore
appear internally inconsistent, if the user does not remember what could have happened
in between. For instance, the listing above would show the Higgs with the momentum
it has before radiation off the two recoiling u and c quarks is considered. When theseshowers are included, the Higgs momentum may shift by the changed recoil. However,this update is not visible in the initial summary, which thus still shows the Higgs before
the showering. When the Higgs decays, on the other hand, it is the real Higgs momentum
further down in the event record that is used, and that thus sets the momenta of the decay products that are also copied up to the summary. Such effects will persist in further
decays; e.g. the b and b shown at the end of the example above are before showers, andmay deviate from the final parton momenta quite significantly. Similar shifts will also
occur e.g. in a t → bW+ → bqq0 decays, when the gluon radiation off the b gives a recoil
to the W that is not visible in the W itself but well in its decay products. In summary,the documentation section should never be mistaken for the physically observable statein the main section of the event record, and never be used as part of any realistic event
analysis.
(An alternative approach would be in the spirit of the Les Houches Accord ‘partonlevel’ event record, section 9.9, where the whole chain of decays normally is carried out
before starting the parton showers. With this approach, one could have an internally65

consistent summary, but then in diverging disagreement with the ”real” particles after
each layer of shower evolution.)
After these lines with the initial information, the event record looks the same as for
MSTP(125) = 0, i.e. first comes the parton configuration to be fragmented and, after
another separator line ‘======’ in the output (but not the event record), the productsof subsequent fragmentation and decay chains. This ordinary listing begins in position
MSTI(4) + 1. The K(I,3) pointers for the partons, as well as leptons and photons produced in the hard interaction, are now pointing towards the documentation lines above,
however. In particular, beam remnants point to 1 or 2, depending on which side they
belong to, and partons emitted in the initial-state parton showers point to 3 or 4. In
the second example above, the partons produced by final-state radiation will be pointing
back to 7 and 8; as usual, it should be remembered that a specific assignment to 7 or 8
need not be unique. For the third example, final-state radiation partons will come bothfrom partons 10 and 11 and from partons 16 and 17, and additionally there will be aneutrino–antineutrino pair pointing to 14 and 15.
A hadronic event may contain several (semi)hard interactions, when multiple interactions are allowed. The hardest interaction of an event is shown in the initial section of
the event record, while further ones are not. Therefore these extra partons, documented
in the main section of the event, do not have a documentation copy to point back to, andso are assigned K(I,3) = 0.
There exists a third documentation option, MSTP(125) = 2. Here the history of initialand final-state parton branchings may be traced, including all details on colour flow. This
information has not been optimized for user-friendliness, and cannot be recommended forgeneral usage. With this option, the initial documentation lines are the same. They
are followed by blank lines, K(I,1) = 0, up to line 100 (can be changed in MSTP(126)).
From line 101 onwards each parton with K(I,1) = 3, 13 or 14 appears with special
colour-flow information in the K(I,4) and K(I,5) positions. For an ordinary 2 → 2
scattering, the two incoming partons at the hard scattering are stored in lines 101 and
102, and the two outgoing in 103 and 104. The colour flow between these partons has tobe chosen according to the proper relative probabilities in cases when many alternatives
are possible, see section 8.2.1. If there is initial-state radiation, the two partons in lines
101 and 102 are copied down to lines 105 and 106, from which the initial-state showers
are reconstructed backwards step by step. The branching history may be read by noting
that, for a branching a → bc, the K(I,3) codes of b and c point towards the line numberof a. Since the showers are reconstructed backwards, this actually means that parton b
would appear in the listing before parton a and c, and hence have a pointer to a position
below itself in the list. Associated time-like partons c may initiate time-like showers, as
may the partons of the hard scattering. Again a showering parton or pair of partons will
be copied down towards the end of the list and allowed to undergo successive branchingsc → de, with d and e pointing towards c.√ The mass of time-like partons is properly
stored in P(I,5); for space-like partons − −m2 is stored instead. After this section,
containing all the branchings, comes the final parton configuration, properly arranged incolour, followed by all subsequent fragmentation and decay products, as usual.

5.4

The HEPEVT Standard

A set of common blocks was developed and agreed on within the framework of the 1989
LEP physics study, see [Sjö89]. This standard defines an event record structure whichshould make the interfacing of different event generators much simpler.
It would be a major work to rewrite Pythia to agree with this standard event record
structure. More importantly, the standard only covers quantities which can be definedunambiguously, i.e. which are independent of the particular program used. There are thus
no provisions for the need for colour-flow information in models based on string fragmen66

tation, etc., so the standard common blocks would anyway have to be supplemented withadditional event information. The adopted approach is therefore to retain the PYJETS
event record, but supply a routine PYHEPC which can convert to or from the standard
event record. Owing to a somewhat different content in the two records, some ambiguities do exist in the translation procedure. PYHEPC has therefore to be used with somejudgement.
In this section, the standard event structure is first presented, i.e. the most important
points in [Sjö89] are recapitulated. Thereafter the conversion routine is described, withparticular attention to ambiguities and limitations.
The standard event record is stored in two common blocks. The second of these isspecifically intended for spin information. Since Pythia never (explicitly) makes use ofspin information, this latter common block is not addressed here. A third common blockfor colour flow information has been discussed, but never formalized. Note that a CALLPYLIST(5) can be used to obtain a simple listing of the more interesting information in
the event record.
In order to make the components of the standard more distinguishable in your programs, the three characters HEP (for High Energy Physics) have been chosen to be a partof all names.
Originally it was not specified whether real variables should be in single or double
precision. At the time, this meant that single precision became the default choice, butsince then the trend has been towards increasing precision. In connection with the 1995
LEP 2 workshop, it was therefore agreed to adopt DOUBLE PRECISION real variables aspart of the standard, and also to extend the size from 2000 to 4000 entries [Kno96]. If, forsome reason, one would want to revert to single precision, this would only require trivial
changes to the code of the PYHEPC conversion routine described below.
PARAMETER (NMXHEP=4000)
COMMON/HEPEVT/NEVHEP,NHEP,ISTHEP(NMXHEP),IDHEP(NMXHEP),&JMOHEP(2,NMXHEP),JDAHEP(2,NMXHEP),PHEP(5,NMXHEP),VHEP(4,NMXHEP)DOUBLE PRECISION PHEP, VHEP
Purpose: to contain an event record in a Monte Carlo-independent format.
NMXHEP: maximum numbers of entries (particles) that can be stored in the common block.
The default value of 4000 can be changed via the parameter construction. In thetranslation, it is checked that this value is not exceeded.
NEVHEP: is normally the event number, but may have special meanings, according to thedescription below:> 0 : event number, sequentially increased by 1 for each call to the main event
generation routine, starting with 1 for the first event generated.
= 0 : for a program which does not keep track of event numbers, as some of the
Pythia routines.
= -1 : special initialization record; not used by Pythia.
= -2 : special final record; not used by Pythia.
NHEP: the actual number of entries stored in the current event. These are found in
the first NHEP positions of the respective arrays below. Index IHEP, 1 ≤ IHEP ≤NHEP, is used below to denote a given entry.
ISTHEP(IHEP): status code for entry IHEP, with the following meanings:= 0 : null entry.
= 1 : an existing entry, which has not decayed or fragmented. This is the mainclass of entries, which represents the ‘final state’ given by the generator.
= 2 : an entry which has decayed or fragmented and is therefore not appearing in
the final state, but is retained for event history information.
67

= 3 :

a documentation line, defined separately from the event history. This could
include the two incoming reacting particles, etc.
= 4 - 10 : undefined, but reserved for future standards.
= 11 - 200 : at the disposal of each model builder for constructs specific to his
program, but equivalent to a null line in the context of any other program.
= 201 - : at the disposal of users, in particular for event tracking in the detector.
IDHEP(IHEP) : particle identity, according to the PDG standard. The four additional
codes 91–94 have been introduced to make the event history more legible, seesection 5.1 and the MSTU(16) description of how daughters can point back to
them.
JMOHEP(1,IHEP) : pointer to the position where the mother is stored. The value is 0 for
initial entries.
JMOHEP(2,IHEP) : pointer to position of second mother. Normally only one motherexists, in which case the value 0 is to be used. In Pythia, entries withcodes 91–94 are the only ones to have two mothers. The flavour contents of these objects, as well as details of momentum sharing, have to befound by looking at the mother partons, i.e. the two partons in positions
JMOHEP(1,IHEP) and JMOHEP(2,IHEP) for a cluster or a shower system, and therange JMOHEP(1,IHEP)–JMOHEP(2,IHEP) for a string or an independent fragmentation parton system.
JDAHEP(1,IHEP) : pointer to the position of the first daughter. If an entry has notdecayed, this is 0.
JDAHEP(2,IHEP) : pointer to the position of the last daughter. If an entry has not decayed, this is 0. It is assumed that daughters are stored sequentially, so thatthe whole range JDAHEP(1,IHEP)–JDAHEP(2,IHEP) contains daughters. Thisvariable should be set also when only one daughter is present, as in K0 → K0S
decays, so that looping from the first daughter to the last one works transparently. Normally daughters are stored after mothers, but in backwards evolutionof initial-state radiation the opposite may appear, i.e. that mothers are foundbelow the daughters they branch into. Also, the two daughters then need notappear one after the other, but may be separated in the event record.
PHEP(1,IHEP) : momentum in the x direction, in GeV/c.
PHEP(2,IHEP) : momentum in the y direction, in GeV/c.
PHEP(3,IHEP) : momentum in the z direction, in GeV/c.
PHEP(4,IHEP) : energy, in GeV.
PHEP(5,IHEP) : mass, in GeV/c2 . For space-like√ partons, it is allowed to use a negative
mass, according to PHEP(5,IHEP)= − −m2 .
VHEP(1,IHEP) : production vertex x position, in mm.
VHEP(2,IHEP) : production vertex y position, in mm.
VHEP(3,IHEP) : production vertex z position, in mm.
VHEP(4,IHEP) : production time, in mm/c (≈ 3.33 × 10−12 s).
This completes the brief description of the standard. In Pythia, the routine PYHEPC
is provided as an interface.
CALL PYHEPC(MCONV)
Purpose: to convert between the PYJETS event record and the HEPEVT event record.
MCONV : direction of conversion.
= 1 : translates the current PYJETS record into the HEPEVT one, while leaving theoriginal PYJETS one unaffected.
= 2 : translates the current HEPEVT record into the PYJETS one, while leaving theoriginal HEPEVT one unaffected.
68The conversion of momenta is trivial: it is just a matter of exchanging the order of theindices. The vertex information is but little more complicated; the extra fifth component
present in PYJETS can be easily reconstructed from other information for particles whichhave decayed. (Some of the advanced features made possible by this component, such as
the possibility to consider decays within expanding spatial volumes in subsequent PYEXEC
calls, cannot be used if the record is translated back and forth, however.) Also, theparticle codes K(I,2) and IDHEP(I) are identical, since they are both based on the PDG
codes.
The remaining, non-trivial areas deal with the status codes and the event history. In
moving from PYJETS to HEPEVT, information on colour flow is lost. On the other hand, theposition of a second mother, if any, has to be found; this only affects lines with K(I,2) =91–94. Also, for lines with K(I,1) = 13 or 14, the daughter pointers have to be found.
By and large, however, the translation from PYJETS to HEPEVT should cause little problem,
and there should never be any need for user intervention. (We assume that Pythia is run
with the default MSTU(16) = 1 mother pointer assignments, otherwise some discrepancies
with respect to the proposed standard event history description will be present.)
In moving from HEPEVT to PYJETS, information on a second mother is lost. Any
codes ISTHEP(I) not equal to 1, 2 or 3 are translated into K(I,1) = 0, and so all entries
with K(I,1) ≥ 30 are effectively lost in a translation back and forth. All entries with
ISTHEP(I) = 2 are translated into K(I,1) = 11, and so entries of type K(I,1) = 12,
13, 14 or 15 are never found. There is thus no colour-flow information available forpartons which have fragmented. For partons with ISTHEP(I) = 1, i.e. which have notfragmented, an attempt is made to subdivide the partonic system into colour singlets, as
required for subsequent string fragmentation. To this end, it is assumed that partons are
stored sequentially along strings. Normally, a string would then start at a q (q) or qq(qq) entry, cover a number of intermediate gluons, and end at a q (q) or qq (qq) entry.
Particles could be interspersed in this list with no adverse effects, i.e. a u − g − γ − usequence would be interpreted as a u − g − u string plus an additional photon. A closed
gluon loop would be assumed to be made up of a sequential listing of the gluons, with thestring continuing from the last gluon up back to the first one. Contrary to the previous,
open string case, the appearance of any particle but a gluon would therefore signal theend of the gluon loop. For example, a g − g − g − g sequence would be interpreted as one
single four-gluon loop, while a g − g − γ − g − g sequence would be seen as composed of
two 2-gluon systems.
If these interpretations, which are not unique, are not to your liking, it is up to you
to correct them, e.g. by using PYJOIN to tell exactly which partons should be joined, inwhich sequence, to give a string. Calls to PYJOIN (or the equivalent) are also necessary if
PYSHOW is to be used to have some partons develop a shower.
For practical applications, one should note that e+ e− events, which have been allowed
to shower but not to fragment, do have partons arranged in the order assumed above,so that a translation to HEPEVT and back does not destroy the possibility to perform
fragmentation by a simple PYEXEC call. Also the hard interactions in hadronic events
fulfil this condition, while problems may appear in the multiple interaction scenario,
where several closed gg loops may appear directly following one another, and thus would
be interpreted as a single multigluon loop after translation back and forth.

69

6

The Old Electron–Positron Annihilation Routines

From the Jetset package, Pythia inherits routines for the dedicated simulation of two
hard processes in e+ e− annihilation. The process of main interest is e+ e− → γ ∗ /Z0 → qq.
The description provided by the PYEEVT routine has been a main staple from PETRA days
up to the LEP1 era. Nowadays it is superseded by process 1 of the main Pythia event
generation machinery, see section 8.4.2. This latter process offers a better description offlavour selection, resonance shape and initial-state radiation. It can also, optionally, beused with the second-order matrix element machinery documented in this section. For
backwards compatibility, however, the old routines have still been retained here. There
are also a few features found in the routines in this section, and not in the other ones,
such as polarized incoming beams.
For the process e+ e− → γ ∗ /Z0 → qq, higher-order QCD corrections can be obtained
either with parton showers or with second-order matrix elements. The details of theparton-shower evolution are given in section 10, while this section contains the matrixelement description, including a summary of the older algorithm for initial-state photon
radiation used here.
The other standalone hard process in this section is Υ decay to ggg or γgg, which isbriefly commented on.
The main sources of information for this chapter are refs. [Sjö83, Sjö86, Sjö89].

6.1

Annihilation Events in the Continuum

The description of e+ e− annihilation into hadronic events involves a number of components: the s dependence of the total cross section and flavour composition, multiparton
matrix elements, angular orientation of events, initial-state photon bremsstrahlung andeffects of initial-state electron polarization. Many of the published formulae have been
derived for the case of massless outgoing quarks. For each of the components described in
the following, we will begin by discussing the massless case, and then comment on what
is done to accommodate massive quarks.
6.1.1

Electroweak cross sections

In the Standard Model, fermions have the following couplings (illustrated here for the
first generation):eν = 0,ee = −1,
eu = 2/3,
ed = −1/3,vν = 1,ve = −1 + 4 sin2 θW ,vu = 1 − 8 sin2 θW /3,
vd = −1 + 4 sin2 θW /3,aν = 1,ae = −1,aν = 1,ad = −1,

with e the electric charge, and v and a the vector and axial couplings to the Z0 . The
relative energy dependence of the weak neutral current to the electromagnetic one is givenbys1,
χ(s) =(19)2
2
2
16 sin θW cos θW s − mZ + imZ ΓZ
2. In this section the electroweak mixing parameter sin2 θW and the Z0 mass
where s = EcmmZ and width ΓZ are considered as constants to be given by you (while the full Pythia
event generation machinery itself calculates an s-dependent width).
Although the incoming e+ and e− beams are normally unpolarized, we have included
the possibility of polarized beams, following the formalism of [Ols80]. Thus the incominge+ and e− are characterized by polarizations P± in the rest frame of the particles:

P± = PT± ŝ± + PL± p̂± ,
70

(20)

where 0 ≤ PT± ≤ 1 and −1 ≤ PL± ≤ 1, with the constraint(P± )2 = (PT± )2 + (PL± )2 ≤ 1 .

(21)

Here ŝ± are unit vectors perpendicular to the beam directions p̂± . To be specific, wechoose a right-handed coordinate frame with p̂± = (0, 0, ∓1), and standard transverse
polarization directions (out of the machine plane for storage rings) ŝ± = (0, ±1, 0), thelatter corresponding to azimuthal
angles ϕ± = ±π/2. As free parameters in the programq
we choose PL+ , PL− , PT = PT+ PT− and ∆ϕ = (ϕ+ + ϕ− )/2.
In the massless QED case, the probability to produce a flavour f is proportional toe2f , i.e up-type quarks are four times as likely as down-type ones. In lowest-order massless QFD (Quantum Flavour Dynamics; part of the Standard Model) the corresponding
relative probabilities are given by [Ols80]onhf (s) = e2e (1 − PL+ PL− ) e2f + 2ee ve (1 − PL+ PL− ) − ae (PL− − PL+ ) <eχ(s) ef vf +onn

+ (ve2 + a2e )(1 − PL+ PL− ) − 2ve ae (PL− − PL+ ) |χ(s)|2 vf2 + a2f

o

,

(22)

where <eχ(s) denotes the real part of χ(s). The hf (s) expression depends both on the svalue and on the longitudinal polarization of the e± beams in a non-trivial way.
The cross section for the process e+ e− → γ ∗ /Z0 → ff may now be written as
σf (s) =2
4παem
Rf (s) ,3s(23)

where Rf gives the ratio to the lowest-order QED cross section for the process e+ e− →µ+ µ− ,
Rf (s) = NC RQCD hf (s) .
(24)
The factor of NC = 3 counts the number of colour states available for the qq pair. The
RQCD factor takes into account QCD loop corrections to the cross section. For nf effective
flavours (normally nf = 5)µαsαs
RQCD ≈ 1 ++ (1.986 − 0.115nf )
π
π

¶2

+ ···

(25)

in the MS renormalization scheme [Din79]. Note that RQCD does not affect the relative
quark-flavour composition, and so is of peripheral interest here. (For leptons the NC
and RQCD factors would be absent, i.e. NC RQCD = 1, but leptonic final states are notgenerated by this routine.)
Neglecting higher-order QCD and QFD effects, the correctionsqfor massive quarks are
given in terms of the velocity βf of a fermion with mass mf , βf = 1 − 4m2f /s, as follows.
The vector quark current terms in hf (proportional to e2f , ef vf , or vf2 ) are multiplied by athreshold factor βf (3 − βf2 )/2, while the axial vector quark current term (proportional toa2f ) is multiplied by βf3 . While inclusion of quark masses in the QFD formulae decreases
the total cross section, first-order QCD corrections tend in the opposite direction [Jer81].
Naı̈vely, one would expect one factor of βf to get cancelled. So far, the available options
are either to include threshold factors in full or not at all.
Given that all five quarks are light at the scale of the Z0 , the issue of quark masses
is not really of interest at LEP. Here, however, purely weak corrections are important, inparticular since they change the b quark partial width differently from that of the otherones [Küh89]. No such effects are included in the program.

71

6.1.2First-order QCD matrix elements

The Born process e+ e− → qq is modified in first-order QCD by the probability for the q or
q to radiate a gluon, i.e. by the process e+ e− → qqg. The matrix element is conveniently
given in terms of scaled energy variables in the c.m. frame of the event, x1 = 2Eq /Ecm ,x2 = 2Eq /Ecm , and x3 = 2Eg /Ecm , i.e. x1 + x2 + x3 = 2. For massless quarks the matrix
element reads [Ell76]
x21 + x22
1
dσ
αs
CF
,
=
σ0 dx1 dx2
2π(1 − x1 )(1 − x2 )(26)

where σ0 is the lowest-order cross section, CF = 4/3 is the appropriate colour factor, andthe kinematically allowed region is 0 ≤ xi ≤ 1, i = 1, 2, 3. By kinematics, the xk variablefor parton k is related to the invariant mass mij of the other two partons i and j by2
= 1 − xk .
yij = m2ij /Ecm
The strong coupling constant αs is in first order given by
αs (Q2 ) =12π.
(33 − 2nf ) ln(Q2 /Λ2 )(27)

2Conventionally Q2 = s = Ecm; we will return to this issue below. The number of flavours
nf is 5 for LEP applications, and so the Λ value determined is Λ5 (while e.g. most Deeply
Inelastic Scattering studies refer to Λ4 , the Q2 scales for these experiments historicallyhaving been below the bottom threshold). The αs values are matched at flavour thresholds, i.e. as nf is changed the Λ value is also changed. It is therefore the derivative of αsthat changes at a threshold, not αs itself.
In order to separate 2-jets from 3-jets, it is useful to introduce jet-resolution parameters. This can be done in several different ways. Most famous are the y and (², δ)
procedures. We will only refer to the y cut, which is the one used in the program. Herea 3-parton configuration is called a 2-jet event ifÃ

m2ij
min(yij ) = min
2
i,j
i,j
Ecm

!

<y .

(28)

The cross section in eq. (26) diverges for x1 → 1 or x2 → 1 but, when first-orderpropagator and vertex corrections are included, a corresponding singularity with opposite
sign appears in the qq cross section, so that the total cross section is finite. In analytical
calculations, the average value of any well-behaved quantity Q can therefore be calculated
as
Ã

Z
1
dσ3partonhQi =lim Q(2parton) σ2parton (y) +Q(x1 , x2 )dx1 dx2
σtot y→0
dx1 dx2
yij >y!

,(29)

where any explicit y dependence disappears in the limit y → 0.
In a Monte Carlo program, it is not possible to work with a negative total 2-jet rate,
and thus it is necessary to introduce a fixed non-vanishing y cut in the 3-jet phase space.
Experimentally, there is evidence for the need of a low y cut, i.e. a large 3-jet rate. For
LEP applications, the recommended value is y = 0.01, which is about as far down as one
can go and still retain a positive 2-jet rate. With αs = 0.12, in full second-order QCD(see below), the 2 : 3 : 4 jet composition is then approximately 11% : 77% : 12%. Since
αs varies only slowly with energy, it is not possible to go much below y = 0.01 even at
future Linear Collider energies.
Note, however, that initial-state QED radiation may occasionally lower the c.m. energy
significantly, i.e. increase αs , and thereby bring the 3-jet fraction above unity if y is kept
72

fixed at 0.01 also in those events. Therefore, at PETRA/PEP energies, y values slightly
above 0.01 are needed. In addition to the y cut, the program contains a cut on theinvariant mass mij between any two partons, which is typically required to be larger than
2 GeV. This cut corresponds to the actual merging of two nearby parton jets, i.e. where atreatment with two separate partons rather than one would be superfluous in view√of thesmearing arising from the subsequent fragmentation. Since the cut-off mass scale yEcm
normally is much larger, this additional cut only enters for events at low energies.
For massive quarks, the amount of QCD radiation is slightly reduced [Iof78]:
1
dσ
αs
CF
=
σ0 dx1 dx2
2π
2m2q
−
s

(

Ã

4m2q
x21 + x22−
(1 − x1 )(1 − x2 )
s

µ

1
1
+
2
(1 − x1 )(1 − x2 )2¶24m4 µ 11+
− 2qs
1 − x1 1 − x2

!

1
1
+
1 − x1 1 − x2¶)

. (30)

Properly, the above expression is only valid for the vector part of the cross section, witha slightly different expression for the axial part, but here the one above is used for it all.
In addition, the phase space for emission is reduced by the requirementm2q(1 − x1 )(1 − x2 )(1 − x3 )
≥
.
x23
s

(31)

For b quarks at LEP energies, these corrections are fairly small.
6.1.3

Four-jet matrix elements

Two new event types are added in second-order QCD, e+ e− → qqgg and e+ e− → qqq0 q0 .
The 4-jet cross section has been calculated by several groups [Ali80a, Gae80, Ell81, Dan82],
which agree on the result. The formulae are too lengthy to be quoted here. In one of thecalculations [Ali80a], quark masses were explicitly included, but here only the massless
expressions are included, as taken from [Ell81]. Here the angular orientation of the event
has been integrated out, so that five independent internal kinematical variables remain.
These may be related to the six yij and the four yijk variables, yij = m2ij /s = (pi + pj )2 /s
and yijk = m2ijk /s = (pi + pj + pk )2 /s, in terms of which the matrix elements are given.
The original calculations were for the pure γ-exchange case; it has been pointed out[Kni89] that an additional contribution to the e+ e− → qqq0 q0 cross section arises from theaxial part of the Z0 . This term is not included in the program, but fortunately it is finite
and small.
Whereas the way the string, i.e. the fragmenting colour flux tube, is stretched isuniquely given in qqg event, for qqgg events there are two possibilities: q − g1 − g2 − q
or q − g2 − g1 − q. A knowledge of quark and gluon colours, obtained by perturbation
theory, will uniquely specify the stretching of the string, as long as the two gluons do nothave the same colour. The probability for the latter is down in magnitude by a factor
1/NC2 = 1/9. One may either choose to neglect these terms entirely, or to keep them for the
choice of kinematical setup, but then drop them at the choice of string drawing [Gus82].
We have adopted the latter procedure. Comparing the two possibilities, differences are
typically 10–20% for a given kinematical configuration, and less for the total 4-jet crosssection, so from a practical point of view this is not a major problem.
In higher orders, results depend on the renormalization scheme; we will use MS
throughout. In addition to this choice, several possible forms can be chosen for αs , allof which are equivalent to that order but differ in higher orders. We have picked therecommended standard [PDG88]
(

2
))
12π
153 − 19nf ln(ln(Q2 /ΛMS
αs (Q ) =1−
62
2
(33 − 2nf ) ln(Q2 /ΛMS )(33 − 2nf )2 ln(Q2 /ΛMS )
2

73

)

.

(32)

6.1.4

Second-order three-jet matrix elements

As for first order, a full second-order calculation consists both of real parton emission
terms and of vertex and propagator corrections. These modify the 3-jet and 2-jet crosssections. Although there was some initial confusion, everybody soon agreed on the sizeof the loop corrections [Ell81, Ver81, Fab82]. In analytic calculations, the procedureof eq. (29), suitably expanded, can therefore be used unambiguously for a well-behaved
variable.
For Monte Carlo event simulation, it is again necessary to impose some finite jetresolution criterion. This means that four-parton events which fail the cuts should bereassigned either to the 3-jet or to the 2-jet event class. It is this area that caused quite a
lot of confusion in the past [Kun81, Got82, Ali82, Zhu83, Gut84, Gut87, Kra88], and wherefull agreement does not exist. Most likely, agreement will never be reached, since there
are indeed ambiguous points in the procedure, related to uncertainties on the theoreticalside, as follows.
2
For the y-cut case, any two partons with an invariant mass m2ij < yEcm
should berecombined into one. If the four-momenta are simply added, the sum will correspondto a parton with a positive mass, namely the original mij . The loop corrections are
given in terms of final massless partons, however. In order to perform the (partial)
cancellation between the four-parton real and the 3-parton virtual contributions, it istherefore necessary to get rid of the bothersome mass in the four-parton states. Several
recombinations are used in practice, which go under names such as ‘E’, ‘E0’, ‘p’ and‘p0’ [OPA91]. In the ‘E’-type schemes, the energy of a recombined parton is given by
Eij = Ei + Ej , and three-momenta may have to be adjusted accordingly. In the ‘p’type schemes, on the other hand, three-momenta are added, pij = pi + pj , and thenenergies may have to be adjusted. These procedures result in different 3-jet topologies,
and therefore in different second-order differential 3-jet cross sections.
Within each scheme, a number of lesser points remain to be dealt with, in particularwhat to do if a recombination of a nearby parton pair were to give an event with a non-qqg
flavour structure.
This code contains two alternative second-order 3-jet implementations, the GKS andthe ERT(Zhu) ones. The latter is the recommended one and default. Other parameterizations have also been made available that run together with Jetset 6 (but not adopted
to the current program), see [Sjö89, Mag89].
The GKS option is based on the GKS [Gut84] calculation, where some of the original
mistakes in FKSS [Fab82] have been corrected. The GKS formulae have the advantage ofgiving the second-order corrections in closed analytic form, as not-too-long functions of
x1 , x2 , and the y cut. However, it is today recognized, also by the authors, that important
terms are still missing, and that the matrix elements should therefore not be taken too
seriously. The option is thus kept mainly for backwards compatibility.
The ERT(Zhu) generator [Zhu83] is based on the ERT matrix elements [Ell81], witha Monte Carlo recombination procedure suggested by Kunszt [Kun81] and developed by
Ali [Ali82]. It has the merit of giving corrections in a convenient, parameterized form.
For practical applications, the main limitation is that the corrections are only given for
discrete values of the cut-off parameter y, namely y = 0.01, 0.02, 0.03, 0.04, and 0.05. At
these y values, the full second-order 3-jet cross section is written in terms of the ‘ratio
function’ R(X, Y ; y), defined by
½

¾

αs
αs
1 dσ3tot= A0 (X, Y ) 1 + R(X, Y ; y) ,(33)
σ0 dX dYππwhere X = x1 − x2 = xq − xq , Y = x3 = xg , σ0 is the lowest-order hadronic crosssection, and A0 (X, Y ) the standard first-order 3-jet cross section, cf. eq. (26). By Monte
Carlo integration, the value of R(X, Y ; y) is evaluated in bins of (X, Y ), and the result
parameterized by a simple function F (X, Y ; y). Further details are found in [Sjö89].
74

6.1.5The matrix-element event generator scheme

The program contains parameterizations, separately, of the total first-order 3-jet rate, thetotal second-order 3-jet rate, and the total 4-jet rate, all as functions of y (with αs as aseparate prefactor). These parameterizations have been obtained as follows:• The first-order 3-jet matrix element is almost analytically integrable; some small
finite pieces were obtained by a truncated series expansion of the relevant integrand.
• The GKS second-order 3-jet matrix elements were integrated for 40 different y-cut
values, evenly distributed in ln y between a smallest value y = 0.001 and the kinematical limit y = 1/3. For each y value, 250 000 phase-space points were generated,evenly in d ln(1 − xi ) = dxi /(1 − xi ), i = 1, 2, and the second-order 3-jet rate in
the point evaluated. The properly normalized sum of weights in each of the 40 ypoints were then fitted to a polynomial in ln(y −1 − 2). For the ERT(Zhu) matrix
elements the parameterizations in eq. (33) were used to perform a corresponding
Monte Carlo integration for the five y values available.
• The 4-jet rate was integrated numerically, separately for qqgg and qqq0 q0 events, bygenerating large samples of 4-jet phase-space points within the boundary y = 0.001.
Each point was classified according to the actual minimum y between any two
partons. The same events could then be used to update the summed weights for
40 different counters, corresponding to y values evenly distributed in ln y between
y = 0.001 and the kinematical limit y = 1/6. In fact, since the weight sums for large
y values only received contributions from few phase-space points, extra (smaller)
subsamples of events were generated with larger y cuts. The summed weights,
properly normalized, were then parameterized in terms of polynomials in ln(y −1 −5).
Since it turned out to be difficult to obtain one single good fit over the whole rangeof y values, different parameterizations are used above and below y = 0.018. As
originally given, the qqq0 q0 parameterization only took into account four q0 flavours,
i.e. secondary bb pairs were not generated, but this has been corrected for LEP.
In the generation stage, each event is treated on its own, which means that the αs andy values may be allowed to vary from event to event. The main steps are the following.
1. The y value to be used in the current event is determined. If possible, this isthe value given by you, but additional constraints exist from the validity of theparameterizations (y ≥ 0.001 for GKS, 0.01 ≤ y ≤ 0.05 for ERT(Zhu)) and an
extra (user-modifiable) requirement of a minimum absolute invariant mass between
jets (which translates into varying y cuts due to the effects of initial-state QEDradiation).
2. The αs value is calculated.
3. For the y and αs values given, the relative two/three/four-jet composition is determined. This is achieved by using the parameterized functions of y for 3- and 4-jet
rates, multiplied by the relevant number of factors of αs . In ERT(Zhu), where thesecond-order 3-jet rate is available only at a few y values, intermediate results are
obtained by linear interpolation in the ratio of second-order to first-order 3-jet rates.
The 3-jet and 4-jet rates are normalized to the analytically known second-order total event rate, i.e. divided by RQCD of eq. (25). Finally, the 2-jet rate is obtainedby conservation of total probability.
4. If the combination of y and αs values is such that the total 3- plus 4-jet fraction islarger than unity, i.e. the remainder 2-jet fraction negative, the y-cut value is raised(for that event), and the process is started over at point 3.
5. The choice is made between generating a 2-, 3- or 4-jet event, according to therelative probabilities.
6. For the generation of 4-jets, it is first necessary to make a choice between qqgg
and qqq0 q0 events, according to the relative (parameterized) total cross sections. A
phase-space point is then selected, and the differential cross section at this point is75

evaluated and compared with a parameterized maximum weight. If the phase-spacepoint is rejected, a new one is selected, until an acceptable 4-jet event is found.
7. For 3-jets, a phase-space point is first chosen according to the first-order cross section. For this point, the weightαs
W (x1 , x2 ; y) = 1 + R(x1 , x2 ; y)(34)
πis evaluated. Here R(x1 , x2 ; y) is analytically given for GKS [Gut84], while it isapproximated by the parameterization F (X, Y ; y) of eq. (33) for ERT(Zhu). Again,
linear interpolation of F (X, Y ; y) has to be applied for intermediate y values. The
weight W is compared with a maximum weightαs
Wmax (y) = 1 + Rmax (y) ,(35)πwhich has been numerically determined beforehand and suitably parameterized. If
the phase-space point is rejected, a new point is generated, etc.
8. Massive matrix elements are not implemented for second-order QCD (but are in thefirst-order option). However, if a 3- or 4-jet event determined above falls outside thephase-space region allowed for massive quarks, the event is rejected and reassigned
to be a 2-jet event. (The way the yij and yijk variables of 4-jet events should
be interpreted for massive quarks is not even unique, so some latitude has beentaken here to provide a reasonable continuity from 3-jet events.) This procedure
is known not to give the expected full mass suppression, but is a reasonable first
approximation.
9. Finally, if the event is classified as a 2-jet event, either because it was initially soassigned, or because it failed the massive phase-space cuts for 3- and 4-jets, thegeneration of 2-jets is trivial.
6.1.6

Optimized perturbation theory

Theoretically, it turns out that the second-order corrections to the 3-jet rate are large. It istherefore not unreasonable to expect large third-order corrections to the 4-jet rate. Indeed,
the experimental 4-jet rate is much larger than second order predicts (when fragmentation
effects have been included), if αs is determined based on the 3-jet rate [Sjö84a, JAD88].
The only consistent way to resolve this issue is to go ahead and calculate the full nextorder. This is a tough task, however, so people have looked at possible shortcuts. For
example, one can try to minimize the higher-order contributions by a suitable choice ofthe renormalization scale [Ste81] — ‘optimized perturbation theory’. This is equivalent
to a different choice for the Q2 scale in αs , a scale which is not unambiguous anyway.
2
Indeed the standard value Q2 = s = Ecmis larger than the natural physical scale ofgluon emission in events, given that most gluons are fairly soft. One could therefore pickanother scale, Q2 = f s, with f < 1. The O(αs ) 3-jet rate would be increased by sucha scale change, and so would the number of 4-jet events, including those which collapse
into 3-jet ones. The loop corrections depend on the Q2 scale, however, and compensate
the changes above by giving a larger negative contribution to the 3-jet rate.
The possibility of picking an optimized scale f is implemented as follows [Sjö89].
Assume that the differential 3-jet rate at scale Q2 = s is given by the expression
R3 = r1 αs + r2 αs2 ,(36)where R3 , r1 and r2 are functions of the kinematical variables x1 and x2 and the y cut,as implied by the second-order formulae above, see e.g. eq. (33). When the coupling ischosen at a different scale, Q02 = f s, the 3-jet rate has to be changed to
R30 = r10 αs0 + r20 αs02 ,
76

(37)

where r10 = r1 ,33 − 2nf
ln f ,(38)12π
and αs0 = αs (f s). Since we only have the Born term for 4-jets, here the effects of a scalechange come only from the change in the coupling constant. Finally, the 2-jet cross section
can still be calculated from the difference between the total cross section and the 3- and4-jet cross sections.
If an optimized scale is used in the program, the default value is f = 0.002, which isfavoured by the studies in ref. [Bet89]. (In fact, it is also possible to use a correspondingly
optimized RQCD factor, eq. (25), but then the corresponding f is chosen independently
and much closer to unity.) The success of describing the jet rates should not hide the factthat one is dabbling in (educated, hopefully) guesswork, and that any conclusions basedon this method have to be taken with a pinch of salt.
One special problem associated with the use of optimized perturbation theory is thatthe differential 3-jet rate may become negative over large regions of the (x1 , x2 ) phasespace. This problem already exists, at least in principle, even for a scale f = 1, since r2 is
not guaranteed to be positive definite. Indeed, depending on the choice of y cut, αs value,
and recombination scheme, one may observe a small region of negative differential 3-jet
rate for the full second-order expression. This region is centred around qqg configurations,where the q and q are close together in one hemisphere and the g is alone in the other, i.e.
x1 ≈ x2 ≈ 1/2. It is well understood why second-order corrections should be negative inthis region [Dok89]: the q and q of a qqg state are in a relative colour octet state, and thus
the colour force between them is repulsive, which translates into a negative second-orderterm.
However, as f is decreased below unity, r20 receives a negative contribution from the ln fterm, and the region of negative differential cross section has a tendency to become larger,
also after taking into account related changes in αs . In an event-generator framework,where all events are supposed to come with unit weight, it is clearly not possible tosimulate negative cross sections. What happens in the program is therefore that no 3-jet
events at all are generated in the regions of negative differential cross section, and thatthe 3-jet rate in regions of positive cross sections is reduced by a constant factor, chosen
so that the total number of 3-jet events comes out as it should. This is a consequenceof the way the program works, where it is first decided what kind of event to generate,
based on integrated 3-jet rates in which positive and negative contributions are added up
with sign, and only thereafter the kinematics is chosen.
Based on our physics understanding of the origin of this negative cross section, theapproach adopted is as sensible as any, at least to that order in perturbation theory (what
one might strive for is a properly exponentiated description of the relevant region). It cangive rise to funny results for low f values, however, as observed by OPAL [OPA92] forthe energy–energy correlation asymmetry.
r20 = r2 + r1

6.1.7

Angular orientation

While pure γ exchange gives a simple 1 + cos2 θ distribution for the q (and q) direction in
qq events, Z0 exchange and γ ∗ /Z0 interference results in a forward–backward asymmetry.
If one introducesnoh0f (s) = 2ee ae (1 − PL+ PL− ) − ve (PL− − PL+ ) <eχ(s)ef afno+ 2ve ae (1 − PL+ PL− ) − (ve2 + a2e )(PL− − PL+ ) |χ(s)|2 vf af ,(39)

then the angular distribution of the quark is given bydσ∝ hf (s)(1 + cos2 θf ) + 2h0f (s) cos θf .
d(cos θf )
77

(40)

The angular orientation of a 3- or 4-jet event may be described in terms of three angles
χ, θ and ϕ; for 2-jet events only θ and ϕ are necessary. From a standard orientation, with
the q along the +z axis and the q in the xz plane with px > 0, an arbitrary orientation may
be reached by the rotations +χ in azimuthal angle, +θ in polar angle, and +ϕ in azimuthal
angle, in that order. Differential cross sections, including QFD effects and arbitrary beampolarizations have been given for 2- and 3-jet events in refs. [Ols80, Sch80]. We use theformalism of ref. [Ols80], with translation from their terminology according to χ → π − χand ϕ− → −(ϕ + π/2). The resulting formulae are tedious, but straightforward to apply,
once the internal jet configuration has been chosen. 4-jet events are approximated by 3-jet
ones, by joining the two gluons of a qqgg event and the q0 and q0 of a qqq0 q0 event into one
effective jet. This means that some angular asymmetries are neglected [Ali80a], but thatweak effects are automatically included. It is assumed that the second-order 3-jet events
have the same angular orientation as the first-order ones, some studies on this issue may
be found in [Kör85]. Further, the formulae normally refer to the massless case; only for
the QED 2- and 3-jet cases are mass corrections available.
The main effect of the angular distribution of multijet events is to smear the lowestorder result, i.e. to reduce any anisotropies present in 2-jet systems. In the parton-shower
option of the program, only the initial qq axis is determined. The subsequent shower
evolution then de facto leads to a smearing of the jet axis, although not necessarily in fullagreement with the expectations from multijet matrix-element treatments.
6.1.8

Initial-state radiation

Initial-state photon radiation has been included using the formalism of ref. [Ber82]. Hereeach event contains either no photon or one, i.e. it is a first-order non-exponentiateddescription. The main formula for the hard radiative photon cross section is
dσ
αem
=
dxγ
π

Ã

!

s
ln 2 − 1me

1 + (1 − xγ )2σ0 (ŝ) ,xγ(41)

where xγ is the photon energy fraction of the beam energy, ŝ = (1 − xγ )s is the squared
reduced hadronic c.m. energy, and σ0 is the ordinary annihilation cross section at the
reduced energy. In particular, the selection of jet flavours should be done according to
expectations at the reduced energy. The cross section is divergent both for xγ → 1 andxγ → 0. The former is related to the fact that σ0 has a 1/ŝ singularity (the real photonpole) for ŝ → 0. An upper cut on xγ can here be chosen to fit the experimental setup.
The latter is a soft photon singularity, which is to be compensated in the no-radiation
cross section. A requirement xγ > 0.01 has therefore been chosen so that the hardphoton fraction is smaller than unity. In the total cross section, effects from photons
with xγ < 0.01 are taken into account, together with vertex and vacuum polarization
corrections (hadronic vacuum polarizations using a simple parameterization of the more
complicated formulae of ref. [Ber82]).
The hard photon spectrum can be integrated analytically, for the full γ ∗ /Z0 structure
including interference terms, provided that no new flavour thresholds are crossed and that
the RQCD term in the cross section can be approximated by a constant over the rangeof allowed ŝ values. In fact, threshold effects can be taken into account by standard
rejection techniques, at the price of not obtaining the exact cross section analytically, but
only by an effective Monte Carlo integration taking place in parallel with the ordinary
event generation. In addition to xγ , the polar angle θγ and azimuthal angle ϕγ of thephotons are also to be chosen. Further, for the orientation of the hadronic system, achoice has to be made whether the photon is to be considered as having been radiatedfrom the e+ or from the e− .
Final-state photon radiation, as well as interference between initial- and final-stateradiation, has been left out of this treatment. The formulae for e+ e− → µ+ µ− cannot
78

be simply taken over for the case of outgoing quarks, since the quarks as such only live
for a short while before turning into hadrons. Another simplification in our treatment isthat effects of incoming polarized e± beams have been completely neglected, i.e. neither
the effective shift in azimuthal distribution of photons nor the reduction in polarization is
included. The polarization parameters of the program are to be thought of as the effective
polarization surviving after initial-state radiation.
6.1.9

Alternative matrix elements

The program contains two sets of ‘toy model’ matrix elements, one for an Abelian vector
gluon model and one for a scalar gluon model. Clearly both of these alternatives are
already excluded by data, and are anyway not viable alternatives for a consistent theoryof strong interactions. They are therefore included more as references to show how well
the characteristic features of QCD can be measured experimentally.
Second-order matrix elements are available for the Abelian vector gluon model. Theseare easily obtained from the standard QCD matrix elements by a substitution of the
Casimir group factors: CF = 4/3 → 1, NC = 3 → 0, and TR = nf /2 → 3nf . First-ordermatrix elements contain only CF ; therefore the standard first-order QCD results may berecovered by a rescaling of αs by a factor 4/3. In second order the change of NC to 0means that g → gg couplings are absent from the Abelian model, while the change of TR
corresponds to an enhancement of the g → q0 q0 coupling, i.e. to an enhancement of theqqq0 q0 4-jet event rate.
The second-order corrections to the 3-jet rate turn out to be strongly negative — if
αs is fitted to get about the right rate of 4-jet events, the predicted differential 3-jet rateis negative almost everywhere in the (x1 , x2 ) plane. Whether this unphysical behaviour
would be saved by higher orders is unclear. It has been pointed out that the rate canbe made positive by a suitable choice of scale, since αs runs in opposite directions in an
Abelian model and in QCD [Bet89]. This may be seen directly from eq. (38), where theterm 33 = 11NC is absent in the Abelian model, and therefore the scale-dependent termchanges sign. In the program, optimized scales have not been implemented for this toy
model. Therefore the alternatives provided for you are either to generate only 4-jet events,
or to neglect second-order corrections to the 3-jet rate, or to have the total 3-jet rate set
vanishing (so that only 2- and 4-jet events are generated). Normally we would expect theformer to be the one of most interest, since it is in angular (and flavour) distributionsof 4-jet events that the structure of QCD can be tested. Also note that the ‘correct’running of αs is not included; you are expected to use the option where αs is just given
as a constant number.
The scalar gluon model is even more excluded than the Abelian vector one, sincedifferences appear already in the 3-jet matrix element [Lae80]:x23
dσ∝
dx1 dx2(1 − x1 )(1 − x2 )(42)when only γ exchange is included. The axial part of the Z0 gives a slightly different
shape; this is included in the program but does not make much difference. The angularorientation does include the full γ ∗ /Z0 interference [Lae80], but the main interest is in the
3-jet topology as such [Ell79]. No higher-order corrections are included. It is recommended
to use the option of a fixed αs also here, since the correct running is not available.

6.2

Decays of Onia Resonances

Many different possibilities are open for the decay of heavy J P C = 1−− onia resonances.
Of special interest are the decays into three gluons or two gluons plus a photon, since79

these offer unique possibilities to study a ‘pure sample’ of gluon jets. A routine for thispurpose is included in the program. It was written at a time where the expectations wereto find toponium at PETRA energies. Given the large value of the top mass, weak decays
dominate, to the extent that the top quark decays weakly even before a bound toponium
state is formed, and thus the routine will be of no use for top. The charm system, on theother hand, is far too low in mass for a jet language to be of any use. The only applicationis therefore likely to be for Υ, which unfortunately also is on the low side in mass.
The matrix element for qq → ggg is (in lowest order) [Kol78]
1

dσggg
1
= 2
σggg dx1 dx2
π −9

(µ

1 − x1
x2 x3

¶2

µ

1 − x2
+
x1 x3

¶2

µ

1 − x3
+
x1 x2

¶2 )

,

(43)

where, as before, xi = 2Ei /Ecm in the c.m. frame of the event. This is a well-defined
expression, without the kind of singularities encountered in the qqg matrix elements. In
principle, no cuts at all would be necessary, but for reasons of numerical simplicity we
implement a y cut as for continuum jet production, with all events not fulfilling this cut
considered as (effective) gg events. For ggg events, each gg invariant mass is required to
be at least 2 GeV.
Another process is qq → γgg, obtained by replacing a gluon in qq → ggg by a photon.
This process has the same normalized cross section as the one above, if e.g. x1 is taken torefer to the photon. The relative rate is [Kol78]
σγgg
36 e2q αem=.
σggg
5 αs (Q2 )(44)

Here eq is the charge of the heavy quark, and the scale in αs has been chosen as the massof the onium state. If the mass of the recoiling gg system is lower than some cut-off (bydefault 2 GeV), the event is rejected.
In the present implementation the angular orientation of the ggg and γgg events isgiven for the e+ e− → γ ∗ → onium case [Kol78] (optionally with beam polarization effects
included), i.e. weak effects have not been included, since they are negligible at around10 GeV.
It is possible to start a perturbative shower evolution from either of the two states
above. However, for Υ the phase space for additional evolution is so constrained that notmuch is to be gained from that. We therefore do not recommend this possibility. The
shower generation machinery, when starting up from a γgg configuration, is constructed
such that the photon energy is not changed. This means that there is currently no
possibility to use showers to bring the theoretical photon spectrum in better agreementwith the experimental one.
In string fragmentation language, a ggg state corresponds to a closed string triangle
with the three gluons at the corners. As the partons move apart from a common origin,
the string triangle expands. Since the photon does not take part in the fragmentation,the γgg state corresponds to a double string running between the two gluons.

6.3

Routines and Common-Block Variables

6.3.1

e+ e− continuum event generation

The only routine a normal user will call to generate e+ e− continuum events is PYEEVT.
The other routines listed below, as well as PYSHOW (see section 10.4), are called by PYEEVT.
CALL PYEEVT(KFL,ECM)

80

Purpose: to generate a complete event e+ e− → γ ∗ /Z0 → qq → parton shower → hadrons
according to QFD and QCD cross sections. As an alternative to parton showers,
second-order matrix elements are available for qq+qqg+qqgg+qqq0 q0 production.
KFL : flavour of events generated.
= 0 : mixture of all allowed flavours according to relevant probabilities.
= 1 - 8 : primary quarks are only of the specified flavour KFL.
ECM : total c.m. energy of system.
Remark: Each call generates one event, which is independent of preceding ones, with
one exception, as follows. If radiative corrections are included, the shape of thehard photon spectrum is recalculated only with each PYXTEE call, which normallyis done only if KFL, ECM or MSTJ(102) is changed. A change of e.g. the Z0 massin mid-run has to be followed either by a user call to PYXTEE or by an internal
call forced e.g. by putting MSTJ(116) = 3.
SUBROUTINE PYXTEE(KFL,ECM,XTOT) : to calculate the total hadronic cross section, including quark thresholds, weak, beam polarization, and QCD effects and radiative corrections. In the process, variables necessary for the treatment of hard
photon radiation are calculated and stored.
KFL, ECM : as for PYEEVT.
XTOT : the calculated total cross section in nb.
SUBROUTINE PYRADK(ECM,MK,PAK,THEK,PHIK,ALPK) : to describe initial-state hard γ
radiation.
SUBROUTINE PYXKFL(KFL,ECM,ECMC,KFLC) : to generate the primary quark flavour incase this is not specified by you.
SUBROUTINE PYXJET(ECM,NJET,CUT) : to determine the number of jets (2, 3 or 4) to begenerated within the kinematically allowed region (characterized by CUT = ycut )in the matrix-element approach; to be chosen such that all probabilities are
between 0 and 1.
SUBROUTINE PYX3JT(NJET,CUT,KFL,ECM,X1,X2) : to generate the internal momentum
variables of a 3-jet event, qqg, according to first- or second-order QCD matrix
elements.
SUBROUTINE PYX4JT(NJET,CUT,KFL,ECM,KFLN,X1,X2,X4,X12,X14) : to generate theinternal momentum variables for a 4-jet event, qqgg or qqq0 q0 , according tosecond-order QCD matrix elements.
SUBROUTINE PYXDIF(NC,NJET,KFL,ECM,CHI,THE,PHI) : to describe the angular orientation of the jets. In first-order QCD the complete QED or QFD formulae areused; in second order 3-jets are assumed to have the same orientation as in first,
and 4-jets are approximated by 3-jets.
6.3.2

A routine for onium decay

In PYONIA we have implemented the decays of heavy onia resonances into three gluons or
two gluons plus a photon, which are the dominant non-background-like decays of Υ.
CALL PYONIA(KFL,ECM)
Purpose: to simulate the process e+ e− → γ ∗ → 1−− onium resonance → (ggg or ggγ) →shower → hadrons.
KFL : the flavour of the quark giving rise to the resonance.
= 0 : generate ggg events alone.
= 1 - 8 : generate ggg and ggγ events in mixture determined by the squared chargeof flavour KFL, see eq. (44). Normally KFL = 5.
ECM : total c.m. energy of system.
81

6.3.3

Common-block variables

The status codes and parameters relevant for the e+ e− routines are found in the common block PYDAT1. This common block also contains more general status codes andparameters, described elsewhere.
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
Purpose: to give access to a number of status codes and parameters regulating theperformance of the e+ e− event generation routines.
MSTJ(101) : (D = 5) gives the type of QCD corrections used for continuum events.
= 0 : only qq events are generated.
= 1 : qq + qqg events are generated according to first-order QCD.
= 2 : qq + qqg + qqgg + qqq0 q0 events are generated according to second-orderQCD.
= 3 : qq + qqg + qqgg + qqq0 q0 events are generated, but without second-ordercorrections to the 3-jet rate.
= 5 : a parton shower is allowed to develop from an original qq pair, see MSTJ(38)- MSTJ(50) for details.
= -1 : only qqg events are generated (within same matrix-element cuts as for = 1).
Since the change in flavour composition from mass cuts or radiative corrections is not taken into account, this option is not intended for quantitative
studies.
= -2 : only qqgg and qqq0 q0 events are generated (as for = 2). The same warning
as for = -1 applies.
= -3 : only qqgg events are generated (as for = 2). The same warning as for = -1
applies.
= -4 : only qqq0 q0 events are generated (as for = 2). The same warning as for = -1
applies.
Note 1: MSTJ(101) is also used in PYONIA, with
≤ 4 : ggg + γgg events are generated according to lowest-order matrix elements.
≥ 5 : a parton shower is allowed to develop from the original ggg or ggγ configuration, see MSTJ(38) - MSTJ(50) for details.
Note 2: the default values of fragmentation parameters have been chosen to work
well with the default parton-shower approach above. If any of the otheroptions are used, or if the parton shower is used in non-default mode, it
is normally necessary to retune fragmentation parameters. As an example,
we note that the second-order matrix-element approach (MSTJ(101) = 2)
at PETRA/PEP energies gives a better description when the a and b parameters of the symmetric fragmentation function are set to a =PARJ(41)= 1, b =PARJ(42) = 0.7, and the width of the transverse momentum distribution to σ =PARJ(21) = 0.40. In principle, one also ought to change
the joining parameter to PARJ(33) = PARJ(35) = 1.1 to preserve a flat rapidity plateau, but if this should be forgotten, it does not make too muchdifference. For applications at TRISTAN or LEP, one has to change thematrix-element approach parameters even more, to make up for additional
soft gluon effects not covered in this approach.
MSTJ(102) : (D = 2) inclusion of weak effects (Z0 exchange) for flavour production, angular orientation, cross sections and initial-state photon radiation in continuum
events.
= 1 : QED, i.e. no weak effects are included.
= 2 : QFD, i.e. including weak effects.
= 3 : as = 2, but at initialization in PYXTEE the Z0 width is calculated from sin2 θW ,82

αem and Z0 and quark masses (including bottom and top threshold factors
for MSTJ(103) odd), assuming three full generations, and the result is storedin PARJ(124).
MSTJ(103) : (D = 7) mass effects in continuum matrix elements, in the form MSTJ(103)= M1 + 2M2 + 4M3 , where Mi = 0 if no mass effects and Mi = 1 if mass effects
should be included. Here;M1 : threshold factor for new flavour production according to QFD result;
M2 : gluon emission probability (only applies for |MSTJ(101)|≤ 1, otherwise no
mass effects anyhow);
M3 : angular orientation of event (only applies for |MSTJ(101)|≤ 1 andMSTJ(102) = 1, otherwise no mass effects anyhow).
MSTJ(104) : (D = 5) number of allowed flavours, i.e. flavours that can be produced ina continuum event if the energy is enough. A change to 6 makes top production
allowed above the threshold, etc. Note that in qqq0 q0 events only the first fiveflavours are allowed in the secondary pair, produced by a gluon breakup.
MSTJ(105) : (D = 1) fragmentation and decay in PYEEVT and PYONIA calls.
= 0 : no PYEXEC calls, i.e. only matrix-element and/or parton-shower treatment,
and collapse of small jet systems into one or two particles (in PYPREP).
= 1 : PYEXEC calls are made to generate fragmentation and decay chain.
= -1 : no PYEXEC calls and no collapse of small jet systems into one or two particles(in PYPREP).
MSTJ(106) : (D = 1) angular orientation in PYEEVT and PYONIA.
= 0 : standard orientation of events, i.e. q along +z axis and q along −z axis or inxz plane with px > 0 for continuum events, and g1 g2 g3 or γg2 g3 in xz planewith g1 or γ along the +z axis for onium events.
= 1 : random orientation according to matrix elements.
MSTJ(107) : (D = 0) radiative corrections to continuum events.
= 0 : no radiative corrections.
= 1 : initial-state radiative corrections (including weak effects for MSTJ(102) = 2
or 3).
MSTJ(108) : (D = 2) calculation of αs for matrix-element alternatives. The MSTU(111)and PARU(112) values are automatically overwritten in PYEEVT or PYONIA calls
accordingly.
= 0 : fixed αs value as given in PARU(111).
= 1 : first-order formula is always used, with ΛQCD given by PARJ(121).
= 2 : first- or second-order formula is used, depending on value of MSTJ(101),with ΛQCD given by PARJ(121) or PARJ(122).
MSTJ(109) : (D = 0) gives a possibility to switch from QCD matrix elements to somealternative toy models. Is not relevant for shower evolution, MSTJ(101) = 5,
where one can use MSTJ(49) instead.
= 0 : standard QCD scenario.
= 1 : a scalar gluon model. Since no second-order corrections are available in this
scenario, one can only use this with MSTJ(101) = 1 or -1. Also note thatthe event-as-a-whole angular distribution is for photon exchange only (i.e. noweak effects), and that no higher-order corrections to the total cross section
are included.
= 2 : an Abelian vector gluon theory, with the colour factors CF = 1 (= 4/3 inQCD), NC = 0 (= 3 in QCD) and TR = 3nf (= nf /2 in QCD). If one
selects αAbelian = (4/3)αQCD , the 3-jet cross section will agree with the QCD
one, and differences are to be found only in 4-jets. The MSTJ(109) = 2
option has to be run with MSTJ(110) = 1 and MSTJ(111) = 0; if need be,
the latter variables will be overwritten by the program.
Warning: second-order corrections give a large negative contribution to the833-jet cross section, so large that the whole scenario is of doubtful use. In
order to make the second-order options work at all, the 3-jet cross sectionis here by hand set exactly equal to zero for MSTJ(101) = 2. It is here
probably better to use the option MSTJ(101) = 3, although this is not aconsistent procedure either.
MSTJ(110) : (D = 2) choice of second-order contributions to the 3-jet rate.
= 1 : the GKS second-order matrix elements.
= 2 : the Zhu parameterization of the ERT matrix elements, based on the programof Kunszt and Ali, i.e. in historical sequence ERT/Kunszt/Ali/Zhu. The
parameterization is available for y = 0.01, 0.02, 0.03, 0.04 and 0.05. Values
outside this range are put at the nearest border, while those inside it are
given by a linear interpolation between the two nearest points. Since thisprocedure is rather primitive, one should try to work at one of the values
given above. Note that no Abelian QCD parameterization is available forthis option.
MSTJ(111) : (D = 0) use of optimized perturbation theory for second-order matrix elements (it can also be used for first-order matrix elements, but here it only
corresponds to a trivial rescaling of the αs argument).
2= 0 : no optimization procedure; i.e. Q2 = Ecm.
222= 1 : an optimized Q scale is chosen as Q = f Ecm, where f =PARJ(128) for the
total cross section R factor, while f =PARJ(129) for the 3- and 4-jet rates.
This f value enters via the αs , and also via a term proportional to αs2 ln f .
Some constraints are imposed; thus the optimized ‘3-jet’ contribution to
R is assumed to be positive (for PARJ(128)), the total 3-jet rate is notallowed to be negative (for PARJ(129)), etc. However, there is no guaranteethat the differential 3-jet cross section is not negative (and truncated to 0)
somewhere (this can also happen with f = 1, but is then less frequent).
The actually obtained f values are stored in PARJ(168) and PARJ(169),
respectively. If an optimized Q2 scale is used, then the ΛQCD (and αs ) should
also be changed. With the value f = 0.002, it has been shown [Bet89] thata ΛQCD = 0.100 GeV gives a reasonable agreement; the parameter to bechanged is PARJ(122) for a second-order running αs . Note that, since theoptimized Q2 scale is sometimes below the charm threshold, the effective
number of flavours used in αs may well be 4 only. If one feels that it is still
appropriate to use 5 flavours (one choice might be as good as the other), it
is necessary to put MSTU(113) = 5.
MSTJ(115) : (D = 1) documentation of continuum or onium events, in increasing orderof completeness.
= 0 : only the parton shower, the fragmenting partons and the generated hadronicsystem are stored in the PYJETS common block.
= 1 : also a radiative photon is stored (for continuum events).
= 2 : also the original e+ e− are stored (with K(I,1) = 21).
= 3 : also the γ or γ ∗ /Z0 exchanged for continuum events, the onium state forresonance events is stored (with K(I,1) = 21).
MSTJ(116) : (D = 1) initialization of total cross section and radiative photon spectrum
in PYEEVT calls.
= 0 : never; cannot be used together with radiative corrections.
= 1 : calculated at first call and then whenever KFL or MSTJ(102) is changed or
ECM is changed by more than PARJ(139).
= 2 : calculated at each call.
= 3 : everything is re-initialized in the next call, but MSTJ(116) is afterwards
automatically put = 1 for use in subsequent calls.
MSTJ(119) : (I) check on need to re-initialize PYXTEE.
84

MSTJ(120) : (R) type of continuum event generated with the matrix-element option(with the shower one, the result is always = 1).
= 1 : qq.
= 2 : qqg.
= 3 : qqgg from Abelian (QED-like) graphs in matrix element.
= 4 : qqgg from non-Abelian (i.e. containing triple-gluon coupling) graphs in matrix element.
= 5 : qqq0 q0 .
MSTJ(121) : (R) flag set if a negative differential cross section was encountered in thelatest PYX3JT call. Events are still generated, but maybe not quite according to
the distribution one would like (the rate is set to zero in the regions of negative
cross section, and the differential rate in the regions of positive cross section isrescaled to give the ‘correct’ total 3-jet rate).
PARJ(121) : (D = 1.0 GeV) Λ value used in first-order calculation of αs in the matrixelement alternative.
PARJ(122) : (D = 0.25 GeV) Λ values used in second-order calculation of αs in thematrix-element alternative.
PARJ(123) : (D = 91.187 GeV) mass of Z0 as used in propagators for the QFD case.
PARJ(124) : (D = 2.489 GeV) width of Z0 as used in propagators for the QFD case.
Overwritten at initialization if MSTJ(102) = 3.
PARJ(125) : (D = 0.01) ycut , minimum squared scaled invariant mass of any two partonsin 3- or 4-jet events; the main user-controlled matrix-element cut. PARJ(126)provides an additional constraint. For each new event, it is additionally checkedthat the total 3- plus 4-jet fraction does not exceed unity; if so the effective y cut
will be dynamically increased. The actual y-cut value is stored in PARJ(150),
event by event.
PARJ(126) : (D = 2. GeV) minimum invariant mass of any two partons in 3- or 4-jet
events; a cut in addition to the one above, mainly for the case of a radiative
photon lowering the hadronic c.m. energy significantly.
PARJ(127) : (D = 1. GeV) is used as a safety margin for small colour-singlet jet systems,
cf. PARJ(32), specifically qq0 masses in qqq0 q0 4-jet events and gg mass in onium
γgg events.
PARJ(128) : (D = 0.25) optimized Q2 scale for the QCD R (total rate) factor for the2
MSTJ(111) = 1 option is given by Q2 = f Ecm, where f =PARJ(128). For various
reasons the actually used f value may be increased compared with the nominal
one; while PARJ(128) gives the nominal value, PARJ(168) gives the actual one
for the current event.
PARJ(129) : (D = 0.002) optimized Q2 scale for the 3- and 4-jet rate for the MSTJ(111)2= 1 option is given by Q2 = f Ecm, where f =PARJ(129). For various reasons
the actually used f value may be increased compared with the nominal one;while PARJ(129) gives the nominal value, PARJ(169) gives the actual one forthe current event. The default value is in agreement with the studies of Bethke[Bet89].
PARJ(131), PARJ(132) : (D = 2*0.) longitudinal polarizations PL+ and PL− of incominge+ and e− .
qPARJ(133) : (D = 0.) transverse polarization PT = PT+ PT− , with PT+ and PT− transverse
polarizations of incoming e+ and e− .
PARJ(134) : (D = 0.) mean of transverse polarization directions of incoming e+ and e− ,∆ϕ = (ϕ+ + ϕ− )/2, with ϕ the azimuthal angle of polarization, leading to a shift
in the ϕ distribution of jets by ∆ϕ.
PARJ(135) : (D = 0.01) minimum photon energy fraction (of beam energy) in initialstate radiation; should normally never be changed (if lowered too much, the85fraction of events containing a radiative photon will exceed unity, leading to
problems).
PARJ(136) : (D = 0.99) maximum photon energy fraction (of beam energy) in initialstate radiation; may be changed to reflect actual trigger conditions of a detector(but must always be larger than PARJ(135)).
PARJ(139) : (D = 0.2 GeV) maximum deviation of Ecm from the corresponding value
at last PYXTEE call, above which a new call is made if MSTJ(116) = 1.
PARJ(141) : (R) value of R, the ratio of continuum cross section to the lowest-ordermuon pair production cross section, as given in massless QED (i.e. three times
the sum of active quark squared charges, possibly modified for polarization).
PARJ(142) : (R) value of R including quark-mass effects (for MSTJ(102) = 1) and/or
weak propagator effects (for MSTJ(102) = 2).
PARJ(143) : (R) value of R as PARJ(142), but including QCD corrections as given by
MSTJ(101).
PARJ(144) : (R) value of R as PARJ(143), but additionally including corrections frominitial-state photon radiation (if MSTJ(107) = 1). Since the effects of heavy
flavour thresholds are not simply integrable, the initial value of PARJ(144) is
updated during the course of the run to improve accuracy.
PARJ(145) - PARJ(148) : (R) absolute cross sections in nb as for the cases PARJ(141)- PARJ(144) above.
PARJ(150) : (R) current effective matrix element cut-off ycut , as given by PARJ(125),
PARJ(126) and the requirements of having non-negative cross sections for 2-, 3and 4-jet events. Not used in parton showers.
PARJ(151) : (R) value of c.m. energy ECM at last PYXTEE call.
PARJ(152) : (R) current first-order contribution to the 3-jet fraction; modified by masseffects. Not used in parton showers.
PARJ(153) : (R) current second-order contribution to the 3-jet fraction; modified bymass effects. Not used in parton showers.
PARJ(154) : (R) current second-order contribution to the 4-jet fraction; modified bymass effects. Not used in parton showers.
PARJ(155) : (R) current fraction of 4-jet rate attributable to qqq0 q0 events rather than
qqgg ones; modified by mass effects. Not used in parton showers.
PARJ(156) : (R) has two functions when using second-order QCD. For a 3-jet event, it
gives the ratio of the second-order to the total 3-jet cross section in the given
kinematical point. For a 4-jet event, it gives the ratio of the modified 4-jet crosssection, obtained when neglecting interference terms whose colour flow is notwell defined, to the full unmodified one, all evaluated in the given kinematicalpoint. Not used in parton showers.
PARJ(157) - PARJ(159) : (I) used for cross-section calculations to include mass threshold effects to radiative photon cross section. What is stored is basic cross section,
number of events generated and number that passed cuts.
PARJ(160) : (R) nominal fraction of events that should contain a radiative photon.
PARJ(161) - PARJ(164) : (I) give shape of radiative photon spectrum including weakeffects.
PARJ(168) : (R) actual f value of current event in optimized perturbation theory for R;
see MSTJ(111) and PARJ(128).
PARJ(169) : (R) actual f value of current event in optimized perturbation theory for 3and 4-jet rate; see MSTJ(111) and PARJ(129).
PARJ(171) : (R) fraction of cross section corresponding to the axial coupling of quark
pair to the intermediate γ ∗ /Z0 state; needed for the Abelian gluon model 3-jet
matrix element.

86

6.4

ExamplesAn ordinary e+ e− annihilation event in the continuum, at a c.m. energy of 91 GeV, may
be generated withCALL PYEEVT(0,91D0)
In this case a qq event is generated, including weak effects, followed by parton-shower
evolution and fragmentation/decay treatment. Before a call to PYEEVT, however, a numberof default values may be changed, e.g. MSTJ(101) = 2 to use second-order QCD matrix
elements, giving a mixture of qq, qqg, qqgg, and qqq0 q0 events, MSTJ(102) = 1 to have
QED only, MSTJ(104) = 6 to allow tt production as well, MSTJ(107) = 1 to include
initial-state photon radiation (including a treatment of the Z0 pole), PARJ(123) = 92.0
to change the Z0 mass, PARJ(81) = 0.3 to change the parton-shower Λ value, or PARJ(82)= 1.5 to change the parton-shower cut-off. If initial-state photon radiation is used, some
restrictions apply to how one can alternate the generation of events at different energies
or with different Z0 mass, etc. These restrictions are not there for efficiency reasons (theextra time for recalculating the extra constants every time is small), but because it ties
in with the cross-section calculations (see PARJ(144)).
Most parameters can be changed independently of each other. However, if just one
or a few parameters/switches are changed, one should not be surprised to find a rather
bad agreement with the data, like e.g. a too low or high average hadron multiplicity. It
is therefore usually necessary to retune one parameter related to the perturbative QCD
description, like αs or Λ, one of the two parameters a and b of the Lund symmetricfragmentation function (since they are so strongly correlated, it is often not necessary
to retune both of them), and the average fragmentation transverse momentum — see
Note 2 of the MSTJ(101) description for an example. For very detailed studies it may benecessary to retune even more parameters.
The three-gluon and gluon–gluon–photon decays of Υ may be simulated by a call
CALL PYONIA(5,9.46D0)
A typical program for analysis of e+ e− annihilation events at 200 GeV might looksomething like
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
INTEGER PYK,PYCHGE,PYCOMPCOMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)COMMON/PYDAT2/KCHG(500,4),PMAS(500,4),PARF(2000),VCKM(4,4)
COMMON/PYDAT3/MDCY(500,3),MDME(8000,2),BRAT(8000),KFDP(8000,5)
MDCY(PYCOMP(111),1)=0! put pi0 stable
MSTJ(107)=1! include initial-state radiation
PARU(41)=1D0! use linear sphericity.....
! other desired changes
CALL PYTABU(10)! initialize analysis statistics
DO 100 IEV=1,1000! loop over events
CALL PYEEVT(0,200D0)! generate new event
IF(IEV.EQ.1) CALL PYLIST(2)! list first event
CALL PYTABU(11)! save particle composition!
statistics
CALL PYEDIT(2)! remove decayed particles
CALL PYSPHE(SPH,APL)! linear sphericity analysis
IF(SPH.LT.0D0) GOTO 100! too few particles in event for87

!
PYSPHE to work on it (unusual)! orient event along axes above! list first treated event! fill analysis statistics! now do thrust analysis! more analysis statistics!
! print particle composition!
statistics! print analysis statistics

CALL PYEDIT(31)IF(IEV.EQ.1) CALL PYLIST(2).....
CALL PYTHRU(THR,OBL).....
100 CONTINUECALL PYTABU(12)
.....
END

88

7

Process Generation

Much can be said about the processes in Pythia and the way they are generated. Therefore the material has been split into three sections. In the current one the philosophy
underlying the event generation scheme is presented. Here we provide a generic description, where some special cases are swept under the carpet. In the next section, theexisting processes are enumerated, with some comments about applications and limitations. Finally, in the third section the generation routines and common-block switches
are described.
The section starts with a survey of parton distributions, followed by a detailed description of the simple 2 → 2 and 2 → 1 hard subprocess generation schemes, including pairsof resonances. This is followed by a few comments on more complicated configurations,and on nonperturbative processes.

7.1

Parton Distributions

The parton distribution function fia (x, Q2 ) parameterizes the probability to find a parton
i with a fraction x of the beam energy when the beam particle a is probed by a hard
scattering at virtuality scale Q2 . Usually the momentum-weighted
combination xfia (x, Q2 )P R1is used, for which the normalization condition i 0 dx xfia (x, Q2 ) ≡ 1 normally applies.
The Q2 dependence of parton distributions is perturbatively calculable, see section 10.3.1.
The parton distributions in Pythia come in many shapes, as shown in the following.
7.1.1

Baryons

For protons, many sets exist on the market. These are obtained by fits to experimental
data, constrained so that the Q2 dependence is in accordance with the standard QCD
evolution equations. The current default in Pythia is CTEQ 5L [Lai00], a leading-orderfit. Several other sets are found in Pythia. The complete list is:• EHLQ sets 1 and 2 [Eic84];• DO sets 1 and 2 [Duk82];• the GRV 92L (updated version) fit [Glü92];• the CTEQ 3L, CTEQ 3M and CTEQ 3D fits [Lai95];• the GRV 94L, GRV 94M and GRV 94D fits [Glü95]; and• the CTEQ 5L and CTEQ 5M1 fits [Lai00].
Of these, EHLQ, DO, GRV 92L, CTEQ 3L, GRV94L and CTEQ5L are leading-orderparton distributions, while CTEQ 3D and GRV94D are in the next-to-leading-order DIS
scheme and the rest in the next-to-leading order MS scheme. The EHLQ and DO sets
are by now rather old, and are kept mainly for backwards compatibility. Since only Bornlevel matrix elements are included in the program, there is no particular reason to use
higher-order parton distributions — the resulting combination is anyway only good toleading-order accuracy. (Some higher-order corrections are effectively included by theparton-shower treatment, but there is no exact match.)
There is a steady flow of new parton-distribution sets on the market. To keep trackof all of them is a major work on its own. Therefore Pythia contains an interface to an
external library of parton distribution functions, Pdflib [Plo93]. This is an encyclopediccollection of almost all proton, pion and photon parton distributions proposed from thelate 70’s to the late 90’s. Three dummy routines come with the Pythia package, so as to
avoid problems with unresolved external references if Pdflib is not linked. One should
also note that Pythia does not check the results, but assumes that sensible answers will
be returned, also outside the nominal (x, Q2 ) range of a set. Only the sets that come withPythia have been suitably modified to provide reasonable answers outside their nominal
domain of validity.
89

Pdflib has been frozen in recent years. Instead a new project has taken over the samerôle, LHAPDF, the Les Houches Accord PDF interface [Gie02], containing all new sets of
the last five years or so. While LHAPDF has a native input/output format different fromthe Pdflib one, the LHAGLUE subpackage allows LHAPDF to be called in exactly thesame way as Pdflib is. Therefore the Pythia external-PFD-interface works for Pdflib
and LHAPDF alike.
From the proton parton distributions, those of the neutron are obtained by isospin
conjugation, i.e. fun = fdp and fdn = fup .
The program does allow for incoming beams of a number of hyperons: Λ0 , Σ−,0,+ ,−,0Ξ and Ω− . Here one has essentially no experimental information. One could imagine
to construct models in which valence s quarks are found at larger average x values thanvalence u and d ones, because of the larger s-quark mass. However, hyperon beams is alittle-used part of the program, included only for a few specific studies. Therefore a simple
approach has been taken, in which an average valence quark distribution is constructed asppfval = (fu,val+ fd,val)/3, according to which each valence quark in a hyperon is assumed tobe distributed. Sea-quark and gluon distributions are taken as in the proton. Any proton
parton distribution set may be used with this procedure.
7.1.2

Mesons and photons

Data on meson parton distributions are scarce, so only very few sets have been constructed, and only for the π ± . Pythia contains the Owens set 1 and 2 parton distributions [Owe84], which for a long time were essentially the only sets on the market, andthe more recent dynamically generated GRV LO (updated version) [Glü92a]. The latter
one is the default in Pythia. Further sets are found in Pdflib and LHAPDF and cantherefore be used by Pythia, just as described above for protons.
Like the proton was used as a template for simple hyperon sets, so also the pion isused to derive a crude ansatz for K± /K0S /K0L . The procedure is the same, except that now
π+
π+
)/2.
fval = (fu,val+ fd,val
Sets of photon parton distributions have been obtained as for hadrons; an additional
complication comes from the necessity to handle the matching of the vector meson dominance (VMD) and the perturbative pieces in a consistent manner. New sets have been
produced where this division is explicit and therefore especially well suited for applications
to event generation[Sch95]. The Schuler and Sjöstand set 1D is the default. Although thevector-meson philosophy is at the base, the details of the fits do not rely on pion data,but only on F2γ data. Here follows a brief summary of relevant details.
Real photons obey a set of inhomogeneous evolution equations, where the inhomogeneous term is induced by γ → qq branchings. The solution can be written as the sum of
two terms,
faγ (x, Q2 ) = faγ,NP (x, Q2 ; Q20 ) + faγ,PT (x, Q2 ; Q20 ) ,(45)
where the former term is a solution to the homogeneous evolution with a (nonperturbative) input at Q = Q0 and the latter is a solution to the full inhomogeneous equation
with boundary condition faγ,PT (x, Q20 ; Q20 ) ≡ 0. One possible physics interpretation is tolet faγ,NP correspond to γ ↔ V fluctuations, where V = ρ0 , ω, φ, . . . is a set of vector
mesons, and let faγ,PT correspond to perturbative (‘anomalous’) γ ↔ qq fluctuations. The
discrete spectrum of vector mesons can be combined with the continuous (in virtualityk 2 ) spectrum of qq fluctuations, to givefaγ (x, Q2 ) =X 4παemVfV2faγ,V (x, Q2 ) +αem X 2 Z Q2 dk 2 γ,qq2eq 2fa (x, Q2 ; k 2 ) ,2π q
Q0 k 2where each component f γ,V and f γ,qq obeys a unit momentum sum rule.
90

(46)

In sets 1 the Q0 scale is picked at a low value, 0.6 GeV, where an identification of thenonperturbative component with a set of low-lying mesons appear natural, while sets 2
use a higher value, 2 GeV, where the validity of perturbation theory is better established.
The data are not good enough to allow a precise determination of ΛQCD . Therefore weuse a fixed value Λ(4) = 200 MeV, in agreement with conventional results for proton
distributions. In the VMD component the ρ0 and ω have been added coherently, so thatuu : dd = 4 : 1 at Q0 .
Unlike the p, the γ has a direct component where the photon acts as an unresolved
probe. In the definition of F2γ this adds a component C γ , symbolicallyF2γ (x, Q2 ) =

X
q

h

i

e2q fqγ + fqγ ⊗ Cq + fgγ ⊗ Cg + C γ .

(47)

Since C γ ≡ 0 in leading order, and since we stay with leading-order fits, it is permissible
to neglect this complication. Numerically, however, it makes a non-negligible difference.
We therefore make two kinds of fits, one DIS type with C γ = 0 and one ms type including
the universal part of C γ .
When jet production is studied for real incoming photons, the standard evolution
approach is reasonable also for heavy flavours, i.e. predominantly the c, but with a lower
cut-off Q0 ≈ mc for γ → cc. Moving to Deeply Inelastic Scattering, eγ → eX, there isan extra kinematical constraint: W 2 = Q2 (1 − x)/x > 4m2c . It is here better to use the‘Bethe-Heitler’ cross section for γ ∗ γ → cc. Therefore each distribution appears in two
variants. For applications to real γ’s the parton distributions are calculated as the sum ofa vector-meson part and an anomalous part including all five flavours. For applications
to DIS, the sum runs over the same vector-meson part, an anomalous part and possiblya C γ part for the three light flavours, and a Bethe-Heitler part for c and b.
In version 2 of the SaS distributions, which are the ones found here, the extension
from real to virtual photons was improved, and further options made available [Sch96].
The resolved components of the photon are dampened by phenomenologically motivatedvirtuality-dependent dipole factors, while the direct ones are explicitly calculable. Thuseq. (46) generalizes to?
faγ (x, Q2 , P 2 )=X 4παem
V

fV2

Ã

m2V
m2V + P 2

αem X 2 Z Q2 dk 2
+
2eq 2
2π q
Q0 k 2!2faγ,V (x, Q2 ; Q̃20 )Ã

k2
k2 + P 2!2

faγ,qq (x, Q2 ; k 2 ) ,(48)

with P 2 the photon virtuality and Q2 the hard-process scale. In addition to the introduction of the dipole form factors, note that the lower input scale for the VMD states is here
shifted from Q20 to some Q̃20 ≥ Q20 . This is based on a study of the evolution equation[Bor93] that shows that the evolution effectively starts ‘later’ in Q2 for a virtual photon.
Equation (48) is one possible answer. It depends on both Q2 and P 2 in a non-trivial way,
however, so that results are only obtained by a time-consuming numerical integration
rather than as a simple parametrization. Therefore several other alternatives are offered,that are in some sense equivalent, but can be given in simpler form.
In addition to the SaS sets, Pythia also contains the Drees–Grassie set of parton distributions [Dre85] and, as for the proton, there is an interface to Pdflib and LHAPDF.
These calls are made with photon virtuality P 2 below the hard-process scale Q2 . Further
author-recommended constrains are implemented in the interface to the GRS set [Glü99]
which, along with SaS, is among the few also to define parton distributions of virtual photons. However, these sets do not allow a subdivision of the photon parton distributions
into one VMD part and one anomalous part. This subdivision is necessary a sophisticated91modelling of γp and γγ events, see above and section 7.7.2. As an alternative, for the
VMD part alone, the ρ0 parton distribution can be found from the assumed equality
00fiρ = fiπ =1 π+−(fi + fiπ ) .
2(49)

Thus any π + parton distribution set, from any library, can be turned into a VMD ρ0 set.
The ω parton distribution is assumed the same, while the φ and J/ψ ones are handled in
φ
π+
φ
π+
the very crude approximation fs,val= fu,val
and fsea= fsea. The VMD part needs to becomplemented by an anomalous part to make up a full photon distribution. The latter isfully perturbatively calculable, given the lower cut-off scale Q0 . The SaS parameterizationof the anomalous part is therefore used throughout for this purpose. The Q0 scale can be
set freely in the PARP(15) parameter.
The fiγ,anom distribution can be further decomposed, by the flavour and the p⊥ ofthe original branching γ → qq. The flavour is distributed according to squared charge(plus flavour thresholds for heavy flavours) and the p⊥ according to dp2⊥ /p2⊥ in the range
Q0 < p⊥ < Q. At the branching scale, the photon only consists of a qq pair, with x
distribution ∝ x2 + (1 − x)2 . A component faγ,qq (x, Q2 ; k 2 ), characterized by its k ≈ p⊥
and flavour, then is evolved homogeneously from p⊥ to Q. For theoretical studies it isconvenient to be able to access a specific component of this kind. Therefore also leadingorder parameterizations of these decomposed distributions are available [Sch95].
7.1.3

Leptons

Contrary to the hadron case, there is no necessity to introduce the parton-distributionfunction concept for leptons. A lepton can be considered as a point-like particle, withinitial-state radiation handled by higher-order matrix elements. However, the parton
distribution function approach offers a slightly simplified but very economical descriptionof initial-state radiation effects for any hard process, also those for which higher-ordercorrections are not yet calculated.
Parton distributions for electrons have been introduced in Pythia, and are used also
for muons and taus, with a trivial substitution of masses. Alternatively, one is free to
use a simple ‘unresolved’ e, fee (x, Q2 ) = δ(x − 1), where the e retains the full original
momentum.
Electron parton distributions are calculable entirely from first principles, but different
levels of approximation may be used. The parton-distribution formulae in Pythia are
based on a next-to-leading-order exponentiated description, see ref. [Kle89], p. 34. The
approximate behaviour isβ(1 − x)β/2−1 ;
2
Ã
!
Q2
2αem
ln 2 − 1 .
β=πmefee (x, Q2 ) ≈(50)

The form is divergent but integrable for x → 1, i.e. the electron likes to keep most of theenergy. To handle the numerical precision problems for x very close to unity, the parton
distribution is set, by hand, to zero for x > 1−10−10 , and is rescaled upwards in the range
1 − 10−7 < x < 1 − 10−10 , in such a way that the total area under the parton distribution
is preserved:³´

fee (x, Q2 )

mod

=

 e
fe (x, Q2 )





β/2

0 ≤ x ≤ 1 − 10−71000
fee (x, Q2 ) 1 − 10−7 < x < 1 − 10−10
β/2

1000
−
1




−10
0

x > 1 − 10

92

.

(51)

A separate issue is that electron beams may not be monochromatic, more so than forother particles because of the small electron mass. In storage rings the main mechanism issynchrotron radiation. For future high-luminosity linear colliders, the beam–beam interactions at the collision vertex may induce a quite significant energy loss — ‘beamstrahlung’.
Note that neither of these are associated with any off-shellness of the electrons, i.e. theresulting spectrum only depends on x and not Q2 . Examples of beamstrahlung spectra
are provided by the Circe program [Ohl97], with a sample interface on the Pythia
webpages.
The branchings e → eγ, which are responsible for the softening of the fee parton
distribution, also gives rise to a flow of photons. In photon-induced hard processes, thefγe parton distribution can be used to describe the equivalent flow of photons. In the next
section, a complete differential photon flux machinery is introduced. Here some simpler
first-order expressions are introduced, for the flux integrated up to a hard interaction scale
Q2 . There is some ambiguity in the choice of Q2 range over which emissions should be
included. The naı̈ve (default) choice is
αem 1 + (1 − x)2fγe (x, Q2 ) =
2π

x

Ã

Q2
ln
m2e

!

(52)

.

Here it is assumed that only one scale enters the problem, namely that of the hardinteraction, and that the scale of the branching e → eγ is bounded from above by the
hard interaction scale. For a pure QCD or pure QED shower this is an appropriate
procedure, cf. section 10.1.3, but in other cases it may not be optimal. In particular, forphotoproduction the alternative that is probably most appropriate is [Ali88]:
Ã

Q2max (1 − x)αem 1 + (1 − x)2fγe (x, Q2 ) =ln
2πx
m2e x2!

.

(53)

Here Q2max is a user-defined cut for the range of scattered electron kinematics that iscounted as photoproduction. Note that we now deal with two different Q2 scales, one
related to the hard subprocess itself, which appears as the argument of the parton distribution, and the other related to the scattering of the electron, which is reflected in
Q2max .
Also other sources of photons should be mentioned. One is the beamstrahlung photons mentioned above, where again Circe provides sample parameterizations. Another,
particularly interesting one, is laser backscattering, wherein an intense laser pulse is shotat an incoming high-energy electron bunch. By Compton backscattering this gives rise
to a photon energy spectrum with a peak at a significant fraction of the original electronenergy [Gin82]. Both of these sources produce real photons, which can be considered as
photon beams of variable energy (see section 9.8), decoupled from the production processproper.
In resolved photoproduction or resolved γγ interactions, one has to include the parton
distributions for quarks and gluons inside the photon inside the electron. This is best doneewith the machinery of the next section. However, as an older and simpler alternative, fq,g
can be obtained by a numerical convolution according toe(x, Q2 ) =
fq,g

Ã

Z 1

x
dxγ e
γ
fγ (xγ , Q2 ) fq,g
, Q2
xγ
x xγ

!

,

(54)

with fγe as discussed above. The necessity for numerical convolution makes this parton
distribution evaluation rather slow compared with the others; one should therefore only
have it switched on for resolved photoproduction studies.

93One can obtain the positron distribution inside an electron, which is also the electron
sea parton distribution, by a convolution of the two branchings e → eγ and γ → e+ e− ;the result is [Che75]
1−fee+ (x, Q2 ) =
2

(

αem
2π

Ã

!)2

Q2
ln 2 − 1
me

1
x

µ

4
4
− x2 − x3 + 2x(1 + x) ln x
3
3

¶

.

(55)

Finally, the program also contains the distribution of a transverse W− inside an electron
Ã
!
αem
1
Q2
1 + (1 − x)2e2
ln 1 + 2
fW (x, Q ) =.
(56)
2π 4 sin2 θW
x
mW
7.1.4

Equivalent photon flux in leptons

With the ’gamma/lepton’ option of a PYINIT call, an ep or e+ e− event (or corresponding
processes with muons) is factorized into the flux of virtual photons and the subsequentinteractions of such photons. While real photons always are transverse (T), the virtualphotons also allow a longitudinal (L) component. This corresponds to cross sectionsdσ(ep → eX) =

X Z Z

ξ
dy dQ2 fγ/e(y, Q2 ) dσ(γξ∗ p → X)

(57)

ξ=T,L

and
dσ(ee → eeX) =

X

Z Z Z

ξ1
ξ2
dy1 dQ21 dy2 dQ22 fγ/e(y1 , Q21 )fγ/e(y2 , Q22 ) dσ(γξ∗1 γξ∗2 → X) .

ξ1 ,ξ2 =T,L

(58)
For ep events, this factorized ansatz is perfectly general, so long as azimuthal distributionsin the final state are not studied in detail. In e+ e− events, it is not a good approximation
when the virtualities Q21 and Q22 of both photons become of the order of the squared
invariant mass W 2 of the colliding photons [Sch98]. In this region the cross section
have terms that depend on the relative azimuthal angle of the scattered leptons, and thetransverse and longitudinal polarizations are non-trivially mixed. However, these terms
are of order Q21 Q22 /W 2 and can be neglected whenever at least one of the photons has low
virtuality compared to W 2 .
When Q2 /W 2 is small, one can derive [Bon73, Bud75, Sch98]Ãαem (1 + (1 − y)2 1
2m2l y
=
−
2π
y
Q2
Q4
αem 2(1 − y) 1
L,fγ/l(y, Q2 ) =
2π
y
Q2
T
fγ/l
(y, Q2 )

!

,

(59)
(60)

T
where l = e± , µ± or τ ± . In fγ/l
the second term, proportional to m2l /Q4 , is not leading log
and is therefore often omitted. Clearly it is irrelevant at large Q2 , but around the lower
cut-off Q2min it significantly dampens the small-y rise of the first term. (Note that Q2min
is y-dependent, so properly the dampening is in a region of the (y, Q2 ) plane.) Overall,under realistic conditions, it reduces event rates by 5–10% [Sch98, Fri93].
The y variable is defined as the light-cone fraction the photon takes of the incoming
lepton momentum. For instance, for l+ l− events,

yi =

qi kj
,
ki kj

j = 2(1) for i = 1(2) ,

94

(61)

where the ki are the incoming lepton four-momenta and the qi the four-momenta of thevirtual photons.
Alternatively, the energy fraction the photon takes in the rest frame of the collision
can be used,qi (k1 + k2 )xi =,i = 1, 2 .
(62)
ki (k1 + k2 )The two are simply related,

Q2i
,
(63)
s
with s = (k1 + k2 )2 . (Here and in the following formulae we have omitted the lepton andhadron mass terms when it is not of importance for the argumentation.) Since the Jacobian d(yi , Q2i )/d(xi , Q2i ) = 1, either variable would be an equally valid choice for covering
the phase space. Small xi values will be of less interest for us, since they lead to small
W 2 , so yi /xi ≈ 1 except in the high-Q2 tail, and often the two are used interchangeably.
T,L
Unless special Q2 cuts are imposed, cross sections obtained with fγ/l(x, Q2 ) dx rather
T,Lthan fγ/l(y, Q2 ) dy differ only at the per mil level. For comparisons with experimental
cuts, it is sometimes relevant to know which of the two is being used in an analysis.
In the ep kinematics, the x and y definitions give thatyi = xi +W 2 = xs = ys − Q2 .

(64)

The W 2 expression for l+ l− is more complicated, especially because of the dependence on
the relative azimuthal angle of the scattered leptons, ϕ12 = ϕ1 − ϕ2 :
s

s

W2

2Q21 Q22
Q2
Q2= x1 x2 s +− 2 1 − x1 − 1 1 − x2 − 2 Q1 Q2 cos ϕ12
s
s
s
2 2
q
q
QQ
= y1 y2 s − Q21 − Q22 + 1 2 − 2 1 − y1 1 − y2 Q1 Q2 cos ϕ12 .
s(65)

The lepton scattering angle θi is related to Q2i as
Ã

!

x2i
2
Q2i =
m2i + (1 − xi ) s −m2 − 2m2j sin2 (θi /2) ,1 − xi(1 − xi )2 i

(66)

with m2i = ki2 = k 0 2i and terms of O(m4 ) neglected. The kinematical limits thus arex2i
m2 ,1 − xi i≈ (1 − xi )s ,(Q2i )min ≈(67)(Q2i )max(68)

unless experimental conditions reduce the θi ranges.
In summary, we will allow the possibility of experimental cuts in the xi , yi , Q2i , θi
and W 2 variables.
Within the allowed region, the phase space is Monte Carlo sampledQ
according to i (dQ2i /Q2i ) (dxi /xi ) dϕi , with the remaining flux factors combined with thecross section factors to give the event weight used for eventual acceptance or rejection.
This cross section in its turn can contain the parton densities of a resolved virtual photon,
thus offering an effective convolution that gives partons inside photons inside electrons.

95

7.2

Kinematics and Cross Section for a Two-body Process

In this section we begin the description of kinematics selection and cross-section calculation. The example is for the case of a 2 → 2 process, with final-state masses assumed to
be vanishing. Later on we will expand to finite fixed masses, and to resonances.
Consider two incoming beam particles in their c.m. frame, each with energy Ebeam .
2
The total squared c.m. energy is then s = 4Ebeam. The two partons that enter the
hard interaction do not carry the total beam momentum, but only fractions x1 and x2 ,respectively, i.e. they have four-momenta
p1 = Ebeam (x1 ; 0, 0, x1 ) ,p2 = Ebeam (x2 ; 0, 0, −x2 ) .

(69)

There is no reason to put the incoming partons on the mass shell, i.e. to have time-likeincoming four-vectors, since partons inside a particle are always virtual and thus spacelike. These space-like virtualities are introduced as part of the initial-state parton-shower
description, see section 10.3.3, but do not affect the formalism of this section, wherefore
massless incoming partons is a sensible ansatz. The one example where it would beappropriate to put a parton on the mass shell is for an incoming lepton beam, but evenhere the massless kinematics description is adequate as long as the c.m. energy is correctly
calculated with masses.
The squared invariant mass of the two partons is defined asŝ = (p1 + p2 )2 = x1 x2 s .

(70)

Instead of x1 and x2 , it is often customary to use τ and either y or xF :
τ = x1 x2 =

ŝ
;
s

1 x1
ln
;
2 x2
= x1 − x2 .

(71)

y =

(72)

xF

(73)

In addition to x1 and x2 , two additional variables are needed to describe the kinematics
of a scattering 1 + 2 → 3 + 4. One corresponds to the azimuthal angle ϕ of the scattering
plane around the beam axis. This angle is always isotropically distributed for unpolarizedincoming beam particles, and so need not be considered further. The other variable canbe picked as θ̂, the polar angle of parton 3 in the c.m. frame of the hard scattering. The
conventional choice is to use the variableŝt̂ = (p1 − p3 )2 = (p2 − p4 )2 = − (1 − cos θ̂) ,(74)
2
with θ̂ defined as above. In the following, we will make use of both t̂ and θ̂. It is also
customary to define û,ŝû = (p1 − p4 )2 = (p2 − p3 )2 = − (1 + cos θ̂) ,2
but û is not an independent variable sinceŝ + t̂ + û = 0 .

(75)

(76)

If the two outgoing particles have masses m3 and m4 , respectively, then the fourmomenta in the c.m. frame of the hard interaction are given by
√
√
!
Ã
ŝ
ŝ
ŝ ± (m23 − m24 )√β34 sin θ̂, 0, ±β34 cos θ̂ ,p̂3,4 =
,±
(77)
2
2
2 ŝ
96

where
β34 =

v
uÃ
u
t

m2 m2
1− 3 − 4ŝŝ

!2m23 m24
.
ŝ ŝ

(78)

o
1n
(ŝ − m23 − m24 ) ∓ ŝ β34 cos θ̂ ,
2

(79)

−4

Then t̂ and û are modified tot̂, û = −withŝ + t̂ + û = m23 + m24 .

(80)

The cross section for the process 1 + 2 → 3 + 4 may be written as
Z Z Z

dx1 dx2 dt̂ f1 (x1 , Q2 ) f2 (x2 , Q2 )

σ =
Z Z Z

=

dσ̂
dt̂

dτ
dσ̂
dy dt̂ x1 f1 (x1 , Q2 ) x2 f2 (x2 , Q2 )
.
τ
dt̂

(81)

The choice of Q2 scale is ambiguous, and several alternatives are available in the
program. For massless outgoing particles the default is the squared transverse momentum
Q2 = p̂2⊥ =

t̂û
ŝ 2
sin θ̂ =
,
4
ŝ

(82)

which is modified tot̂û − m23 m24
1 2
1 2
1 2
2
2
2
2
Q = (m⊥3 + m⊥4 ) = (m3 + m4 ) + p̂⊥ = (m3 + m4 ) +
2
2
2
ŝ
2

(83)

when masses are introduced in the final state. The mass term is selected such that, form3 = m4 = m, the expression reduces to the squared transverse mass, Q2 = m̂2⊥ = m2 +p̂2⊥ . For cases with space-like virtual incoming photons, of virtuality Q2i = −m2i = |p2i |, afurther generalization to
1
Q2 = (Q21 + Q22 + m23 + m24 ) + p̂2⊥
2

(84)

is offered.
The dσ̂/dt̂ expresses the differential cross section for a scattering, as a function of thekinematical quantities ŝ, t̂ and û, and of the relevant masses. It is in this function thatthe physics of a given process resides.
The performance of a machine is measured in terms of its luminosity L, which is
directly proportional to the number of particles in each bunch and to the bunch crossing
frequency, and inversely proportional to the area of the bunches at the collision point.
For a process with a σ as given by eq. (81), the differential event rate is given by σL, andthe number of events collected over a given period of timeZ

N =σ

L dt .

(85)

The program does not calculate the number of events, but only the integrated crosssections.

97

7.3

Resonance Production

The simplest way to produce a resonance is by a 2 → 1 process. If the decay of theresonance is not considered, the cross-section formula does not depend on t̂, but takes the
form
Z Z
dτ
σ=
dy x1 f1 (x1 , Q2 ) x2 f2 (x2 , Q2 ) σ̂(ŝ) .
(86)τHere the physics is contained in the cross section σ̂(ŝ). The Q2 scale is usually taken to
be Q2 = ŝ.
In published formulae, cross sections are often given in the zero-width approximation,i.e. σ̂(ŝ) ∝ δ(ŝ − m2R ), where mR is the mass of the resonance. Introducing the scaled
mass τR = m2R /s, this corresponds to a delta function δ(τ − τR ), which can be used toeliminate the integral over τ .
However, what we normally want to do is replace the δ function by the appropriate
Breit–Wigner shape. For a resonance width ΓR this is achieved by the replacementδ(τ − τR ) →

s
mR ΓR
.
π (sτ − m2R )2 + m2R Γ2R(87)

In this formula the resonance width ΓR is a constant.
An improved description of resonance shapes is obtained if the width is made ŝdependent (occasionally also referred to as mass-dependent width, since ŝ is not always theresonance mass), see e.g. [Ber89]. To first approximation, this means that the expressionmR ΓR is to be replaced by ŝΓR /mR , both in the numerator and the denominator. An
intermediate step is to perform this replacement only in the numerator. This is convenientwhen not only s-channel resonance production is simulated but also non-resonance t- or
u-channel graphs are involved, since mass-dependent widths in the denominator here may
give an imperfect cancellation of divergences. (More about this below.)
To be more precise, in the program the quantity HR (ŝ) is introduced, and the Breit–
Wigner is written ass
HR (sτ )δ(τ − τR ) →.
(88)
π (sτ − m2R )2 + HR2 (sτ )P(f )The HR factor is evaluated as a sum over all possible final-state channels, HR = f HR .
Each decay channel may have its own ŝ dependence, as follows.
A decay to a fermion pair, R → ff, gives no contribution below threshold, i.e. for(f )ŝ < 4m2f . Above threshold, HR is proportional to ŝ, multiplied by a threshold factor
2β(3 − β )/2 for the vector part of a spin 1 resonance, by β 3 for the axialqvector part,by β 3 for a scalar resonance and by β for a pseudoscalar one. Here β = 1 − 4m2f /ŝ.
For the decay into unequal masses, e.g. of the W+ , corresponding but more complicatedexpressions are used.
For decays into a quark pair, a first-order strong correction factor 1 + αs (ŝ)/π is(f )included in HR . This is the correct choice for all spin 1 colourless resonances, but
is here used for all resonances where no better knowledge is available. Currently the
major exception is top decay, where the factor 1 − 2.5 αs (ŝ)/π is used to approximate loop
corrections [Jeż89]. The second-order corrections are often known, but then are specific toeach resonance, and are not included. An option exists for the γ/Z0 /Z00 resonances, where
threshold effects due to qq bound-state formation are taken into account in a smeared-out,
average sense, see eq. (136).
For other decay channels, not into fermion pairs, the ŝ dependence is typically more
complicated. An example would be the decay h0 → W+ W− , with a nontrivial threshold
and a subtle energy dependence above that [Sey95a]. Since a Higgs with mh < 2mW
could still decay in this channel, it is in fact necessary to perform a two-dimensional
98integral over the W ± Breit–Wigner mass distributions to obtain the correct result (andthis has to be done numerically, at least in part). Fortunately, a Higgs particle lighter than2mW is sufficiently narrow that the integral only needs to be performed once and for all
at initialization (whereas most other partial widths are recalculated whenever needed).
Channels that proceed via loops, such as h → gg, also display complicated threshold
behaviours.
The coupling structure within the electroweak sector is usually (re)expressed in termsof gauge boson masses, αem and sin2 θW , i.e. factors of GF are replaced according to
√
π αem
2GF =
.
(89)
sin2 θW m2W
Having done that, αem is allowed to run [Kle89], and is evaluated at the ŝ scale. Thereby
the relevant electroweak loop correction factors are recovered at the mW /mZ scale. However, the option exists to go the other way and eliminate αem in favour of GF . Currently
sin2 θW is not allowed to run.
For Higgs particles and technipions, fermion masses enter not only in the kinematics
but also as couplings. The latter kind of quark masses (but not the former, at least not in
the program) are running with the scale of the process, i.e. normally the resonance mass.
The expression used is [Car96]
Ã2m(Q ) = m0ln(k 2 m20 /Λ2 )ln(Q2 /Λ2 )!12/(33−2nf )

.

(90)

Here m0 is the input mass at a reference scale km0 , defined in the ms scheme. Typicalchoices are either k = 1 or k = 2; the latter would be relevant if the reference scale ischosen at the QQ threshold. Both Λ and nf are as given in αs .
In summary, we see that an ŝ dependence may enter several different ways into the(f )HR expressions from which the total HR is built up.
When only decays to a specific final state f are considered, the HR in the denominatorremains the sum over all allowed decay channels, but the numerator only contains the(f )HR term of the final state considered.
If the combined production and decay process i → R → f is considered, the same ŝ
dependence is implicit in the coupling structure of i → R as one would have had in R → i,i.e. to first approximation there is a symmetry between couplings of a resonance to theinitial and to the final state. The cross section σ̂ is therefore, in the program, written in
the form(f )(i)
π HR (ŝ) HR (ŝ)σ̂i→R→f (ŝ) ∝(91).
ŝ (ŝ − m2R )2 + HR2 (ŝ)
As a simple example, the cross section for the process e− ν e → W− → µ− ν µ can be written
as
(i)
(f )
π
HW (ŝ) HW (ŝ)σ̂(ŝ) = 12,(92)2ŝ (ŝ − m2W )2 + HW
(ŝ)
where
(i)

(f )

HW (ŝ) = HW (ŝ) =αem (ŝ)ŝ .
24 sin2 θW(93)

If the effects of several initial and/or final states are studied, it is straightforward to
introduce an appropriate summation in the numerator.
(f )(i)
The analogy between the HR and HR cannot be pushed too far, however. The
two differ in several important aspects. Firstly, colour factors appear reversed: the decay
R → qq contains a colour factor NC = 3 enhancement, while qq → R is instead suppressed99

by a factor 1/NC = 1/3. Secondly, the 1 + αs (ŝ)/π first-order correction factor for the
final state has to be replaced by a more complicated K factor for the initial state. Thisfactor is not known usually, or it is known (to first non-trivial order) but too lengthy to
be included in the program. Thirdly, incoming partons as a rule are space-like. All thethreshold suppression factors of the final-state expressions are therefore irrelevant when(f )(i)production is considered. In sum, the analogy between HR and HR is mainly usefulas a consistency cross-check, while the two usually are calculated separately. Exceptions
include the rather messy loop structure involved in gg → h0 and h0 → gg, which is only
coded once.
It is of some interest to consider the observable resonance shape when the effects ofparton distributions are included. In a hadron collider, to first approximation, parton
distributions tend to have a behaviour roughly like f (x) ∝ 1/x for small x — this is why
f (x) is replaced by xf (x) in eq. (81). Instead, the basic parton-distribution behaviour
is shifted into the factor of 1/τ in the integration phase space dτ /τ , cf. eq. (86). When
convoluted with the Breit–Wigner shape, two effects appear. One is that the overall
resonance is tilted: the low-mass tail is enhanced and the high-mass one suppressed. The
other is that an extremely long tail develops on the low-mass side of the resonance: when
τ → 0, eq. (91) with HR (ŝ) ∝ ŝ gives a σ̂(ŝ) ∝ ŝ ∝R τ , which exactly cancels the 1/τ
factor mentioned above. Naı̈vely, the integral over y, dy = − ln τ , therefore gives a net
logarithmic divergence of the resonance shape when τ → 0. Clearly, it is then necessary
to consider the shape of the parton distributions in more detail. At not-too-small Q2 , theevolution equations in fact lead to parton distributions more strongly peaked than 1/x,
typically with xf (x) ∝ x−0.3 , and therefore a divergence like τ −0.3 in the cross-section
expression. Eventually this divergence is regularized by a closing of the phase space,i.e. that HR (ŝ) vanishes faster than ŝ, and by a less drastic small-x parton-distribution
behaviour when Q2 ≈ ŝ → 0.
The secondary peak at small τ may give a rather high cross section, which can evenrival that of the ordinary peak around the nominal mass. This is the case, for instance,with W production. Such a peak has never been observed experimentally, but this is not
surprising, since the background from other processes is overwhelming at low ŝ. Thus a
lepton of one or a few GeV of transverse momentum is far more likely to come from thedecay of a charm or bottom hadron than from an extremely off-shell W of a mass of afew GeV. When resonance production is studied, it is therefore important to set limits
on the mass of the resonance, so as to agree with the experimental definition, at least to
first approximation. If not, cross-section information given by the program may be very
confusing.
Another problem is that often the matrix elements really are valid only in the resonanceregion. The reason is that one usually includes only the simplest s-channel graph in thecalculation. It is this ‘signal’ graph that has a peak at the position of the resonance, whereit (usually) gives much larger cross sections than the other ‘background’ graphs. Away
from the resonance position, ‘signal’ and ‘background’ may be of comparable order, orthe ‘background’ may even dominate. There is a quantum mechanical interference whensome of the ‘signal’ and ‘background’ graphs have the same initial and final state, and thisinterference may be destructive or constructive. When the interference is non-negligible,
it is no longer meaningful to speak of a ‘signal’ cross section. As an example, consider the
scattering of longitudinal W’s, WL+ WL− → WL+ WL− , where the ‘signal’ process is s-channel
exchange of a Higgs. This graph by itself is ill-behaved away from the resonance region.
Destructive interference with ‘background’ graphs such as t-channel exchange of a Higgs
and s- and t-channel exchange of a γ/Z is required to save unitarity at large energies.
In e+ e− colliders, the fee parton distribution is peaked at x = 1 rather than at x = 0.
The situation therefore is the opposite, if one considers e.g. Z0 production in a machinerunning at energies above mZ : the resonance-peak tail towards lower masses is suppressed
and the one towards higher masses enhanced, with a sharp secondary peak at around the100nominal energy of the machine. Also in this case, an appropriate definition of cross sections
therefore is necessary — with additional complications due to the interference between γ ∗and Z0 . When other processes are considered, problems of interference with backgroundappears also here. Numerically the problems may be less pressing, however, since thesecondary peak is occurring in a high-mass region, rather than in a more complicatedlow-mass one. Further, in e+ e− there is little uncertainty from the shape of the parton
distributions.
In 2 → 2 processes where a pair of resonances are produced, e.g. e+ e− → Z0 h0 , crosssection are almost always given in the zero-width approximation for the resonances. Heretwo substitutions of the type
Z

Z

1=

2

δ(m

− m2R ) dm2 →1
mR Γ R
dm2
2 2
2 2
2
π (m − mR ) + mR ΓR(94)

are used to introduce mass distributions for the two resonance masses, i.e. m23 and m24 . In
the formula, mR is the nominal mass and m the actually selected one. The phase-space
integral over x1 , x1 and t̂ in eq. (81) is then extended to involve also m23 and m24 . The effectsof the mass-dependent width is only partly taken into account, by replacing the nominal
masses m23 and m24 in the dσ̂/dt̂ expression by the actually generated ones (also e.g. in
the relation between t̂ and cos θ̂), while the widths are evaluated at the nominal masses.
This is the equivalent of a simple replacement of mR ΓR by ŝΓR /mR in the numerator ofeq. (87), but not in the denominator. In addition, the full threshold dependence of thewidths, i.e. the velocity-dependent factors, is not reproduced.
There is no particular reason why the full mass-dependence could not be introduced,
except for the extra work and time consumption needed for each process. In fact, thematrix elements for several γ ∗ /Z0 and W± production processes do contain the full expressions. On the other hand, the matrix elements given in the literature are often valid
only when the resonances are almost on the mass shell, since some graphs have been
omitted. As an example, the process qq → e− ν e µ+ νµ is dominated by qq → W− W+
when each of the two lepton pairs is close to mW in mass, but in general also receives
contributions e.g. from qq → Z0 → e+ e− , followed by e+ → ν e W+ and W+ → µ+ νµ . The
latter contributions are neglected in cross sections given in the zero-width approximation.
Widths may induce gauge invariance problems, in particular when the s-channel graphinterferes with t- or u-channel ones. Then there may be an imperfect cancellation of contributions at high energies, leading to an incorrect cross section behaviour. The underlying
reason is that a Breit–Wigner corresponds to a resummation of terms of different orders
in coupling constants, and that therefore effectively the s-channel contributions are calculated to higher orders than the t- or u-channel ones, including interference contributions.
A specific example is e+ e− → W+ W− , where s-channel γ ∗ /Z∗ exchange interferes witht-channel νe exchange. In such cases, a fixed width is used in the denominator. One
could also introduce procedures whereby the width is made to vanish completely at highenergies, and theoretically this is the cleanest, but the fixed-width approach appears goodenough in practice.
Another gauge invariance issue is when two particles of the same kind are produced
in a pair, e.g. gg → tt. Matrix elements are then often calculated for one common
mt mass, even though in real life the masses m3 6= m4 . The proper gauge invariant
procedure to handle this would be to study the full six-fermion state obtained after thetwo t → bW → bfi f j decays, but that may be overkill if indeed the t’s are close tomass shell. Even when only equal-mass matrix elements are available, Breit–Wigners are
therefore used to select two separate masses m3 and m4 . From these two masses, anaverage mass m is constructed so that the β34 velocity factor of eq. (78) is retained,
β34 (ŝ, m2 , m2 ) = β34 (ŝ, m23 , m24 )

⇒
101

m2 =

m23 + m24 (m23 − m24 )2
−
.
2
4ŝ

(95)

This choice certainly is not unique, but normally should provide a sensible behaviour,also around threshold. Of course, the differential cross section is no longer guaranteed
to be gauge invariant when gauge bosons are involved, or positive definite. The programautomatically flags the latter situation as unphysical. The approach may well break downwhen either or both particles are far away from mass shell. Furthermore, the preliminary
choice of scattering angle θ̂ is also retained. Instead of the correct t̂ and û of eq. (79),
modifiedo1n(m23 − m24 )22t̂, û = − (ŝ − 2m ) ∓ ŝ β34 cos θ̂ = (t̂, û) −
(96)
2
4ŝ
can then be obtained. The m2 , t̂ and û are now used in the matrix elements to decide
whether to retain the event or not.
Processes with one final-state resonance and another ordinary final-state product, e.g.
qg → W+ q0 , are treated in the same spirit as the 2 → 2 processes with two resonances,
except that only one mass need be selected according to a Breit–Wigner.

7.4

Cross-section Calculations

In the program, the variables used in the generation of a 2 → 2 process are τ , y andz = cos θ̂. For a 2 → 1 process, the z variable can be integrated out, and need therefore
not be generated as part of the hard process, except when the allowed angular range of
decays is restricted. In unresolved lepton beams, i.e. when fee (x) = δ(x − 1), the variables
τ and/or y may be integrated out. We will cover all these special cases towards the endof the section, and here concentrate on ‘standard’ 2 → 2 and 2 → 1 processes.
7.4.1

The simple two-body processes

In the spirit of section 4.1, we want to select simple functions such that the true τ , y andz dependence of the cross sections is approximately modelled. In particular, (almost) allconceivable kinematical peaks should be represented by separate terms in the approximate
formulae. If this can be achieved, the ratio of the correct to the approximate cross sections
will not fluctuate too much, but allow reasonable Monte Carlo efficiency.
Therefore the variables are generated according to the distributions hτ (τ ), hy (y) andhz (z), where normallyc1 1 c2 1
c3
1
c4
1
+
+
+
2 2
2
I1 τ
I2 τ
I3 τ (τ + τR ) I4 (sτ − mR ) + m2R Γ2R
c5
1
c6
1
+
,
+
I5 τ (τ + τR0 ) I6 (sτ − m2R0 )2 + m2R0 Γ2R0
c1
c2
c3 1
hy (y) =(y − ymin ) +(ymax − y) +,I1
I2
I3 cosh y
c2 1
c3 1
c4
1
c5
1
c1
+
+
+
+
.
hz (z) =2
I1 I2 a − z I3 a + z I4 (a − z)
I5 (a + z)2hτ (τ ) =

(97)
(98)
(99)

Here each term is separately integrable, with an invertible primitive function, such thatgeneration of τ , y and z separately is a standard task, as described in section 4.1. In thefollowing we describe the details of the scheme, including the meaning of the coefficientsci and Ii , which are separate for τ , y and z.
The first variable to be selected is τ . The range of allowed values, τmin ≤ τ ≤ τmax ,is generally constrained by a number of user-defined requirements. A cut on the allowed
mass range is directly reflected in τ , a cut on the p⊥ range indirectly. The first two termsof hτ are intended to represent a smooth τ dependence, as generally obtained in processes
102which do not receive contributions from s-channel resonances. Also s-channel exchangeof essentially massless particles (γ, g, light quarks and leptons) are accounted for, sincethese do not produce any separate peaks at non-vanishing τ . The last four terms ofhτ are there to catch the peaks in the cross section from resonance production. These
terms are only included when needed. Each resonance is represented by two pieces, a firstto cover the interference with graphs which peak at τ = 0, plus the variation of parton
distributions, and a second to approximate the Breit–Wigner shape of the resonance itself.
The subscripts R and R0 denote values pertaining to the two resonances, with τR = m2R /s.
Currently there is only one process where the full structure with two resonances is used,
namely ff → γ ∗ /Z0 /Z00 . Otherwise either one or no resonance peak is taken into account.
The kinematically allowed range of y values is constrained by τ , |y| ≤ − 21 ln τ , andyou may impose additional cuts. Therefore the allowed range ymin ≤ y ≤ ymax is only
constructed after τ has been selected. The first two terms of hy give a fairly flat y dependence — for processes which are symmetric in y ↔ −y, they will add to give a completely
flat y spectrum between the allowed limits. In principle, the natural subdivision would
have been one term flat in y and one forward–backward asymmetric, i.e. proportional toy. The latter is disallowed by the requirement of positivity, however. The y − ymin andymax − y terms actually used give the same amount of freedom, but respect positivity.
The third term is peaked at around y = 0, and represents the bias of parton distributions
towards this region.
The allowed z = cos θ̂ range is naı̈vely −1 ≤ z ≤ 1. However, most cross sections aredivergent for z → ±1, so some kind of regularization is necessary. Normally one requiresp⊥ ≥ p⊥min , which translates into z 2 ≤ 1 − 4p2⊥min /(τ s) for massless outgoing particles.
Since again the limits depend on τ , the selection of z is done after that of τ . Additional
requirements may constrain the range further. In particular, a p⊥max constraint may split
the allowed z range into two, i.e. z−min ≤ z ≤ z−max or z+min ≤ z ≤ z+max . An un-split
range is represented by z−max = z+min = 0. For massless outgoing particles the parametera = 1 in hz , such that the five terms represent a piece flat in angle and pieces peaked as 1/t̂,
1/û, 1/t̂2 , and 1/û2 , respectively. For non-vanishing masses one has a = 1 + 2m23 m24 /ŝ2 .
In this case, the full range −1 ≤ z ≤ 1 is therefore available — physically, the standard t̂
and û singularities are regularized by the masses m3 and m4 .
For each of the terms, the Ii coefficients represent the integral over the quantity
multiplying the coefficient ci ; thus, for instance:
µ
¶
Z
dτ
τmax
hτ :
I1 =
= ln
,
τ
τmin
Z
dτ
1
1
I2 =
=
−
;
2
τ
τmin τmax
Z1hy :I1 = (y − ymin ) dy = (ymax − ymin )2 ;
2
Z
hz :

I1 =

dz = (z−max − z−min ) + (z+max − z+min ),
Z

I2 =

Ã

dz
(a − z−min )(a − z+min )= lna−z(a − z−max )(a − z−min )

!

.

(100)

The ci coefficients are normalized to unit sum for hτ , hy and hz separately. They
have a simple interpretation, as the probability for each of the terms to be used in the
preliminary selection of τ , y and z, respectively. The variation of the cross section over
the allowed phase space is explored in the initialization procedure of a Pythia run, andbased on this knowledge the ci are optimized so as to give functions hτ , hy and hz thatclosely follow the general behaviour of the true cross section. For instance, the coefficient
c4 in hτ is to be made larger the more the total cross section is dominated by the region
around the resonance mass.
103

The phase-space points tested at initialization are put on a grid, with the number ofpoints in each dimension given by the number of terms in the respective h expression,
and with the position of each point given by the median value of the distributionof one√of the terms. For instance, the dτ /τ distribution gives a median point at τmin τmax , anddτ /τ 2 has the median 2τmin τmax /(τmin + τmax ). Since the allowed y and z ranges dependon the τ value selected, then so do the median points defined for these two variables.
With only a limited set of phase-space points studied at the initialization, the ‘optimal’
set of coefficients is not uniquely defined. To be on the safe side, 40% of the total weight istherefore assigned evenly between all allowed ci , whereas the remaining 60% are assigned
according to the relative importance surmised, under the constraint that no coefficient isallowed to receive a negative contribution from this second piece.
After a preliminary choice has been made of τ , y and z, it is necessary to find theweight of the event, which is to be used to determine whether to keep it or generate
another one. Using the relation dt̂ = ŝ β34 dz/2, eq. (81) may be rewritten as
Z Z Z

dτ
ŝβ34
dσ̂
dy
dz x1 f1 (x1 , Q2 ) x2 f2 (x2 , Q2 )
τ
2
dt̂
Z
Z
Z
π
x1 f1 (x1 , Q2 ) x2 f2 (x2 , Q2 ) ŝ2 dσ̂=hτ (τ ) dτ hy (y) dy hz (z) dz β34sτ 2 hτ (τ ) hy (y) 2hz (z) π dt̂

σ =

*

=

2
β34
π
2
2 ŝ dσ̂
x
f
(x
,
Q
)
x
f
(x
,
Q
)
1
1
1
2
2
2
s τ 2 hτ (τ ) hy (y) 2hz (z)
π dt̂

+

.

(101)

In the middle line, a factor of 1 = hτ /hτ Rhas been introduced to rewrite the τ integral interms of a phase space of unit volume: hτ (τ ) dτ = 1 according to the relations above.
Correspondingly for the y and z integrals. In addition, factors of 1 = ŝ/(τ s) and 1 = π/π
are used to isolate the dimensionless cross section (ŝ2 /π) dσ̂/dt̂. The content of the lastline is that, with τ , y and z selected according to the expressions hτ (τ ), hy (y) and hz (z),respectively, the cross section is obtained as the average of the final expression over all
events. Since the h’s have been picked to give unit volume, there is no need to multiply
by the total phase-space volume.
As can be seen, the cross section for a given Monte Carlo event is given as the productof four factors, as follows:
1. The π/s factor, which is common to all events, gives the overall dimensions of thecross section, in GeV−2 . Since the final cross section is given in units of mb, theconversion factor of 1 GeV−2 = 0.3894 mb is also included here.
2. Next comes the Jacobian, which compensates for the change from the original to
the final phase-space volume.
3. The parton-distribution function weight is obtained by making use of the parton
distribution libraries in Pythia or externally.
The x1 and x2 values are obtained√
from τ and y via the relations x1,2 = τ exp(±y).
4. Finally, the dimensionless cross section (ŝ2 /π) dσ̂/dt̂ is the quantity that has to be
coded for each process separately, and where the physics content is found.
Of course, the expression in the last line is not strictly necessary to obtain the crosssection by Monte Carlo integration. One could also have used eq. (81) directly, selecting
phase-space points evenly in τ , y and t̂, and averaging over those Monte Carlo weights.
Clearly this would be much simpler, but the price to be paid is that the weights ofindividual events could fluctuate wildly. For instance, if the cross section contains anarrow resonance, the few phase-space points that are generated in the resonance region
obtain large weights, while the rest do not. With our procedure, a resonance would
be included in the hτ (τ ) factor, so that more events would be generated at around theappropriate τR value (owing to the hτ numerator in the phase-space expression), but with104

these events assigned a lower, more normal weight (owing to the factor 1/hτ in the weight
expression). Since the weights fluctuate less, fewer phase-space points need be selected toget a reasonable cross-section estimate.
In the program, the cross section is obtained as the average over all phase-space points
generated. The events actually handed on to you should have unit weight, however (anoption with weighted events exists, but does not represent the mainstream usage). At
initialization, after the ci coefficients have been determined, a search inside the allowed
phase-space volume is therefore made to find the maximum of the weight expression in
the last line of eq. (101). In the subsequent generation of events, a selected phase-spacepoint is then retained with a probability equal to the weight in the point divided by
the maximum weight. Only the retained phase-space points are considered further, andgenerated as complete events.
The search for the maximum is begun by evaluating the weight in the same grid ofpoints as used to determine the ci coefficients. The point with highest weight is used as
starting point for a search towards the maximum. In unfortunate cases, the convergence
could be towards a local maximum which is not the global one. To somewhat reduce
this risk, also the grid point with second-highest weight is used for another search. Afterinitialization, when events are generated, a warning message will be given by default at
any time a phase-space point is selected where the weight is larger than the maximum,
and thereafter the maximum weight is adjusted to reflect the new knowledge. This meansthat events generated before this time have a somewhat erroneous distribution in phasespace, but if the maximum violation is rather modest the effects should be negligible. The
estimation of the cross section is not affected by any of these considerations, since themaximum weight does not enter into eq. (101).
For 2 → 2 processes with identical final-state particles, the symmetrization factor of
1/2 is explicitly included at the end of the dσ̂/dt̂ calculation. In the final cross section,a factor of 2 is retrieved because of integration over the full phase space (rather than
only half of it). That way, no special provisions are needed in the phase-space integration
machinery.
7.4.2

Resonance production

We have now covered the simple 2 → 2 case. In a 2 → 1 process, the t̂ integral is absent,
and the differential cross section dσ̂/dt̂ is replaced by σ̂(ŝ). The cross section may now
be written as
Z Zdτdy x1 f1 (x1 , Q2 ) x2 f2 (x2 , Q2 ) σ̂(ŝ)τ
Z
x1 f1 (x1 , Q2 ) x2 f2 (x2 , Q2 ) ŝπZhτ (τ ) dτ hy (y) dyσ̂(ŝ)=sτ 2 hτ (τ ) hy (y)
π
*
+
π
1
2
2 ŝ
=
x1 f1 (x1 , Q ) x2 f2 (x2 , Q ) σ̂(ŝ) .
s τ 2 hτ (τ ) hy (y)
π

σ =

(102)

The structure is thus exactly the same, but the z-related pieces are absent, and the rôleof the dimensionless cross section is played by (ŝ/π)σ̂(ŝ).
If the range of allowed decay angles of the resonance is restricted, e.g. by requiring
the decay products to have a minimum transverse momentum, effectively this translates
into constraints on the z = cos θ̂ variable of the 2 → 2 process. The difference is that theangular dependence of a resonance decay is trivial, and that therefore the z-dependent
factor can be easily evaluated. For a spin-0 resonance, which decays isotropically, therelevant weight is simply (z−max −z−min )/2+(z+max −z+min )/2. For a transversely polarized105

spin-1 resonance the expression is, instead,3
3
1
1(z−max − z−min ) + (z+max − z+min ) + (z−max − z−min )3 + (z+max − z+min )3 . (103)8888Since the allowed z range could depend on τ and/or y (it does for a p⊥ cut), the factor
has to be evaluated for each individual phase-space point and included in the expressionof eq. (102).
For 2 → 2 processes where either of the final-state particles is a resonance, or both, an
additional choice has to be made for each resonance mass, eq. (94). Since the allowed τ ,y and z ranges depend on m23 and m24 , the selection of masses have to precede the choiceof the other phase-space variables. Just as for the other variables, masses are not selected
uniformly over the allowed range, but are rather distributed according to a function
hm (m2 ) dm2 , with a compensating factor 1/hm (m2 ) in the Jacobian. The functional form
picked is normallyhm (m2 ) =c1 1
mR Γ R
c2
c3 1
c4 1
+
+
+
.
2 2
2 2
2
2
I1 π (m − mR ) + mR ΓR I2 I3 m
I4 m 4(104)

The definition of the Ii integrals is analogous to the one before. The ci coefficients are notfound by optimization, but predetermined, normally to c1 = 0.8, c2 = c3 = 0.1, c4 = 0.
Clearly, had the phase space and the cross section been independent of m23 and m24 , theoptimal choice would have been to put c1 = 1 and have all other ci vanishing — then the
1/hm factor of the Jacobian would exactly have cancelled the Breit–Wigner of eq. (94)in the cross section. The second and the third terms are there to cover the possibility
that the cross section does not die away quite as fast as given by the naı̈ve Breit–Wigner
shape. In particular, the third term covers the possibility of a secondary peak at small
m2 , in a spirit slightly similar to the one discussed for resonance production in 2 → 1
processes.
The fourth term is only used for processes involving γ ∗ /Z0 production, where the γ
propagator guarantees that the cross section does have a significant secondary peak for
m2 → 0. Therefore here the choice is c1 = 0.4, c2 = 0.05, c3 = 0.3 and c4 = 0.25.
A few special tricks have been included to improve efficiency when the allowed mass
range of resonances is constrained by kinematics or by user cuts. For instance, if a pairof equal or charge-conjugate resonances are produced, such as in e+ e− → W+ W− , use ismade of the constraint that the lighter of the two has to have a mass smaller than half
the c.m. energy.
7.4.3

Lepton beams

Lepton beams have to be handled slightly differently from what has been described so far.
One also has to distinguish between a lepton for which parton distributions are included
and one which is treated as an unresolved point-like particle. The necessary modifications
are the same for 2 → 2 and 2 → 1 processes, however, since the t̂ degree of freedom isunaffected.
If one incoming beam is an unresolved lepton, the corresponding parton-distribution
piece collapses to a δ function. This function can be used to integrate out the y variable:δ(x1,2 − 1) = δ(y ± (1/2) ln τ ). It is therefore only necessary to select the τ and the z
variables according to the proper distributions, with compensating weight factors, andonly one set of parton distributions has to be evaluated explicitly.
If both incoming beams are unresolved leptons, both the τ and the y variables are
trivially given: τ = 1 and y = 0. Parton-distribution weights disappear completely. For
a 2 → 2 process, only the z selection remains to be performed, while a 2 → 1 process iscompletely specified, i.e. the cross section is a simple number that only depends on thec.m. energy.
106For a resolved electron, the fee parton distribution is strongly peaked towards x = 1.
This affects both the τ and the y distributions, which are not well described by eitherof the pieces in hτ (τ ) or hy (y) in processes with interacting e± . (Processes which involvee.g. the γ content of the e are still well simulated, since fγe is peaked at small x.)
If both parton distributions are peaked close to 1, the hτ (τ ) expression in eq. (99)is therefore increased with one additional term of the form hτ (τ ) ∝ 1/(1 − τ ), withcoefficients c7 and I7 determined as before. The divergence when τ → 1 is cut off by ourregularization procedure for the fee parton distribution; therefore we only need consider
τ < 1 − 2 × 10−10 .
Correspondingly, the hy (y) expression is expanded with a term 1/(1−exp(y−y0 )) whenincoming beam number 1 consists of a resolved e± , and with a term 1/(1 − exp(−y − y0 ))when incoming beam number 2 consists of a resolved e± . Both terms are present foran e+ e− collider, only one for an ep one. The coefficient y0 = −(1/2) ln τ is the naı̈ve
kinematical limit of the y range, |y| < y0 . From the definitions of y and y0 it is easy
to see that the two terms above correspond to 1/(1 − x1 ) and 1/(1 − x2 ), respectively,
and thus are again regularized by our parton-distribution function cut-off. Therefore theintegration ranges are y < y0 − 10−10 for the first term and y > −y0 + 10−10 for the second
one.
7.4.4

Mixing processes

In the cross-section formulae given so far, we have deliberately suppressed a summation
over the allowed incoming flavours. For instance, the process ff → Z0 in a hadron collider
receives contributions from uu → Z0 , dd → Z0 , ss → Z0 , and so on. These contributions
share the same basic form, but differ in the parton-distribution weights and (usually) in a
few coupling constants in the hard matrix elements. It is therefore convenient to generate
the terms together, as follows:
1. A phase-space point is picked, and all common factors related to this choice are
evaluated, i.e. the Jacobian and the common pieces of the matrix elements (e.g. fora Z0 the basic Breit–Wigner shape, excluding couplings to the initial flavour).
2. The parton-distribution-function library is called to produce all the parton distributions, at the relevant x and Q2 values, for the two incoming beams.
3. A loop is made over the two incoming flavours, one from each beam particle. For eachallowed set of incoming flavours, the full matrix-element expression is constructed,using the common pieces and the flavour-dependent couplings. This is multipliedby the common factors and the parton-distribution weights to obtain a cross-section
weight.
4. Each allowed flavour combination is stored as a separate entry in a table, together
with its weight. In addition, a summed weight is calculated.
5. The phase-space point is kept or rejected, according to a comparison of the summed
weight with the maximum weight obtained at initialization. Also the cross-section
Monte Carlo integration is based on the summed weight.
6. If the point is retained, one of the allowed flavour combinations is picked according
to the relative weights stored in the full table.
Generally, the flavours of the final state are either completely specified by those of
the initial state, e.g. as in qg → qg, or completely decoupled from them, e.g. as in
0
ff → Z0 → f 0 f . In neither case need therefore the final-state flavours be specified in the
cross-section calculation. It is only necessary, in the latter case, to include an overall
weight factor, which takes into account the summed contribution of all final states thatare to be simulated. For instance, if only the process Z0 → e+ e− is studied, the relevant
weight factor is simply Γee /Γtot . Once the kinematics and the incoming flavours havebeen selected, the outgoing flavours can be picked according to the appropriate relative107

probabilities.
In some processes, such as gg → gg, several different colour flows are allowed, eachwith its own kinematical dependence of the matrix-element weight, see section 8.2.1. Eachcolour flow is then given as a separate entry in the table mentioned above, i.e. in total an
entry is characterized by the two incoming flavours, a colour-flow index, and the weight.
For an accepted phase-space point, the colour flow is selected in the same way as theincoming flavours.
The program can also allow the mixed generation of two or more completely different
processes, such as ff → Z0 and qq → gg. In that case, each process is initialized separately,with its own set of coefficients ci and so on. The maxima obtained for the individual crosssections are all expressed in the same units, even when the dimensionality of the phasespace
is different. (This is because we always transform to a phase space of unit volume,Rhτ (τ ) dτ ≡ 1, etc.) The above generation scheme need therefore only be generalized asfollows:
1. One process is selected among the allowed ones, with a relative probability given by
the maximum weight for this process.
2. A phase-space point is found, using the distributions hτ (τ ) and so on, optimized forthis particular process.
3. The total weight for the phase-space point is evaluated, again with Jacobians, matrix
elements and allowed incoming flavour combinations that are specific to the process.
4. The point is retained with a probability given by the ratio of the actual to the
maximum weight of the process. If the point is rejected, one has to go back to step
1 and pick a new process.
5. Once a phase-space point has been accepted, flavours may be selected, and the event
generated in full.
It is clear why this works: although phase-space points are selected among the allowed
processes according to relative probabilities given by the maximum weights, the probability that a point is accepted is proportional to the ratio of actual to maximum weight. In
total, the probability for a given process to be retained is therefore only proportional tothe average of the actual weights, and any dependence on the maximum weight is gone.
In γp and γγ physics, the different components of the photon give different final states,
see section 7.7.2. Technically, this introduces a further level of administration, since each
event class contains a set of (partly overlapping) processes. From an ideological point ofview, however, it just represents one more choice to be made, that of event class, before
the selection of process in step 1 above. When a weighting fails, both class and process
have to be picked anew.

7.5

Three- and Four-body Processes

The Pythia machinery to handle 2 → 1 and 2 → 2 processes is fairly sophisticated andgeneric. The same cannot be said about the generation of hard-scattering processes withmore than two final-state particles. The number of phase-space variables is larger, and itis therefore more difficult to find and transform away all possible peaks in the cross section
by a suitably biased choice of phase-space points. In addition, matrix-element expressionsfor 2 → 3 processes are typically fairly lengthy. Therefore Pythia only contains a very
limited number of 2 → 3 and 2 → 4 processes, and almost each process is a special case of
its own. It is therefore less interesting to discuss details, and we only give a very generic
overview.
If the Higgs mass is not light, interactions among longitudinal W and Z gauge bosons
are of interest. In the program, 2 → 1 processes such as WL+ WL− → h0 and 2 → 2ones such as WL+ WL− → Z0L Z0L are included. The former are for use when the h0 still isreasonably narrow, such that a resonance description is applicable, while the latter are
intended for high energies, where different contributions have to be added up. Since the108program does not contain WL or ZL distributions inside hadrons, the basic hard scattering
has to be convoluted with the q → q0 WL and q → qZL branchings, to yield effective 2 → 3
and 2 → 4 processes. However, it is possible to integrate out the scattering angles of thequarks analytically, as well as one energy-sharing variable [Cha85]. Only after an event
has been accepted are these other kinematical variables selected. This involves further
choices of random variables, according to a separate selection loop.
In total, it is therefore only necessary to introduce one additional variable in the basic
phase-space selection, which is chosen to be ŝ0 , the squared invariant mass of the full2 → 3 or 2 → 4 process, while ŝ is used for the squared invariant mass of the inner
2 → 1 or 2 → 2 process. The y variable is coupled to the full process,√ since partondistribution weights have to be given for the original quarks at x1,2 = τ 0 exp (±y). Thet̂ variable is related to the inner process, and thus not needed for the 2 → 3 processes.
The selection of the τ 0 = ŝ0 /s variable is done after τ , but before y has been chosen. To
improve
R the efficiency, the selection is made according to a weighted phase space of theform hτ 0 (τ 0 ) dτ 0 , wherehτ 0 (τ 0 ) =c1 1
c2 (1 − τ /τ 0 )3
c3
1
+
+
,
0
02
0
I1 τ
I2
τ
I3 τ (1 − τ 0 )(105)

in conventional notation. The ci coefficients are optimized at initialization. The c3 term,
peaked at τ 0 ≈ 1, is only used for e+ e− collisions. The choice of hτ 0 is roughly matchedto the longitudinal gauge-boson flux factor, which is of the form
µ

1+

τ
τ0

¶

µ

ln

¶

µ

τ
τ
−2 1− 0
0
τ
τ

¶

.

(106)

For a light h the effective W approximation above breaks down, and it is necessary
to include the full structure of the qq0 → qq0 h0 (i.e. ZZ fusion) and qq0 → q00 q000 h0 (i.e.
WW fusion) matrix elements. The τ 0 , τ and y variables are here retained, and selectedaccording to standard procedures. The Higgs mass is represented by the τ choice; normallythe h0 is so narrow that the τ distribution effectively collapses to a δ function. In addition,
the three-body final-state phase space is rewritten as
Ã 5
Y

1 d3 pi3i=3 (2π) 2Ei!

1π2
dϕ3 2 dϕ4
√
dp⊥4
dy5 ,
dp2⊥3
5
(2π) 4 λ⊥34
2π
2π
(107)
where λ⊥34 = (m2⊥34 −m2⊥3 −m2⊥4 )2 −4m2⊥3 m2⊥4 . The outgoing quarks are labelled 3 and 4,
and the outgoing Higgs 5. The ϕ angles are selected isotropically, while the two transverse
momenta are picked, with some foreknowledge of the shape of the W/Z propagators in
the cross sections, according to h⊥ (p2⊥ ) dp2⊥ , where(2π)4 δ (4) (p3 +p4 +p5 −p1 −p2 ) =h⊥ (p2⊥ ) =

c1
c2
1
c3
1
+
+
,
2
2
2
I1 I2 mR + p⊥ I3 (mR + p2⊥ )2(108)

with mR the W or Z mass, depending on process, and c1 = c2 = 0.05, c3 = 0.9. Within
the limits given by the other variable choices, the rapidity y5 is chosen uniformly. A final
choice remains to be made, which comes from a twofold ambiguity of exchanging thelongitudinal momenta of partons 3 and 4 (with minor modifications if they are massive).
Here the relative weight can be obtained exactly from the form of the matrix element
itself.

7.6

Resonance Decays

Resonances (see section 2.1.2) can be made to decay in two different routines. One
is the standard decay treatment (in PYDECY) that can be used for any unstable particle,
109where decay channels are chosen according to fixed probabilities, and decay angles usuallyare picked isotropically in the rest frame of the resonance, see section 13.3. The moresophisticated treatment (in PYRESD) is the default one for resonances produced in Pythia,
and is described here. The ground rule is that everything in mass up to and including b
hadrons is decayed with the simpler PYDECY routine, while heavier particles are handledwith PYRESD. This also includes the γ ∗ /Z0 , even though here the mass in principle could
be below the b threshold. Other resonances include, e.g., t, W± , h0 , Z00 , W0± , H0 , A0 ,H± , and technicolor and supersymmetric particles.
7.6.1

The decay scheme

In the beginning of the decay treatment, either one or two resonances may be present,
the former represented by processes such as qq0 → W+ and qg → W+ q0 , the latter by
qq → W+ W− . If the latter is the case, the decay of the two resonances is considered inparallel (unlike PYDECY, where one particle at a time is made to decay).
First the decay channel of each resonance is selected according to the relative weights(f )HR , as described above, evaluated at the actual mass of the resonance, rather than at
the nominal one. Threshold factors are therefore fully taken into account, with channels
automatically switched off below the threshold. Normally the masses of the decay products are well-defined, but e.g. in decays like h0 → W+ W− it is also necessary to selectthe decay product masses. This is done according to two Breit–Wigners of the type ineq. (94), multiplied by the threshold factor, which depends on both masses.
Next the decay angles of the resonance are selected isotropically in its rest frame.
Normally the full range of decay angles is available, but in 2 → 1 processes the decay
angles of the original resonance may be restrained by user cuts, e.g. on the p⊥ of the decay
products. Based on the angles, the four-momenta of the decay products are constructed
and boosted to the correct frame. As a rule, matrix elements are given with quark andlepton masses assumed vanishing. Therefore the four-momentum vectors constructed at
this stage are actually massless for all quarks and leptons.
The matrix elements may now be evaluated. For a process such as qq → W+ W− →e+ νe µ− ν µ , the matrix element is a function of the four-momenta of the two incoming
fermions and of the four outgoing ones. An upper limit for the event weight can beconstructed from the cross section for the basic process qq → W+ W− , as already used to
select the two W momenta. If the weighting fails, new resonance decay angles are picked
and the procedure is iterated until acceptance.
Based on the accepted set of angles, the correct decay product four-momenta are constructed, including previously neglected fermion masses. Quarks and, optionally, leptons
are allowed to radiate, using the standard final-state showering machinery, with maximum
virtuality given by the resonance mass.
In some decays new resonances are produced, and these are then subsequently allowed
to decay. Normally only one resonance pair is considered at a time, with the possibility offull correlations. In a few cases triplets can also appear, but such configurations currently
are considered without inclusion of correlations. Also note that, in a process like qq →Z0 h0 → Z0 W+ W− → 6 fermions, the spinless nature of the h0 ensures that the W± decays
are decoupled from that of the Z0 (but not from each other).
7.6.2

Cross-section considerations

The cross section for a process which involves the production of one or several resonancesis always reduced to take into account channels not allowed by user flags. This is trivial
for a single s-channel resonance, cf. eq. (91), but can also be included approximately ifseveral layers of resonance decays are involved. At initialization, the ratio between theuser-allowed width and the nominally possible one is evaluated and stored, starting from110

the lightest resonances and moving upwards. As an example, one first finds the reduction
factors for W+ and for W− decays, which need not be the same if e.g. W+ is allowed todecay only to quarks and W− only to leptons. These factors enter together as a weight
for the h0 → W+ W− channel, which is thus reduced in importance compared with otherpossible Higgs decay channels. This is also reflected in the weight factor of the h0 itself,
where some channels are open in full, others completely closed, and finally some (like theone above) open but with reduced weight. Finally, the weight for the process qq → Z0 h0is evaluated as the product of the Z0 weight factor and the h0 one. The standard crosssection of the process is multiplied with this weight.
Since the restriction on allowed decay modes is already included in the hard-processcross section, mixing of different event types is greatly simplified, and the selection of decay
channel chains is straightforward. There is a price to be paid, however. The reduction
factors evaluated at initialization all refer to resonances at their nominal masses. For
instance, the W reduction factor is evaluated at the nominal W mass, even when thatfactor is used, later on, in the description of the decay of a 120 GeV Higgs, where at least
one W would be produced below this mass. We know of no case where this approximation
has any serious consequences, however.
The weighting procedure works because the number of resonances to be produced,
directly or in subsequent decays, can be derived recursively already from the start. It
does not work for particles which could also be produced at later stages, such as theparton-shower evolution and the fragmentation. For instance, D0 mesons can be produced
fairly late in the event generation chain, in unknown numbers, and so weights could not
be introduced to compensate, e.g. for the forcing of decays only into π + K− .
One should note that this reduction factor is separate from the description of theresonance shape itself, where the full width of the resonance has to be used. This widthis based on the sum of all possible decay modes, not just the simulated ones. Pythia
does allow the possibility to change also the underlying physics scenario, e.g. to include
the decay of a Z0 into a fourth-generation neutrino.
Normally the evaluation of the reduction factors is straightforward. However, fordecays into a pair of equal or charge-conjugate resonances, such as Z0 Z0 or W+ W− , it ispossible to pick combinations in such a way that the weight of the pair does not factorize
into a product of the weight of each resonance itself. To be precise, any decay channel
can be given seven different status codes:• −1: a non-existent decay mode, completely switched off and of no concern to us;• 0: an existing decay channel, which is switched off;
• 1: a channel which is switched on;• 2: a channel switched on for particles, but off for antiparticles;• 3: a channel switched on for antiparticles, but off for particles;• 4: a channel switched on for one of the particles or antiparticles, but not for both;• 5: a channel switched on for the other of the particles or antiparticles, but not for
both.
The meaning of possibilities 4 and 5 is exemplified by the statement ‘in a W+ W− pair,
one W decays hadronically and the other leptonically’, which thus covers the cases where
either W+ or W− decays hadronically.
Neglecting non-existing channels, each channel belongs to either of the classes above.
If we denote the total branching ratio into channels of type i by ri , this then translates
into the requirement r0 + r1 + r2 + r3 + r4 + r5 = 1. For a single particle the weight factor
is r1 + r2 + r4 , and for a single antiparticle r1 + r3 + r4 . For a pair of identical resonances,
the joint weight is instead(r1 + r2 )2 + 2(r1 + r2 )(r4 + r5 ) + 2r4 r5 ,

111

(109)

and for a resonance–antiresonance pair
(r1 + r2 )(r1 + r3 ) + (2r1 + r2 + r3 )(r4 + r5 ) + 2r4 r5 .

(110)

If some channels come with a reduced weight because of restrictions on subsequent decay
chains, this may be described in terms of properly reduced ri , so that the sum is less thanunity. For instance, in a tt → bW+ bW− process,the W decay P
modes may be restrictedP+−−
to W → qq and W → e ν̄e , in which case ( ri )t ≈ 2/3 and ( ri )t ≈ 1/9. With index± denoting resonance/antiresonance, eq. (110) then generalizes to(r1 + r2 )+ (r1 + r3 )− + (r1 + r2 )+ (r4 + r5 )− + (r4 + r5 )+ (r1 + r3 )− + r4+ r5− + r5+ r4− . (111)7.7

Nonperturbative Processes

A few processes are not covered by the discussion so far. These are the ones that dependon the details of hadronic wave functions, and therefore are not strictly calculable perturbatively (although perturbation theory may often provide some guidance). What we haveprimarily in mind is elastic scattering, diffractive scattering and low-p⊥ ‘minimum-bias’
events in hadron–hadron collisions, but one can also find corresponding processes in γpand γγ interactions. The description of these processes is rather differently structured
from that of the other ones, as is explained below. Models for ‘minimum-bias’ events are
discussed in detail in sections 11.2–11.4, to which we refer for details on this part of the
program.
7.7.1

Hadron–hadron interactions

AB
In hadron–hadron interactions, the total hadronic cross section for AB → anything, σtot,
is calculated using the parameterization of Donnachie and Landshoff [Don92]. In thisapproach, each cross section appears as the sum of one pomeron term and one reggeon
one
AB
σtot
(s) = X AB s² + Y AB s−η ,(112)
2where s = Ecm. The powers ² = 0.0808 and η = 0.4525 are expected to be universal,
whereas the coefficients X AB and Y AB are specific to each initial state. (In fact, the highenergy behaviour given by the pomeron term is expected to be the same for particle andantiparticle interactions, i.e. X AB = X AB .) Parameterizations not provided in [Don92]
have been calculated in the same spirit, making use of quark counting rules [Sch93a].
The total cross section is subdivided according to
AB
AB
AB
AB
AB
σtot
(s) = σelAB (s) + σsd(XB)(s) + σsd(AX)(s) + σdd(s) + σnd
(s) .

(113)

Here ‘el’ is the elastic process AB → AB, ‘sd(XB)’ the single diffractive AB → XB,‘sd(AX)’ the single diffractive AB → AX, ‘dd’ the double diffractive AB → X1 X2 , and‘nd’ the non-diffractive ones. Higher diffractive topologies, such as central diffraction, arecurrently neglected. In the following, the elastic and diffractive cross sections and event
characteristics are described, as given in the model by Schuler and Sjöstrand [Sch94,
Sch93a]. The non-diffractive component is identified with the ‘minimum bias’ physics
already mentioned, a practical but not unambiguous choice. Its cross section is given by‘whatever is left’ according to eq. (113), and its properties are discussed in section 11.2.
At not too large squared momentum transfers t, the elastic cross section can be approximated by a simple exponential fall-off. If one neglects the small real part of the crosssection, the optical theorem then givesdσel
σ2= tot exp(Bel t) ,
dt
16π
112

(114)

2
and σel = σtot
/16πBel . The elastic slope parameter is parameterized by

Bel = BelAB (s) = 2bA + 2bB + 4s² − 4.2 ,(115)

with s given in units of GeV and Bel in GeV−2 . The constants bA,B are bp = 2.3, bπ,ρ,ω,φ =
1.4, bJ/ψ = 0.23. The increase of the slope parameter with c.m. energy is faster thanthe logarithmically one conventionally assumed; that way the ratio σel /σtot remains wellbehaved at large energies.
The diffractive cross sections are given bydσsd(XB) (s)
g3IP
1
2
=
βAIP βBIP
exp(Bsd(XB) t) Fsd ,
2
dt dM
16π
M2
dσsd(AX) (s)g3IP 21
β=βexp(Bsd(AX) t) Fsd ,BIP
AIP
dt dM 2
16π
M2
2
g3IP
dσdd (s)1 1=βAIP βBIP 2 2 exp(Bdd t) Fdd .
2
2
dt dM1 dM2
16π
M1 M2

(116)

AB ²
The couplings βAIP are related to the pomeron term
√ X s of the total cross section
parameterization, eq. (112). Picking a reference scale sref = 20 GeV, the couplings are
given by βAIP βBIP = X AB s²ref . The triple-pomeron coupling is determined from singlediffractive data to be g3IP ≈ 0.318 mb1/2 ; within the context of the formulae in thissection.
The spectrum of diffractive masses M is taken to begin 0.28 GeV ≈ 2mπ above themass of the respective incoming particle and extend to the kinematical limit. The simple
dM 2 /M 2 form is modified by the mass-dependence in the diffractive slopes and in the Fsd
and Fdd factors (see below).
The slope parameters are assumed to beµ¶s
Bsd(XB) (s) = 2bB + 2α ln
,
M2¶
µ
s
Bsd(AX) (s) = 2bA + 2α0 ln
,
M2 !
Ã
ss0
Bdd (s) = 2α0 ln e4 + 2 2 .
M1 M2
0

(117)

Here α0 = 0.25 GeV−2 and conventionally s0 is picked as s0 = 1/α0 . The term e4 in Bdd is
added by hand to avoid a breakdown of the standard expression for large values of M12 M22 .
The bA,B terms protect Bsd from breaking down; however a minimum value of 2 GeV−2
is still explicitly required for Bsd , which comes into play e.g. for a J/ψ state (as part of a
VMD photon beam).
The kinematical range in t depends on all the masses of the problem. In terms of2/s (= m2A /s when A scattersthe scaled variables µ1 = m2A /s, µ2 = m2B /s, µ3 = M(1)22elastically), µ4 = M(2) /s (= mB /s when B scatters elastically), and the combinations
C1 = 1 − (µ1 + µ2 + µ3 + µ4 ) + (µ1 − µ2 )(µ3 − µ4 ) ,qq(1 − µ1 − µ2 )2 − 4µ1 µ2 (1 − µ3 − µ4 )2 − 4µ3 µ4 ,C2 =C3 = (µ3 − µ1 )(µ4 − µ2 ) + (µ1 + µ4 − µ2 − µ3 )(µ1 µ4 − µ2 µ3 ) ,(118)

one has tmin < t < tmax withstmin = − (C1 + C2 ) ,
2
s 4C3
s2 C3
s
=
.
tmax = − (C1 − C2 ) = −2
2 C1 + C2
tmin
113

(119)

The Regge formulae above for single- and double-diffractive events are supposed tohold in certain asymptotic regions of the total phase space. Of course, there will bediffraction also outside these restrictive regions. Lacking a theory which predicts differential cross sections at arbitrary t and M 2 values, the Regge formulae are used everywhere,
but fudge factors are introduced in order to obtain ‘sensible’ behaviour in the full phasespace. These factors are:
Ã

Fsd
Fdd

M2
1−
s

!Ã

!

2
cres Mres
1+ 2,=
Mres + M 2
!
Ã
!Ã
s m2p
(M1 + M2 )2=1−s
s m2p + M12 M22Ã×2cres Mres
1+ 2
Mres + M12!Ã2cres Mres
1+ 2
Mres + M22

!

.

(120)

The first factor in either expression suppresses production close to the kinematical limit.
The second factor in Fdd suppresses configurations where the two diffractive systems
overlap in rapidity space. The final factors give an enhancement of the low-mass region,
where a resonance structure is observed in the data. Clearly a more detailed modelling
would have to be based on a set of exclusive states rather than on this smeared-out
averaging procedure. A reasonable fit to pp/pp data is obtained for cres = 2 and Mres =A
2 GeV, for an arbitrary particle A which is diffractively excited we use Mres
= mA − mp +2 GeV.
The diffractive cross-section formulae above have been integrated for a set of c.m.
energies, starting at 10 GeV, and the results have been parameterized. The form of
these parameterizations is given in ref. [Sch94], with explicit numbers for the pp/pp case.
Pythia also contains similar parameterizations for πp (assumed to be same as ρp andωp), φp, J/ψp, ρρ (ππ etc.), ρφ, ρJ/ψ, φφ, φJ/ψ and J/ψJ/ψ.
The processes above do not obey the ordinary event mixing strategy. First of all,since their total cross sections are known, it is possible to pick the appropriate processfrom the start, and then remain with that choice. In other words, if the selection ofkinematical variables fails, one would not go back and pick a new process, the way it
was done in section 7.4.4. Second, it is not possible to impose any cuts or restrain
allowed incoming or outgoing flavours; especially for minimum-bias events the production
at different transverse momenta is interrelated by the underlying formalism. Third, it is
not recommended to mix generation of these processes with that of any of the other ones:
normally the other processes have so small cross sections that they would almost never be
generated anyway. (We here exclude the cases of ‘underlying events’ and ‘pile-up events’,
where mixing is provided for, and even is a central part of the formalism, see sections 11.2
and 11.5.)
Once the cross-section parameterizations has been used to pick one of the processes,
the variables t and M are selected according to the formulae given above.
A ρ0 formed by γ → ρ0 in elastic or diffractive scattering is polarized, and therefore
its decay angular distribution in ρ0 → π + π − is taken to be proportional to sin2 θ, where
the reference axis is given by the ρ0 direction of motion.
A light diffractive system, with a mass less than 1 GeV above the mass of the incoming particle, is allowed to decay isotropically into a two-body state. Single-resonancediffractive states, such as a ∆+ , are therefore not explicitly generated, but are assumed
described in an average, smeared-out sense.
A more massive diffractive system is subsequently treated as a string with the quantum
numbers of the original hadron. Since the exact nature of the pomeron exchanged between
the hadrons is unknown, two alternatives are included. In the first, the pomeron isassumed to couple to (valence) quarks, so that the string is stretched directly between the114struck quark and the remnant diquark (antiquark) of the diffractive state. In the second,
the interaction is rather with a gluon, giving rise to a ‘hairpin’ configuration in which thestring is stretched from a quark to a gluon and then back to a diquark (antiquark). Bothof these scenarios could be present in the data; the default choice is to mix them in equal
proportions.
There is experimental support for more complicated scenarios [Ing85], wherein thepomeron has a partonic substructure, which e.g. can lead to high-p⊥ jet production in
the diffractive system. The full machinery, wherein a pomeron spectrum is convoluted
with a pomeron-proton hard interaction, is not available in Pythia. (But is found in the
PomPyt program [Bru96].)7.7.2

Photoproduction and γγ physics

The photon physics machinery in Pythia has been largely expanded in recent years.
Historically, the model was first developed for photoproduction, i.e. a real photon ona hadron target [Sch93, Sch93a]. Thereafter γγ physics was added in the same spirit[Sch94a, Sch97]. More recently also virtual photons have been added to the description[Fri00], including the nontrivial transition region between real photons and Deeply Inelastic Scattering (DIS). In this section we partly trace this evolution towards more complex
configurations.
The total γp and γγ cross sections can again be parameterized in a form like eq. (112),
which is not so obvious since the photon has more complicated structure than an ordinary
hadron. In fact, the structure is still not so well understood. The model we outline is theone studied by Schuler and Sjöstrand [Sch93, Sch93a], and further updated in [Fri00]. In
this model the physical photon is represented by
q

|γi =

Z3 |γB i +

X

X e
X e
e
|V i +|qqi +|`+ `− i .
f
f
f
V
qq
``
0
q
`=e,µ,τ
V =ρ ,ω,φ,J/ψ

(121)

By virtue of this superposition, one is led to a model of γp interactions, where threedifferent kinds of events may be distinguished:• Direct events, wherein the bare photon |γB i interacts directly with a parton fromthe proton. The process is perturbatively calculable, and no parton distributionsof the photon are involved. The typical event structure is two high-p⊥ jets and a
proton remnant, while the photon does not leave behind any remnant.
• VMD events, in which the photon fluctuates into a vector meson, predominantly a
ρ0 . All the event classes known from ordinary hadron–hadron interactions may thus
occur here, such as elastic, diffractive, low-p⊥ and high-p⊥ events. For the latter,
one may define (VMD) parton distributions of the photon, and the photon also
leaves behind a beam remnant. This remnant is smeared in transverse momentum
by a typical ‘primordial k⊥ ’ of a few hundred MeV.
• Anomalous or GVMD (Generalized VMD) events, in which the photon fluctuates
into a qq pair of larger virtuality than in the VMD class. The initial parton distribution is perturbatively calculable, as is the subsequent QCD evolution. It gives riseto the so-called anomalous part of the parton distributions of the photon, whence
one name for the class. As long as only real photons were considered, it made sense
to define the cross section of this event class to be completely perturbatively calculable, given some lower p⊥ cut-off. Thus only high-p⊥ events could occur. However,alternatively, one may view these states as excited higher resonances (ρ0 etc.), thus
the GVMD name. In this case one is lead to a picture which also allows a low-p⊥
cross section, uncalculable in perturbation theory. The reality may well interpolate between these two extreme alternatives, but the current framework more leans
towards the latter point of view. Either the q or the q plays the rôle of a beam115

remnant, but this remnant has a larger p⊥ than in the VMD case, related to thevirtuality of the γ ↔ qq fluctuation.
The |`+ `− i states can only interact strongly with partons inside the hadron at higher
orders, and can therefore be neglected in the study of hadronic final states.
In order that the above classification is smooth and free of double counting, one
has to introduce scales that separate the three components. The main one is k0 , which
separates the low-mass vector meson region from the high-mass |qqi one, k0 ≈ mφ /2 ≈ 0.5
GeV. Given this dividing line to VMD states, the anomalous parton distributions are
perturbatively calculable. The total cross section of a state is not, however, since thisinvolves aspects of soft physics and eikonalization of jet rates. Therefore an ansatz is2chosen where the total cross section of a state scales like kV2 /k⊥, where the adjustableparameter kV ≈ mρ /2 for light quarks. The k⊥ scale is roughly equated with half themass of the GVMD state. The spectrum of GVMD states is taken to extend over a range
k0 < k⊥ < k1 , where k1 is identified with the p⊥min (s) cut-off of the perturbative jet
spectrum in hadronic interactions, p⊥min (s) ≈ 1.5 GeV at typical energies, see section
11.2 and especially eq. (205). Above that range, the states are assumed to be sufficiently
weakly interacting that no eikonalization procedure is required, so that cross sections canbe calculated perturbatively without any recourse to pomeron phenomenology. There issome arbitrariness in that choice, and some simplifications are required in order to obtaina manageable description.
The VMD and GVMD/anomalous events are together called resolved ones. In termsof high-p⊥ jet production, the VMD and anomalous contributions can be combined into
a total resolved one, and the same for parton-distribution functions. However, the twoclasses differ in the structure of the underlying event and possibly in the appearance ofsoft processes.
In terms of cross sections, eq. (121) corresponds to
γp
γp
γp
γp
(s) + σanom
(s) .
σtot
(s) = σdir(s) + σVMD

(122)

The direct cross section is, to lowest order, the perturbative cross section for the twoprocesses γq → qg and γg → qq, with a lower cut-off p⊥ > k1 , in order to avoid doublecounting with the interactions of the GVMD states. Properly speaking, this should be
multiplied by the Z3 coefficient,ÃX

Z3 = 1 −V =ρ0 ,ω,φ,J/ψ

e
fV

!2

−

X

Ã

q

e
fqq

!2

−

Ã

X
`=e,µ,τ

e
f``

!2

,

(123)

but normally Z3 is so close to unity as to make no difference.
The VMD factor (e/fV )2 = 4παem /fV2 gives the probability for the transition γ → V .
The coefficients fV2 /4π are determined from data to be (with a non-negligible amount of
uncertainty) 2.20 for ρ0 , 23.6 for ω, 18.4 for φ and 11.5 for J/ψ. Together these numbers
imply that the photon can be found in a VMD state about 0.4% of the time, dominatedby the ρ0 contribution. All the properties of the VMD interactions can be obtained byappropriately scaling down V p physics predictions. Thus the whole machinery developedin the previous section for hadron–hadron interactions is directly applicable. Also parton
distributions of the VMD component inside the photon are obtained by suitable rescaling.
The contribution from the ‘anomalous’ high-mass fluctuations to the total cross section
is obtained by a convolution of the fluctuation rate
X
q

Ã

e
fqq

!2

αem
≈
2π

Ã

2

X
q

!Z

e2q

k1 dk 2⊥
2
k0 k ⊥,(124)

2
for the total
which is to be multiplied by the abovementioned reduction factor kV2 /k⊥cross section, and all scaled by the assumed real vector meson cross section.

116

As an illustration of this scenario, the phase space of γp events may be represented
by a (k⊥ , p⊥ ) plane. Two transverse momentum scales are distinguished: the photon resolution scale k⊥ and the hard interaction scale p⊥ . Here k⊥ is a measure of the virtualityof a fluctuation of the photon and p⊥ corresponds to the most virtual rung of the ladder,possibly apart from k⊥ . As we have discussed above, the low-k⊥ region corresponds to
VMD and GVMD states that encompasses both perturbative high-p⊥ and nonperturbative low-p⊥ interactions. Above k1 , the region is split along the line k⊥ = p⊥ . Whenp⊥ > k⊥ the photon is resolved by the hard interaction, as described by the anomalouspart of the photon distribution function. This is as in the GVMD sector, except that weshould (probably) not worry about multiple parton–parton interactions. In the complementary region k⊥ > p⊥ , the p⊥ scale is just part of the traditional evolution of the parton
distributions of the proton up to the scale of k⊥ , and thus there is no need to introduce aninternal structure of the photon. One could imagine the direct class of events as extending
below k1 and there being the low-p⊥ part of the GVMD class, only appearing when a hardinteraction at a larger p⊥ scale would not preempt it. This possibility is implicit in thestandard cross section framework.
In γγ physics [Sch94a, Sch97], the superposition in eq. (121) applies separately foreach of the two incoming photons. In total there are therefore 3 × 3 = 9 combinations.
However, trivial symmetry reduces this to six distinct classes, written in terms of the total
cross section (cf. eq. (122)) as
γγ
γγ
γγ
γγ
(s)
(s) + σGVMD×GVMD
σtot(s) = σdir×dir(s) + σVMD×VMDγγγγγγ+ 2σdir×VMD (s) + 2σdir×GVMD (s) + 2σVMD×GVMD (s) .

(125)

A parameterization of the total γγ cross section is found in [Sch94a, Sch97].
The six different kinds of γγ events are thus:• The direct×direct events, which correspond to the subprocess γγ → qq (or `+ `− ).
The typical event structure is two high-p⊥ jets and no beam remnants.
• The VMD×VMD events, which have the same properties as the VMD γp events.
There are four by four combinations of the two incoming vector mesons, with one
VMD factor for each meson.
• The GVMD×GVMD events, wherein each photon fluctuates into a qq pair of larger
virtuality than in the VMD class. The ‘anomalous’ classification assumes that one
parton of each pair gives a beam remnant, whereas the other (or a daughter parton thereof) participates in a high-p⊥ scattering. The GVMD concept implies thepresence also of low-p⊥ events, like for VMD.
• The direct×VMD events, which have the same properties as the direct γp events.
• The direct×GVMD events, in which a bare photon interacts with a parton fromthe anomalous photon. The typical structure is then two high-p⊥ jets and a beam
remnant.
• The VMD×GVMD events, which have the same properties as the GVMD γp events.
Like for photoproduction events, this can be illustrated in a parameter space, but now
three-dimensional, with axes given by the k⊥1 , k⊥2 and p⊥ scales. Here each k⊥i is ameasure of the virtuality of a fluctuation of a photon, and p⊥ corresponds to the mostvirtual rung on the ladder between the two photons, possibly excepting the endpointk⊥i ones. So, to first approximation, the coordinates along the k⊥i axes determine thecharacters of the interacting photons while p⊥ determines the character of the interaction
process. Double-counting should be avoided by trying to impose a consistent classification.
Thus, for instance, p⊥ > k⊥i with k⊥1 < k0 and k0 < k⊥2 < k1 gives a hard interaction
between a VMD and a GVMD photon, while k⊥1 > p⊥ > k⊥2 with k⊥1 > k1 and k⊥2 <k0 is a single-resolved process (direct×VMD; with p⊥ now in the parton distribution
evolution).
In much of the literature, where a coarser classification is used, our direct×direct is117called direct, our direct×VMD and direct×GVMD is called single-resolved since theyboth involve one resolved photon which gives a beam remnant, and the rest are called
double-resolved since both photons are resolved and give beam remnants.
If the photon is virtual, it has a reduced probability to fluctuate into a vector mesonstate, and this state has a reduced interaction probability. This can be modelled by a
traditional dipole factor (m2V /(m2V +Q2 ))2 for a photon of virtuality Q2 , where mV → 2k⊥
for a GVMD state. Putting it all together, the cross section of the GVMD sector ofphotoproduction then scales like
Z k2
1

k02

2
dk⊥
kV2
2
2
k⊥
k⊥

Ã

2
4k⊥
2
4k⊥
+ Q2

!2

(126)

.

For a virtual photon the DIS process γ ∗ q → q is also possible, but by gauge invariance
its cross section must vanish in the limit Q2 → 0. At large Q2 , the direct processes canbe considered as the O(αs ) correction to the lowest-order DIS process, but the directones survive for Q2 → 0. There is no unique prescription for a proper combination at
all Q2 , but we have attempted an approach that gives the proper limits and minimizes
double-counting. For large Q2 , the DIS γ ∗ p cross section is proportional to the structure
function F2 (x, Q2 ) with the Bjorken x = Q2 /(Q2 + W 2 ). Since normal parton distribution
parameterizations are frozen below some Q0 scale and therefore do not obey the gauge
invariance condition, an ad hoc factor (Q2 /(Q2 + m2ρ ))2 is introduced for the conversionγ∗p:from the parameterized F2 (x, Q2 ) to a σDISÃ!2

o
4π 2 αem
4π 2 αem Q2 X 2 n
2
2
2
F
(x,
Q
)
=
Q
)
.
e
xq(x,
Q
)
+
xq(x,
2
q
Q2
(Q2 + m2ρ )2 q
(127)
Here mρ is some nonperturbative hadronic mass parameter, for simplicity identified withγp
the ρ mass. One of the Q2 /(Q2 + m2ρ ) factors is required already to give finite σtot
forconventional parton distributions, and could be viewed as a screening of the individual
partons at small Q2 . The second factor is chosen to give not only a finite but actually aγ∗pfor Q2 → 0 in order to retain the pure photoproduction description there.
vanishing σDISThis latter factor thus is more a matter of convenience, and other approaches could havebeen pursued.
In order to avoid double-counting between DIS and direct events, a requirement p⊥ >max(k1 , Q) is imposed on direct events. In the remaining DIS ones, denoted lowest order
γ∗p
γ∗p
γ∗p
− σdirect, with(LO) DIS, thus p⊥ < Q. This would suggest a subdivision σLO= σDISDISγ∗pγ∗pσDIS given by eq. (127) and σdirect by the perturbative matrix elements. In the limit
Q2 → 0, the DIS cross section is now constructed to vanish while the direct is not, so thisγ∗pwould give σLO
DIS < 0. However, here we expect the correct answer not to be a negative
number but an exponentially suppressed one, by a Sudakov form factor. This modifies
the cross section:
γ∗p
'
σDIS

Q2
Q2 + m2ρ

Ã

γ∗p
γ∗p
γ∗p
σLO
DIS = σDIS − σdirect

−→

γ∗p
σDIS

∗

σγ p
exp − direct
γ∗p
σDIS

!

.

(128)

Since we here are in a region where the DIS cross section is no longer the dominant one,this change of the total DIS cross section is not essential.
The overall picture, from a DIS perspective, now requires three scales to be kept2
À p2⊥ , wheretrack of. The traditional DIS region is the strongly ordered one, Q2 À k⊥
DGLAP-style evolution [Alt77, Gri72] is responsible for the event structure. As always,
ideology wants strong ordering, while the actual classification is based on ordinary ordering
2
2> max(Q2 , p2⊥ ) is also DIS, but of the O(αs ) direct kind.
> p2⊥ . The region k⊥
Q2 > k ⊥118

The region where k⊥ is the smallest scale corresponds to non-ordered emissions, that then2
go beyond DGLAP validity, while the region p2⊥ > k⊥
> Q2 cover the interactions of aresolved virtual photon. Comparing with the plane of real photoproduction, we conclude
that the whole region p⊥ > k⊥ involves no double-counting, since we have made no
attempt at a non-DGLAP DIS description but can choose to cover this region entirely by
the VMD/GVMD descriptions. Actually, it is only in the corner p⊥ < k⊥ < min(k1 , Q)
that an overlap can occur between the resolved and the DIS descriptions. Some further
considerations show that usually either of the two is strongly suppressed in this region,
except in the range of intermediate Q2 and rather small W 2 . Typically, this is the region
where x ≈ Q2 /(Q2 + W 2 ) is not close to zero, and where F2 is dominated by the valencequark contribution. The latter behaves roughly ∝ (1 − x)n , with an n of the order of 3
or 4. Therefore we will introduce a corresponding damping factor to the VMD/GVMD
terms.
In total, we have now arrived at our ansatz for all Q2 :
Ã

γ∗p
γ∗p
σtot
= σDIS∗γ pσdirect
exp − γ ∗ p
σDIS

!

Ã
γ∗p
+ σdirect+

W2
Q2 + W 2

!n

³

∗

∗

´

γ p
γ p
σVMD
+ σGVMD,(129)

with four main components. Most of these in their turn have a complicated internal
structure, as we have seen.
Turning to γ ∗ γ ∗ processes, finally, the parameter space is now five-dimensional: Q1 ,Q2 , k⊥1 , k⊥2 and p⊥ . As before, an effort is made to avoid double-counting, by having
a unique classification of each region in the five-dimensional space. Remaining doublecounting is dealt with as above. In total, our ansatz for γ ∗ γ ∗ interactions at all Q2 contains
13 components: 9 when two VMD, GVMD or direct photons interact, as is already allowed
for real photons, plus a further 4 where a ‘DIS photon’ from either side interacts with a
VMD or GVMD one. With the label resolved used to denote VMD and GVMD, one can
write
Ã

γ∗γ∗
σtot
(W 2 , Q21 , Q22 )

∗ ∗

γ γ
σdir×resexp − γ ∗ γ ∗σDIS×res=γ∗γ∗
σDIS×res+γ∗γ∗
σres×DIS+γ∗γ∗
σdir×dir
+

Ã

∗ ∗

γ γ
σres×direxp − γ ∗ γ ∗σres×DIS

Ã

!
∗ ∗

γ γ
+ σdir×res!

W2
Q21 + Q22 + W 2

∗ ∗

γ γ
+ σres×dir

!3

(130)

∗ ∗

γ γ
σres×res

Most of the 13 components in their turn have a complicated internal structure, as we haveseen.
An important note is that the Q2 dependence of the DIS and direct photon interactions
is implemented in the matrix element expressions, i.e. in processes such as γ ∗ γ ∗ → qq orγ ∗ q → qg the photon virtuality explicitly enters. This is different from VMD/GVMD,
where dipole factors are used to reduce the total cross sections and the assumed flux ofpartons inside a virtual photon relative to those of a real one, but the matrix elements
themselves contain no dependence on the virtuality either of the partons or of the photon
itself. Typically results are obtained with the SaS 1D parton distributions for the virtualtransverse photons [Sch95, Sch96], since these are well matched to our framework, e.g.
allowing a separation of the VMD and GVMD/anomalous components. Parton distributions of virtual longitudinal photons are by default given by some Q2 -dependent factor
times the transverse ones. The set by Chýla [Chý00] allows more precise modelling here,
but indications are that many studies will not be sensitive to the detailed shape.
The photon physics machinery is of considerable complexity, and so the above is onlya brief summary. Further details can be found in the literature quoted above. Some topics
119are also covered in other places in this manual, e.g. the flux of transverse and longitudinal
photons in section 7.1.4, scale choices for parton density evaluation in section 7.2, andfurther aspects of the generation machinery and switches in section 8.3.

120

Table 16: Subprocess codes, part 1. First column is ‘+’ for processes implemented andblank for those that are/were only foreseen. Second is the subprocess number ISUB, andthird the description of the process. The final column gives references from which thecross sections have been obtained. See text for further information.
In No.
+
1
+
2
+
3
4
+
5
6
7
+
8
+
10
+
11
+
12
+
13
+
14
+
15
+
16
17
+
18
+
19
+
20
21
+
22
+
23
+
24
+
25
+
26
27
+
28
+
29
+
30
+
31
+
32
+
33
+
34
+
35
+
36
37

Subprocess
fi f i → γ ∗ /Z0
fi f j → W +fi f i → h0
γW+ → W+Z0 Z0 → h0
Z0 W+ → W+
W+ W− → Z0
W+ W− → h0
fi fj → fk fl (QFD)
fi fj → fi fj (QCD)
fi f i → fk f k
fi f i → gg
fi f i → gγ
fi f i → gZ0
fi f j → gW+
fi f i → gh0
fi f i → γγ
fi f i → γZ0
fi f j → γW+
fi f i → γh0
fi f i → Z0 Z0fi f j → Z0 W+
fi f i → Z0 h0fi f i → W + W −fi f j → W+ h0
fi f i → h0 h0
fi g → fi g
fi g → fi γ
fi g → fi Z0
fi g → fk W +fi g → fi h0
fi γ → fi g
fi γ → fi γ
fi γ → fi Z0
fi γ → fk W +fi γ → fi h0121

Reference[Eic84][Eic84][Eic84][Eic84, Cha85][Eic84, Cha85][Ing87a][Com77, Ben84, Eic84][Com77, Ben84, Eic84][Com77, Ben84][Hal78, Ben84][Eic84][Eic84][Ber84][Eic84][Eic84, Sam91][Eic84, Gun86][Eic84, Gun86][Ber85][Bar94, Gun86][Eic84][Com77, Ben84][Hal78, Ben84][Eic84][Eic84][Bar88][Duk82][Duk82][Gab86][Gab86]

Table 17: Subprocess codes, part 2. Comments as before.
In

+
+

+

+
+
+
+
+
+

No.
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75

Subprocess
fi Z0 → fi g
fi Z0 → fi γ
fi Z0 → fi Z0
fi Z0 → fk W+
fi Z0 → fi h0
fi W+ → fk g
fi W+ → fk γ
fi W+ → fk Z0
fi W+ → fk W+
fi W+ → fk h0
fi h0 → fi g
fi h0 → fi γ
fi h0 → fi Z0
fi h0 → fk W+
fi h0 → fi h0
gg → fk f kgγ → fk f k
gZ0 → fk f k
gW+ → fk f l
gh0 → fk f kγγ → fk f k
γZ0 → fk f k
γW+ → fk f l
γh0 → fk f k
Z0 Z0 → fk f k
Z0 W+ → fk f l
Z0 h0 → fk f k
W+ W− → fk f k
W+ h0 → fk f l
h0 h0 → fk f kgg → ggγγ → W+ W−
γW+ → Z0 W+
Z0 Z0 → Z0 Z0 (longitudinal)
Z0 Z0 → W+ W− (longitudinal)
Z0 W+ → Z0 W+ (longitudinal)
Z0 h0 → Z0 h0
W+ W− → γγ

122

Reference[Com77, Ben84][Duk82][Bar90][Com77, Ben84][Kat83][Kun87][Abb87][Abb87][Dob91]

Table 18: Subprocess codes, part 3. Comments as before
In No.
+
76
+
77
78
79
+
80
+
81
+
82
+
83
+
84
+
85
+
86
+
87
+
88
+
89
+
91
+
92
+
93
+
94
+
95
+
96
+
99
101
+ 102
+ 103
+ 104
+ 105
+ 106
+ 107
+ 108
+ 110
+ 111
+ 112
+ 113
+ 114
+ 115
116
117
118

Subprocess
W+ W− → Z0 Z0 (longitudinal)
W+ W± → W+ W± (longitudinal)
W+ h0 → W+ h0
h0 h0 → h0 h0
qi γ → qk π ±fi f i → Qk Qk
gg → Qk Qk
qi fj → Qk fl
gγ → Qk Qkγγ → Fk Fk
gg → J/ψg
gg → χ0c g
gg → χ1c g
gg → χ2c gelastic scattering
single diffraction (AB → XB)
single diffraction (AB → AX)
double diffraction
low-p⊥ production
semihard QCD 2 → 2γ ∗q → q
gg → Z0
gg → h0γγ → h0
gg → χ0c
gg → χ2c
gg → J/ψγ
gγ → J/ψgγγ → J/ψγ
fi f i → γh0
fi f i → gh0
fi g → fi h0
gg → gh0
gg → γγ
gg → gγ
gg → γZ0
gg → Z0 Z0
gg → W+ W−

123

Reference[Ben87b][Dun86, Bar90a][Bag82][Com79][Com79][Dic86][Fon81][Bar90][Bai83][Gas87][Gas87][Gas87][Sch94][Sch94][Sch94][Sch94][Sjö87a][Sjö87a][Fri00][Eic84][Dre89][Bai83][Bai83][Dre91][Ber81][Jun97][Ber85a][Ell88][Ell88][Ell88][Con71, Ber84, Dic88][Con71, Ber84, Dic88]

Table 19: Subprocess codes, part 4. Comments as before.
In No.
119
+ 121
+ 122
+ 123
+ 124
+ 131
+ 132
+ 133
+ 134
+ 135
+ 136
+ 137
+ 138
+ 139
+ 140
+ 141
+ 142
+ 143
+ 144
+ 145
+ 146
+ 147
+ 148
+ 149
+ 151
+ 152
+ 153
+ 156
+ 157
+ 158
+ 161
+ 162
+ 163
+ 164
+ 165
+ 166
+ 167
+ 168

Subprocess
γγ → gg
gg → Qk Qk h0
qi qi → Qk Qk h0
fi fj → fi fj h0 (ZZ fusion)fi fj → fk fl h0 (W+ W− fusion)
fi γT∗ → fi g
fi γL∗ → fi g
fi γT∗ → fi γ
fi γL∗ → fi γ
gγT∗ → fi f i
gγL∗ → fi f iγT∗ γT∗ → fi f iγT∗ γL∗ → fi f iγL∗ γT∗ → fi f iγL∗ γL∗ → fi f i
fi f i → γ/Z0 /Z00
fi f j → W0+
fi f j → H+
fi f j → R
qi `j → LQ
eγ → e∗dg → d∗ug → u∗gg → ηtc
fi f i → H0
gg → H0γγ → H0fi f i → A0
gg → A0γγ → A0fi g → fk H+
qi g → `k LQ
gg → LQ LQ
qi qi → LQ LQ
fi f i → fk f k (via γ ∗ /Z0 )fi f j → fk f l (via W± )qi qj → qk d∗
qi qj → qk u∗

124

Reference[Kun84][Kun84][Cah84][Cah84][Alt78][Alt78][Alt78][Alt78][Alt78][Alt78][Bai81][Bai81][Bai81][Bai81][Alt89][Alt89][Gun87][Ben85a][Wud86][Bau90][Bau90][Bau90][Eic84, App92][Eic84][Eic84][Dre89][Eic84][Eic84][Dre89][Bar88][Hew88][Hew88, Eic84][Hew88][Eic84, Lan91][Eic84, Lan91][Bau90][Bau90]Table 20: Subprocess codes, part 5. Comments as before.
In
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+

No.
169
171
172
173
174
176
177
178
179
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
201
202
203
204
205
206
207
208
209
210
211
212
213
214

Subprocess
qi qi → e± e∗∓fi f i → Z0 H0fi f j → W+ H0
fi fj → fi fj H0 (ZZ fusion)fi fj → fk fl H0 (W+ W− fusion)fi f i → Z0 A0fi f j → W+ A0
fi fj → fi fj A0 (ZZ fusion)fi fj → fk fl A0 (W+ W− fusion)
gg → Qk Qk H0
qi qi → Qk Qk H0
fi f i → gH0
fi g → fi H0
gg → gH0
gg → Qk Qk A0
qi qi → Qk Qk A0
fi f i → gA0
fi g → fi A0
gg → gA0
fi f i → ρ0tc
fi f j → ρ±tc
0fi f i → ωtc
fi f i → fk f k
fi f j → fk f l
fi f i → ẽL ẽ∗Lfi f i → ẽR ẽ∗Rfi f i → ẽL ẽ∗R + ẽ∗L ẽRfi f i → µ̃L µ̃∗Lfi f i → µ̃R µ̃∗R
fi f i → µ̃L µ̃∗R + µ̃∗L µ̃Rfi f i → τ̃1 τ̃1∗
fi f i → τ̃2 τ̃2∗
fi f i → τ̃1 τ̃2∗ + τ̃1∗ τ̃2
fi f j → `˜L ν̃`∗ + `˜∗L ν̃`fi f j → τ̃1 ν̃τ∗ + τ̃1∗ ν̃τ
fi f j → τ̃2 ν̃τ ∗ + τ̃2∗ ν̃τ
fi f i → ν̃` ν̃`∗fi f i → ν̃τ ν̃τ∗

125

Reference[Bau90][Eic84][Eic84][Cah84][Cah84][Eic84][Eic84][Cah84][Cah84][Kun84][Kun84][Ell88][Ell88][Ell88][Kun84][Kun84][Ell88][Ell88][Ell88][Eic96][Eic96][Eic96][Eic96, Lan99][Eic96, Lan99][Bar87, Daw85][Bar87, Daw85][Bar87][Bar87, Daw85][Bar87, Daw85][Bar87][Bar87, Daw85][Bar87, Daw85][Bar87][Daw85][Daw85][Daw85][Bar87, Daw85][Bar87, Daw85]

Table 21: Subprocess codes, part 6. Comments as before.
In
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+

No.
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
246
247
248
249
250
251
252
253
254

Subprocess
fi f i → χ̃1 χ̃1
fi f i → χ̃2 χ̃2fi f i → χ̃3 χ̃3fi f i → χ̃4 χ̃4fi f i → χ̃1 χ̃2
fi f i → χ̃1 χ̃3
fi f i → χ̃1 χ̃4
fi f i → χ̃2 χ̃3fi f i → χ̃2 χ̃4fi f i → χ̃3 χ̃4∓fi f i → χ̃±1 χ̃1∓fi f i → χ̃±
2 χ̃2
± ∓
fi f i → χ̃1 χ̃2
fi f j → χ̃1 χ̃±
1
fi f j → χ̃2 χ̃±
1
fi f j → χ̃3 χ̃±
1
fi f j → χ̃4 χ̃±
1
fi f j → χ̃1 χ̃±
2
fi f j → χ̃2 χ̃±
2fi f j → χ̃3 χ̃±
2fi f j → χ̃4 χ̃±
2fi f i → g̃χ̃1
fi f i → g̃χ̃2
fi f i → g̃χ̃3fi f i → g̃χ̃4
fi f j → g̃χ̃±
1
fi f j → g̃χ̃±2fi f i → g̃g̃
gg → g̃g̃
fi g → q̃iL χ̃1
fi g → q̃iR χ̃1
fi g → q̃iL χ̃2fi g → q̃iR χ̃2
fi g → q̃iL χ̃3fi g → q̃iR χ̃3fi g → q̃iL χ̃4fi g → q̃iR χ̃4fi g → q̃j L χ̃±
1

126

Reference[Bar86a][Bar86a][Bar86a][Bar86a][Bar86a][Bar86a][Bar86a][Bar86a][Bar86a][Bar86a][Bar86b][Bar86b][Bar86b][Bar86a, Bar86b][Bar86a, Bar86b][Bar86a, Bar86b][Bar86a, Bar86b][Bar86a, Bar86b][Bar86a, Bar86b][Bar86a, Bar86b][Bar86a, Bar86b][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85]

Table 22: Subprocess codes, part 7. Comments as before.
In No.
+ 256
+ 258
+ 259
+ 261
+ 262
+ 263
+ 264
+ 265
+ 271
+ 272
+ 273
+ 274
+ 275
+ 276
+ 277
+ 278
+ 279
+ 280
+ 281
+ 282
+ 283
+ 284
+ 285
+ 286
+ 287
+ 288
+ 289
+ 290
+ 291
+ 292
+ 293
+ 294
+ 295
+ 296
+ 297
+ 298
+ 299
+ 300

Subprocess
fi g → q̃j L χ̃±2fi g → q̃iL g̃
fi g → q̃iR g̃
fi f i → t̃1 t̃∗1
fi f i → t̃2 t̃∗2
fi f i → t̃1 t̃∗2 + t̃∗1 t̃2
gg → t̃1 t̃∗1
gg → t̃2 t̃∗2
fi fj → q̃iL q̃j L
fi fj → q̃iR q̃j R
fi fj → q̃iL q̃j R + q̃iR q̃j L
fi f j → q̃iL q̃∗j L
fi f j → q̃iR q̃∗j R
fi f j → q̃iL q̃∗j R + q̃iR q̃∗j L
fi f i → q̃j L q̃∗j L
fi f i → q̃j R q̃∗j R
gg → q̃iL q̃∗i L
gg → q̃iR q̃∗i R
bq → b̃1 q̃L (q not b)
bq → b̃2 q̃R
bq → b̃1 q̃R + b̃2q̃L
bq → b̃1 q̃∗L
bq → b̃2 q̃∗R
bq → b̃1 q̃∗R + b̃2 q̃∗Lfi f i → b̃1 b̃∗1
fi f i → b̃2 b̃∗2
gg → b̃1 b̃∗1
gg → b̃2 b̃∗2
bb → b̃1 b̃1bb → b̃2 b̃2bb → b̃1 b̃2
bg → b̃1 g̃
bg → b̃2 g̃
bb → b̃1 b̃∗2 + b̃∗1 b̃2
fi f j → H± h0
fi f j → H± H0
fi f i → Ah0
fi f i → AH0

127

Reference[Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a][Daw85a]Table 23: Subprocess codes, part 8. Comments as before.
In No.
+ 301
+ 341
+ 342
+ 343
+ 344
+ 345
+ 346
+ 347
+ 348
+ 349
+ 350
+ 351
+ 352
+ 353
+ 354
+ 361
+ 362
+ 363
+ 364
+ 365
+ 366
+ 367
+ 368
+ 370
+ 371
+ 372
+ 373
+ 374
+ 375
+ 376
+ 377
+ 381
+ 382
+ 383
+ 384
+ 385
+ 386
+ 387

Subprocess
fi f i → H+ H−`i `j → H±±
L`i `j → H±±R∓`i γ → H±±
L e±± ∓`i γ → HR e∓`i γ → H±±L µ±± ∓`i γ → HR µ∓`i γ → H±±L τ±± ∓`i γ → HR τ−−fi f i → H++
L HL−−
fi f i → H++
R HR±±
fi fj → fk fl HL (WW) fusion)fi fj → fk fl H±±
R (WW) fusion)
0
fi f i → ZR+
fi f j → WR+fi f i → WL WL−∓
fi f i → WL± πtc+ −fi f i → πtc πtc
0
fi f i → γπtc
fi f i → γπ 0 0tc
0fi f i → Z0 πtc
fi f i → Z0 π 0 0tc∓
fi f i → W± πtc
fi f j → WL± Z0L0fi f j → WL± πtc± 0
fi f j → πtc ZL± 0
fi f j → πtc
πtc±
fi f j → γπtc±
fi f j → Z0 πtc± 0fi f j → W πtc
fi f j → W± π 0 0tc
qi qj → qi qj (QCD+TC)
qi qi → qk qk (QCD+TC)
qi qi → gg (QCD+TC)
fi g → fi g (QCD+TC)
gg → qk qk (QCD+TC)
gg → gg (QCD+TC)
fi f i → Qk Qk (QCD+TC)

128

Reference[Daw85a][Hui97][Hui97][Hui97][Hui97][Hui97][Hui97][Hui97][Hui97][Hui97][Hui97][Hui97][Hui97][Eic84][Eic84][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Lan99][Chi90, Lan02a][Chi90, Lan02a][Lan02a][Lan02a][Lan02a][Lan02a][Lan02a]Table 24: Subprocess codes, part 9. Comments as before.
In
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+

No.
388
391
392
393
394
395
401
402
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
461
462
463
464
465
466
467
468
469

Subprocess
gg → Qk Qk (QCD+TC)
ff → G∗
gg → G∗qq → gG∗
qg → qG∗
gg → gG∗
gg → tbH+
qq → tbH+(1)
gg → cc[3 S1 ] g(8)gg → cc[3 S1 ] g(8)gg → cc[1 S0 ] g(8)gg → cc[3 PJ ] g(8)gq → q cc[3 S1 ](8)
gq → q cc[1 S0 ](8)
gq → q cc[3 PJ ](8)
qq → g cc[3 S1 ](8)
qq → g cc[1 S0 ](8)
qq → g cc[3 PJ ](1)
gg → cc[3 P0 ] g(1)
gg → cc[3 P1 ] g(1)gg → cc[3 P2 ] g(1)gq → q cc[3 P0 ](1)
gq → q cc[3 P1 ](1)
gq → q cc[3 P2 ](1)
qq → g cc[3 P0 ](1)
qq → g cc[3 P1 ](1)
qq → g cc[3 P2 ](1)
gg → bb[3 S1 ] g(8)gg → bb[3 S1 ] g(8)gg → bb[1 S0 ] g(8)gg → bb[3 PJ ] g(8)gq → q bb[3 S1 ](8)
gq → q bb[1 S0 ](8)
gq → q bb[3 PJ ](8)
qq → g bb[3 S1 ](8)
qq → g bb[1 S0 ]129

Reference[Lan02a][Ran99][Ran99][Ran99, Bij01][Ran99, Bij01][Ran99, Bij01][Bor99][Bor99][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95]

Table 25: Subprocess codes, part 10. Comments as before.
In
+
+
+
+
+
+
+
+
+
+

8

No.
470
471
472
473
474
475
476
477
478
479

Subprocess(8)
qq → g bb[3 PJ ](1)
gg → bb[3 P0 ] g(1)
gg → bb[3 P1 ] g(1)gg → bb[3 P2 ] g(1)gq → q bb[3 P0 ](1)
gq → q bb[3 P1 ](1)
gq → q bb[3 P2 ](1)
qq → g bb[3 P0 ](1)
qq → g bb[3 P1 ](1)
qq → g bb[3 P2 ]Reference[Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95][Bod95]

Physics Processes

In this section we enumerate the physics processes that are available in Pythia, introducing the ISUB code that can be used to select desired processes. A number of comments
are made about the physics scenarios involved, with emphasis on the underlying assumptions and domain of validity. The section closes with a survey of interesting processes
by machine. Note that ISUB is a dummy index introduced to allow simple referencing ofprocesses. There are no global variables in Pythia named ISUB.

8.1

The Process Classification Scheme

A wide selection of fundamental 2 → 1 and 2 → 2 tree processes of the Standard Model(electroweak and strong) has been included in Pythia, and slots are provided for somemore, not (yet) implemented. In addition, ‘minimum-bias’-type processes (like elastic
scattering), loop graphs, box graphs, 2 → 3 tree graphs and many non-Standard Model
processes are included. The classification is not always unique. A process that proceeds
only via an s-channel state is classified as a 2 → 1 process (e.g. qq → γ ∗ /Z0 → e+ e− ).
A generic 2 → 2 process may have contributions from s-, t− and u-channel diagrams.
Also, in the program, 2 → 1 and 2 → 2 graphs may sometimes be convoluted with two
1 → 2 splittings to form effective 2 → 3 or 2 → 4 processes (W+ W− → h0 is folded withq → q00 W+ and q0 → q000 W− to give qq0 → q00 q000 h0 ).
The original classification and numbering scheme is less relevant today than when
originally conceived. The calculation of 2 → 3 or 2 → 4 matrix elements by hand issufficiently complicated that approximation schemes were employed, such as the effective W-approximation which factored W bosons into an effective parton density. Today,
improvements in computational techniques and increases in computing power make theexact calculation manageable. Given the large top mass and large Higgs boson mass
limits, there is also a natural subdivision, such that the b quark is the heaviest object
for which the parton-distribution concept makes sense at current or near-future colliders. Therefore most of the prepared but empty slots are likely to remain empty, or bereclaimed for other processes.
It is possible to select a combination of subprocesses and also to know which subprocess
was actually selected in each event. For this purpose, all subprocesses are numberedaccording to an ISUB code. The list of possible codes is given in Tables 16 through 25, and130

summarized in Appendix A. Only processes marked with a ‘+’ sign in the first column
have been implemented in the program to date. Although ISUB codes were originallydesigned in a logical fashion, subsequent developments of the program have obscured thestructure. For instance, the process numbers for Higgs production are spread out, inpart as a consequence of the original classification, in part because further production
mechanisms have been added one at a time, in whatever free slots could be found. In thethematic descriptions that follow the main tables, the processes of interest are repeatedin a more logical order. If you want to look for a specific process, it will be easier to findit there.
In the following, fi represents a fundamental fermion of flavour i, i.e. d, u, s, c, b, t,
b0 , t0 , e− , νe , µ− , νµ , τ − , ντ , τ 0 − or ν 0 τ . A corresponding antifermion is denoted by f i . In
several cases, some classes of fermions are explicitly excluded, since they do not couple
to the g or γ (no e+ e− → gg, e.g.). When processes have only been included for quarks,
while leptons might also have been possible, the notation qi is used. A lepton is denotedby `; in a few cases neutrinos are also lumped under this heading. In processes wherefermion masses are explicitly included in the matrix elements, an F or Q is used to denotean arbitrary fermion or quark. Flavours appearing already in the initial state are denotedby indices i and j, whereas new flavours in the final state are denoted by k and l.
In supersymmetric processes, antiparticles of sfermions are denoted by ∗ , i.e. t̃∗ .
Charge-conjugate channels are always assumed included as well (where separate), andprocesses involving a W+ also imply those involving a W− . Wherever Z0 is written, it isunderstood that γ ∗ and γ ∗ /Z0 interference should be included as well (with possibilities toswitch off either, if so desired). In practice, this means that fermion pairs produced fromγ ∗ /Z0 decay will have invariant masses as small as the program cutoff, and not regulatedby the large Z mass. The cutoff is set by an appropriate CKIN variable. In some cases,γ ∗ /Z0 interference is not implemented; see further below. Correspondingly, Z00 denotes
the complete set γ ∗ /Z0 /Z00 (or some subset of it). Thus the notation γ is only used for a
photon on the mass shell.
In the last column of the tables below, references are given to works from which
formulae have been taken. Sometimes these references are to the original works on thesubject, sometimes only to the place where the formulae are given in the most convenientor accessible form, or where chance lead us. Apologies to all matrix-element calculators
who are not mentioned. However, remember that this is not a review article on physics
processes, but only a way for readers to know what is actually found in the program, forbetter or worse. In several instances, errata have been obtained from the authors. Often
the formulae given in the literature have been generalized to include trivial radiative
corrections, Breit–Wigner line shapes with ŝ-dependent widths (see section 7.3), etc.
The following sections contain some useful comments on the processes included in the
program, grouped by physics interest rather than sequentially by ISUB or MSEL code (see
9.2 for further information on the MSEL code). The different ISUB and MSEL codes that canbe used to simulate the different groups are given. ISUB codes within brackets indicate
the kind of processes that indirectly involve the given physics topic, although only as partof a larger whole. Some obvious examples, such as the possibility to produce jets in justabout any process, are not spelled out in detail.
The text at times contains information on which special switches or parameters are ofparticular interest to a given process. All these switches are described in detail in sections
9.3 9.4 and 9.5, but are alluded to here so as to provide a more complete picture of thepossibilities available for the different subprocesses. However, the list of possibilities iscertainly not exhausted by the text below.

131

8.2

QCD Processes

Obviously most processes in Pythia contain QCD physics one way or another, so theabove title should not be overstressed. One example: a process like e+ e− → γ ∗ /Z0 → qq
is also traditionally called a QCD event, but is here book-kept as γ ∗ /Z0 production. In
this section we discuss scatterings between coloured partons, plus a few processes thatare close relatives to other processes of this kind.
8.2.1

QCD jets

MSEL = 1, 2
ISUB = 11 qi qj → qi qj
12 qi qi → qk qk
13 qi qi → gg
28 qi g → qi g
53 gg → qk qk
68 gg → gg
96 semihard QCD 2 → 2
These are all tree-level, 2 → 2 process with a cross section ∝ αs2 . No higher-order loop
corrections are explicitly included. However, initial- and final-state QCD radiation added
to the above processes generates multijet events. In general, the rate of multijet (> 2)production through the parton shower mechanism is less certain for jets at high-p⊥ andfor well-separated pairs of jets.
A string-based fragmentation scheme such as the Lund model needs cross sections forthe different colour flows. The planar approximation allows such a subdivision, with crosssections, calculated in [Ben84], that differ from the usual ones by interference terms of theorder 1/NC2 . By default, the standard colour-summed QCD expressions for the differential
cross sections are used with the interference terms distributed among the various colour
flows according to the pole structure of the terms. However, the interference terms canbe excluded by changing MSTP(34).
As an example, consider subprocess 28, qg → qg. The differential cross section forthis process, obtained by summing and squaring the Feynman graphs and employing theidentity of the Mandelstam variables for the massless case, ŝ + t̂ + û = 0, is proportional
to [Com77]Ã!
ŝ2 + û2 4 ŝ û
−
+
.
(131)
9 û ŝt̂2
On the other hand, the cross sections for the two possible colour flows of this subprocess
are [Ben84]
Ã

A:
B:

!

û2 û
4
2 2 −
;
9
ŝ
t̂
Ã
!
4
ŝ2
ŝ
2 2 −
.
9
û
t̂

(132)

Colour configuration A is one in which the original colour of the q annihilates with theanticolour of the g, the g colour flows through, and a new colour–anticolour is createdbetween the final q and g. In colour configuration B, the gluon anticolour flows through,but the q and g colours are interchanged. Note that these two colour configurations
have different kinematics dependence. In principle, this has observable consequences. For
MSTP(34) = 0, these are the cross sections actually used.
The sum of the A and B contributions isÃ!
8 ŝ2 + û2 4 ŝ û
−
+
.
(133)
9 t̂2
9 û ŝ
132

The difference between this expression and that of [Com77], corresponding to the interference between the two colour-flow configurations, is then1 ŝ2 + û2
,
9 t̂2

(134)

which can be naturally divided between colour flows A and B:
A:
B:

1 û2
;
9 t̂2
1 ŝ2
.
9 t̂2

(135)

For MSTP(34) = 1, the standard QCD matrix element is used, with the same relative
importance of the two colour configurations as above. Similar procedures are followed
also for the other QCD subprocesses.
All the matrix elements in this group are for massless quarks (although final-state
quarks are of course put on the mass shell). As a consequence, cross sections are divergent
for p⊥ → 0, and some kind of regularization is required. Normally you are expected to
set the desired p⊥min value in CKIN(3).
The new flavour produced in the annihilation processes (ISUB = 12 and 53) is determined by the flavours allowed for gluon splitting into quark–antiquark; see switch MDME.
Subprocess 96 is special among all the ones in the program. In terms of the basic
cross section, it is equivalent to the sum of the other ones, i.e. 11, 12, 13, 28, 53 and68. The phase space is mapped differently, however, and allows p⊥ as the input variable.
This is especially useful in the context of the multiple interactions machinery (see section
11.2) where potential scatterings are considered in order of decreasing p⊥ , with a form
factor related to the probability of not having another scattering with a p⊥ larger thanthe considered one. You are not expected to access process 96 yourself. Instead it isautomatically initialized and used either if process 95 is included or if multiple interactions
are switched on. The process will then appear in the maximization information output,
but not in the cross section table at the end of a run. Instead, the hardest scattering
generated within the context of process 95 is reclassified as an event of the 11, 12, 13, 28,
53 or 68 kinds, based on the relative cross section for these in the point chosen. Further
multiple interactions, subsequent to the hardest one, also do not show up in cross section
tables.
8.2.2

Heavy flavours

MSEL = 4, 5, 6, 7, 8
ISUB =
81 qi qi → Qk Qk
82 gg → Qk Qk(83) qi fj → Qk fl(84) gγ → Qk Qk(85) γγ → Fk Fk(1) fi f i → γ ∗ /Z0 → Fk Fk(2) fi f j → W+ → Fk Fl(142) fi f j → W0+ → Fk Fl
The matrix elements in this group differ from the corresponding ones in the group
above in that they correctly take into account the quark masses. As a consequence, thecross sections are finite for p⊥ → 0 and require no special cuts.
The two first processes that appear here are the dominant lowest-order QCD graphs
in hadron colliders — a few other graphs will be mentioned later, such as process 83.
The flavour produced is selected according to a hierarchy of options:133

1. if MSEL = 4 - 8 then the flavour is set by the MSEL value;
2. else if MSTP(7) = 1 - 8 then the flavour is set by the MSTP(7) value;
3. else the flavour is determined by the heaviest flavour allowed for gluon splitting intoquark–antiquark; see switch MDME.
Note that only one heavy flavour is allowed at a time; if more than one is turned on in
MDME, only the heaviest will be produced (as opposed to the case for ISUB = 12 and 53
above, where more than one flavour is allowed simultaneously).
The lowest-order processes listed above just represent one source of heavy-flavour production. Heavy quarks can also be present in the parton distributions at the Q2 scaleof the hard interaction, leading to processes like Qg → Qg, so-called flavour excitation,or they can be created by gluon splittings g → QQ in initial- or final-state shower evolution. The implementation and importance of these various production mechanisms is
discussed in detail in [Nor98]. In fact, as the c.m. energy is increased, these other processes gain in importance relative to the lowest-order production graphs above. As as
example, only 10%–20% of the b production at LHC energies come from the lowest-ordergraphs. The figure is even smaller for charm, while it is well above 50% for top. At LHC
energies, the specialized treatment described in this section is therefore only of interestfor top (and potential fourth-generation quarks) — the higher-order corrections can here
be approximated by an effective K factor, except possibly in some rare corners of phasespace.
For charm and bottom, on the other hand, it is necessary to simulate the full event
sample (within the desired kinematics cuts), and then only keep those events that containb/c, be that either from lowest-order production, or flavour excitation, or gluon splitting.
Obviously this may be a time-consuming enterprise — although the probability for a highp⊥ event at collider energies to contain (at least) one charm or bottom pair is fairly large,
most of these heavy flavours are carrying a small fraction of the total p⊥ flow of the jets,
and therefore do not survive normal experimental cuts. We note that the lowest-orderproduction of charm or bottom with processes 12 and 53, as part of the standard QCDmix, is now basically equivalent with that offered by processes 81 and 82. For 12 vs. 81
this is rather trivial, since only s-channel gluon exchange is involved, but for process 53
it requires a separate evaluation of massive matrix elements for c and b in the flavour
sum. This is performed by retaining the ŝ and θ̂ values already preliminarily selected forthe massless kinematics, and recalculating t̂ and û with mass effects included. Some ofthe documentation information in PARI does not properly reflect this recalculation, butthat is purely a documentation issue. Also process 96, used internally for the total QCD
jet cross section, includes c and b masses. Only the hardest interaction in a multiple
interactions scenario may contain c/b, however, for technical reasons, so that the total
rate may be underestimated. (Quite apart from other uncertainties, of course.)
As an aside, it is not only for the lowest-order graphs that events may be generated
with a guaranteed heavy-flavour content. One may also generate the flavour excitation
process by itself, in the massless approximation, using ISUB = 28 and setting the KFIN
array appropriately. No trick exists to force the gluon splittings without introducing
undesirable biases, however. In order to have it all, one therefore has to make a full QCD
jets run, as already noted.
Also other processes can generate heavy flavours, all the way up to top, but thenwithout a proper account of masses. By default, top production is switched off in those
processes where a new flavour pair is produced at a gluon or photon vertex, i.e. 12, 53, 54,
58, 96 and 135–140, while charm and bottom is allowed. These defaults can be changed
by setting the MDME(IDC,1) values of the appropriate g or γ ‘decay channels’; see further
below.
The cross section for heavy quark pair production close to threshold can be modified
according to the formulae of [Fad90]; see MSTP(35). Here threshold effects due to QQ
bound-state formation are taken into account in a smeared-out, average sense. Then134

the naı̈ve cross section is multiplied by the squared wave function at the origin. In acolour-singlet channel this gives a net enhancement of the form
|Ψ(s) (0)|2 =X(s),1 − exp(−X(s) )where X(s) =

4 παs
,
3 β

(136)

where β is the quark velocity, while in a colour octet channel there is a net suppression
given byX(8)1 παs
|Ψ(8) (0)|2 =, where X(8) =.
(137)
exp(X(8) ) − 1
6 β
The αs factor in this expression is related to the energy scale of bound-state formationand is selected independently from the factor in the standard production cross section.
The presence of a threshold factor affects the total rate and also kinematic distributions.
Heavy flavours can also be produced by secondary decays of gauge bosons or new
exotic particles. We have listed 1, 2 and 142 above as among the most important ones.
There is a special point to including W0 in this list. Imagine that you want to study theelectroweak s-channel production of a single top, ud → W+ → tb, and therefore decide toforce this particular decay mode of the W. But then the same decay channel is required
for the W+ produced in the decay t → bW+ , i.e. you have set up an infinite recursion
W → t → W → t → . . .. The way out is to use the W0 , which has default couplings justlike the normal W, only a different mass, which then can be changed to agree, PMAS(34,1)= PMAS(24,1). The W0 is now forced to decay to tb, while the W can decay freely (or also
be forced, e.g. to have a leptonic decay, if desired). (Additionally, it may be necessary
to raise CKIN(1) to be at least around the top mass, so that the program does not get
stuck in a region of phase space where the cross section is vanishing.) Alternatively, a fullrun (after raising CKIN(1) to be just below the single top threshold) can be used if one iswilling to select the desired events by hand.
Heavy flavours, i.e. top and fourth generation, are assumed to be so short-lived thatthey decay before they have time to hadronize. This means that the light quark in the
decay Q → W± q inherits the colour of the heavy one. The current Pythia description
represents a change of philosophy compared to much earlier versions, formulated at atime when the top was thought to live long enough to form hadrons. For event shapes
the difference between the two time orderings normally has only marginal effects [Sjö92a].
In practical terms, the top (or a fourth generation fermion) is treated like a resonance inthe sense of section 7.6.2, i.e. the cross-section is reduced so as only to correspond to thechannels left open by you. This also includes the restrictions on secondary decays, i.e. on
the decays of a W+ or a H+ produced in the top decay. For b and c quarks, which are
long-lived enough to form hadrons, no such reduction takes place. Branching ratios thenhave to be folded in by hand to get the correct cross sections.
This rule about cross-section calculations applies to all the processes explicitly set up
to handle heavy flavour creation. In addition to the ones above, this means all the ones inTables 16–23 where the fermion final state is given as capital letters (‘Q’ and ‘F’) and also
flavours produced in resonance decays (Z0 , W± , h0 , etc., including processes 165 and 166).
However, heavy flavours could also be produced in a process such as 31, qi g → qk W± ,where qk could be a top quark. In this case, the thrust of the description is clearly onlight flavours — the kinematics of the process is formulated in the massless fermion limit— so any top production is purely incidental. Since here the choice of scattered flavour
is only done at a later stage, the top branching ratios are not correctly folded in to thehard-scattering cross section. So, for applications like these, it is not recommended torestrict the allowed top decay modes. Often one might like to get rid of the possibility of
producing top together with light flavours. This can be done by switching off (i.e. setting
MDME(I,1) = 0) the ‘channels’ d → W− t, s → W− t, b → W− t, g → tt and γ → tt. Also
any heavy flavours produced by parton-shower evolution would not be correctly weighted
135into the cross section. However, currently top production is switched off both as a beam
remnant (see MSTP(9)) and in initial (see KFIN array) and final (see MSTJ(45)) stateradiation.
In pair production of heavy flavour (top) in processes 81, 82, 84 and 85, matrix elements are only given for one common mass, although Breit–Wigner distributions areused to select two separate masses. As described in section 7.3, an average mass value
is constructed for the matrix element evaluation so that the β34 kinematics factor can beretained.
Because of its large mass, it is possible that the top quark can decay to some not yet
discovered particle. Some such alternatives are included in the program, such as t → bH+
or t → G̃t̃. These decays are not obtained by default, but can be included as discussed
for the respective physics scenario.
8.2.3

J/ψ and other Hidden Heavy Flavours

ISUB =

86
87
88
89
104
105
106
107
108

gg → J/ψg
gg → χ0c g
gg → χ1c g
gg → χ2c g
gg → χ0c
gg → χ2c
gg → J/ψγ
gγ → J/ψgγγ → J/ψγ

MSEL = 61,62,63
ISUB = cc bb(1)
421 461 gg → QQ[3 S1 ] g(8)
422 462 gg → QQ[3 S1 ] g(8)
423 463 gg → QQ[1 S0 ] g(8)424 464 gg → QQ[3 PJ ] g(8)425 465 gq → q QQ[3 S1 ](8)
426 466 gq → q QQ[1 S0 ](8)
427 467 gq → q QQ[3 PJ ](8)
428 468 qq → g QQ[3 S1 ](8)
429 469 qq → g QQ[1 S0 ](8)
430 470 qq → g QQ[3 PJ ](1)
431 471 gg → QQ[3 P0 ] g(1)
432 472 gg → QQ[3 P1 ] g(1)
433 473 gg → QQ[3 P2 ] g(1)
434 474 gq → q QQ[3 P0 ](1)
435 475 gq → q QQ[3 P1 ](1)
436 476 gq → q QQ[3 P2 ](1)
437 477 qq → g QQ[3 P0 ](1)
438 478 qq → g QQ[3 P1 ](1)
439 479 qq → g QQ[3 P2 ]In Pythia one may distinguish between three main sources of J/ψ production.
1. Decays of B mesons and baryons.
2. Parton-shower evolution, wherein a c and a c quark produced in adjacent branchings136(e.g. g → gg → cccc) turn out to have so small an invariant mass that the paircollapses to a single particle.
3. Direct production, where a c quark loop gives a coupling between a set of gluons
and a cc bound state. Higher-lying states, like the χc ones, may subsequently decay
to J/ψ.
The first two sources are implicit in the production of b and c quarks, although theforcing specifically of J/ψ production is difficult. In this section are given the mainprocesses for the third source, intended for applications at hadron colliders.
The traditional ‘colour singlet’ approach is encapsulated in the above processes in therange 86 – 108. Processes 104 and 105 are the equivalents of 87 and 89 in the limit ofp⊥ → 0; note that gg → J/ψ and gg → χ1c are forbidden and thus absent. As always one
should beware of double-counting between 87 and 104, and between 89 and 105, and thus
use either the one or the other depending on the kinematic domain to be studied. The
cross sections depend on wave function values at the origin, see PARP(38) and PARP(39).
A review of the physics issues involved may be found in [Glo88] (note, however, that the
choice of Q2 scale is different in Pythia).
While programmed for the charm system, it would be straightforward to apply theseprocesses instead to bottom mesons, i.e. for the production of Υ.One needs to change thecodes of states produced, which is achieved by KFPR(ISUB,1) = KFPR(ISUB,1) + 110
for the processes ISUB above, and changing the values of the wave functions at the origin,
PARP(38) and PARP(39).
It is known that the above sources are not enough to explain the full J/ψ rate, andfurther production mechanisms have been proposed, extending on the more conventional
treatment here [Can97]. The most common extension is the ‘colour octet’ production
mechanism, in the framework of nonrelativistic QCD (NRQCD) [Bod95]. In this language, production in part proceeds via intermediate colour octet states that collapse to
singlet states by the emission of soft (and thus nonperturbative) gluons. In the current
implementation [Wol02], three new colour octet states are introduced for each of the cc(8)
and bb systems, with spectroscopic notation QQ[2S+1 LJ ], where the (8) is a reminderof the colour octet nature of these states. These new ‘particles’ are assumed to ‘decay’
exclusively to J/ψ + g or Υ + g, respectively. Their masses have been chosen to allow this,without too much excess phase space, so that the emitted gluon is always very soft.
Unlike the first set of processes above, the NRQCD processes have been explicitly
duplicated for the cc and bb sectors. Further, several processes already present in thecolour singlet framework are repeated here, only differing by the way wave function andmatrix element normalization factors are defined, so as to provide a coherent framework.
For this reason, obviously the processes above 420 should not be combined with thelower-number ones, or else one would doublecount.
The rates for these new processes are regulated by 10 new NRQCD matrix element
values, PARP(141) - PARP(150). The switches MSTP(145) - MSTP(149) can be used tomodify the behaviour of the processes, but are mainly intended for experts.
8.2.4

Minimum bias

MSEL = 1, 2
ISUB = 91 elastic scattering
92 single diffraction (AB → XB)
93 single diffraction (AB → AX)
94 double diffraction
95 low-p⊥ production
These processes are briefly discussed in section 7.7. They are mainly intended forinteractions between hadrons, although one may also consider γp and γγ interactions in
the options where the incoming photon(s) is (are) assumed resolved.
137

Uncertainties come from a number of sources, e.g. from the parameterizations of thevarious cross sections and slope parameters.
In diffractive scattering, the structure of the selected hadronic system may be regulatedwith MSTP(101). No high-p⊥ jet production in diffractive events is included so far; one
would have to use add-on programs like PomPyt [Bru96] for that.
The subprocess 95, low-p⊥ events, is somewhat unique in that no meaningful physicalborder-line to high-p⊥ events can be defined. Even if the QCD 2 → 2 high-p⊥ processes
are formally switched off, some of the generated events will be classified as belonging tothis group, with a p⊥ spectrum of interactions to match the ‘minimum-bias’ event sample.
The generation of such jets is performed with the help of the auxiliary subprocess 96, seesection 8.2.1. Only with the option MSTP(82) = 0 will subprocess 95 yield strictly low-p⊥
events, events which will then probably not be compatible with any experimental data.
A number of options exist for the detailed structure of low-p⊥ events, see in particular
MSTP(81) and MSTP(82). Further details on the model(s) for minimum-bias events arefound in sections 11.2–11.4.

8.3

Physics with Incoming Photons

With recent additions, the machinery for photon physics has become rather extensive[Fri00]. The border between the physics of real photon interactions and of virtual photon
ones is now bridged by a description that continuously interpolates between the two
extremes, as summarized in section 7.7.2. Furthermore, the ’gamma/lepton’ option (wherelepton is to be replaced by e-, e+, mu-, mu+, tau- or tau+ as the case may be) in a PYINIT
call gives access to an internally generated spectrum of photons of varying virtuality. The
CKIN(61) - CKIN(78) variables can be used to set experimentally motivated x and Q2
limits on the photon fluxes. With this option, and the default MSTP(14) = 30, one
automatically obtains a realistic first approximation to ‘all’ QCD physics of γ ∗ p and γ ∗ γ ∗interactions. The word ‘all’ clearly does not mean that a perfect description is guaranteed,or that all issues are addressed, but rather that the intention is to simulate all processes
that give a significant contribution to the total cross section in whatever Q2 range is being
studied: jets, low-p⊥ events, elastic and diffractive scattering, etc.
The material to be covered encompasses many options, several of which have beensuperseded by further developments but have been retained for backwards compatibility.
Therefore it is here split into three sections. The first covers the physics of real photons
and the subsequent one that of (very) virtual ones. Thereafter, in the final section, thethreads are combined into a machinery applicable at all Q2 .
8.3.1

Photoproduction and γγ physics

MSEL = 1, 2, 4, 5, 6, 7, 8
ISUB = 33 qi γ → qi g
34 fi γ → fi γ
54 gγ → qk qk
58 γγ → fk f k
80 qi γ → qk π ±84 gγ → Qk Qk
85 γγ → Fk Fk
An (almost) real photon has both a point-like component and a hadron-like one. Thismeans that several classes of processes may be distinguished, see section 7.7.2.
1. The processes listed above are possible when the photon interacts as a point-likeparticle, i.e. couples directly to quarks and leptons.
2. When the photon acts like a hadron, i.e. is resolved in a partonic substructure,then high-p⊥ parton–parton interactions are possible, as described in sections 8.2.1
138and 8.4.1. These interactions may be further subdivided into VMD and anomalous(GVMD) ones [Sch93, Sch93a].
3. A hadron-like photon can also produce the equivalent of the minimum-bias processesof section 8.2.4. Again, these can be subdivided into VMD and GVMD (anomalous)ones.
For γp events, we believe that the best description can be obtained when three separate
event classes are combined, one for direct, one for VMD and one for GVMD/anomalous
events, see the detailed description in [Sch93, Sch93a]. These correspond to MSTP(14)being 0, 2 and 3, respectively. The direct component is high-p⊥ only, while VMD and
GVMD contain both high-p⊥ and low-p⊥ events. The option MSTP(14) = 1 combines the
VMD and GVMD/anomalous parts of the photon into one single resolved photon concept,
which therefore is less precise than the full subdivision.
When combining three runs to obtain the totality of γp interactions, to the best of
our knowledge, it is necessary to choose the p⊥ cut-offs with some care, so as to represent
the expected total cross section.
• The direct processes by themselves only depend on the CKIN(3) cut-off of the generation. In older program versions the preferred value was 0.5 GeV [Sch93, Sch93a]. In
the more recent description in [Fri00], also eikonalization of direct with anomalousinteractions into the GVMD event class is considered. That is, given a branchingγ → qq, direct interactions are viewed as the low-p⊥ events and anomalous ones as
high-p⊥ events that have to merge smoothly. Then the CKIN(3) cut-off is increased
to the p⊥min of multiple interactions processes, see PARP(81) (or PARP(82), depending on minijet unitarization scheme). See MSTP(18) for a possibility to switch backto the older behaviour. However, full backwards compatibility cannot be assured,
so the older scenarios are better simulated by using an older Pythia version.
• The VMD processes work as ordinary hadron–hadron ones, i.e. one obtains bothlow- and high-p⊥ events by default, with dividing line set by p⊥min above.
• Also the GVMD processes work like the VMD ones. Again this is a change fromprevious versions, where the anomalous processes only contained high-p⊥ physics
and the low-p⊥ part was covered in the direct event class. See MSTP(15) = 5 for a
possibility to switch back to the older behaviour, with comments as above for the
direct class. A GVMD state is book-kept as a diffractive state in the event listing,even when it scatters ‘elastically’, since the subsequent hadronization descriptions
are very similar.
The processes in points 1 and 2 can be simulated with a photon beam, i.e. when’gamma’ appears as argument in the PYINIT call. It is then necessary to use option
MSTP(14) to switch between a point-like and a resolved photon — it is not possible tosimulate the two sets of processes in a single run. This would be the normal mode ofoperation for beamstrahlung photons, which have Q2 = 0 but with a nontrivial energy
spectrum that would be provided by some external routine.
For bremsstrahlung photons, the x and Q2 spectrum can be simulated internally, withthe ’gamma/lepton’ argument in the PYINIT call. This is the recommended procedure,
wherein direct and resolved processes can be mixed. An older — now not recommended —alternative is to use a parton-inside-electron structure function concept, obtainable witha simple ’e-’ (or other lepton) argument in PYINIT. To access these quark and gluon
distributions inside the photon (itself inside the electron), MSTP(12) = 1 must then be
used. Also the default value MSTP(11) = 1 is required for the preceding step, that offinding photons inside the electron. Also here the direct and resolved processes may begenerated together. However, this option only works for high-p⊥ physics. It is not possible
to have also the low-p⊥ physics (including multiple interactions in high-p⊥ events) for anelectron beam. Kindly note that subprocess 34 contains both the scattering of an electron
off a photon and the scattering of a quark (inside a photon inside an electron) off a photon;
the former can be switched off with the help of the KFIN array.
139

If you are only concerned with standard QCD physics, the option MSTP(14) = 10or the default MSTP(14) = 30 gives an automatic mixture of the VMD, direct and
GVMD/anomalous event classes. The mixture is properly given according to the relative cross sections. Whenever possible, this option is therefore preferable in terms ofuser-friendliness. However, it can only work because of a completely new layer of administration, not found anywhere else in Pythia. For instance, a subprocess like qg → qg
is allowed in several of the classes, but appears with different sets of parton distributions
and different p⊥ cut-offs in each of these, so that it is necessary to switch gears between
each event in the generation. It is therefore not possible to avoid a number of restrictions
on what you can do in this case:• The MSTP(14) = 10 and = 30 options can only be used for incoming photon beams,
with or without convolution with the bremsstrahlung spectrum, i.e. when ’gamma’or ’gamma/lepton’ is the argument in the PYINIT call.
• The machinery has only been set up to generate standard QCD physics, specificallyeither ‘minimum-bias’ one or high-p⊥ jets. There is thus no automatic mixing ofprocesses only for heavy-flavour production, say, or of some exotic particle. For
minimum bias, you are not allowed to use the CKIN variables at all. This is not amajor limitation, since it is in the spirit of minimum-bias physics not to impose any
constraints on allowed jet production. (If you still do, these cuts will be ineffective
for the VMD processes but take effect for the other ones, giving inconsistencies.)
The minimum-bias physics option is obtained by default; by switching from MSEL =1 to MSEL = 2 also the elastic and diffractive components of the VMD and GVMD
parts are included. High-p⊥ jet production is obtained by setting the CKIN(3)cut-off larger than the p⊥min (W 2 ) of the multiple interactions scenario. For lower
input CKIN(3) values the program will automatically switch back to minimum-bias
physics.
• Multiple interactions become possible in both the VMD and GVMD sector, with theaverage number of interactions given by the ratio of the jet to the total cross section.
Currently only the simpler scenario MSTP(82) = 1 in the old model is implemented,
however, i.e. the more sophisticated variable-impact-parameter ones need further
physics studies and model development.
• Some variables are internally recalculated and reset, notably CKIN(3). This is because it must have values that depend on the component studied. It can therefore
not be modified without changing PYINPR and recompiling the program, which obviously is a major exercise.
• Pileup events are not at all allowed.
Also, a warning about the usage of Pdflib/LHAPDF for photons. So long as
MSTP(14) = 1, i.e. the photon is not split up, Pdflib is accessed by MSTP(56) = 2 andMSTP(55) as the parton distribution set. However, when the VMD and anomalous pieces
are split, the VMD part is based on a rescaling of pion distributions by VMD factors(except for the SaS sets, that already come with a separate VMD piece). Therefore, toaccess Pdflib for MSTP(14) = 10, it is not correct to set MSTP(56) = 2 and a photon
distribution in MSTP(55). Instead, one should put MSTP(56) = 2, MSTP(54) = 2 and apion distribution code in MSTP(53), while MSTP(55) has no function. The anomalous partis still based on the SaS parameterization, with PARP(15) as main free parameter.
Currently, hadrons are not defined with any photonic content. None of the processes
are therefore relevant in hadron–hadron collisions. In ep collisions, the electron can emit
an almost real photon, which may interact directly or be resolved. In e+ e− collisions, one
may have direct, singly-resolved or doubly-resolved processes.
The γγ equivalent to the γp description involves six different event classes, see section
7.7.2. These classes can be obtained by setting MSTP(14) to 0, 2, 3, 5, 6 and 7, respectively.
If one combines the VMD and anomalous parts of the parton distributions of the photon,
in a more coarse description, it is enough to use the MSTP(14) options 0, 1 and 4. The
140

cut-off procedures follows from the ones used for the γp ones above.
As with γp events, the options MSTP(14) = 10 or MSTP(14) = 30 give a mixture of
the six possible γγ event classes. The same complications and restrictions exist here as
already listed above.
Process 54 generates a mixture of quark flavours; allowed flavours are set by the gluon
MDME values. Process 58 can generate both quark and lepton pairs, according to the
MDME values of the photon. Processes 84 and 85 are variants of these matrix elements,
with fermion masses included in the matrix elements, but where only one flavour canbe generated at a time. This flavour is selected as described for processes 81 and 82
in section 8.2.2, with the exception that for process 85 the ‘heaviest’ flavour allowed forphoton splitting takes to place of the heaviest flavour allowed for gluon splitting. Since
lepton KF codes come after quark ones, they are counted as being ‘heavier’, and thus takeprecedence if they have been allowed.
Process 80 is a higher twist one. The theory for such processes is rather shaky, so
results should not be taken too literally. The messy formulae given in [Bag82] havenot been programmed in full, instead the pion form factor has been parameterized as
Q2 Fπ (Q2 ) ≈ 0.55/ ln Q2 , with Q in GeV.
8.3.2

Deeply Inelastic Scattering and γ ∗ γ ∗ physics

MSEL = 1, 2, 35, 36, 37, 38
ISUB = 10 fi fj → fk fl
83 qi fj → Qk fl
99 γ ∗ q → q
131 fi γT∗ → fi g
132 fi γL∗ → fi g
133 fi γT∗ → fi γ
134 fi γL∗ → fi γ
135 gγT∗ → fi f i
136 gγL∗ → fi f i
137 γT∗ γT∗ → fi f i
138 γT∗ γL∗ → fi f i
139 γL∗ γT∗ → fi f i
140 γL∗ γL∗ → fi f iAmong the processes in this section, 10 and 83 are intended to stand on their own,
while the rest are part of the newer machinery for γ ∗ p and γ ∗ γ ∗ physics. We therefore
separate the description in this section into these two main parts.
The Deeply Inelastic Scattering (DIS) processes, i.e. t-channel electroweak gauge boson
exchange, are traditionally associated with interactions between a lepton or neutrino anda hadron, but processes 10 and 83 can equally well be applied for qq scattering in hadroncolliders (with a cross section much smaller than corresponding QCD processes, however).
If applied to incoming e+ e− beams, process 10 corresponds to Bhabha scattering.
For process 10 both γ, Z0 and W± exchange contribute, including interference between
γ and Z0 . The switch MSTP(21) may be used to restrict to only some of these, e.g. neutralor charged current only.
The option MSTP(14) = 10 (see previous section) has now been extended so that it
also works for DIS of an electron off a (real) photon, i.e. process 10. What is obtained is a
mixture of the photon acting as a vector meson and it acting as an anomalous state. Thisshould therefore be the sum of what can be obtained with MSTP(14) = 2 and = 3. It isdistinct from MSTP(14) = 1 in that different sets are used for the parton distributions —in MSTP(14) = 1 all the contributions to the photon distributions are lumped together,while they are split in VMD and anomalous parts for MSTP(14) = 10. Also the beamremnant treatment is different, with a simple Gaussian distribution (at least by default)
141

for MSTP(14) = 1 and the VMD part of MSTP(14) = 10, but a powerlike distribution
2
2
dk⊥
/k⊥
between PARP(15) and Q for the anomalous part of MSTP(14) = 10.
To access this option for e and γ as incoming beams, it is only necessary to set
MSTP(14) = 10 and keep MSEL at its default value. Unlike the corresponding option
for γp and γγ, no cuts are overwritten, i.e. it is still your responsibility to set theseappropriately.
Cuts especially appropriate for DIS usage include either CKIN(21) - CKIN(22) or
CKIN(23) - CKIN(24) for the x range (former or latter depending on which side is the incoming real photon), CKIN(35) - CKIN(36) for the Q2 range, and CKIN(39) - CKIN(40)for the W 2 range.
In principle, the DIS x variable of an event corresponds to the x value stored in
PARI(33) or PARI(34), depending on which side the incoming hadron is on, while the
DIS Q2 = −t̂ =-PARI(15). However, just like initial- and final-state radiation can shift
jet momenta, they can modify the momentum of the scattered lepton. Therefore the DIS
x and Q2 variables are not automatically conserved. An option, on by default, exists in
MSTP(23), where the event can be ‘modified back’ so as to conserve x and Q2 , but thisoption is rather primitive and should not be taken too literally.
Process 83 is the equivalent of process 10 for W± exchange only, but with the heavyquark mass included in the matrix element. In hadron colliders it is mainly of interest forthe production of very heavy flavours, where the possibility of producing just one heavy
quark is kinematically favoured over pair production. The selection of the heavy flavour
is already discussed in section 8.2.2.
Turning to the other processes, part of the γ ∗ p and γ ∗ γ ∗ process-mixing machineries,
99 has close similarities with the above discussed 10 one. Whereas 10 would simulate thefull process eq → eq, 99 assumes a separate machinery for the flux of virtual photons,e → eγ ∗ and only covers the second half of the process, γ ∗ q → q. One limitation of thisfactorization is that only virtual photons are considered in process 99, not contributions
from the Z0 neutral current or the W± charged current.
Note that 99 has no correspondence in the real-photon case, but has to vanish inthis limit by gauge invariance, or indeed by simple kinematics considerations. This, plus
the desire to avoid double-counting with real-photon physics processes, is why the crosssection for this process is explicitly made to vanish for photon virtuality Q2 → 0, eq. (127),
also when parton distributions have not been constructed to fulfil this, see MSTP(19). (Nosuch safety measures are present in 10, again illustrating how the two are intended mainly
to be used at large or at small Q2 , respectively.)
For a virtual photon, processes 131–136 may be viewed as first-order corrections to
99. The three with a transversely polarized photon, 131, 133 and 135, smoothly reduceto the real-photon direct (single-resolved for γγ) processes 33, 34 and 54. The otherthree, corresponding to the exchange of a longitudinal photon, vanish like Q2 for Q2 → 0.
The double-counting issue with process 99 is solved by requiring the latter process not to
contain any shower branchings with a p⊥ above the lower p⊥ cut-off of processes 131-136.
The cross section is then to be reduced accordingly, see eq. (128) and the discussion there,
and again MSTP(19).
We thus see that process 99 by default is a low-p⊥ process in about the same sense
as process 95, giving ‘what is left’ of the total cross section when jet events have been
removed. Therefore, it will be switched off in event class mixes such as MSTP(14) = 30
if CKIN(3) is above p⊥min (W 2 ) and MSEL is not 2. There is a difference, however, in thatprocess 99 events still are allowed to contain shower evolution (although currently only
the final-state kind has been implemented), since the border to the other processes is atp⊥ = Q for large Q and thus need not be so small. The p⊥ scale of the ‘hard process’,
stored e.g. in PARI(17) always remains 0, however. (Other PARI variables defined fornormal 2 → 2 and 2 → 1 processes are not set at all, and may well contain irrelevant junkleft over from previous events.)
142

Processes 137–140, finally, are extensions of process 58 from the real-photon limit to
the virtual-photon case, and correspond to the direct process of γ ∗ γ ∗ physics. The fourcases correspond to either of the two photons being either transversely or longitudinallypolarized. As above, the cross section of a longitudinal photon vanishes when its virtuality
approaches 0.
8.3.3

Photon physics at all virtualities

ISUB = direct×direct:
137, 138, 139, 140
direct×resolved:
131, 132, 135, 136
DIS×resolved:
99
resolved×resolved, high-p⊥ : 11, 12, 13, 28, 53, 68
resolved×resolved, low-p⊥ : 91, 92, 93, 94, 95
where ‘resolved’ is a hadron or a VMD or GVMD photon.
At intermediate photon virtualities, processes described in both of the sections above
are allowed, and have to be mixed appropriately. The sets are of about equal importanceat around Q2 ∼ m2ρ ∼ 1 GeV2 , but the transition is gradual over a larger Q2 range. The
ansatz for this mixing is given by eq. (129) for γ ∗ p events and eq. (130) for γ ∗ γ ∗ ones. In
short, for direct and DIS processes the photon virtuality explicitly enters in the matrix
element expressions, and thus is easily taken into account. For resolved photons, perturbation theory does not provide a unique answer, so instead cross sections are suppressedby dipole factors, (m2 /(m2 + Q2 ))2 , where m = mV for a VMD state and m = 2k⊥ for a
GVMD state characterized by a k⊥ scale of the γ ∗ → qq branching. These factors appear
explicitly for total, elastic and diffractive cross sections, and are also implicitly used e.g. inderiving the SaS parton distributions for virtual photons. Finally, some double-counting
need to be removed, between direct and DIS processes as mentioned in the previous
section, and between resolved and DIS at large x.
Since the mixing is not trivial, it is recommended to use the default MSTP(14) = 30
to obtain it in one go and hopefully consistently, rather than building it up by combining
separate runs. The main issues still under your control include, among others• The CKIN(61) - CKIN(78) should be used to set the range of x and Q2 values
emitted from the lepton beams. That way one may decide between almost real orvery virtual photons, say. Also some other quantities, like W 2 , can be constrained
to desirable ranges.
• Whether or not minimum-bias events are simulated depends on the CKIN(3) value,
just like in hadron physics. The only difference is that the initialization energy scale
Winit is selected in the allowed W range rather than to be the full c.m. energy.
2
For a high CKIN(3), CKIN(3) > p⊥min (Winit), only jet production is included. Thenfurther CKIN values can be set to constrain e.g. the rapidity of the jets produced.
2
For a low CKIN(3), CKIN(3) < p⊥min (Winit), like the default value CKIN(3) = 0,
low-p⊥ physics is switched on together with jet production, with the latter properly
eikonalized to be lower than the total one. The ordinary CKIN cuts, not related tothe photon flux, cannot be used here.
For a low CKIN(3), when MSEL = 2 instead of the default = 1, also elastic anddiffractive events are simulated.
• The impact of resolved longitudinal photons is not unambiguous, e.g. only recently
the first parameterization of parton distributions appeared [Chý00]. Different simple
alternatives can be probed by changing MSTP(17) and associated parameters.
• The choice of scales to use in parton distributions for jet rates is always ambiguous,
but depends on even more scales for virtual photons than in hadronic collisions.
MSTP(32) allows a choice between several alternatives.
• The matching of p⊥ generation by shower evolution to that by primordial k⊥ is a
general problem, for photons with an additional potential source in the γ ∗ → qq
143

vertex. MSTP(66) offer some alternatives.
• PARP(15) is the k0 parameter separating VMD from GVMD.
• PARP(18) is the kρ parameter in GVMD total cross sections.
• MSTP(16) selects the momentum variable for an e → eγ ∗ branching.
• MSTP(18) regulates the choice of p⊥min for direct processes.
• MSTP(19) regulates the choice of partonic cross section in process 99, γ ∗ q → q.
• MSTP(20) regulates the suppression of the resolved cross section at large x.
The above list is not complete, but gives some impression what can be done.

8.4

Electroweak Gauge Bosons

This section covers the production and/or exchange of γ, Z0 and W± gauge bosons, singly
and in pairs. The topic of longitudinal gauge-boson scattering at high energies is deferredto the Higgs section, since the presence or absence of a Higgs boson here makes a big
difference.
8.4.1

Prompt photon production

MSEL = 10
ISUB = 14 qi qi → gγ
18 fi f i → γγ
29 qi g → qi γ
114 gg → γγ
115 gg → gγ
In hadron colliders, processes ISUB = 14 and 29 give the main source of single-γ
production, with ISUB = 115 giving an additional contribution which, in some kinematics
regions, may become important. For γ-pair production, the process ISUB = 18 is often
overshadowed in importance by ISUB = 114.
Another source of photons is bremsstrahlung off incoming or outgoing quarks. This has
to be treated on an equal footing with QCD parton showering. For time-like parton-shower
evolution, i.e. in the final-state showering and in the side branches of the initial-state
showering, photon emission may be switched on or off with MSTJ(41). Photon radiation
off the space-like incoming quark or lepton legs is similarly regulated by MSTP(61).
Warning: the cross sections for the box graphs 114 and 115 become very complicated,numerically unstable and slow when the full quark mass dependence is included. For quark
masses much below the ŝ scale, the simplified massless expressions are therefore used
— a fairly accurate approximation. However, there is another set of subtle numericalcancellations between different terms in the massive matrix elements in the region ofsmall-angle scattering. The associated problems have not been sorted out yet. There
are therefore two possible solutions. One is to use the massless formulae throughout.
The program then becomes faster and numerically stable, but does not give, for example,
the characteristic dip (due to destructive interference) at top threshold. This is thecurrent default procedure, with five flavours assumed, but this number can be changedin MSTP(38). The other possibility is to impose cuts on the scattering angle of the hard
process, see CKIN(27) and CKIN(28), since the numerically unstable regions are when| cos θ̂| is close to unity. It is then also necessary to change MSTP(38) to 0.

144

8.4.2Single W/Z production

MSEL = 11, 12, 13, 14, 15, (21)ISUB =
1 fi f i → γ ∗ /Z0
2 fi f j → W +15 fi f i → g(γ ∗ /Z0 )16 fi f j → gW+
19 fi f i → γ(γ ∗ /Z0 )20 fi f j → γW+
30 fi g → fi (γ ∗ /Z0 )31 fi g → fk W+
35 fi γ → fi (γ ∗ /Z0 )36 fi γ → fk W+(141) fi f i → γ/Z0 /Z00
(142) fi f j → W0+
This group consists of 2 → 1 processes, i.e. production of a single resonance, and
2 → 2 processes, where the resonance is recoiling against a jet or a photon. The processes
141 and 142, which also are listed here, are described further elsewhere.
With initial-state showers turned on, the 2 → 1 processes also generate additional jets;in order to avoid double-counting, the corresponding 2 → 2 processes should therefore not
be turned on simultaneously. The basic rule is to use the 2 → 1 processes for inclusive
generation of W/Z, i.e. where the bulk of the events studied have p⊥ ¿ mW/Z . With theintroduction of explicit matrix-element-inspired corrections to the parton shower [Miu99],
also the high-p⊥ tail is well described in this approach, thus offering an overall good
description of the full p⊥ spectrum of gauge bosons [Bál01].
If one is interested in the high-p⊥ tail only, however, the generation efficiency will be
low. It is here better to start from the 2 → 2 matrix elements and add showers to these.
However, the 2 → 2 matrix elements are divergent for p⊥ → 0, and should not be used
down to the low-p⊥ region, or one may get unphysical cross sections. As soon as thegenerated 2 → 2 cross section corresponds to a non-negligible fraction of the total 2 → 1
one, say 10%–20%, Sudakov effects are likely to be affecting the shape of the p⊥ spectrum
to a corresponding extent, and results should not be trusted.
The problems of double-counting and Sudakov effects apply not only to W/Z production in hadron colliders, but also to a process like e+ e− → Z0 γ, which clearly is part ofthe initial-state radiation corrections to e+ e− → Z0 obtained for MSTP(11) = 1. As is thecase for Z production in association with jets, the 2 → 2 process should therefore only be
used for the high-p⊥ region.
The Z0 of subprocess 1 includes the full interference structure γ ∗ /Z0 ; via MSTP(43)you can select to produce only γ ∗ , only Z0 , or the full γ ∗ /Z0 . The same holds true for theZ00 of subprocess 141; via MSTP(44) any combination of γ ∗ , Z0 and Z00 can be selected.
Thus, subprocess 141 with MSTP(44) = 4 is essentially equivalent to subprocess 1 with
MSTP(43) = 3; however, process 141 also includes the possibility of a decay into Higgs
bosons. Also processes 15, 19, 30 and 35 contain the full mixture of γ ∗ /Z0 , with MSTP(43)available to change this. Note that the γ ∗ /Z0 decay products can have an invariant mass
as small as the program cutoff. This can be changed using CKIN.
Note that process 1, with only qq → γ ∗ → `+ `− allowed, and studied in the region
well below the Z0 mass, is what is conventionally called Drell–Yan. This latter process
therefore does not appear under a separate heading, but can be obtained by a suitablesetting of switches and parameters.
A process like fi f j → γW+ requires some comment. When the W boson decays,
photons can be radiated off the decay products. The full interference between photon
radiation off the incoming fermions, the intermediate W boson, and the decay products is
not included in the Pythia treatment. If such effects are important, a full matrix element145

calculation is preferred. Some caution must therefore be exercised; see also section 8.4.3
for related comments.
For the 2 → 1 processes, the Breit–Wigner includes an ŝ-dependent width, whichshould provide an improved description of line shapes. In fact, from a line-shape pointof view, process 1 should provide a more accurate simulation of e+ e− annihilation events
than the dedicated e+ e− generation scheme of PYEEVT (see section 6.1). Another differenceis that PYEEVT only allows the generation of γ ∗ /Z0 → qq, while process 1 additionallycontains γ ∗ /Z0 → `+ `− and νν. The parton-shower and fragmentation descriptions are
the same, but the process 1 implementation only contains a partial interface to the firstand second-order matrix-element options available in PYEEVT, see MSTP(48).
All processes in this group have been included with the correct angular distribution
in the subsequent W/Z → ff decays. In process 1 also fermion mass effects have been included in the angular distributions, while this is not the case for the other ones. Normally
mass effects are not large anyway.
As noted earlier, some approximations can be used to simulate higher-order processes.
The process e+ e− → e+ e− Z0 can be simulated in two different ways. One is to make useof the e ‘sea’ distribution inside e, i.e. have splittings e → γ → e. This can be obtained,
together with ordinary Z0 production, by using subprocess 1, with MSTP(11) = 1 andMSTP(12) = 1. Then the contribution of the type above is 5.0 pb for a 500 GeV e+ e−
collider, compared with the correct 6.2 pb [Hag91]. Alternatively one may use process 35,
with MSTP(11) = 1 and MSTP(12) = 0, relying on the splitting e → γ. This process hasa singularity in the forward direction, regularized by the electron mass and also sensitive
to the virtuality of the photon. It is therefore among the few where the incoming masses
have been included in the matrix element expression. Nevertheless, it may be advisable
to set small lower cut-offs, e.g. CKIN(3) = CKIN(5) = 0.01, if one should experienceproblems (e.g. at higher energies).
Process 36, fγ → f 0 W± may have corresponding problems; except that in e+ e− theforward scattering amplitude for eγ → νW is killed (radiation zero), which means thatthe differential cross section is vanishing for p⊥ → 0. It is therefore feasible to use thedefault CKIN(3) and CKIN(5) values in e+ e− , and one also comes closer to the correctcross section.
The process gg → Z0 bb, formerly available as process 131, has been removed from thecurrent version, since the implementation turned out to be slow and unstable. However,process 1 with incoming flavours set to be bb (by KFIN(1,5) = KFIN(1,-5) = KFIN(2,5)= KFIN(2,-5) = 1 and everything else = 0) provides an alternative description, where
the additional bb are generated by g → bb branchings in the initial-state showers. (Awayfrom the low-p⊥ region, process 30 with KFIN values as above except that also incoming
gluons are allowed, offers yet another description. Here it is in terms of gb → Z0 b,
with only one further g → bb branching constructed by the shower.) At first glance, theshower approach would seem less reliable than the full 2 → 3 matrix element. The relative
lightness of the b quark will generate large logs of the type ln(m2Z /m2b ), however, that ought
to be resummed [Car00]. This is implicit in the parton-density approach of incoming b
quarks but absent from the lowest-order gg → Z0 bb matrix elements. Therefore actually
the shower approach may be the more accurate of the two in the region of intermediatetransverse momenta.

146

8.4.3W/Z pair production

MSEL = 15
ISUB = 22 fi f i → (γ ∗ /Z0 )(γ ∗ /Z0 )23 fi f j → Z0 W+
25 fi f i → W+ W−
69 γγ → W+ W−
70 γW+ → Z0 W+
In this section we mainly consider the production of W/Z pairs by fermion–antifermion
annihilation, but also include two processes which involve γ/W beams. Scatterings between gauge-boson pairs, i.e. processes like W+ W− → Z0 Z0 , depend so crucially on theassumed Higgs scenario that they are considered separately in section 8.5.2.
The cross sections used for the above processes are those derived in the narrow-width
limit, but have been extended to include Breit–Wigner shapes with mass-dependent
widths for the final-state particles. In process 25, the contribution from Z0 exchange
to the cross section is now evaluated with the fixed nominal Z0 mass and width in the
propagator. If instead the actual mass and the running width were to be used, it would
give a diverging cross section at large energies, by imperfect gauge cancellation.
However, one should realize that other graphs, not included here, can contribute inregions away from the W/Z mass. This problem is especially important if several flavours
coincide in the four-fermion final state. Consider, as an example, e+ e− → µ+ µ− νµ ν µ . Not
only would such a final state receive contributions from intermediate Z0 Z0 and W+ W−
states, but also from processes e+ e− → Z0 → µ+ µ− , followed either by µ+ → µ+ Z0 →µ+ νµ ν µ , or by µ+ → ν µ W+ → ν µ µ+ νµ . In addition, all possible interferences should
be considered. Since this is not done, the processes have to be used with some soundjudgement. Very often, one may wish to constrain a lepton pair mass to be close to mZ ,in which case a number of the possible ‘other’ processes are negligible.
For the W pair production graph, one experimental objective is to do precision measurements of the cross section near threshold. Then also other effects enter. One such is
Coulomb corrections, induced by photon exchange between the two W’s and their decay
products. The gauge invariance issues induced by the finite W lifetime are not yet fully
resolved, and therefore somewhat different approximate formulae may be derived [Kho96].
The options in MSTP(40) provide a reasonable range of uncertainty.
Of the above processes, the first contains the full fi f i → (γ ∗ /Z0 )(γ ∗ /Z0 ) structure,obtained by a straightforward generalization of the formulae in ref. [Gun86] (done by oneof the Pythia authors). Of course, the possibility of there being significant contributions
from graphs that are not included is increased, in particular if one γ ∗ is very light andtherefore could be a bremsstrahlung-type photon. It is possible to use MSTP(43) to recover
the pure Z0 case, i.e. fi f i → Z0 Z0 exclusively. In processes 23 and 70, only the pure Z0
contribution is included.
Full angular correlations are included for the first three processes, i.e. the full 2 → 2 →4 matrix elements are included in the resonance decays, including the appropriate γ ∗ /Z0
interference in process 22. In the latter two processes, 69 and 70, no spin information iscurrently preserved, i.e. the W/Z bosons are allowed to decay isotropically.
We remind you that the mass ranges of the two resonances may be set with the
CKIN(41) - CKIN(44) parameters; this is particularly convenient, for instance, to pick
one resonance almost on the mass shell and the other not.

8.5

Higgs Production

A fair fraction of all the processes in Pythia deal with Higgs production in one formor another. This multiplication is caused by the need to consider production by several
different mechanisms, depending on Higgs mass and machine type. Further, the program147

contains a full two-Higgs-multiplet scenario, as predicted for example in the Minimal
Supersymmetric extension of the Standard Model (MSSM). Therefore the continued discussion is, somewhat arbitrarily, subdivided into a few different scenarios. Doubly-charged
Higgs particles appear in left–right symmetric models, and are covered in section 8.6.3.
8.5.1

Light Standard Model Higgs

MSEL = 16, 17, 18
ISUB =
3 f i f i → h0
24 fi f i → Z0 h0
26 fi f j → W+ h0
32 fi g → fi h0
102 gg → h0
103 γγ → h0
110 fi f i → γh0
111 fi f i → gh0
112 fi g → fi h0
113 gg → gh0
121 gg → Qk Qk h0
122 qi qi → Qk Qk h0
123 fi fj → fi fj h0 (Z0 Z0 fusion)
124 fi fj → fk fl h0 (W+ W− fusion)
In this section we discuss the production of a reasonably light Standard Model Higgs,
below 700 GeV, say, so that the narrow width approximation can be used with someconfidence. Below 400 GeV there would certainly be no trouble, while above that thenarrow width approximation is gradually starting to break down.
In a hadron collider, the main production processes are 102, 123 and 124, i.e. gg, Z0 Z0and W+ W− fusion. In the latter two processes, it is also necessary to take into accountthe emission of the space-like W/Z bosons off quarks, which in total gives the 2 → 3
processes above.
Other processes with lower cross sections may be of interest because they provide
signals with less background. For instance, processes 24 and 26 give associated productionof a Z or a W together with the h0 . There is also the processes 3 (see below), 121 and122, which involve production of heavy flavours.
Process 3 contains contributions from all flavours, but is completely dominated by
the subprocess tt → h0 , i.e. by the contribution from the top sea distributions. This
assumes that parton densities for top quarks are provided, which is no longer the case incurrent parameterizations of PDF’s. This process is by now known to overestimate thecross section for Higgs production as compared with a more careful calculation based on
the subprocess gg → tth0 , process 121. The difference between the two is that in process
3 the t and t are added by the initial-state shower, while in 121 the full matrix element
is used. The price to be paid is that the complicated multi-body phase space in process
121 makes the program run slower than with most other processes. As usual, it would be
double-counting to include the same flavour both with 3 and 121. An intermediate step— in practice probably not so useful — is offered by process 32, qg → qh0 , where thequark is assumed to be a b one, with the antiquark added by the showering activity.
Process 122 is similar in structure to 121, but is less important. In both process 121
and 122 the produced quark is assumed to be a t; this can be changed in KFPR(121,2) andKFPR(122,2) before initialization, however. For b quarks it could well be that process 3
with bb → h0 is more reliable than process 121 with gg → bbh0 [Car00]; see the discussion
on Z0 bb final states in section 8.4.2. Thus it would make sense to run with all quarks
up to and including b simulated in process 3 and then consider t quarks separately in148

process 121. Assuming no t parton densities, this would actually be the default behaviour,meaning that the two could be combined in the same run without double-counting.
The two subprocess 112 and 113, with a Higgs recoiling against a quark or gluon
jet, are also effectively generated by initial-state corrections to subprocess 102. Thus, inorder to avoid double-counting, just as for the case of Z0 /W+ production, section 8.4.2,
these subprocesses should not be switched on simultaneously. Process 111, qq → gh0 is
different, in the sense that it proceeds through an s-channel gluon coupling to a heavyquark loop, and that therefore the emitted gluon is necessary in the final state in order toconserve colours. It is not to be confused with a gluon-radiation correction to the Bornlevel process 3, like in process 32, since processes 3 and 32 vanish for massless quarks while
process 111 is mainly intended for such. The lack of a matching Born-level process shows
up by process 111 being vanishing in the p⊥ → 0 limit. Numerically it is of negligible
importance, except at very large p⊥ values. Process 102, possibly augmented by 111,
should thus be used for inclusive production of Higgs, and 111–113 for the study of the
Higgs subsample with high transverse momentum.
A warning is that the matrix-element expressions for processes 111–113 are very
lengthy and the coding therefore more likely to contain some errors and numerical instabilities than for most other processes. Therefore the full expressions are only available
by setting the non-default value MSTP(38) = 0. Instead the default is based on the simplified expressions obtainable if only the top quark contribution is considered, in the
mt → ∞ limit [Ell88]. As a slight improvement, this expression is rescaled by the ratioof the gg → h0 cross sections (or, equivalently, the h → gg partial widths) of the full calculation and that in the mt → ∞ limit. Simple checks show that this approach normallyagrees with the full expressions to within ∼ 20%, which is small compared with otheruncertainties. The agreement is worse for process 111 alone, about a factor of 2, but thisprocess is small anyway. We also note that the matrix element correction factors, usedin the initial-state parton shower for process 102, section 10.3.5, are based on the same
mt → ∞ limit expressions, so that the high-p⊥ tail of process 102 is well matched to thesimple description of process 112 and 113.
In e+ e− annihilation, associated production of an h0 with a Z0 , process 24, is usually
the dominant one close to threshold, while the Z0 Z0 and W+ W− fusion processes 123 and124 win out at high energies. Process 103, γγ fusion, may also be of interest, in particularwhen the possibilities of beamstrahlung photons and backscattered photons are included(see section 7.1.3). Process 110, which gives an h0 in association with a γ, is a loop
process and is therefore suppressed in rate. It would have been of interest for a h0 mass
above 60 GeV at LEP 1, since its phase space suppression there is less severe than for the
associated production with a Z0 . Now it is not likely to be of any further interest.
The branching ratios of the Higgs are very strongly dependent on the mass. In principle, the program is set up to calculate these correctly, as a function of the actual Higgs
mass, i.e. not just at the nominal mass. However, higher-order corrections may at times
be important and not fully unambiguous; see for instance MSTP(37).
Since the Higgs is a spin-0 particle it decays isotropically. In decay processes suchas h0 → W+ W− /Z0 Z0 → 4 fermions angular correlations are included [Lin97]. Also inprocesses 24 and 26, Z0 and W± decay angular distributions are correctly taken into
account.

149

8.5.2

Heavy Standard Model Higgs

5 Z0 Z0 → h0
8 W + W − → h0
71 Z0 Z0 → Z0 Z0 (longitudinal)
72 Z0 Z0 → W+ W− (longitudinal)
73 Z0 W+ → Z0 W+ (longitudinal)
76 W+ W− → Z0 Z0 (longitudinal)
77 W+ W± → W+ W± (longitudinal)
Processes 5 and 8 are the simple 2 → 1 versions of what is now available in 123 and124 with the full 2 → 3 kinematics. For low Higgs masses processes 5 and 8 overestimate
the correct cross sections and should not be used, whereas good agreement between the
2 → 1 and 2 → 3 descriptions is observed when heavy Higgs production is studied.
The subprocesses 5 and 8, V V → h0 , which contribute to the processes V V → V 0 V 0 ,show a bad high-energy behaviour. Here V denotes a longitudinal intermediate gaugeboson, Z0 or W± . This can be cured only by the inclusion of all V V → V 0 V 0 graphs, as
is done in subprocesses 71, 72, 73, 76 and 77. In particular, subprocesses 5 and 8 giverise to a fictitious high-mass tail of the Higgs. If this tail is thrown away, however, theagreement between the s-channel graphs only (subprocesses 5 and 8) and the full set ofgraphs (subprocesses 71 etc.) is very good: for a Higgs of nominal mass 300 (800) GeV, a
cut at 600 (1200) GeV retains 95% (84%) of the total cross section, and differs from theexact calculation, cut at the same values, by only 2% (11%) (numbers for SSC energies).
With this prescription there is therefore no need to use subprocesses 71 etc. rather thansubprocesses 5 and 8.
For subprocess 77, there is an option, see MSTP(45), to select the charge combinationof the scattering W’s: like-sign, opposite-sign (relevant for Higgs), or both.
Process 77 contains a divergence for p⊥ → 0 due to γ-exchange contributions. This
leads to an infinite total cross section, which is entirely fictitious, since the simple partondistribution function approach to the longitudinal W flux is not appropriate in this limit.
For this process, it is therefore necessary to make use of a cut, e.g. p⊥ > mW .
For subprocesses 71, 72, 76 and 77, an option is included (see MSTP(46)) whereby
you can select only the s-channel Higgs graph; this will then be essentially equivalent
to running subprocess 5 or 8 with the proper decay channels (i.e. Z0 Z0 or W+ W− ) set
via MDME. The difference is that the Breit–Wigner distributions in subprocesses 5 and 8
contain a mass-dependent width, whereas the width in subprocesses 71–77 is calculated
at the nominal Higgs mass; also, higher-order corrections to the widths are treated moreaccurately in subprocesses 5 and 8. Further, processes 71–77 assume the incoming W/Z
to be on the mass shell, with associated kinematics factors, while processes 5 and 8 have
W/Z correctly space-like. All this leads to differences in the cross sections by up to afactor of 1.5.
In the absence of a Higgs, the sector of longitudinal Z and W scattering will become
strongly interacting at energies above 1 TeV. The models proposed by Dobado, Herrero
and Terron [Dob91] to describe this kind of physics have been included as alternative
matrix elements for subprocesses 71, 72, 73, 76 and 77, selectable by MSTP(46). Fromthe point of view of the general classification scheme for subprocesses, this model should
appropriately be included as separate subprocesses with numbers above 100, but thecurrent solution allows a more efficient reuse of existing code. By a proper choice ofparameters, it is also here possible to simulate the production of a techni-ρ (see section
8.6.7).
Currently, the scattering of transverse gauge bosons has not been included, neither thatof mixed transverse–longitudinal scatterings. These are expected to be less important athigh energies, and do not contain an h0 resonance peak, but need not be entirely negligible
in magnitude. As a rule of thumb, processes 71–77 should not be used for V V invariant
ISUB =

150

masses below 500 GeV.
The decay products of the longitudinal gauge bosons are correctly distributed in angle.
8.5.3

Extended neutral Higgs sector

MSEL = 19
ISUB = h0 H0 A03 151 156 fi f i → X
102 152 157 gg → X
103 153 158 γγ → X
111 183 188 qq → gX
112 184 189 qg → qX
113 185 190 gg → gX
24 171 176 fi f i → Z0 X
26 172 177 fi f j → W+ X
123 173 178 fi fj → fi fj X (ZZ fusion)
124 174 179 fi fj → fk fl X (W+ W− fusion)
121 181 186 gg → Qk Qk X
122 182 187 qi qi → Qk Qk X
In Pythia, the particle content of a two-Higgs-doublet scenario is included: twoneutral scalar particles, 25 and 35, one pseudoscalar one, 36, and a charged doublet, ±37.
(Of course, these particles may also be associated with corresponding Higgs states inlarger multiplets.) By convention, we choose to call the lighter scalar Higgs h0 and theheavier H0 . The pseudoscalar is called A0 and the charged H± . Charged-Higgs production
is covered in section 8.5.4.
A number of h0 processes have been duplicated for H0 and A0 . The correspondence
between ISUB numbers is shown in the table above: the first column of ISUB numbers
corresponds to X = h0 , the second to X = H0 , and the third to X = A0 . Note that severalof these processes are not expected to take place at all, owing to vanishing Born term
couplings. We have still included them for flexibility in simulating arbitrary couplingsat the Born or loop level, or for the case of mixing between the scalar and pseudoscalar
sectors.
A few Standard Model Higgs processes have no correspondence in the scheme above.
These include• 5 and 8, which anyway have been superseded by 123 and 124;
• 71, 72, 73, 76 and 77, which deal with what happens if there is no light Higgs, andso is a scenario complementary to the one above, where several light Higgs bosons
are assumed; and• 110, which is mainly of interest in Standard Model Higgs searches.
The processes 102–103, 111–113, 152–153, 157–158, 183–185 and 188–190 have only
been worked out in full detail for the Standard Model Higgs case, and not when other(e.g. squark loop) contributions need be considered. For processes 102–103, 152–153, and
157–158, the same approximation mainly holds true for the decays, since these production
processes are proportional to the partial decay width for the gg and γγ channels. The γγchannel does include H+ in the loop. In some corners of SUSY parameter space, the effects
of squarks and gauginos in loops can be relevant. The approximate procedure outlined
in section 8.5.1, based on combining the kinematics shape from simple expressions in
the mt → ∞ limit with a normalization derived from the gg → X cross section, should
therefore be viewed as a first ansatz only. In particular, it is not recommended to try thenon-default MSTP(38) = 0 option, which is incorrect beyond the Standard Model.
In processes 121, 122, 181, 182, 186 and 187 the recoiling heavy flavour is assumed to
be top, which is the only one of interest in the Standard Model, and the one where the151parton-distribution-function approach invoked in processes 3, 151 and 156 is least reliable.
However, it is possible to change the quark flavour in 121 etc.; for each process ISUB thisflavour is given by KFPR(ISUB,2). This may become relevant if couplings to bb states areenhanced, e.g. if tan β À 1 in the MSSM. The matrix elements in this group are basedon scalar Higgs couplings; differences for a pseudoscalar Higgs remains to be worked out,
but are proportional to the heavy quark mass relative to other kinematic quantities.
By default, the h0 has the couplings of the Standard Model Higgs, while the H0
and A0 have couplings set in PARU(171) - PARU(178) and PARU(181) - PARU(190),
respectively. The default values for the H0 and A0 have no deep physics motivation,
but are set just so that the program will not crash due to the absence of any couplings
whatsoever. You should therefore set the above couplings to your desired values if you
want to simulate either H0 or A0 . Also the couplings of the h0 particle can be modified,in PARU(161) - PARU(165), provided that MSTP(4) = 1.
For MSTP(4) = 2, the mass of the h0 (PMAS(25,1)) and the tan β value (PARU(141))
are used to derive the masses of the other Higgs bosons, as well as all Higgs couplings.
PMAS(35,1) - PMAS(37,1) and PARU(161) - PARU(195) are overwritten accordingly.
The relations used are the ones of the Born-level MSSM [Gun90].
Note that not all combinations of mh and tan β are allowed; for MSTP(4) = 2 therequirement of a finite A0 mass imposes the constraint
mh < m Z

tan2 β − 1,tan2 β + 1

or, equivalently,(138)

mZ + mh.
(139)
mZ − mh
If this condition is not fulfilled, the program will print a diagnostic message and stop.
A more realistic approach to the Higgs mass spectrum is to include radiative corrections to the Higgs potential. Such a machinery has never been implemented as such in
Pythia, but appears as part of the Supersymmetry framework described in sections 8.7
and 9.5. At tree level, the minimal set of inputs would be IMSS(1) = 1 to switch on
SUSY, RMSS(5) to set the tan β value (this overwrites the PARU(141) value when SUSY
is switched on) and RMSS(19) to set A0 mass. However, the significant radiative corrections depend on the properties of all particles that couple to the Higgs boson, and theuser may want to change the default values of the relevant RMSS inputs. In practice, themost important are those related indirectly to the physical masses of the third generation
supersymmetric quarks and the Higgsino: RMSS(10) to set the left-handed doublet SUSY
mass parameter, RMSS(11) to set the right stop mass parameter, RMSS(12) to set the rightsbottom mass parameter, RMSS(4) to set the Higgsino mass and a portion of the squark
mixing, and RMSS(16) and RMSS(17) to set the stop and bottom trilinear couplings, respectively, which specifies the remainder of the squark mixing. From these inputs, theHiggs masses and couplings would be derived. Note that switching on SUSY also implies that Supersymmetric decays of the Higgs particles become possible if kinematicallyallowed. If you do not want this to happen, you may want to increase the SUSY mass
parameters. (Use CALL PYSTAT(2) after initialization to see the list of branching ratios.)
Pair production of Higgs states may be a relevant source, see section 8.5.5 below.
Finally, heavier Higgs bosons may decay into lighter ones, if kinematically allowed, inprocesses like A0 → Z0 h0 or H+ → W+ h0 . Such modes are included as part of the general
mixture of decay channels, but they can be enhanced if the uninteresting channels are
switched off.
tan2 β >152

8.5.4Charged Higgs sector

MSEL = 23
ISUB = 143 fi f j → H+
161 fi g → fk H+
401 gg → tbH+
402 qq → tbH+
A charged Higgs doublet, H± , is included in the program. This doublet may be theone predicted in the MSSM scenario, see section 8.5.3, or in any other scenario. The tan βparameter, which is relevant also for neutral Higgs couplings, is set via PARU(141) or, if
SUSY is switched on, via RMSS(5).
The basic subprocess for charged Higgs production in hadron colliders is ISUB = 143.
However, this process is dominated by tb → H+ , and so depends on the choice of t parton
distribution, if non-vanishing. A better representation is provided by subprocess 161,
fg → f 0 H+ ; i.e. actually bg → tH+ . It is therefore recommended to use 161 and not 143;
to use both would be double-counting.
A further step is to include the initial state gluon splitting g → bb as part of the matrix
element, as in process 401. Using both 161 and 401 again would involve doublecounting,but now the issue is more complicated, since 401 may be expected to give the better
description at large p⊥b and 161 the better at small p⊥b , so some matching may be thebest solution [Bor99]. Process 402 gives a less important contribution, that can be added
without doublecounting. (The situation is similar to that for processes 3, 32, 121 and 122
for neutral Higgs production.)
A major potential source of charged Higgs production is top decay. It is possible toswitch on the decay channel t → bH+ . Top will then decay to H+ a fraction of the time,
whichever way it is produced. The branching ratio is automatically calculated, based on
the tan β value and masses. It is possible to only have the H+ decay mode switched on,in which case the cross section is reduced accordingly.
Pair production of the charged Higgs is also possible through its electromagnetic
charge; see section 8.5.5 below.
8.5.5

Higgs pairs

ISUB = (141) fi f i → γ/Z0 /Z00
297 fi f j → H± h0
298 fi f j → H± H0
299 fi f i → Ah0
300 fi f i → AH0
301 fi f i → H+ H−
The subprocesses 297–301 give the production of a pair of Higgs bosons via the schannel exchange of a γ ∗ /Z0 or a W± state.
Note that Higgs pair production is possible alternatively through subprocess 141, aspart of the decay of a generic combination of γ ∗ /Z0 /Z00 . Thus it can be used to simulate
Z0 → h0 A0 and Z0 → H0 A0 for associated neutral Higgs production. The fact that wehere make use of the Z00 can easily be discounted, either by letting the relevant couplings
vanish, or by the option MSTP(44) = 4.
Similarly the decay γ ∗ /Z0 /Z00 → H+ H− allows the production of a pair of charged
Higgs particles. This process is especially important in e+ e− colliders. The coupling ofthe γ ∗ to H+ H− is determined by the charge alone (neglecting loop effects), while the Z0
coupling is regulated by PARU(142), and that of the Z00 by PARU(143). The Z00 piece canbe switched off, e.g. by MSTP(44) = 4. An ordinary Z0 , i.e. particle code 23, cannot bemade to decay into a Higgs pair, however.

153

The advantage of the explicit pair production processes is the correct implementationof the pair threshold.

8.6

Non-Standard Physics

Many extensions of the Standard Model have been proposed, and likely this represents
only a small number of what is possible. Despite different underlying assumptions, many
Beyond-the-Standard-Model scenarios predict some common interactions or particles. For
instance, new W0 and Z0 gauge bosons can arise in a number of different ways. Therefore
it makes sense to cover a few basic classes of particles and interactions, with enough generality that many kinds of detailed scenarios can be accommodated by suitable parameterchoices. We have already seen one example of this, in the extended Higgs sector above.
In this section a few other kinds of non-standard generic physics scenarios are discussed.
Supersymmetry is covered separately in the following section, since it is such a large sector
by itself.
8.6.1

Fourth-generation fermions

MSEL = 7, 8, 37, 38
ISUB =
1 fi f i → γ ∗ /Z0
2 fi f j → W +81 qi qi → Qk Qk
82 gg → Qk Qk
83 qi fj → Qk fl
84 gγ → Qk Qk
85 γγ → Fk Fk
141 fi f i → γ/Z0 /Z00
142 fi f j → W0+
While the existence of a fourth generation currently seems unlikely, the appropriate
flavour content is still found in the program. In fact, the fourth generation is included onan equal basis with the first three, provided MSTP(1) = 4. Also processes other than theones above can therefore be used, e.g. all other processes with gauge bosons, including
non-standard ones such as the Z00 . We therefore do not repeat the descriptions foundelsewhere, such as how to set only the desired flavour in processes 81–85. Note that it
may be convenient to set CKIN(1) and other cuts such that the mass of produced gaugebosons is enough for the wanted particle production — in principle the program will copeeven without that, but possibly at the expense of very slow execution.
8.6.2

New gauge bosons

MSEL = 21, 22, 24
ISUB = 141 fi f i → γ/Z0 /Z00
142 fi f j → W0+
144 fi f j → R
The Z00 of subprocess 141 contains the full γ ∗ /Z0 /Z00 interference structure for couplings to fermion pairs. With MSTP(44) it is possible to pick only a subset, e.g. only thepure Z00 piece. The couplings of the Z00 to quarks and leptons in the first generation canbe set via PARU(121) - PARU(128), in the second via PARJ(180) - PARJ(187) and in
the third via PARJ(188) - PARJ(195). The eight numbers correspond to the vector andaxial couplings of down-type quarks, up-type quarks, leptons and neutrinos, respectively.
The default corresponds to the same couplings as that of the Standard Model Z0 , with axial couplings af = ±1 and vector couplings vf = af − 4ef sin2 θW . This implies a resonance154

width that increases linearly with the mass. By a suitable choice of the parameters, it ispossible to simulate just about any imaginable Z00 scenario, with full interference effects incross sections and decay angular distributions and generation-dependent couplings. The
conversion from the coupling conventions in a set of different Z00 models in the literature
to those used in Pythia can be found in [Cio05].
The coupling to the decay channel Z00 → W+ W− is regulated by PARU(129) PARU(130). The former gives the strength of the coupling, which determines the rate. The
default, PARU(129) = 1., corresponds to the ‘extended gauge model’ of [Alt89], wherein
the Z0 → W+ W− coupling is used, scaled down by a factor m2W /m2Z0 , to give a Z00 partial
width into this channel that again increases linearly. If this factor is cancelled, by having
PARU(129) proportional to m2Z0 /m2W , one obtains a partial width that goes like the fifth
power of the Z00 mass, the ‘reference model’ of [Alt89]. In the decay angular distribution
one could imagine a much richer structure than is given by the one parameter PARU(130).
Other decay modes include Z00 → Z0 h0 , predicted in left–right symmetric models (seePARU(145) and ref. [Coc91]), and a number of other Higgs decay channels, see sections
8.5.3 and 8.5.4.
The W0± of subprocess 142 so far does not contain interference with the Standard
Model W± — in practice this should not be a major limitation. The couplings of the W0
to quarks and leptons are set via PARU(131) - PARU(134). Again one may set vector
and axial couplings freely, separately for the qq0 and the `ν` decay channels. The defaults
correspond to the V − A structure of the Standard Model W, but can be changed tosimulate a wide selection of models. One possible limitation is that the same Cabibbo–
Kobayashi–Maskawa quark mixing matrix is assumed as for the standard W.
The coupling W0 → Z0 W can be set via PARU(135) - PARU(136). Further commentson this channel as for Z0 ; in particular, default couplings again agree with the ‘extended
gauge model’ of [Alt89]. A W0 → Wh0 channel is also included, in analogy with the
Z00 → Z0 h0 one, see PARU(146).
The R boson (particle code 41) of subprocess 144 represents one possible scenario[Ben85a] for a horizontal gauge boson, i.e. a gauge boson that couples between the generations, inducing processes like sd → R0 → µ− e+ . Experimental limits on flavour-changing
neutral currents forces such a boson to be fairly heavy.
A further example of new gauge groups follows right after this.
8.6.3

Left–Right Symmetry and Doubly Charged Higgs Bosons

ISUB = 341 `i `j → H±±L342 `i `j → H±±R∓343 `i γ → H±±
L e∓
344 `i γ → H±±R e±± ∓345 `i γ → HL µ∓346 `i γ → H±±R µ±± ∓347 `i γ → HL τ∓
348 `i γ → H±±
R τ
++ −−
349 fi f i → HL HL−−
350 fi f i → H++
R HR
351 fi fj → fk fl H±±
L (WW fusion)
352 fi fj → fk fl H±±
R (WW fusion)
353 fi f i → Z0R±
354 fi f i → WR
At current energies, the world is left-handed, i.e. the Standard Model contains an
SU(2)L group. Left–right symmetry at some larger scale implies the need for an SU(2)R±and rightgroup. Thus the particle content is expanded by right-handed Z0R and WR
155

handed neutrinos. The Higgs fields have to be in a triplet representation, leading todoubly-charged Higgs particles, one set for each of the two SU(2) groups. Also thenumber of neutral and singly-charged Higgs states is increased relative to the Standard
Model, but a search for the lowest-lying states of this kind is no different from e.g. thefreedom already accorded by the MSSM Higgs scenarios.
Pythia implements the scenario of [Hui97]. The expanded particle content withdefault masses is:
KF
name m (GeV)
9900012 νRe
500
9900014 νRµ500
9900016 νRτ
500
9900023 Z0R
1200+
9900024 WR
750
9900041 H++
200
L
9900042 H++
200
R+
The main decay modes implemented are H++→ WL+ WL+ , `+i `j (i, j generation indices)
L
+
+ + +
and H++
R → WR WR , `i `j . The physics parameters of the scenario are found in PARP(181)- PARP(192).
The WR± has been implemented as a simple copy of the ordinary W± , with the exception that it couple to right-handed neutrinos instead of the ordinary left-handed ones.
Thus the standard CKM matrix is used in the quark sector, and the same vector andaxial coupling strengths, leaving only the mass as free parameter. The Z0R implementation (without interference with γ or the ordinary Z0 ) allows decays both to left- andright-handed neutrinos, as well as other fermions, according to one specific model ansatz[Fer00]. Obviously both the WR± and the Z0R descriptions are likely to be simplifications,but provide a starting point.
The right-handed neutrinos can be allowed to decay further [Riz81, Fer00]. Assuming+
them to have a mass below that of WR, they decay to three-body states via a virtual0+WR, νR` → `+ ff and νR` → `− ff 0 , where both choices are allowed owing to the Majorana
character of the neutrinos. If there is a significant mass splitting, also sequential decays
νR` → `± `0 ∓ ν 0 R` are allowed. Currently the decays are isotropic in phase space. If theneutrino masses are close to or above the WR ones, this description has to be substituted
by a sequential decay via a real WR (not implemented, but actually simpler to do thanthe one here).
8.6.4

Leptoquarks

MSEL = 25
ISUB = 145 qi `j → LQ
162 qg → `LQ
163 gg → LQ LQ
164 qi qi → LQ LQ
Several leptoquark production processes are included. Currently only one leptoquark
species has been implemented, as particle 42, denoted LQ . The leptoquark is assumed tocarry specific quark and lepton quantum numbers, by default u quark plus electron. Theseflavour numbers are conserved, i.e. a process such as ue− → LQ → dνe is not allowed.
This may be a bit restrictive, but it represents one of many leptoquark possibilities. The
spin of the leptoquark is assumed to be zero, so its decay is isotropic. Vector leptoquarks
have not yet been implemented.
Although only one leptoquark is implemented, its flavours may be changed arbitrarily
to study the different possibilities. The flavours of the leptoquark are defined by thequark and lepton flavours in the decay mode list. Since only one decay channel is allowed,
this means that the quark flavour is stored in KFDP(MDCY(42,2),1) and the lepton one
156

in KFDP(MDCY(42,2),2). The former must always be a quark, while the latter could
be a lepton or an antilepton; a charge-conjugate partner is automatically defined by the
program. At initialization, the charge is recalculated as a function of the flavours defined;
also the leptoquark name is redefined to be of the type ’LQ (q)(l)’, where actual quark(q) and lepton (l) flavours are displayed.
The LQ → q` vertex contains an undetermined Yukawa coupling strength, which
fixes both the width of the leptoquark and the cross section for many of the production
graphs. This strength may be changed in PARU(151) which corresponds to the k factor of[Hew88], i.e. to λ2 /(4παem ), where λ is the Yukawa coupling strength of [Wud86]. Notethat PARU(151) is thus quadratic in the coupling.
The leptoquark is likely to be fairly long-lived, in which case it has time to fragment
into a mesonic- or baryonic-type state, which would decay later on. This is a bit tedious tohandle; therefore the leptoquark is always assumed to decay before fragmentation. This
simplification should not have a major impact on experimental analyses [Fri97].
Inside the program, the leptoquark is treated as a resonance. Since it carries colour,some extra care is required. In particular, the leptoquark should not be made stableby modifying either MDCY(42,1) or MSTP(41): then the leptoquark would be handed
undecayed to Pythia, which would try to fragment it (as it does with any other coloured
object) unsuccessfully, leading to error messages and a premature conclusion of the run.
8.6.5

Compositeness and anomalous couplings

20 fi f j → γW+
165 fi f i → fk f k (via γ ∗ /Z0 )166 fi f j → fk f l (via W± )381 fi fj → fi fj
382 fi f i → fk f kSome processes are implemented to allow the introduction of anomalous coupling, inaddition to the Standard Model ones. These can be switched on by ITCM(5) ≥ 1; thedefault ITCM(5) = 0 corresponds to the Standard Model behaviour.
In processes 381 and 382, the quark substructure is included in the left–left isoscalarmodel [Eic84, Chi90] for ITCM(5) = 1, with compositeness scale Λ given in RTCM(41)(default 1000 GeV) and sign η of interference term in RTCM(42) (default +1; only otheralternative −1). The above model assumes that only u and d quarks are composite (atleast at the scale studied); with ITCM(5) = 2 compositeness terms are included in theinteractions between all quarks. When ITCM(5) = 0, the two processes are equivalent
with 11 and 12. A consistent set of high-p⊥ jet production processes in compositenessscenarios is thus obtained by combining 381 and 382 with 13, 28, 53 and 68.
The processes 165 and 166 are basically equivalent to 1 and 2, i.e. γ ∗ /Z0 and W± exchange, respectively, but with less detail (no mass-dependent width, etc.). The reason forthis duplication is that the resonance treatment formalism of processes 1 and 2 could noteasily be extended to include other than s-channel graphs. In processes 165 and 166, only
one final-state flavour is generated at the time; this flavour should be set in KFPR(165,1)
and KFPR(166,1), respectively. For process 166 one gives the down-type flavour, and theprogram will associate the up-type flavour of the same generation. Defaults are 11 inboth cases, i.e. e+ e− and e+ νe (e− ν e ) final states. While ITCM(5) = 0 gives the Standard
Model results, ITCM(5) = 1 contains the left–left isoscalar model (which does not affectprocess 166), and ITCM(5) = 3 the helicity-non-conserving model (which affects both)[Eic84, Lan91]. Both models above assume that only u and d quarks are composite; with
ITCM(5) = 2 or 4, respectively, contact terms are included for all quarks in the initialstate. The relevant parameters are RTCM(41) and RTCM(42), as above.
Note that processes 165 and 166 are book-kept as 2 → 2 processes, while 1 and 2 are
2 → 1 ones. This means that the default Q2 scale in parton distributions is p2⊥ for the
ISUB =

157

former and ŝ for the latter. To make contact between the two, it is recommended to set
MSTP(32) = 4, so as to use ŝ as scale also for processes 165 and 166.
In process 20, for Wγ pair production, it is possible to set an anomalous magnetic
moment for the W in RTCM(46) (= η = κ − 1; where κ = 1 is the Standard Model value).
The production process is affected according to the formulae of [Sam91], while W decay
currently remains unaffected. It is necessary to set ITCM(5) = 1 to enable this extension.
8.6.6

Excited fermions

ISUB = 146 eγ → e∗
147 dg → d∗
148 ug → u∗
167 qi qj → qk d∗
168 qi qj → qk u∗
169 qi qi → e± e∗∓
Compositeness scenarios may also give rise to sharp resonances of excited quarks andleptons. An excited copy of the first generation is implemented, consisting of spin 1/2
particles d∗ (code 4000001), u∗ (4000002), e∗ (4000011) and νe∗ (4000012). A treatmentof other generations is not currently possible.
The current implementation contains gauge interaction production by quark–gluon
fusion (processes 147 and 148) or lepton–photon fusion (process 146) and contact interaction production by quark–quark or quark–antiquark scattering (processes 167–169) . The
couplings f , f 0 and fs to the SU(2), U(1) and SU(3) groups are stored in RTCM(43)- RTCM(45), the scale parameter Λ in RTCM(41); you are also expected to change thef ∗ masses in accordance with what is desired — see [Bau90] for details on conventions.
Decay processes are of the types q∗ → qg, q∗ → qγ, q∗ → qZ0 or q∗ → q0 W± , with thelatter three (two) available also for e∗ (νe∗ ). A non-trivial angular dependence is includedin the q∗ decay for processes 146–148, but has not been included for processes 167–169.

158

8.6.7

Technicolor

MSEL = 50, 51
ISUB = 149 gg → ηtc (obsolete)
191 fi f i → ρ0tc (obsolete)
192 fi f j → ρ+tc (obsolete)0
193 fi f i → ωtc(obsolete)
194 fi f i → fk f k
195 fi f j → fk f l
361 fi f i → WL+ WL−∓
362 fi f i → WL± πtc+ −363 fi f i → πtc
πtc
0364 fi f i → γπtc
365 fi f i → γπ 0 0tc
0366 fi f i → Z0 πtc
367 fi f i → Z0 π 0 0tc∓
368 fi f i → W± πtc± 0
370 fi f j → WL ZL
0371 fi f j → WL± πtc± 0
372 fi f j → πtc ZL± 0
373 fi f j → πtc
πtc±
374 fi f j → γπtc±
375 fi f j → Z0 πtc
0376 fi f j → W± πtc± 00
377 fi f j → W π tc
381 qi qj → qi qj
382 qi qi → qk qk
383 qi qi → gg
384 fi g → fi g
385 gg → qk qk
386 gg → gg
387 fi f i → Qk Qk
388 gg → Qk Qk
Technicolor (TC) uses strong dynamics instead of weakly-coupled fundamental scalars
to manifest the Higgs mechanism for giving masses to the W and Z bosons. In TC, thebreaking of a chiral symmetry in a new, strongly interacting gauge theory generates theGoldstone bosons necessary for electroweak symmetry breaking (EWSB). Thus three ofthe technipions assume the rôle of the longitudinal components of the W and Z bosons, butother states can remain as separate particles depending on the gauge group: technipions(πtc ), technirhos (ρtc ), techniomegas (ωtc ), etc.
No fully-realistic model of strong EWSB has been found so far, and some of theassumptions and simplifications used in model-building may need to be discarded in the
future. The processes represented here correspond to several generations of development.
Processes 149, 191, 192 and 193 should be considered obsolete and superseded by theother processes 194, 195 and 361–377. The former processes are kept for cross-checks andbackward compatibility. In section 8.5.2 it is discussed how processes 71–77 can be used
to simulate a scenario with techni-ρ resonances in longitudinal gauge boson scattering.
Process 149 describes the production of a spin–0 techni-η meson (particle code KF =3000331), which is an electroweak singlet and a QCD colour octet. It is one of the possible
techni-π particles; the name ‘techni-η’ is not used universally in the literature. The
techni-η couples to ordinary fermions proportional to fermion mass. The dominant decay
159

mode is therefore tt, if kinematically allowed. An effective gg–coupling arises throughan anomaly, and is roughly comparable in size with that to bb. Techni-η production at
hadron colliders is therefore predominantly through gg fusion, as implemented in process
149. In topcolor-assisted technicolor (discussed below), particles like the techni-η should
not have a predominant coupling to t quarks. In this sense, the process is consideredobsolete.
(The following discussion borrows liberally from the introduction to Ref. [Lan99a] withthe author’s permission.) Modern technicolor models require walking technicolor [Hol81]
to prevent large flavor-changing neutral currents and the assistance of topcolor (TC2)interactions that are strong near 1 TeV [Nam88, Hil95, Lan95] to provide the large massof the top quark. Both additions to the basic technicolor scenario [Wei79, Eic80] tend torequire a large number ND of technifermion doublets to make the β-function of walking
technicolor small. They are needed in TC2 to generate the hard masses of quarks andleptons, to induce the right mixing between heavy and light quarks, and to break topcolor
symmetry down to ordinary colour. A large number of techni-doublets implies a relatively
low √technihadron mass scale [Lan89, Eic96], set by the technipion decay constant FT 'Fπ / ND , where Fπ = 246 GeV.
The model adopted in Pythia is the ‘Technicolor Straw Man Model’ (TCSM)[Lan99a, Lan02a]. The TCSM describes the phenomenology of color-singlet vector andpseudoscalar technimesons and their interactions with SM particles. These technimesons
are expected to be the lowest-lying bound states of the lightest technifermion doublet,(TU , TD ), with components that transform under technicolor SU(NTC ) as fundamentals,
but are QCD singlets; they have electric charges QU and QD = QU − 1. The vector
technimesons form a spin-one isotriplet ρ±,0
tc and an isosinglet ωtc . Since techni-isospin is
likely to be a good approximate symmetry, ρtc and ωtc should be approximately massdegenerate. The pseudoscalars, or technipions, also comprise an isotriplet Π±,0
tc and an
isosinglet Π00.
However,
these
are
not
mass
eigenstates.
In
thismodel,
theyaresimple,tc±
0
two-state mixtures of the longitudinal weak bosons WL , ZL — the true Goldstone bosonsof dynamical electroweak symmetry breaking in the limit that the SU(2)⊗U(1) couplings±
0g, g 0 vanish — and mass-eigenstate pseudo-Goldstone technipions πtc
, πtc
:
0
00
|Πtc i = sin χ |WL i + cos χ |πtc i ; |Π00
tc i = cos χ |πtc i + · · · ,(140)

where sin χ = FT /Fπ ¿ 1, χ0 is another mixing angle and the ellipsis refer to othertechnipions needed to eliminate the TC anomaly from the Π00
tc chiral current. Thesemassive technipions are also expected to be approximately degenerate.
The coupling of technipions to quarks and leptons are induced mainly by extended
technicolor (ETC) interactions [Eic80]. These couplings are proportional to fermion mass,
except for the case of the top quark, which has most of its mass generation through TC2
interactions. The coupling to electroweak gauge boson pairs vanishes at tree-level, andis assumed to be negligible. Thus the ordinary mechanisms for producing Higgs-likebosons through enhanced couplings to heavy fermions or heavy gauge bosons is absent
for technipions. In the following, we will concentrate on how technipions decay once
they are produced. Besides coupling to fermions proportional to mass (except for the00
case of top quarks where the coupling strength should be much less than mt ), the πtc
can decay to gluon or photon pairs through technifermion loops. However, there may be000
mixing [Eic96]. If that happens, the lightest neutral technipions are–πtc
appreciable πtc
ideally-mixed T̄U TU and T̄D TD bound states. To simulate this effect, there are separate
00
partial widths for gg decays. The
factors Cπtc0 →gg and Cπtc00 →gg to weight the πtc and πtc+
0→ tt̄, bb̄, cc̄,relevant technipion decay modes are πtc → tb̄, cb̄, ub̄, cs̄, cd̄ and τ + ντ ; πtc
00→ gg, tt̄, bb̄, cc̄, and τ + τ − . In the numerical evaluation of partialand τ + τ − ; and πtc
widths, the running mass (see PYMRUN) is used, and all fermion pairs are considered as+
final states. The decay πtc→ W+ bb̄ is also included, with the final-state kinematics
160

distributed according to phase space (i.e. not weighted by the squared matrix element).
The πtc couplings to fermions can be weighted by parameters Cc , Cb , Ct and Cτ dependingon the heaviest quark involved in the decay.
The technivector mesons have direct couplings to the technipion interaction states. In
the limit of vanishing gauge couplings g, g 0 = 0, the ρtc and ωtc coupling to technipionsare:ρtc → Πtc Πtc = cos2 χ (πtc πtc ) + 2 sin χ cos χ (WL πtc ) + sin2 χ (WL WL ) ;ωtc → Πtc Πtc Πtc = cos3 χ (πtc πtc πtc ) + · · · .
(141)The ρtc → πtc πtc decay amplitude, then, is given simply byM(ρtc (q) → πA (p1 )πB (p2 )) = gρtc CAB ²(q) · (p1 − p2 ) ,(142)

where the technirho coupling αρtc ≡ gρ2tc /4π = 2.91(3/NT C ) is scaled naı̈vely from QCD(NT C = 4 by default) and CAB = cos2 χ for πtc πtc , sin χ cos χ for πtc WL , and sin2 χ for
WL WL . While the technirho couples to WL WL , the coupling is suppressed. Technivector
production will be addressed shortly; here, we concentrate on technivector decays.
Walking technicolor enhancements of technipion masses are assumed to close off thechannel ωtc → πtc πtc πtc (which is not included) and to kinematically suppress the channels
ρtc → πtc πtc and the isospin-violating ωtc → πtc πtc (which are allowed with appropriate
choices of mass parameters). The rates for the isospin-violating decays ωtc → πA+ πB− =∓+ −WL+ WL− , WL± πtc, πtc
πtc are given by Γ(ωtc → πA+ πB− ) = |²ρω |2 Γ(ρ0tc → πA+ πB− ) where ²ρω isthe isospin-violating ρtc -ωtc mixing. Based on analogy with QCD, mixing of about 5%
is expected. Additionally, this decay mode is dynamically suppressed, but it is included
as a possibility. While a light technirho can decay to WL πtc or WL WL through TC
0
dynamics, a light techniomega decays mainly through electroweak dynamics, ωtc → γπtc
,
2
0 0
± ∓
Z πtc , W πtc , etc., where Z and W may are transversely polarized. Since sin χ ¿ 1, theelectroweak decays of ρtc to the transverse gauge bosons γ, W, Z plus a technipion may
be competitive with the open-channel strong decays.
Note, the exact meaning of longitudinal or transverse polarizations only makes sense athigh energies, where the Goldstone equivalence theorem can be applied. At the moderateenergies considered in the TCSM, the decay products of the W and Z bosons are distributed according to phase space, regardless of their designation as longitudinal WL /ZL
or ordinary transverse gauge bosons.
To calculate the rates for transverse gauge boson decay, an effective Lagrangian fortechnivector interactions was constructed [Lan99a], exploiting gauge invariance, chiralsymmetry, and angular momentum and parity conservation. As an example, the lowest00
dimensional operator mediating the decay ωtc (q) → γ(p1 )πtc
(p2 )) is (e/MV ) Fρtc · Feγ πtc,
where the mass parameter MV is expected to be of order several 100 GeV. This leads tothe decay amplitude:
0(p2 )) =M(ωtc (q) → γ(p1 )πtc

e cos χ µνλρ²²µ (q)²∗ν (p1 )qλ p1ρ .
MV

(143)

Similar expressions exist for the other amplitudes involving different technivectors and/or
different gauge bosons [Lan99a], where the couplings are derived in the valence technifermion approximation [Eic96, Lan99]. In a similar fashion, decays to fermionantifermion pairs are included. These partial widths are typically small, but can haveimportant phenomenological consequences, such as narrow lepton-antilepton resonances
produced with electroweak strength.
Next, we address the issue of techniparticle production. Final states containing Standard Model particles and/or pseudo-Goldstone bosons (technipions) can be produced atcolliders through two mechanisms: technirho and techniomega mixing with gauge bosons
161through a vector-dominance mechanism, and anomalies [Lan02] involving technifermionsin loops. Processes 191, 192 and 193 are based on s-channel production of the respective
resonance [Eic96] in the narrow width approximation. All decay modes implemented canbe simulated separately or in combination, in the standard fashion. These include pairsof fermions, of gauge bosons, of technipions, and of mixtures of gauge bosons and technipions. Processes 194, 195 and 361–377, instead, include interference, a correct treatmentof kinematic thresholds and the anomaly contribution, all of which can be important effects, but also are limited to specific final states. Therefore, several processes need to be
simulated at once to determine the full effect of TC.
Process 194 is intended to more accurately represent the mixing between the γ ∗ , Z0 , ρ0tc
0particles in the Drell–Yan process [Lan99]. Process 195 is the analogous charged
and ωtc
channel process including W± and ρ±
tc mixing. By default, the final-state fermions aree+ e− and e± νe , respectively. These can be changed through the parameters KFPR(194,1)
and KFPR(195,1), respectively (where the KFPR value should represent a charged fermion).
Processes 361–368 describe the pair production of technipions and gauge bosons
0through ρ0tc /ωtc
resonances and anomaly contributions. Processes 370–377 describe pair
production through the ρ±
tc resonance and anomalies. It is important to note that processes 361, 362, 370, 371, 372 include final states with only longitudinally-polarized
W and Z bosons, whereas the others include final states with only transverse W and Z
bosons. Again, all processes must be simulated to get the full effect of the TC model
under investigation. All processes 361–377 are obtained by setting MSEL = 50.
The vector dominance mechanism is implemented using the full γ–Z0 –ρtc –ωtc propagator matrix,∆0 (s), including the effects of kinetic mixing. With the notation M2V =√MV2 − i s ΓV (s) and ΓV (s) the energy-dependent width for V = Z0 , ρtc , ωtc , this matrix
is the inverse of




s
0
sfγρtcsfγωtc
2


0
s
−
M
sf
sf
Zρ
Zωtc


tc
Z
∆−1
(s)
=

.
2
0
sf
sf
s
−
M
0
0


γρtc
Zρtc
ρtc
sfγωtc sfZωtc
0s − M2ωtc(144)

The parameters fγρtc =qξ, fγωtc = ξ (QU + QD ), fZρtc = ξ cot 2θW , and fZωtc = −ξ (QU +QD ) tan θW , and ξ = α/αρtc determine the strength of the kinetic mixing, and are
fixed by the quantum numbers of the technifermions in the theory. Because of the offdiagonal entries, the propagators resonate at mass values shifted from the nominal MV
values. Thus, while users input the technihadron masses using PMAS values, these will
not represent exactly the resulting mass spectrum of pair-produced particles. Note thatspecial care is taken in the limit of very heavy technivectors to reproduce the canonical+ −γ ∗ /Z∗ → πtc
πtc couplings. In a similar fashion, cross sections for charged final statesrequire the W± –ρ±
tc matrix ∆± :
Ã∆−1
± (s) =s − M2W sfWρtc
sfWρtc s − M2ρ±

!

,

(145)

tc

where fWρtc = ξ/(2 sin θW ).
By default, the TCSM Model has the parameters NT C = 4, sin χ = 13 , QU = 43 , QD =QU − 1 = 13 , Cb = Cc = Cτ = 1, Ct = mb /mt , Cπtc = 43 , Cπtc0 →gg =0, Cπtc00 →gg =1, |²ρω | =0.05, FT = Fπ sin χ = 82 GeV, Mρ±tc = Mρ0tc = Mωtc = 210 GeV, Mπtc± = Mπtc0 = Mπtc00= 110 GeV, MV = MA = 200 GeV. The techniparticle mass parameters are set throughthe usual PMAS array. Parameters regulating production and decay rates are stored in
the RTCM array in PYTCSM. This concludes the discussion of the electroweak sector of the
strawman model.
162

In the original TCSM outlined above, the existence of top-color interactions only
affected the coupling of technipions to top quarks, which is a significant effect only forhigher masses. In general, however, TC2 requires some new and possibly light coloured
particles. In most TC2 models, the existence of a large tt̄, but not bb̄, condensate and massis due to SU(3)1 ⊗ U(1)1 gauge interactions which are strong near 1 TeV. The SU(3)1
interaction is t–b symmetric while U(1)1 couplings are t–b asymmetric. There are weaker
SU(3)2 ⊗ U(1)2 gauge interactions in which light quarks (and leptons) may [Hil95], or
may not [Chi96], participate. The two U(1)’s must be broken to weak hypercharge U(1)Yat an energy somewhat higher than 1 TeV by electroweak-singlet condensates. The full
phenomenology of even such a simple model can be quite complicated, and many (possibly
unrealistic) simplifications are made to reduce the number of free parameters [Lan02a].
Nonetheless, it is useful to have some benchmark to guide experimental searches.
The two TC2 SU(3)’s can be broken to their diagonal SU(3) subgroup by using
technicolor and U(1)1 interactions, both strong near 1 TeV. This can be explicitly accomplished [Lan95] using two electroweak doublets of technifermions, T1 = (U1 , D1 )and T2 = (U2 , D2 ), which transform respectively as (3, 1, NT C ) and (1, 3, NT C ) under
the two colour groups and technicolor. The desired pattern of symmetry breaking
occurs if SU(NTC ) and U(1)1 interactions work together to induce electroweak andSU(3)1 ⊗ SU(3)2 non-invariant condensates h ŪiL UjR i and h D̄iL DjR i, (i, j = 1, 2) . This
minimal TC2 scenario leads to a rich spectrum of colour-nonsinglet states readily accessible in hadron collisions. The lowest-lying ones include eight ‘colorons’, V8 , the massive
gauge bosons of broken topcolor SU(3); four isosinglet ρtc8 formed from T̄i Tj and theisosinglet pseudo-Goldstone technipions formed from T̄2 T2 . In this treatment, the isovector technipions are ignored, because they must be pair produced in ρtc8 decays, and suchdecays are assumed to be kinematically suppressed.
The colorons are new fundamental particles with couplings to quarks. In standard
TC2 [Hil95], top and bottom quarks couple to SU(3)1 and the four light quarks to
SU(3)2 . Because the SU(3)1 interaction is strong and acts exclusively on the third
generation, the residual V8 coupling can be enhanced for t and b quarks. The couplingga = gc cot θ3 for t and b and ga = −gc tan θ3 for u, d, c, s, where gc is the QCD coupling
and cot θ3 is related to the original g1 and g2 couplings. In flavor-universal TC2 [Chi96]
all quarks couple to SU(3)1 , not SU(3)2 , so that colorons couple equally and strongly to
all flavors: ga = gc cot θ3 .
Assuming that techni-isospin is not badly broken by ETC interactions, the ρtc8 areAAA
isosinglets labeled by the technifermion content and colour index A: ρA
11 , ρ22 , ρ12 , ρ120 . The
first two of these states, ρ11 and ρ22 , mix with V8 and g. The topcolor-breaking condensate,hT̄1L T2R i 6= 0, causes them to also mix with ρ12 and ρ120 . Technifermion condensation
also leads to a number of (pseudo)Goldstone boson technipions. The lightest technipionsA0
are expected to be the isosinglet SU(3) octet and singlet T̄2 T2 states π22
and π22.
These technipions can decay into either fermion-antifermion pairs or two gluons;presently, they are assumed to decay only into gluons. As noted, walking technicolor
enhancement of technipion masses very likely close off the ρtc8 → πtc πtc channels. Thenthe ρtc8 decay into qq̄ and gg. The rate for the former are proportional to the amount of0,Akinetic mixing, set by ξg = gc /gρtc . Additionally, the ρ22 decays to gπ22.
The V8 colorons are expected to be considerably heavier than the ρtc8 , with mass in
the range 0.5–1 TeV. In both the standard and flavor-universal models, colorons couple
strongly to T̄1 T1 , but with only strength gc to T̄2 T2 . Since relatively light technipions areT̄2 T2 states, it is estimated that Γ(V8 → πtc πtc ) = O(αc ) and Γ(V8 → gπtc ) = O(αc2 ).
Therefore, these decay modes are ignored, so that the V8 decay rate is the sum over open
channels ofÃ!
´12m2a ³αa1+
s − 4m2a 2 ,Γ(V8 → qa q̄a ) =
(146)
6
s

163

where αa = ga2 /4π.
The phenomenological effect of this techniparticle structure is to modify the gluon
propagator in ordinary QCD processes, because of mixing between the gluon, V8 and theρtc8 ’s. The g–V8 –ρ11 –ρ22 –ρ12 –ρ120 propagator is the inverse of the symmetric matrix


s



 0




 s ξg−1
D (s) = 

 sξ
g




 0



0

0

s ξg

s ξg

0

0

s − M2V8s ξρ11s ξρ22s ξρ12s ξρ11s − M211

2−M11,222−M11,12s ξρ222−M11,22s − M222

2−M22,12s ξρ122−M11,122−M22,12s − M212s ξρ1202
2
2−M22,12−M12,12s − M2120−M11,12
0
0
0










2
−M11,12
0 

.

2
−M22,120 




2
−M12,120 s ξρ120(147)√
Here, M2V = MV2 −i s ΓV (s) uses the energy-dependent widths of the octet vector bosons,and the ξρij are proportional to ξg and elements of matrices that describe the pattern of2
technifermion condensation. The mixing terms Mij,kl, induced by T̄1 T2 condensation are
assumed to be real.
This extension of the TCSM is still under development, and any results should be
carefully scrutinized. The main effects are indirect, in that they modify the underlying
two-parton QCD processes much like compositeness terms, except that a resonant structure is visible. Similar to compositeness, the effects of these colored technihadrons are
simulated by setting ITCM(5) = 5 for processes 381–388. By default, these processes areequivalent to the 11, 12, 13, 28, 53, 68, 81 and 82 ones, respectively. The last two are
specific for heavy-flavour production, while the first six could be used to describe standard or non-standard high-p⊥ jet production. These six are simulated by MSEL = 51.
The parameter dependence of the ‘model’ is encoded in tan θ3 (RTCM(21)) and a mass
parameter M8 (RTCM(27)), which determines the decay width ρ22 → gπ22 analogously
to MV for ωtc → γπtc . For ITCM(2) equal to 0 (1), the standard (flavor universal) TC2
couplings are used. The mass parameters are set by the PMAS array using the codes: V818(3100021), π22(3100111), π22(3200111), ρ11 (3100113), ρ12 (3200113), ρ21 (3300113), andρ22 (3400113). The mixing parameters Mij,kl take on the (arbitrary) values M11,22 = 100
GeV, M11,12 = M11,21 = M22,12 = 150 GeV, M22,21 = 75 GeV and M12,21 = 50 GeV, while
the kinetic mixing terms ξρij are calculated assuming the technicolor condensates are fully√
mixed, i.e. hTi T̄j i ∝ 1/ 2.
8.6.8

Extra Dimensions

ISUB = 391 ff → G∗
392 gg → G∗
393 qq → gG∗
394 qg → qG∗
395 gg → gG∗
In recent years, the area of observable consequences of extra dimensions has attracted
a strong interest. The field is still in rapid development, so there still does not exist a‘standard phenomenology’. The topic is also fairly new in Pythia, and currently only afirst scenario is available.
The G∗ , introduced as new particle code 5000039, is intended to represent the lowest
excited graviton state in a Randall-Sundrum scenario [Ran99] of extra dimensions. The
lowest-order production processes by fermion or gluon fusion are found in 391 and 392.
164

The further processes 393–395 are intended for the high-p⊥ tail in hadron colliders. As
usual, it would be double-counting to have both sets of processes switched on at the sametime. Processes 391 and 392, with initial-state showers switched on, are appropriate forthe full cross section at all p⊥ values, and gives a reasonable description also of the high-p⊥
tail. Processes 393–395 could be useful e.g. for the study of invisible decays of the G∗ ,where a large p⊥ imbalance would be required. It also serves to test/confirm the shower
expectations of different p⊥ spectra for different production processes [Bij01].
Decay channels of the G∗ to ff, gg, γγ, Z0 Z0 and W+ W− contribute to the total width.
The correct angular distributions are included for decays to a fermion, g or γ pair in thelowest-order processes, whereas other decays currently are taken to be isotropic.
The G∗ mass is to be considered a free parameter. The other degree of freedom in this
scenario is a dimensionless coupling; see PARP(50).

8.7Supersymmetry

MSEL = 39–45
ISUB = 201–296 (see tables at the beginning of this chapter)
Pythia includes the possibility of simulating a large variety of production and decay
processes in the Minimal Supersymmetric extension of the Standard Model (MSSM). The
simulation is based on an effective Lagrangian of softly-broken SUSY with parameters
defined at the weak scale, typically between mZ and 1 TeV. The relevant parameters
should either be supplied directly by the user or they should be read in from a SUSY Les
Houches Accord (SLHA) spectrum file [Ska03] (see below). Some other possibilities for
obtaining the SUSY parameters also exist in the code, as described below, but these are
only intended for backwards compatibility and debugging purposes.
8.7.1

General Introduction

In any (N = 1) supersymmetric version of the SM there exists a partner to each SM
state with the same gauge quantum numbers but whose spin differs by one half unit.
Additionally, the dual requirements of generating masses for up- and down-type fermions
while preserving SUSY and gauge invariance, require that the SM Higgs sector be enlarged
to two scalar doublets, with corresponding spin-partners.
After Electroweak symmetry breaking (EWSB), the bosonic Higgs sector contains aquintet of physical states: two CP-even scalars, h0 and H0 , one CP-odd pseudoscalar, A0 ,and a pair of charged scalar Higgs bosons, H± (naturally, this classification is only correctwhen CP violation is absent in the Higgs sector. Non-trivial phases between certain softbreaking parameters will induce mixing between the CP eigenstates). The fermionic Higgs(called ‘Higgsino’) sector is constituted by the superpartners of these fields, but these are
not normally exact mass eigenstates, so we temporarily postpone the discussion of them.
In the gauge sector, the spin-1/2 partners of the U(1)Y and SU(2)L gauge bosons
e the neutral Wino, W
f , and the charged Winos, W
f(called ‘gauginos’) are the Bino, B,
3
1
f , while the partner of the gluon is the gluino, g̃. After EWSB, the Be and W
f
and W
2
3
e
e
mix with the neutral Higgsinos, H1 , H2 , to form four neutral Majorana fermion masse ± , mix with the
eigenstates, the neutralinos, χe01−4 . In addition, the charged Higgsinos, H
f and Wf , resulting in two charged Dirac fermion mass eigenstates, thecharged Winos, W
12
charginos, χe±
1,2 . Note that the γ̃ and Z̃, which sometimes occur in the literature, are linear
e and Wf , by exact analogy with the mixing giving the γ and Z0 , but
combinations of the B
3these are not normally mass eigenstates after EWSB, due to the enlarged mixing causedby the presence of the Higgsinos.
The spin-0 partners of the SM fermions (so-called ‘scalar fermions’, or ‘sfermions’) are˜ and sneutrinos ν̃. Each fermion (except possibly the neutrinos)
the squarks q̃, sleptons `,165

has two scalar partners, one associated with each of its chirality states. These are named
left-handed and right-handed sfermions, respectively. Due to their scalar nature, it is ofcourse impossible for these particles to possess any intrinsic ‘handedness’ themselves, butthey inherit their couplings to the gauge sector from their SM partners, so that e.g. a d̃Rdoes not couple to SU(2)L while a d̃L does.
Generically, the KF code numbering scheme used in Pythia reflects the relationship between particle and sparticle, so that e.g. for sfermions, the left-handed (right-handed) superpartners have codes 1000000 (2000000) plus the code of the corresponding SM fermion.
A complete list of the particle partners and their KF codes is given in Table 11. Note that,
antiparticles of scalar particles are denoted by ∗ , i.e. t̃∗ . A gravitino is also included
with KF=1000039. The gravitino is only relevant in Pythia when simulating models ofgauge-mediated SUSY breaking, where the gravitino becomes the lightest superpartner.
In practice, the gravitino simulated here is the spin- 12 Goldstino components of the spin- 32
gravitino.
The MSSM Lagrangian contains interactions between particles and sparticles, withcouplings fixed by SUSY. There are also a number of soft SUSY-breaking mass parameters.
‘Soft’ here means that they break the mass degeneracy between SM particles and theirSUSY partners without reintroducing quadratic divergences in the theory or destroying
its gauge invariance. In the MSSM, the soft SUSY-breaking parameters are extra massterms for gauginos and sfermions and trilinear scalar couplings. Further soft terms may
arise, for instance in models with broken R-parity, but we here restrict our attention to
the minimal case (for RPV in Pythia see section 8.7.7).
The exact number of independent parameters depends on the detailed mechanismof SUSY breaking. The general MSSM model in Pythia assumes only a few relations
between these parameters which seem theoretically difficult to avoid. Thus, the first twogenerations of sfermions with otherwise similar quantum numbers, e.g. d̃L and s̃L , have thesame masses. Despite such simplifications, there are a fairly large number of parametersthat appear in the SUSY Lagrangian and determine the physical masses and interactions
with Standard Model particles, though far less than the > 100 which are allowed in all
generality. The Lagrangian (and, hence, Feynman rules) follows the conventions set down
by Kane and Haber in their Physics Report article [Hab85] and the papers of Gunion and
Haber [Gun86a]. Once the parameters of the softly-broken SUSY Lagrangian are specified,the interactions are fixed, and the sparticle masses can be calculated. Note that, whenusing SUSY Les Houches Accord input, Pythia automatically translates between the
SLHA conventions and the above, with no action required on the part of the user.
8.7.2

Extended Higgs Sector

Pythia already simulates a Two Higgs Doublet Model (2HDM) obeying tree-level relations fixed by two parameters, which can be conveniently taken as the ratio of doublet
vacuum expectation values tan β, and the pseudoscalar mass MA (as noted earlier, forthe non-SUSY implementation of a 2HDM, the input parameters are Mh and tan β). The
Higgs particles are considered Standard Model fields, since a 2HDM is a straightforward
extension of the Standard Model. The MSSM Higgs sector is more complicated than that
described above in section 8.5, and includes important radiative corrections to the treelevel relations. The CP-even Higgs mixing angle α is shifted as well as the full Higgs mass
spectrum. The properties of the radiatively-corrected Higgs sector in Pythia are derived
in the effective potential approach [Car95]. The effective potential contains an all-orders
resummation of the most important radiative corrections, but makes approximations to
the virtuality of internal propagators. This is to be contrasted with the diagrammatic
technique, which performs a fixed-order calculation without approximating propagators.
In practice, both techniques can be systematically corrected for their respective approximations, so that there is good agreement between their predictions, though sometimes the166agreement occurs for slightly different values of SUSY-breaking parameters. The calculation of the Higgs spectrum in Pythia is based on the FORTRAN code SubHpole [Car95],
which is also used in HDecay [Djo97], except that certain corrections that are particularly
important at large values of tan β are included rigorously in Pythia.
There are several notable properties of the MSSM Higgs sector. As long as the soft
SUSY-breaking parameters are less than about 1.5 TeV, a number which represents a fair,albeit subjective, limit for where the required degree of fine-tuning of MSSM parameters
becomes unacceptably large, there is an upper bound of about 135 GeV on the mass of
the CP-even Higgs boson most like the Standard Model one, i.e. the one with the largest
couplings to the W and Z bosons, be it the h or H. If it is h that is the SM-like Higgs
boson, then H can be significantly heavier. On the other hand, if H is the SM-like Higgs
boson, then h must be even lighter. If all SUSY particles are heavy, but MA is small, thenthe low-energy theory would look like a two-Higgs-doublet model. For sufficiently large
MA , the heavy Higgs doublet decouples, and the effective low-energy theory has only one
light Higgs doublet with SM-like couplings to gauge bosons and fermions.
The Standard Model fermion masses are not fixed by SUSY, but their Yukawa couplings become a function of tan β. For the up- and down-quark and leptons, mu =hu v sin β, md = hd v cos β, and m` = h` v cos β, where hf =u,d,` is the corresponding Yukawa
coupling and v ≈ 246 GeV is the order parameter of Electroweak symmetry breaking.
At large tan β, significant corrections can occur to these relations. These are included
for the b quark, which appears to have the most sensitivity to them, and the t quark inthe subroutine PYPOLE, based on an updated version of SubHpole, which also includes
some bug fixes, so that it is generally better behaved. The array values RMSS(40) andRMSS(41) are used for temporary storage of the corrections ∆mt and ∆mb .
The input parameters that determine the MSSM Higgs sector in Pythia are RMSS(5)(tan β), RMSS(19) (MA ), RMSS(10-12) (the third generation squark mass parameters),
RMSS(15-16) (the third generation squark trilinear couplings), and RMSS(4) (the Higgsino
mass µ). Additionally, the large tan β corrections related to the b Yukawa coupling dependon RMSS(3) (the gluino mass). Of course, these calculations also depend on SM parameters(mt , mZ , αs , etc.). Any modifications to these quantities from virtual MSSM effects are
not taken into account. In principle, the sparticle masses also acquire loop corrections
that depend on all MSSM masses.
If IMSS(4) = 0, an approximate version of the effective potential calculation can beused. It is not as accurate as that available for IMSS(4) = 1, but it useful for demonstrating the effects of higher orders. Alternatively, for IMSS(4) = 2, the physical Higgs
masses are set by their PMAS values while the CP-even Higgs boson mixing angle α is set
by RMSS(18). These values and tan β (RMSS(5)) are enough to determine the couplings,
provided that the same tree-level relations are used.
See section 8.7.5 for a description how to use the loop-improved RGE’s of Isasusy todetermine the SUSY mass and mixing spectrum (including also loop corrections to the
Higgs mass spectrum and couplings) with Pythia.
Finally, a run-time interface to FeynHiggs [Hei99], for the diagrammatic calculationof the h0 , H0 , A0 , and H+ masses and the mixing angle α in the MSSM, has been introduced, available through the option IMSS(4) = 3. For the time being, it can be invokedeither when using an SLHA SUSY spectrum, i.e. for IMSS(1) = 11, or when using the
run-time interface to Isasusy, i.e. for IMSS(1) = 12 or 13. The interface calls three
FeynHiggs routines, in the following order:• FHSETFLAGS(IERR,4,0,0,2,0,2,1,1) : these are the ‘default’ settings recommended for FeynHiggs [Hei99].
• FHSETPARA : to set the MSSM parameters.
• FHHIGGSCORR : to get the corrected Higgs parameters.
Note that, for Pythia to compile properly without the FeynHiggs library, three dummy
routines have been added to the Pythia source code, corresponding to the three listed167

above. To obtain proper linking with FeynHiggs, these dummy routines should firstbe removed/renamed and the Pythia source recompiled without them. The interface
has been tested to work with FeynHiggs-2.2.8. Differences in the FeynHiggs andSubHpole predictions represent, to some degree, the theoretical uncertainty in the MSSM
Higgs sector.
8.7.3

Superpartners of Gauge and Higgs Bosons

The chargino and neutralino masses and their mixing angles (that is, their gaugino and
Higgsino composition) are determined by the SM gauge boson masses (MW and MZ ),tan β, two soft SUSY-breaking parameters (the SU(2)L gaugino mass M2 and the U(1)Ygaugino mass M1 ), together with the Higgsino mass parameter µ, all evaluated at theelectroweak scale ∼ MZ . Pythia assumes that the input parameters are evaluated atthe ‘correct’ scale. Obviously, more care is needed to set precise experimental limits or to
make a connection to higher-order calculations.
Explicit solutions of the chargino and neutralino masses and their mixing angles (whichappear in Feynman rules) are found by diagonalizing the 2 × 2 chargino MC and 4 × 4
neutralino MN mass matrices:
Ã
!
Ã
!
√
M
Z
M
2M
sβ
i
2
W
MC = √; MN =(148)
ZT M µ2MW cβ
µ
µ

Mi =

M1 0
0 M2

¶

µ

; Mµ =

0 −µ
−µ 0

¶

µ

;Z =

−MZ cβsW MZ sβsW
MZ cβcW −MZ sβcW¶

f +, He + ) basis, M in the (B,e W
f 3, He ,He ) basis, with the notation
MC is written in the (WN12sβ = sin β, cβ = cos β, sW = sin θW and cW = cos θW . Different sign conventions andbases sometimes appear in the literature. In particular, Pythia agrees with the Isasusy[Bae93] convention for µ, but uses a different basis of fields and has different-looking
mixing matrices.
In general, the soft SUSY-breaking parameters can be complex valued, resulting in
CP violation in some sector of the theory, but more directly expanding the possible
masses and mixings of sparticles. Presently, the consequences of arbitrary phases are only
considered in the chargino and neutralino sector, though it is well known that they canhave a significant impact on the Higgs sector. A generalization of the Higgs sector isamong the plans for the future development of the program. The chargino and neutralino
input parameters are RMSS(5) (tan β), RMSS(1) (the modulus of M1 ) and RMSS(30) (thephase of M1 ), RMSS(2) and RMSS(31) (the modulus and phase of M2 ), and RMSS(4) andRMSS(33) (the modulus and phase of µ). To simulate the case of real parameters (whichis CP-conserving), the phases are zeroed by default. In addition, the moduli parameters
can be signed, to make a simpler connection to the CP-conserving case. (For example,
RMSS(5) = -100.0 and RMSS(30) = 0.0 represents µ = −100 GeV.)
The expressions for the production cross sections and decay widths of neutralino andchargino pairs contain the phase dependence, but ignore possible effects of the phasesin the sfermion masses appearing in propagators. The production cross sections havebeen updated to include the dependence on beam polarization through the parameters
PARJ(131,132) (see Sect. 8.8). There are several approximations made for three-bodydecays. The numerical expressions for three-body decay widths ignore the effects of finite
fermion masses in the matrix element, but include them in the phase space. No three−00body decays χ0i → tt̄χ0j are simulated, nor χ+i (χi ) → tb̄χj (χj ). Finally, the effects ofmixing between the third generation interaction and mass eigenstates for sfermions isignored, except that the physical sfermion masses are used. The kinematic distributionsof the decay products are spin-averaged, but include the correct matrix-element weighting.

168

Note that for the R-parity-violating decays (see below), both sfermion mixing effects andmasses of b, t, and τ are fully included.
In some corners of SUSY parameter space, special decay modes must be implemented
to capture important phenomenology. Three different cases are distinguished here: (1)In the most common models of Supergravity-mediated SUSY breaking, small values ofM1 , M2 , µ and tan β can lead to a neutralino spectrum with small mass splittings. For
this case, the radiative decays χ0i → χ0j γ can be relevant, which are the SUSY analoge → γe γ, for example.
of h → γγ with two particles switched to superpartners to yield h
These decays are calculated approximately for all neutralinos when tan β ≤ 2, or the
decay χ02 → χ01 γ can be forced using IMSS(10) = 1; (2) In models of gauge-mediatede is light and phenomenologically relevant at
SUSY breaking (GMSB), the gravitino G
colliders. For IMSS(11) = 1, the two-body decays of sparticle to particle plus gravitino
e with V = γ, Z
are allowed. The most relevant of these decay modes are likely χ01 → VG,
or a Higgs boson; (3) In models of anomaly-mediated SUSY breaking (AMSB), the winomass parameter M2 is much smaller than M1 or µ. As a result, the lightest chargino andneutralino are almost degenerate in mass. At tree level, it can be shown analytically thatthe chargino should be heavier than the neutralino, but this is hard to achieve numerically.
Furthermore, for this case, radiative corrections are important in increasing the mass
splitting more. Currently, if ever the neutralino is heavier than the chargino when solving
the eigenvalue problem numerically, the chargino mass is set to the neutralino mass plus
± e0
2 times the charged pion mass, thus allowing the decay χe±
1 → π χ1.
Since the SU(3)C symmetry of the SM is not broken, the gluinos have masses determined by the SU(3)C gaugino mass parameter M3 , input through the parameter RMSS(3).
The physical gluino mass is shifted from the value of the gluino mass parameter M3 because of radiative corrections. As a result, there is an indirect dependence on the squark
masses. Nonetheless, it is sometimes convenient to input the physical gluino mass, assuming that there is some choice of M3 which would be shifted to this value. This canbe accomplished through the input parameter IMSS(3). A phase for the gluino mass canbe set using RMSS(32), and this can influence the gluino decay width (but no effect isincluded in the g̃ + χe production). Three-body decays of the gluino to tt̄ and bb̄ andtb̄ plus the appropriate neutralino or chargino are allowed and include the full effects ofsfermion mixing. However, they do not include the effects of phases arising from complex
neutralino or chargino parameters.
8.7.4

Superpartners of Standard Model Fermions

The mass eigenstates of squarks and sleptons are, in principle, mixtures of their left- andright-handed components, given by:
Mf2˜L = m22 + m2f + Df˜L

Mf2˜R = m21 + m2f + Df˜R

(149)

where m2 are soft SUSY-breaking parameters for superpartners of SU(2)L doublets, andm1 are parameters for singlets. The D-terms associated with Electroweak symmetry
breaking are Df˜L = MZ2 cos(2β)(T3f − Qf sin2 θW ) and Df˜R = MZ2 cos(2β)Qf sin2 θW ,where T3f is the weak isospin eigenvalue (= ±1/2) of the fermion and Qf is the electriccharge. Taking the D-terms into account, one easily sees that the masses of sfermions in
SU(2)L doublets are related by a sum rule: Mf2˜ ,T3 =1/2 − Mf2˜ ,T3 =−1/2 = MZ2 cos(2β).
L
L
In many high-energy models, the soft SUSY-breaking sfermion mass parameters aretaken to be equal at the high-energy scale, but, in principle, they can be different for eachgeneration or even within a generation. However, the sfermion flavor dependence can haveimportant effects on low-energy observables, and it is often strongly constrained. The
suppression of flavor changing neutral currents (FCNC’s), such as KL → π ◦ ν ν̄, requiresthat either (i) the squark soft SUSY-breaking mass matrix is diagonal and degenerate,169

or (ii) the masses of the first- and second-generation sfermions are very large. Thus we
make the data-motivated simplification of setting MũL = Mc̃L , Md̃L = Ms̃L , MũR = Mc̃R ,Md̃R = Ms̃R .
The left–right sfermion mixing is determined by the product of soft SUSY-breaking
parameters and the mass of the corresponding fermion. Unless the soft SUSY-breaking
parameters for the first two generations are orders of magnitude greater than for the third
generation, the mixing in the first two generations can be neglected. This simplifying
assumption is also made in Pythia: the sfermions q̃L,R , with q̃ = ũ, d̃, c̃, s̃, and `˜L,R , ν̃` ,with ` = e, µ, are the real mass eigenstates with masses mq̃L,R and m`˜L,R , mν̃` , respectively.
For the third generation sfermions, due to weaker experimental constraints, the left–rightmixing can be nontrivial. The tree-level mass matrix for the top squarks (stops) in the(t̃L , t̃R ) basis is given byÃ

Mt̃2 =

m2Q3 + m2t + Dt̃L mt (At − µ/ tan β)mt (At − µ/ tan β) m2U3 + m2t + Dt̃R

!

,

(150)

where At is a trilinear coupling. Different sign conventions for At occur in the literature;
Pythia and Isasusy use opposite signs. Unless there is a cancellation between At andµ/ tan β, left–right mixing occurs for the stop squarks because of the large top quark
mass. The stop mass eigenstates are then given byt̃1 = cos θt̃ t̃L + sin θt̃ t̃Rt̃2 = − sin θt̃ t̃L + cos θt̃ t̃R ,(151)

where the masses and mixing angle θt̃ are fixed by diagonalizing the squared-mass matrix
Eq. (150). Note that different conventions exist also for the mixing angle θt̃ , and that
Pythia here agrees with Isasusy. When translating Feynman rules from the (L,R) to(1,2) basis, we use:t̃L = cos θt̃ t̃1 − sin θt̃ t̃2t̃R = sin θt̃ t̃1 + cos θt̃ t̃2 .

(152)

Because of the large mixing, the lightest stop t̃1 can be one of the lightest sparticles.
For the sbottom, an analogous formula for the mass matrix holds with mU3 → mD3 ,At → Ab , Dt̃L,R → Db̃L,R , mt → mb , and tan β → 1/ tan β. For the stau, the substitutions
mQ3 → mL3 , mU3 → mE3 , At → Aτ , Dt̃L,R → Dτ̃L,R , mt → mτ and tan β → 1/tan β areappropriate. The parameters At , Ab , and Aτ can be independent, or they might berelated by some underlying principle. When mb tan β or mτ tan β is large (O(mt )), left–right mixing can also become relevant for the sbottom and stau.
Most of the SUSY input parameters are needed to specify the properties of thesfermions. As mentioned earlier, the effects of mixing between the interaction and mass
eigenstates are assumed negligible for the first two generations. Furthermore, sleptons
and squarks are treated slightly differently. The physical slepton masses `˜L and `˜R are set
by RMSS(6) and RMSS(7). By default, the τ̃ mixing is set by the parameters RMSS(13),RMSS(14) and RMSS(17), which represent ML3 , ME3 and Aτ , respectively, i.e. neither Dterms nor mτ is included. However, for IMSS(8) = 1, the τ̃ masses will follow the same
pattern as for the first two generations. Previously, it was assumed that the soft SUSYbreaking parameters associated with the stau included D-terms. This is no longer thecase, and is more consistent with the treatment of the stop and sbottom. For the first twogenerations of squarks, the parameters RMSS(8) and RMSS(9) are the mass parameters
m2 and m1 , i.e. without D-terms included. For more generality, the choice IMSS(9) = 1
means that m1 for ũR is set instead by RMSS(22), while m1 for d̃R is RMSS(9). Note that170

the left-handed squark mass parameters must have the same value since they reside in
the same SU(2)L doublet. For the third generation, the parameters RMSS(10), RMSS(11),RMSS(12), RMSS(15) and RMSS(16) represent MQ3 , MD3 , MU3 , Ab and At , respectively.
There is added flexibility in the treatment of stops, sbottoms and staus. With the flag
IMSS(5) = 1, the properties of the third generation sparticles can be specified by theirmixing angle and mass eigenvalues (instead of being derived from the soft SUSY-breaking
parameters). The parameters RMSS(26) - RMSS(28) specify the mixing angle (in radians)
for the sbottom, stop, and stau. The parameters RMSS(10) - RMSS(14) specify the twostop masses, the one sbottom mass (the other being fixed by the other parameters) andthe two stau masses. Note that the masses RMSS(10) and RMSS(13) correspond to theleft-left entries of the diagonalized matrices, while RMSS(11), RMSS(12) and RMSS(14)correspond to the right-right entries. These entries need not be ordered in mass.
8.7.5

ModelsAt present, the exact mechanism of SUSY breaking is unknown. It is generally assumed
that the breaking occurs spontaneously in a set of fields that are almost entirely disconnected from the fields of the MSSM; if SUSY is broken explicitly in the MSSM, thensome superpartners must be lighter than the corresponding Standard Model particle, aphenomenological disaster. The breaking of SUSY in this ‘hidden sector’ is then communicated to the MSSM fields through one or several mechanisms: gravitational interactions,gauge interactions, anomalies, etc. While any one of these may dominate, it is also possiblethat all contribute at once.
We may parametrize our ignorance of the exact mechanism of SUSY breaking by simply setting each of the soft SUSY breaking parameters in the Lagrangian by hand. In
Pythia this approach can be effected by setting IMSS(1) = 1, although some simplifications have already been made to greatly reduce the number of parameters from the initial
more than 100.
As to specific models, several exist which predict the rich set of measurable mass andmixing parameters from the assumed soft SUSY breaking scenario with a much smaller
set of free parameters. One example is Supergravity (Sugra) inspired models, where
the number of free parameters is reduced by imposing universality at some high scale,motivated by the apparent unification of gauge couplings. Five parameters fixed at thegauge coupling unification scale, tan β, M0 , m1/2 , A0 , and sign(µ), are then related to themass parameters at the scale of Electroweak symmetry breaking by renormalization groupequations (see e.g. [Pie97]).
The user who wants to study this and other models in detail can use spectrum calculation programs (e.g. Isasusy [Bae93], Softsusy [All02], SPheno [Por03], or Suspect[Djo02]), which numerically solve the renormalization group equations (RGE) to determine the mass and mixing parameters at the weak scale. These may then be input toPythia via a SLHA spectrum file [Ska03] using IMSS(1) = 11 and IMSS(21) equal tothe unit number where the spectrum file has been opened. All of Pythia’s own internalmSUGRA machinery (see below) is then switched off. This means that none of the other
IMSS switches can be used, except for IMSS(51:53) (R-parity violation), IMSS(10) (forceχ̃2 → χ˜1 γ), and IMSS(11) (gravitino is the LSP). Note that the dependence of the b andt quark Yukawa couplings on tan β and the gluino mass is at present ignored when using
IMSS(1) = 11.
As an alternative, a run-time interface to Isasusy can be accessed by the options
IMSS(1) = 12 and IMSS(1) = 13, in which case the SUGRA routine of Isasusy is calledby PYINIT. This routine then calculates the spectrum of SUSY masses and mixings (CPconservation, i.e. real-valued parameters, is assumed) and passes the information run-timerather than in a file.
For IMSS(1) = 12, only the mSUGRA model of Isasusy can be accessed. The
171

mSUGRA model input parameters should then be given in RMSS as for IMSS(1) = 2, i.e.:
RMSS(1)= M1/2 , RMSS(4) = sign(µ), RMSS(5)= tan β, RMSS(8)= M0 , and RMSS(16)= A0 .
For IMSS(1) = 13, the full range of Isasusy models can be interfaced, but the input
parameters must then be given in the form of an Isajet input file which Pythia reads
during initialization and passes to Isasusy. The contents of the input file should be
identical to what would normally be typed when using the Isajet RGE executable standalone (normally isasugra.x). The input file should be opened by the user in his/her main
program and the Logical Unit Number should be stored in IMSS(20), where Pythia will
look for it during initialization.
The routine PYSUGI handles the conversion between the conventions of Pythia andIsasusy, so that conventions are self-consistent inside Pythia. In the call to PYSUGI,
the RMSS array is filled with the values produced by Isasusy as for IMSS(1) = 1. In
particular, this means that when using the IMSS(1) = 12 option, the mSUGRA input
parameters mentioned above will be overwritten during initialization. Cross sections anddecay widths are then calculated by Pythia. Note that, since Pythia cannot always
be expected to be linked with Isajet, two dummy routines and a dummy function havebeen added to the Pythia source. These are SUBROUTINE SUGRA, SUBROUTINE SSMSSM
and FUNCTION VISAJE. These must first be given other names and Pythia recompiled
before proper linking with Isajet can be achieved.
A problem is that the size of some Isasusy common blocks has been expanded in more
recent versions. Corresponding changes have been implemented in the PYSUGI interface
routine. Currently Pythia is matched to Isajet 7.71, and thus assumes the SSPAR,SUGPAS, SUGMG and SUGXIN common blocks to have the forms:
COMMON/SSPAR/AMGLSS,AMULSS,AMURSS,AMDLSS,AMDRSS,AMSLSS&,AMSRSS,AMCLSS,AMCRSS,AMBLSS,AMBRSS,AMB1SS,AMB2SS&,AMTLSS,AMTRSS,AMT1SS,AMT2SS,AMELSS,AMERSS,AMMLSS,AMMRSS&,AMLLSS,AMLRSS,AML1SS,AML2SS,AMN1SS,AMN2SS,AMN3SS&,TWOM1,RV2V1,AMZ1SS,AMZ2SS,AMZ3SS,AMZ4SS,ZMIXSS(4,4)&,AMW1SS,AMW2SS&,GAMMAL,GAMMAR,AMHL,AMHH,AMHA,AMHC,ALFAH,AAT,THETAT&,AAB,THETAB,AAL,THETAL,AMGVSS,MTQ,MBQ,MLQ,FBMA,&VUQ,VDQREAL AMGLSS,AMULSS,AMURSS,AMDLSS,AMDRSS,AMSLSS&,AMSRSS,AMCLSS,AMCRSS,AMBLSS,AMBRSS,AMB1SS,AMB2SS&,AMTLSS,AMTRSS,AMT1SS,AMT2SS,AMELSS,AMERSS,AMMLSS,AMMRSS&,AMLLSS,AMLRSS,AML1SS,AML2SS,AMN1SS,AMN2SS,AMN3SS&,TWOM1,RV2V1,AMZ1SS,AMZ2SS,AMZ3SS,AMZ4SS,ZMIXSS&,AMW1SS,AMW2SS&,GAMMAL,GAMMAR,AMHL,AMHH,AMHA,AMHC,ALFAH,AAT,THETAT&,AAB,THETAB,AAL,THETAL,AMGVSS,MTQ,MBQ,MLQ,FBMA,VUQ,VDQCOMMON /SUGPAS/ XTANB,MSUSY,AMT,MGUT,MU,G2,GP,V,VP,XW,&A1MZ,A2MZ,ASMZ,FTAMZ,FBMZ,B,SIN2B,FTMT,G3MT,VEV,HIGFRZ,&FNMZ,AMNRMJ,NOGOOD,IAL3UN,ITACHY,MHPNEG,ASM3,&VUMT,VDMT,ASMTP,ASMSS,M3Q
REAL XTANB,MSUSY,AMT,MGUT,MU,G2,GP,V,VP,XW,&A1MZ,A2MZ,ASMZ,FTAMZ,FBMZ,B,SIN2B,FTMT,G3MT,VEV,HIGFRZ,&FNMZ,AMNRMJ,ASM3,VUMT,VDMT,ASMTP,ASMSS,M3Q
INTEGER NOGOOD,IAL3UN,ITACHY,MHPNEGCOMMON /SUGMG/ MSS(32),GSS(31),MGUTSS,GGUTSS,AGUTSS,FTGUT,&FBGUT,FTAGUT,FNGUTREAL MSS,GSS,MGUTSS,GGUTSS,AGUTSS,FTGUT,FBGUT,FTAGUT,FNGUTCOMMON /SUGXIN/ XISAIN(24),XSUGIN(7),XGMIN(14),XNRIN(4),&XAMIN(7)172

REAL XISAIN,XSUGIN,XGMIN,XNRIN,XAMIN
Isasusy users are warned to check that no incompatibilities arise between the versions
actually used. Unfortunately there is no universal solution to this problem: the Fortranstandard does not allow you dynamically to vary the size of a (named) common block.
So if you use an earlier Isasusy version, you have to shrink the size accordingly, and fora later you may have to check that the above common blocks have not been expanded
further.
As a cross check, the option IMSS(1) = 2 uses approximate analytical solutions of therenormalization group equations [Dre95], which reproduce the output of Isasusy within' 10% (based on comparisons of masses, decay widths, production cross sections, etc.).
This option is intended for debugging only, and does not represent the state-of-the-art.
In Sugra and in other models with the SUSY breaking scale of order MGUT , the spin–e (code 1000039), has a mass of order
3/2 superpartner of the graviton, the gravitino G
MW and interacts only gravitationally. In models of gauge-mediated SUSY breaking[Din96], however, the gravitino can play a crucial role in the phenomenology, and can be
the lightest superpartner (LSP). Typically, sfermions decay to fermions and gravitinos,
and neutralinos, chargino, and gauginos decay to gauge or Higgs bosons and gravitinos.
Depending on the gravitino mass, the decay lengths can be substantial on the scale ofcolliders. Pythia correctly handles finite decay lengths for all sparticles.
R-parity is a possible symmetry of the SUSY Lagrangian that prevents problemsof rapid proton decay and allows for a viable dark matter candidate. However, it isalso possible to allow a restricted amount of R-parity violation. At present, there is no
theoretical consensus that R-parity should be conserved, even in string models. In the
production of superpartners, Pythia assumes R-parity conservation (at least on the timeand distance scale of a typical collider experiment), and only lowest order, sparticle pair
production processes are included. Only those processes with e+ e− , µ+ µ− , or quark andgluon initial states are simulated. Tables 20, 21 and 22 list available SUSY processes. In
processes 210 and 213, `˜ refers to both ẽ and µ̃. For ease of readability, we have removed
the subscript L on ν̃. t̃i t̃∗i , τ̃i τ̃j∗ and τ̃i ν̃τ∗ production correctly account for sfermion mixing.
Several processes are conspicuously absent from the table. For example, processes 255
and 257 would simulate the associated production of right-handed squarks with charginos.
Since the right-handed squark only couples to the higgsino component of the chargino, theinteraction strength is proportional to the quark mass, so these processes can be ignored.
By default, only R-parity conserving decays are allowed, so that one sparticle is stable,
either the lightest neutralino, the gravitino, or a sneutrino. SUSY decays of the top quark
are included, but all other SM particle decays are unaltered.
Generally, the decays of the superpartners are calculated using the formulae ofrefs. [Gun88, Bar86a, Bar86b, Bar95]. All decays are spin averaged. Decays involvingb̃ and t̃ use the formulae of [Bar95], so they are valid for large values of tan β. The one
loop decays χ̃j → χ̃i γ and t̃ → cχ̃1 are also included, but only with approximate formula.
Typically, these decays are only important when other decays are not allowed because of
mixing effects or phase space considerations.
One difference between the SUSY simulation and the other parts of the program isthat it is not beforehand known which sparticles may be stable. Normally this would
mean either the χ̃01 or the gravitino G̃, but in principle also other sparticles could bestable. The ones found to be stable have their MWID(KC) and MDCY(KC,1) values set zeroat initialization. If several PYINIT calls are made in the same run, with different SUSY
parameters, the ones set zero above are not necessarily set back to nonzero values, sincemost original values are not saved anywhere. As an exception to this rule, the PYMSINSUSY initialization routine, called by PYINIT, does save and restore the MWID(KC) andMDCY(KC,1) values of the lightest SUSY particle. It is therefore possible to combine
several PYINIT calls in a single run, provided that only the lightest SUSY particle is173stable. If this is not the case, MWID(KC) and MDCY(KC,1) values may have to be reset byhand, or else some particles that ought to decay will not do that.
8.7.6

SUSY examples

The SUSY routines and common-block variables are described in section 9.5. To illustrate
the usage of the switches and parameters, we give six simple examples.
Example 1: Light Stop
The first example is an MSSM model with a light neutralino χ̃1 and a light stop t̃1 , sothat t → t̃1 χ̃1 can occur. The input parameters are
IMSS(1) = 1, RMSS(1) = 70., RMSS(2) = 70., RMSS(3) = 225., RMSS(4) = -40.,
RMSS(5) = 1.5, RMSS(6) = 100., RMSS(7) = 125., RMSS(8) = 250.,
RMSS(9) = 250., RMSS(10) = 1500., RMSS(11) = 1500., RMSS(12) = -128.,
RMSS(13) = 100., RMSS(14) = 125., RMSS(15) = 800., RMSS(16) = 800.,
RMSS(17) = 0., and RMSS(19) = 400.0.
The top mass is fixed at 175 GeV, PMAS(6,1) = 175.0. The resulting model has Mt̃1 = 55
GeV and Mχ̃1 = 38 GeV. IMSS(1) = 1 turns on the MSSM simulation. By default, there
are no intrinsic relations between the gaugino masses, so M1 = 70 GeV, M2 = 70 GeV,
and M3 = 225 GeV. The pole mass of the gluino is slightly higher than the parameter
M3 , and the decay g̃ → t̃∗1 t + t̃1 t occurs almost 100% of the time.
Example 2: SUSY Les Houches Accord spectrum
The second example shows how to input a spectrum file in the SUSY Les Houches Accord
format [Ska03] to Pythia. First, you should set IMSS(1) = 11 and open the spectrum
file you want to use on some unused Logical Unit Number. Then, set IMSS(21) equal
to that number, to tell Pythia where to read the spectrum file from. This should bedone somewhere in your main program before calling PYINIT. During the call to PYINIT,
Pythia will read the spectrum file, perform a number of consistency checks and issue
warning messages if it finds something it does not understand or which seems inconsistent.
E.g. BLOCK GAUGE will normally be present in the spectrum file, but since Pythia currently cannot use the information in that block, it will issue a warning that the block willbe ignored. In case a decay table is also desired to be read in, the Logical Unit Number onwhich the decay table is opened should be put in IMSS(22). To avoid inconsistencies, thespectrum and the decay table should normally go together, so IMSS(22) should normallybe equal to IMSS(21).
Example 3: Calling Isasusy 7.71 at runtime using IMSS(1) = 12
The third example shows how to use the built-in run-time interface to Isasusy with the
IMSS(1) = 12 option. First, the Pythia source code needs to be changed. Rename thefunction VISAJE to, for example, FDUMMY, rename the subroutines SUGRA and SSMSSM toe.g. SDUMM1 and SDUMM2, and recompile. In the calling program, set IMSS(1) = 12 and the
RMSS input parameters exactly as in example 5, and compile the executable while linked
to both Isajet and the modified Pythia. The resulting mass and mixing spectrum isprinted in the Pythia output.
Example 4: Calling Isasusy 7.71 at runtime using IMSS(1) = 13
The fourth example shows how to use the built-in run-time interface to Isasusy with
the IMSS(1) = 13 option. First, the Pythia source code needs to be changed, cf. theprevious example. In the calling program, set IMSS(1) = 13 and open an Isajet SUSY
model input file on any available Logical Unit Number. The contents of the file should be
exactly identical to what would normally be typed when using the Isajet RGE executable
stand-alone (normally isasugra.x). Then, store that Unit Number in IMSS(20), thatwill enable Pythia to access the correct file during initialization. Compile the executable
while linked to both Isajet and the modified Pythia. The resulting mass and mixing
spectrum is printed in the Pythia output.
Example 5: Approximate SUGRA
174

This example shows you how to get a (very) approximate SUGRA model. Note that thisway of obtaining the SUSY spectrum should never be used for serious studies. The input
parameters are
IMSS(1) = 2, RMSS(1) = 200., RMSS(4) = 1., RMSS(5) = 10., RMSS(8) = 800., andRMSS(16) = 0.0.
The resulting model has Md̃L = 901 GeV, MũR = 890 GeV, Mt̃1 = 538 GeV, MẽL = 814
GeV, Mg̃ = 560 GeV, Mχ̃1 = 80 GeV, Mχ̃±1 = 151 GeV, Mh = 110 GeV, and MA = 883
GeV. It corresponds to the choice M0 =800 GeV, M1/2 =200 GeV, tan β = 10, A0 = 0, andsign(µ)> 0. The output is similar to an Isasusy run, but there is not exact agreement.
Example 6: Isasusy 7.71 Model
The final example demonstrates how to convert the output of an Isasusy run directly
into the Pythia format, i.e. if SLHA output is not available. This assumes that you
already made an Isasusy run, e.g. with the equivalents of the input parameters above.
From the output of this run you can now extract those physical parameters that need tobe handed to Pythia, in the above example
IMSS(1) = 1, IMSS(3) = 1, IMSS(8) = 0, IMSS(9) = 1, RMSS(1) = 79.61,
RMSS(2) = 155.51, RMSS(3) = 533.1, RMSS(4) = 241.30, RMSS(5) = 10.,
RMSS(6) = 808.0, RMSS(7) = 802.8, RMSS(8) = 878.4, RMSS(9) = 877.1,
RMSS(10) = 743.81, RMSS(11) = 871.26, RMSS(12) = 569.87, RMSS(13) = 803.20,
RMSS(14) = 794.71,
RMSS(15) = -554.96,
RMSS(16) = -383.23,
RMSS(17) =-126.11,
RMSS(19) = 829.94 and RMSS(22) = 878.5.
8.7.7

R-Parity Violation

R-parity, defined as R = (−1)2S+3B+L , is a discrete multiplicative symmetry where S
is the particle spin, B is the baryon number, and L is the lepton number. All SM
particles have R = 1, while all superpartners have R = −1, so a single SUSY particle
cannot decay into just SM particles if R-parity is conserved. In this case, the lightest
superpartner (LSP) is absolutely stable. Astrophysical considerations imply that a stable
LSP should be electrically neutral. Viable candidates are the lightest neutralino, thelightest sneutrino, or alternatively the gravitino. Since the LSP can carry away energy
without interacting in a detector, the apparent violation of momentum conservation is an
important part of SUSY phenomenology. Also, when R-parity is conserved, superpartners
must be produced in pairs from a SM initial state. The breaking of the R-parity symmetry
would result in lepton- and/or baryon-number-violating processes. While there are strong
experimental constraints on some classes of R-parity-violating interactions, others are
hardly constrained at all.
One simple extension of the MSSM is to break the multiplicative R-parity symmetry.
Presently, neither experiment nor any theoretical argument demand R-parity conservation, so it is natural to consider the most general case of R-parity breaking. It is convenient
to introduce a function of superfields called the superpotential, from which the Feynman
rules for R-parity-violating processes can be derived. The R-parity-violating (RPV) terms
which can contribute to the superpotential are:00

0

WRP V = λijk Li Lj Ē k + λijk Li Qj D̄k + λijk Ū i D̄j D̄k + ²i Li H2(153)

where i, j, k are generation indices (1,2,3), Li1 ≡ νLi , Li2 = `iL and Qi1 = uiL , Qi2 = diL are
lepton and quark components of SU(2)L doublet superfields, and E i = eiR , Di = diR and
U i = uiR are lepton, down- and up-quark SU(2)L singlet superfields, respectively. The
unwritten SU(2)L and SU(3)C indices imply that the first term is antisymmetric underi ↔ j, and the third term is antisymmetricunderj ↔ k. Therefore, i 6= j in Li Lj Ē k and
0
00
i j k
j 6= k in Ū D̄ D̄ . The coefficients λijk , λijk , λijk , and ²i are Yukawa couplings, and there175is no a priori generic prediction for their values. In principle, WRP V contains 48 extra
parameters over the R-parity-conserving MSSM case. In Pythia the effects of the lastterm in eq. (153) are not included.
Expanding eq. (153) as a function of the superfield components, the interaction Lagrangian derived from the first term isno

LLLE = λijk ν̃Li ejL ēkR + ẽiL νLj ēkR + (ẽkR )∗ νLi ejL + h.c.

(154)

and from the second term,n0
LLQD = λijk ν̃Li djL d¯kR − ẽiL ujL d¯kR + d˜jL νLi d¯kR − ũjL eiL d¯kR +(d˜kR )∗ νLi djL − (d˜kR )∗ eiL ujL + h.c.

o

(155)

Both of these sets of interactions violate lepton number. The Ū D̄D̄ term, instead, violates
baryon number. In principle, all types of R-parity-violating terms may co-exist, but thiscan lead to a proton with a lifetime shorter than the present experimental limits. The
simplest way to avoid this is to allow only operators which conserve baryon-number but
violate lepton-number or vice versa.
There are several effects on the SUSY phenomenology due to these new couplings:(1) lepton- or baryon-number-violating processes are allowed, including the productionof single sparticles (instead of pair production), (2) the LSP is no longer stable, but can
decay to SM particles within a collider detector, and (3) because it is unstable, the LSP
need not be the neutralino or sneutrino, but can be charged and/or coloured.
In the current version of Pythia, decays of supersymmetric particles to SM particles
via two different types of lepton-number-violating couplings and one type of baryonnumber-violating couplings can be invoked [Ska01, Sjö03].
Complete matrix elements (including L − R mixing for all sfermion generations) for all
two-body sfermion and three-body neutralino, chargino, and gluino decays are included(as given in [Dre00]). The final-state fermions are treated as massive in the phase space
integrations and in the matrix elements for b, t, and τ .
The existence of R-odd couplings also allows for single sparticle production, i.e. there
is no requirement that SUSY particles should be produced in pairs. Single sparticle
production cross sections are not yet included in the program, and it may require somerethinking of the parton shower to do so. For low-mass sparticles, the associated error
is estimated to be negligible, as long as the R-violating couplings are smaller than thegauge couplings. For higher-mass sparticles, the reduction of the phase space for pair
production becomes an important factor, and single sparticle production could dominate
even for very small values of the R-violating couplings. The total SUSY production crosssections, as calculated by Pythia in its current form are thus underestimated, possiblyquite severely for heavy-mass sparticles.
Three possibilities exist for the initializations of the couplings, representing a fairbut not exhaustive range of models. The first, selected by setting IMSS(51) = 1 for
LLE, IMSS(52) = 1 for LQD, and/or IMSS(53) = 1 for UDD type couplings, sets all
the couplings, independent of generation, to a common value of 10−RMSS(51) , 10−RMSS(52) ,and/or 10−RMSS(53) , depending on which couplings are activated.
Taking now LLE couplings as an example, setting IMSS(51) = 2 causes the LLE
couplings to be initialized (in PYINIT) to so-called ‘natural’ generation-hierarchical values,
as proposed in [Hin93]. These values, inspired by the structure of the Yukawa couplingsin the SM, are defined by:|λijk |2 = (RMSS(51))2 m̂ei m̂ej m̂ek
|λ0ijk |2 = (RMSS(52))2 m̂ei m̂qj m̂dk
|λ00ijk |2 = (RMSS(53))2 m̂qi m̂qj m̂qk
176

;

m̂ ≡

m
m
=
v
126GeV

(156)

where mqi is the arithmetic mean of mui and mdi .
The third option available is to set IMSS(51) = 3, IMSS(52) = 3, and/or IMSS(53)= 3, in which case all the relevant couplings are zero by default (but the corresponding
lepton- or baryon-number-violating processes are turned on) and the user is expected toenter the non-zero coupling values by hand. (Where antisymmetry is required, half of theentries are automatically derived from the other half, see IMSS(51) = 3 and IMSS(53)= 3.) RVLAM(i,j,k) contains the λijk , RVLAMP(i,j,k) contains the λ0ijk couplings, andRVLAMB(i,j,k) contains the λ00ijk couplings.
8.7.8

NMSSM

In the Next-to-Minimal Supersymmetric Standard Model (NMSSM), three new particles
appear: a new CP-even Higgs boson H03 (code 45), a new CP-odd Higgs boson A02 (code46), and an additional neutralino χ05 (code 1000045), where the particle codes are the ones
tentatively adopted by the SUSY Les Houches Accord (SLHA) community [All06].
Pythia does not contain any internal machinery for doing calculations in the NMSSM.
Thus, the basic scattering processes should be generated by an external program (e.g.
CompHEP/CalcHEP [Puk99]) and handed to Pythia via the Les Houches Accord
interface for parton-level events (LHA). This should then be combined with either setting
the NMSSM resonance decays by hand, or by reading in an SLHA decay table preparedby an external decay package (e.g. NMHDecay [Ell05]). One possible chain of steps
for generating fully simulated events for the NMSSM, starting from a high-scale model
definition, would thus be (see e.g. [Puk05]):
1. Obtain the EW scale masses and couplings for the model (e.g. by running NMHDecay), and store the results as a SLHA spectrum file.
2. Compute decay widths and branching ratios for all relevant particles (by hand or
using some code), and store the resulting numbers in the SLHA decay table format.
3. Pass the spectrum to an NMSSM Matrix Element level generator (e.g. CompHEP/CalcHEP), and obtain a set of elementary 2 → 2 (or 2 → ‘a few’) scatterings.
4. Read in the SLHA spectrum and decay table into Pythia using IMSS(1) = 11 andIMSS(13) = 1 (you need to set IMSS(21) and possibly IMSS(22) as well).
5. Read in the ME level events into Pythia using the LHA interface routines UPINIT
and UPEVNT.
8.7.9

Long-lived coloured sparticles

In SUSY scenarios, the coloured sparticles — squarks and gluinos — typically have widthsof several GeV, and thus decay well before they would have had time to hadronize. There
are specific cases in which one of them is more long-lived or even (quasi-)stable, however.
A recent example is the split SUSY scenarios, wherein the gauginos are rather light andthe sfermions very heavy [Ark05]. Then the gluino decay is strongly suppressed, since
it has to go via a virtual squark. In such cases, hadronic states may have time to formaround them. These are called R-hadrons, since they carry negative R-parity.
In the long-lived-gluino case, the set of possible R-hadrons include ‘gluino-balls’ g̃g,‘gluino-mesons’ g̃qq and ‘gluino-baryons’ g̃qqq, in multiplets similarly to normal hadrons,
but of course with differences in the mass spectra and other properties. In the caseof a long-lived squark, such as the stop, there would be ‘squark-mesons’ q̃q and ‘squarkbaryons’ q̃qq. When they pass through a detector, they may undergo charge- and baryonnumber-changing interactions [Kra04], giving rise to quite spectacular and characteristic
experimental signals [Kra04a].
Owing to the somewhat special nature of their production, these R-hadrons do not
fit exactly into the current Pythia event generation chain, although all the separate
177

tools exist. Therefore the production of gluino- and stop-hadrons is simulated by two
separate add-on programs, available on the Pythia webpage, where the calling sequence
is modified as appropriate. In future versions with a modified administrative structure,
the intention is to include this capability in the standard distribution.
Finally, we note that a numbering scheme for R-hadrons is under development, foreventual inclusion in the PDG standard. The abovementioned programs comply withthe draft proposal, but the absence of an approved standard is another reason why the
programs have not yet been fully integrated into Pythia.

8.8

Polarization

In most processes, incoming beams are assumed unpolarized. However, especially for e+ e−
linear collider studies, polarized beams would provide further important information on
many new physics phenomena, and at times help to suppress backgrounds. Therefore a few
process cross sections are now available also for polarized incoming beams. The averagepolarization of the two beams is then set by PARJ(131) and PARJ(132), respectively. In
some cases, noted below, MSTP(50) need also be switched on to access the formulae forpolarized beams.
Process 25, W+ W− pair production, allows polarized incoming lepton beam particles.
The polarization effects are included both in the production matrix elements and in the
angular distribution of the final four fermions. Note that the matrix element used [Mah98]
is for on-shell W production, with a suppression factor added for finite width effects. This
polarized cross section expression, evaluated at vanishing polarization, disagrees withthe standard unpolarized one, which presumably is the more accurate of the two. The
difference can be quite significant below threshold and at very high energies. This canbe traced to the simplified description of off-shell W’s in the polarized formulae. Goodagreement is obtained either by switching off the W width with MSTP(42) = 0 or by
restricting the W mass ranges (with CKIN(41) - CKIN(44)) to be close to on-shell. It istherefore necessary to set MSTP(50) = 1 to switch from the default standard unpolarized
formulae to the polarized ones.
Also many SUSY production processes now include the effects from polarization of theincoming fermion beams. This applies for scalar pair production, with the exception ofsneutrino pair production and h0 A0 and H0 A0 production, this omission being an oversightat the time of this release, but easily remedied in the future.
The effect of polarized photons is included in the process γγ → Fk Fk , process 85. Herethe array values PARJ(131) and PARJ(132) are used to define the average longitudinalpolarization of the two photons.

8.9

Main Processes by Machine

In the previous section we have already commented on which processes have limited
validity, or have different meanings (according to conventional terminology) in different
contexts. Let us just repeat a few of the main points to be remembered for different
machines.
8.9.1

e+ e− collisions

The main annihilation process is number 1, e+ e− → Z0 , where in fact the full γ ∗ /Z0
interference structure is included. This process can be used, with some confidence, for c.m.
energies from about 4 GeV upwards, i.e. at DORIS/CESR/PEP-II/KEKB, PETRA/PEP,
TRISTAN, LEP, and any future linear colliders. (To get below 10 GeV, you have to change
PARP(2), however.) This is the default process obtained when MSEL = 1, i.e. when you
do not change anything yourself.
178

Process 141 contains a Z00 , including full interference with the standard γ ∗ /Z0 . Withthe value MSTP(44) = 4 in fact one is back at the standard γ ∗ /Z0 structure, i.e. the Z00
piece has been switched off. Even so, this process may be useful, since it can simulate
e.g. e+ e− → h0 A0 . Since the h0 may in its turn decay to Z0 Z0 , a decay channel ofthe ordinary Z0 to h0 A0 , although physically correct, would be technically confusing. In
particular, it would be messy to set the original Z0 to decay one way and the subsequentones another. So, in this sense, the Z00 could be used as a copy of the ordinary Z0 , butwith a distinguishable label.
The process e+ e− → Υ does not exist as a separate process in Pythia, but can be
simulated by using PYONIA, see section 6.2.
At LEP 2 and even higher energy machines, the simple s-channel process 1 loses out
to other processes, such as e+ e− → Z0 Z0 and e+ e− → W+ W− , i.e. processes 22 and 25.
The former process in fact includes the structure e+ e− → (γ ∗ /Z0 )(γ ∗ /Z0 ), which meansthat the cross section is singular if either of the two γ ∗ /Z0 masses is allowed to vanish. A
mass cut therefore needs to be introduced, and is actually also used in other processes,
such as e+ e− → W+ W− .
For practical applications, both with respect to cross sections and to event shapes,
it is imperative to include initial-state radiation effects. Therefore MSTP(11) = 1 is thedefault, wherein exponentiated electron-inside-electron distributions are used to give themomentum of the actually interacting electron. By radiative corrections to process 1,
such processes as e+ e− → γZ0 are therefore automatically generated. If process 19 were
to be used at the same time, this would mean that radiation were to be double-counted.
In the alternative MSTP(11) = 0, electrons are assumed to deposit their full energy in thehard process, i.e. initial-state QED radiation is not included. This option is very useful,since it often corresponds to the ‘ideal’ events that one wants to correct back to.
Resolved electrons also means that one may have interactions between photons. This
opens up the whole field of γγ processes, which is described in section 8.3. In particular,
with ’gamma/e+’,’gamma/e-’ as beam and target particles in a PYINIT call, a flux ofphotons of different virtualities is convoluted with a description of direct and resolved
photon interaction processes, including both low-p⊥ and high-p⊥ processes. This machinery is directed to the description of the QCD processes, and does e.g. not address the
production of gauge bosons or other such particles by the interactions of resolved photons. For the latter kind of applications, a simpler description of partons inside photons
inside electrons may be obtained with the MSTP(12) = 1 options and e± as beam andtarget particles.
The thrust of the Pythia programs is towards processes that involve hadron production, one way or another. Because of generalizations from other areas, also a
few completely non-hadronic processes are available. These include Bhabha scattering,e+ e− → e+ e− in process 10, and photon pair production, e+ e− → γγ in process 18. However, note that the precision that could be expected in a Pythia simulation of those
processes is certainly far less than that of dedicated programs. For one thing, electroweak
loop effects are not included. For another, nowhere is the electron mass taken into account,which means that explicit cut-offs at some minimum p⊥ are always necessary.
8.9.2

Lepton–hadron collisions

The main option for photoproduction and Deeply Inelastic Scattering (DIS) physics isprovided by the ’gamma/lepton’ option as beam or target in a PYINIT call, see section
8.3. The Q2 range to be covered, and other kinematics constraints, can be set by CKIN
values. By default, when the whole Q2 range is populated, obviously photoproduction
dominates.
The older DIS process 10, `q → `0 q0 , includes γ 0 /Z0 /W± exchange, with full interference, as described in section 8.3.2. The Z0 /W± contributions are not implemented in the179

’gamma/lepton’ machinery. Therefore process 10 is still the main option for physics atvery high Q2 , but has been superseded for lower Q2 . Radiation off the incoming lepton
leg is included by MSTP(11) = 1 and off the outgoing one by MSTJ(41) = 2 (both are
default). Note that both QED and QCD radiation (off the e and the q legs, respectively)
are allowed to modify the x and Q2 values of the process, while the conventional approach in the literature is to allow only the former. Therefore an option (on by default)
has been added to preserve these values by a post-facto rescaling, MSTP(23) = 1. Further
comments on HERA applications are found in [Sjö92b, Fri00].
8.9.3

Hadron–hadron collisions

The default is to include QCD jet production by 2 → 2 processes, see section 8.2.1. Since
the differential cross section is divergent for p⊥ → 0, a lower cut-off has to be introduced.
Normally that cut-off is given by the user-set p⊥min value in CKIN(3). If CKIN(3) ischosen smaller than a given value of the order of 2 GeV (see PARP(81) and PARP(82)),
then low-p⊥ events are also switched on. The jet cross section is regularized at low p⊥ , so
as to obtain a smooth joining between the high-p⊥ and the low-p⊥ descriptions, see further
section 11.2. As CKIN(3) is varied, the jump from one scenario to another is abrupt, in
terms of cross section: in a high-energy hadron collider, the cross section for jets down
to a p⊥min scale of a few GeV can well reach values much larger than the total inelastic,
non-diffractive cross section. Clearly this is nonsense; therefore either p⊥min should be
picked so large that the jet cross section be only a fraction of the total one, or else one
should select p⊥min = 0 and make use of the full description.
If one switches to MSEL = 2, also elastic and diffractive processes are switched on, seesection 8.2.4. However, the simulation of these processes is fairly primitive, and should
not be used for dedicated studies, but only to estimate how much they may contaminate
the class of non-diffractive minimum-bias events.
Most processes can be simulated in hadron colliders, since the bulk of Pythia processes can be initiated by quarks or gluons. However, there are limits. Currently we include no photon or lepton parton distributions, which means that a process like γq → γq
is not accessible. Further, the possibility of having Z0 and W± interacting in processes
such as 71–77 has been hardwired process by process, and does not mean that there is a
generic treatment of Z0 and W± distributions.
The emphasis in the hadron–hadron process description is on high energy hadron colliders. The program can be used also at fixed-target energies, but the multiple interaction
model for underlying events then may break down and has to be used with caution. The
limit of ‘safe’ applicability is somewhere at around 100 GeV. Only with the simpler modelobtained for MSTP(82) = 1 can one go arbitrarily low.

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9

The Process Generation Program Elements

In the previous two sections, the physics processes and the event-generation schemesof Pythia have been presented. Here, finally, the event-generation routines and thecommon-block variables are described. However, routines and variables related to initialand final-state showers, beam remnants and underlying events, and fragmentation anddecay are relegated to subsequent sections on these topics.
In the presentation in this section, information less important for an efficient use of
Pythia has been put closer to the end. We therefore begin with the main event generation
routines, and follow this by the main common-block variables.
It is useful to distinguish three phases in a normal run with Pythia. In the firstphase, the initialization, the general character of the run is determined. At a minimum,
this requires the specification of the incoming hadrons and the energies involved. At thediscretion of the user, it is also possible to select specific final states, and to make anumber of decisions about details in the subsequent generation. This step is finished by a
PYINIT call, at which time several variables are initialized in accordance with the values
set. The second phase consists of the main loop over the number of events, with each
new event being generated by a call to either PYEVNT or PYEVNW (depending on which
underlying-event and parton-shower framework is desired; below we shall often not makethe distinction explicit, referring to both routines by PYEVNT generically). This event may
then be analysed, using information stored in some common blocks, and the statistics
accumulated. In the final phase, results are presented. This may often be done without
the invocation of any Pythia routines. From PYSTAT, however, it is possible to obtain auseful list of cross sections for the different subprocesses.

9.1

The Main Subroutines

There are two routines that you must know: PYINIT for initialization, and either PYEVNTor PYEVNW for the subsequent generation of each new event. In addition, the cross section
and other kinds of information available with PYSTAT are frequently useful. The othertwo routines described here, PYFRAM and PYKCUT, are of more specialized interest.
CALL PYINIT(FRAME,BEAM,TARGET,WIN)
Purpose: to initialize the generation procedure. Normally it is foreseen that this call will
be followed by many PYEVNT (or PYEVNW) ones, to generate a sample of the event
kind specified by the PYINIT call. (For problems with cross section estimates inruns of very few events per PYINIT call, see the description for PYSTAT(1) below
in this section.)
FRAME : a character variable used to specify the frame of the experiment. Upper-caseand lower-case letters may be freely mixed.
= ’CMS’ : colliding beam experiment in c.m. frame, with beam momentum in +z
direction and target momentum in −z direction.
= ’FIXT’ : fixed-target experiment, with beam particle momentum pointing in +z
direction.
= ’3MOM’ : full freedom to specify frame by giving beam momentum in P(1,1),
P(1,2) and P(1,3) and target momentum in P(2,1), P(2,2) and P(2,3)
in common block PYJETS. Particles are assumed on the mass shell, andenergies are calculated accordingly.
= ’4MOM’ : as ’3MOM’, except also energies should be specified, in P(1,4) andP(2,4), respectively. The particles need not be on the mass shell; effective masses are calculated from energy and momentum. (But note that181

numerical precision may suffer; if you know the masses the option ’5MOM’
below is preferable.)= ’5MOM’ : as ’3MOM’, except also energies and masses should be specified, i.e
the full momentum information in P(1,1) - P(1,5) and P(2,1) - P(2,5)
should be given for beam and target, respectively. Particles need√ not be on
the mass shell. Space-like virtualities should be stored as − −m2 . Especially useful for physics with virtual photons. (The virtuality could bevaried from one event to the next, but then it is convenient to initialize forthe lowest virtuality likely to be encountered.) Four-momentum and massinformation must match.
= ’USER’ : a run primarily intended to involve Les Houches Accord external, userdefined processes, see section 9.9. Information on incoming beam particles
and energies is read from the HEPRUP common block. In this option, theBEAM, TARGET and WIN arguments are dummy.
= ’NONE’ : there will be no initialization of any processes, but only of resonancewidths and a few other process-independent variables. Subsequent to sucha call, PYEVNT cannot be used to generate events, so this option is mainly
intended for those who will want to construct their own events afterwards,
but still want to have access to some of the Pythia facilities. In this option,
the BEAM, TARGET and WIN arguments are dummy.
BEAM, TARGET : character variables to specify beam and target particles. Upper-caseand lower-case letters may be freely mixed. An antiparticle can be denoted by‘bar’ at the end of the name (‘∼’ is a valid alternative for reasons of backwards
compatibility). It is also possible to leave out the underscore (‘ ’) directly after‘nu’ in neutrino names, and the charge for proton and neutron. The arguments
are dummy when the FRAME argument above is either ’USER’ or ’NONE’.
= ’e-’ : electron.
= ’e+’ : positron.
= ’nu e’ : νe .
= ’nu ebar’ : ν e .
= ’mu-’ : µ− .
= ’mu+’ : µ+ .
= ’nu mu’ : νµ .
= ’nu mubar’ : ν µ .
= ’tau-’ : τ − .
= ’tau+’ : τ + .
= ’nu tau’ : ντ .
= ’nu taubar’ : ν τ .
= ’gamma’ : photon (real, i.e. on the mass shell).
= ’gamma/e-’ : photon generated by the virtual-photon flux in an electron beam;
WIN below refers to electron, while photon energy and virtuality varies between events according to what is allowed by CKIN(61) - CKIN(78).
= ’gamma/e+’ : as above for a positron beam.
= ’gamma/mu-’ : as above for a µ− beam.
= ’gamma/mu+’ : as above for a µ+ beam.
= ’gamma/tau-’ : as above for a τ − beam.
= ’gamma/tau+’ : as above for a τ + beam.
= ’pi0’ : π 0 .
= ’pi+’ : π + .
= ’pi-’ : π − .
= ’n0’ : neutron.
= ’nbar0’ : antineutron.
= ’p+’ : proton.
182

= ’pbar-’ : antiproton.
= ’K+’ : K+ meson; since parton distributions for strange hadrons are not available,
very simple and untrustworthy recipes are used for this and subsequent
hadrons, see section 7.1.
= ’K-’ : K− meson.
= ’KS0’ : K0S meson.
= ’KL0’ : K0L meson.
= ’Lambda0’ : Λ baryon.
= ’Sigma-’ : Σ− baryon.
= ’Sigma0’ : Σ0 baryon.
= ’Sigma+’ : Σ+ baryon.
= ’Xi-’ : Ξ− baryon.
= ’Xi0’ : Ξ0 baryon.
= ’Omega-’ : Ω− baryon.
= ’pomeron’ : the pomeron IP; since pomeron parton distribution functions havenot been defined this option can not be used currently.
= ’reggeon’ : the reggeon IR, with comments as for the pomeron above.
WIN : related to energy of system, exact meaning depends on FRAME.
FRAME = ’CMS’ : total energy of system (in GeV).
FRAME = ’FIXT’ : momentum of beam particle (in GeV/c).
FRAME = ’3MOM’, ’4MOM’, ’5MOM’ : dummy (information is taken from the P vectors, see above).
FRAME = ’USER’ : dummy (information is taken from the HEPRUP common block,see above).
FRAME = ’NONE’ : dummy (no information required).
CALL PYEVNT
Purpose: to generate one event of the type specified by the PYINIT call, using the traditional ‘old’ underlying-event and parton-shower model. It is also possible to have
PYEVNT call PYEVNW to access the ‘new’ model that way, see further MSTP(81).
(This is the main routine, which calls a number of other routines for specifictasks.)CALL PYEVNW
Purpose: to generate one event of the type specified by the PYINIT call, using the ‘new’
underlying-event and parton-shower model. (This is the main routine, whichcalls a number of other routines for specific tasks.)
Warning: this routine can be used in exactly the same way as PYEVNT. Technically,
one can freely mix calls to the two routines in the same run, after the PYINIT
call. However, several of the multiple interactions and shower parameters havea different meaning, or at least a different proposed best value, which meansthat caution is recommended. For instance, a change of p⊥0 in PARP(82) isalmost certainly necessary between PYEVNT and PYEVNW, and this require a reinitialization to take effect, so in the end one cannot mix.
CALL PYSTAT(MSTAT)
Purpose: to print out cross-sections statistics, decay widths, branching ratios, status
codes and parameter values. PYSTAT may be called at any time, after the PYINIT

183

call, e.g. at the end of the run, or not at all.
MSTAT : specification of desired information.
= 1 : prints a table of how many events of the different kinds that have been
generated and the corresponding cross sections. All numbers already include
the effects of cuts required by you in PYKCUT.
At the bottom of the listing is also given the total number of warnings anderrors in the current run. (These numbers are reset at each PYINIT call.)
By default only the ten first warnings and errors are written explicitly; here
one may easily see whether many further occured but were not written in
the output. The final number is the fraction of events that have failed
the fragmentation cuts, i.e. where, for one reason or another, the program
has had problems fragmenting the system and has asked for a new hard
subprocess.
Note that no errors are given on the cross sections. In most cases a crosssection is obtained by Monte Carlo integration during the course of the run.
(Exceptions include e.g. total and elastic hadron–hadron cross sections,which are parameterized and thus known from the very onset.) A rule ofthumb would then√ be that the statistical error of a given subprocess scales
like δσ/σ ≈ 1/ n, where n is the number of events generated of this kind. In
principle, the numerator of this relation could be decreased by making use of
the full information accumulated during the run, i.e. also on the cross sectionin those phase space points that are eventually rejected. This is actually
the way the cross section itself is calculated. However, once you introduce
further cuts so that only some fraction of the generated√ events survive to thefinal analysis, you would be back to the simple 1/ n scaling rule for that
number of surviving events. Statistical errors are therefore usually better
evaluated within the context of a specific analysis. Furthermore, systematic
errors often dominate over the statistical ones.
Also note that runs with very few events, in addition to having large errors,
tend to have a bias towards overestimating the cross sections. In a typicalcase, the average cross section obtained with many runs of only one event
each may be twice that of the correct answer of a single run with many
events. The reason is a ‘quit while you are ahead’ phenomenon, that an
upwards fluctuation in the differential cross section in an early try gives anacceptable event and thus terminates the run, while a downwards one leads
to rejection and a continuation of the run.
= 2 : prints a table of the resonances defined in the program, with their particle
codes (KF), and all allowed decay channels. (If the number of generationsin MSTP(1) is 3, however, channels involving fourth-generation particles are
not displayed.) For each decay channel is shown the sequential channel
number (IDC) of the Pythia decay tables, the decay products (usually two
but sometimes three), the partial decay width, branching ratio and effective
branching ratio (in the event some channels have been excluded by you).
= 3 : prints a table with the allowed hard interaction flavours KFIN(I,J) for beamand target particles.
= 4 : prints a table of the kinematical cuts CKIN(I) set by you in the current run.
= 5 : prints a table with all the values of the status codes MSTP(I) and the parameters PARP(I) used in the current run.
= 6 : prints a table of all subprocesses implemented in the program.
= 7 : prints two tables related to R-violating supersymmetry, where lepton and/or
baryon number is not conserved. The first is a collection of semi-inclusive
branching ratios where the entries have a form like ~chi 10 --> nu + q +q, where a sum has been performed over all lepton and quark flavours. In
184the rightmost column of the table, the number of modes that went into thesum is given. The purpose of this table is to give a quick overview of the
branching fractions, since there are currently more than 1500 individual Rviolating processes included in the generator. Note that only the pure 1 → 3parts of the 3-body modes are included in this sum. If a process can also
proceed via two successive 1 → 2 branchings (i.e. the intermediate resonanceis on shell) the product of these branchings should be added to the number
given in this table. A small list at the bottom of the table shows the total
number of R-violating processes in the generator, the number with non-zerobranching ratios in the current run, and the number with branching ratios
larger than 10−3 . The second table which is printed by this call merely lists
the R-violating λ, λ0 , and λ00 couplings.
CALL PYFRAM(IFRAME)
Purpose: to transform an event listing between different reference frames, if so desired.
The use of this routine assumes you do not do any boosts yourself.
IFRAME : specification of frame the event is to be boosted to.
= 1 : frame specified by you in the PYINIT call.
= 2 : c.m. frame of incoming particles.
= 3 : hadronic c.m. frame of lepton–hadron interaction events. Mainly intended
for Deeply Inelastic Scattering, but can also be used in photoproduction. Is
not guaranteed to work with the ’gamma/lepton’ options, however, and soof limited use. Note that both the lepton and any photons radiated off the
lepton remain in the event listing, and have to be removed separately if you
only want to study the hadronic subsystem.
CALL PYKCUT(MCUT)
Purpose: to enable you to reject a given set of kinematic variables at an early stage ofthe generation procedure (before evaluation of cross sections), so as not to spend
unnecessary time on the generation of events that are not wanted. The routine
will not be called unless you require is by setting MSTP(141) = 1, and never if‘minimum-bias’-type events (including elastic and diffractive scattering) are to
be generated as well. Furthermore it is never called for user-defined external
processes. A dummy routine PYKCUT is included in the program file, so as to
avoid unresolved external references when the routine is not used.
MCUT : flag to signal effect of user-defined cuts.
= 0 : event is to be retained and generated in full.
= 1 : event is to be rejected and a new one generated.
Remark : at the time of selection, several variables in the MINT and VINT arrays in
the PYINT1 common block contain information that can be used to make thedecision. The routine provided in the program file explicitly reads the variables
that have been defined at the time PYKCUT is called, and also calculates somederived quantities. The information available includes subprocess type ISUB,
Ecm , ŝ, t̂, û, p̂⊥ , x1 , x2 , xF , τ , y, τ 0 , cos θ̂, and a few more. Some of these may
not be relevant for the process under study, and are then set to zero.

9.2

Switches for Event Type and Kinematics Selection

By default, if Pythia is run for a hadron collider, only QCD 2 → 2 processes are generated, composed of hard interactions above p⊥min =PARP(81), with low-p⊥ processes added185

on so as to give the full (parameterized) inelastic, non-diffractive cross section. In an e+ e−
collider, γ ∗ /Z0 production is the default, and in an ep one it is Deeply Inelastic Scattering.
With the help of the common block PYSUBS, it is possible to select the generation of another process, or combination of processes. It is also allowed to restrict the generation to
specific incoming partons/particles at the hard interaction. This often automatically also
restricts final-state flavours but, in processes such as resonance production or QCD/QED
production of new flavours, switches in the Pythia program may be used to this end; seesection 14.4.
The CKIN array may be used to impose specific kinematics cuts. You should here be
warned that, if kinematical variables are too strongly restricted, the generation time per
event may become very long. In extreme cases, where the cuts effectively close the fullphase space, the event generation may run into an infinite loop. The generation of 2 → 1
resonance production is performed in terms of the ŝ and y variables, and so the ranges
CKIN(1) - CKIN(2) and CKIN(7) - CKIN(8) may be arbitrarily restricted without asignificant loss of speed. For 2 → 2 processes, cos θ̂ is added as a third generation variable,
and so additionally the range CKIN(27) - CKIN(28) may be restricted without any lossof efficiency.
Effects from initial- and final-state radiation are not included, since they are not knownat the time the kinematics at the hard interaction is selected. The sharp kinematical cutoffs that can be imposed on the generation process are therefore smeared, both by QCD
radiation and by fragmentation. A few examples of such effects follow.
• Initial-state radiation implies that each of the two incoming partons has a nonvanishing p⊥ when they interact. The hard scattering subsystem thus receives a
net transverse boost, and is rotated with respect to the beam directions. In a
2 → 2 process, what typically happens is that one of the scattered partons receives
an increased p⊥ , while the p⊥ of the other parton can be reduced or increased,
depending on the detailed topology.
• Since the initial-state radiation machinery assigns space-like virtualities to the incoming partons, the definitions of x in terms of energy fractions and in terms of
momentum fractions no longer coincide, and so the interacting subsystem may receive a net longitudinal boost compared with naı̈ve expectations, as part of theparton-shower machinery.
• Initial-state radiation gives rise to additional jets, which in extreme cases may be
mistaken for either of the jets of the hard interaction.
• Final-state radiation gives rise to additional jets, which smears the meaning of thebasic 2 → 2 scattering. The assignment of soft jets is not unique. The energy of ajet becomes dependent on the way it is identified, e.g. what jet cone size is used.
• The beam-remnant description assigns primordial k⊥ values, which also gives a
net p⊥ shift of the hard-interaction subsystem; except at low energies this effectis overshadowed by initial-state radiation, however. Beam remnants may also add
further activity under the ‘perturbative’ event.
• Fragmentation will further broaden jet profiles, and make jet assignments and energy
determinations even more uncertain.
In a study of events within a given window of experimentally defined variables, it is up
to you to leave such liberal margins that no events are missed. In other words, cuts haveto be chosen such that a negligible fraction of events migrate from outside the simulated
region to inside the interesting region. Often this may lead to low efficiency in terms ofwhat fraction of the generated events are actually of interest to you. See also section 3.6.
In addition to the variables found in PYSUBS, also those in the PYPARS common block
may be used to select exactly what one wants to have simulated. These possibilities will
be described in the following section.
The notation used above and in the following is that ‘ˆ’ denotes internal variables in186the hard-scattering subsystem, while ‘∗ ’ is for variables in the c.m. frame of the event as
a whole.
COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)
Purpose: to allow you to run the program with any desired subset of processes, or restrictflavours or kinematics. If the default values, denoted below by (D = . . . ), arenot satisfactory, they must be changed before the PYINIT call.
MSEL : (D = 1) a switch to select between full user control and some preprogrammed
alternatives.
= 0 : desired subprocesses have to be switched on in MSUB, i.e. full user control.
= 1 : depending on incoming particles, different alternatives are used.
Lepton–lepton: Z or W production (ISUB = 1 or 2).
Lepton–hadron: Deeply Inelastic Scattering (ISUB = 10; this option is now
out of date for most applications, superseded by the ’gamma/lepton’ machinery).
Hadron–hadron: QCD high-p⊥ processes (ISUB = 11, 12, 13, 28, 53, 68);
additionally low-p⊥ production if CKIN(3) < PARP(81) or PARP(82), depending on MSTP(82) (ISUB = 95). If low-p⊥ is switched on, the other CKIN
cuts are not used.
A resolved photon counts as hadron. When the photon is not resolved, thefollowing cases are possible.
Photon–lepton: Compton scattering (ISUB = 34).
Photon–hadron: photon-parton scattering (ISUB = 33, 34, 54).
Photon–photon: fermion pair production (ISUB = 58).
When photons are given by the ’gamma/lepton’ argument in the PYINIT
call, the outcome depends on the MSTP(14) value. Default is a mixture ofmany kinds of processes, as described in section 8.3.
= 2 : as MSEL = 1 for lepton–lepton, lepton–hadron and unresolved photons. For
hadron–hadron (including resolved photons) all QCD processes, including
low-p⊥ , single and double diffractive and elastic scattering, are included(ISUB = 11, 12, 13, 28, 53, 68, 91, 92, 93, 94, 95). The CKIN cuts are here
not used.
For photons given with the ’gamma/lepton’ argument in the PYINIT call,the above processes are replaced by other ones that also include the photon
virtuality in the cross sections. The principle remains to include both highand low-p⊥ processes, however.
= 4 : charm (cc) production with massive matrix elements (ISUB = 81, 82, 84,
85).
= 5 : bottom (bb) production with massive matrix elements (ISUB = 81, 82, 84,85).
= 6 : top (tt) production with massive matrix elements (ISUB = 81, 82, 84, 85).
0
= 7 : fourth generation b0 (b0 b ) production with massive matrix elements (ISUB= 81, 82, 84, 85).
0
= 8 : fourth generation t0 (t0 t ) production with massive matrix elements (ISUB =81, 82, 84, 85).
= 10 : prompt photons (ISUB = 14, 18, 29).
= 11 : Z0 production (ISUB = 1).
= 12 : W± production (ISUB = 2).
= 13 : Z0 + jet production (ISUB = 15, 30).
= 14 : W± + jet production (ISUB = 16, 31).
= 15 : pair production of different combinations of γ, Z0 and W± (except γγ; see
187

MSEL = 10) (ISUB = 19, 20, 22, 23, 25).
= 16 : h0 production (ISUB = 3, 102, 103, 123, 124).
= 17 : h0 Z0 or h0 W± (ISUB = 24, 26).
= 18 : h0 production, combination relevant for e+ e− annihilation (ISUB = 24, 103,
123, 124).
= 19 : h0 , H0 and A0 production, excepting pair production (ISUB = 24, 103, 123,
124, 153, 158, 171, 173, 174, 176, 178, 179).
= 21 : Z00 production (ISUB = 141).
= 22 : W0± production (ISUB = 142).
= 23 : H± production (ISUB = 143).
= 24 : R0 production (ISUB = 144).
= 25 : LQ (leptoquark) production (ISUB = 145, 162, 163, 164).
= 35 : single bottom production by W exchange (ISUB = 83).
= 36 : single top production by W exchange (ISUB = 83).
= 37 : single b0 production by W exchange (ISUB = 83).
= 38 : single t0 production by W exchange (ISUB = 83).
= 39 : all MSSM processes except Higgs production.
= 40 : squark and gluino production (ISUB = 243, 244, 258, 259, 271–280).
= 41 : stop pair production (ISUB = 261–265).
= 42 : slepton pair production (ISUB = 201–214).
= 43 : squark or gluino with chargino or neutralino, (ISUB = 237–242, 246–256).
= 44 : chargino–neutralino pair production (ISUB = 216–236).
= 45 : sbottom production (ISUB = 281–296).
0,±0= 50 : pair production of technipions and gauge bosons by πtc/ωtc
exchange (ISUB= 361–377).
= 51 : standard QCD 2 → 2 processes 381–386, with possibility to introduce compositeness/technicolor modifications, see ITCM(5).
= 61 : charmonimum production in the NRQCD framework, (ISUB = 421–439).
= 62 : bottomonimum production in the NRQCD framework, (ISUB = 461–479).
= 63 : both charmonimum and bottomonimum production in the NRQCD framework, (ISUB = 421–439, 461–479).
MSUB : (D = 500*0) array to be set when MSEL = 0 (for MSEL ≥ 1 relevant entries are set
in PYINIT) to choose which subset of subprocesses to include in the generation.
The ordering follows the ISUB code given in section 8.1 (with comments as given
there).
MSUB(ISUB) = 0 : the subprocess is excluded.
MSUB(ISUB) = 1 : the subprocess is included.
Note: when MSEL = 0, the MSUB values set by you are never changed by Pythia.
If you want to combine several different ‘subruns’, each with its own PYINIT
call, into one single run, it is up to you to remember not only to switch on
the new processes before each new PYINIT call, but also to switch off theold ones that are no longer desired.
KFIN(I,J) : provides an option to selectively switch on and off contributions to the crosssections from the different incoming partons/particles at the hard interaction. In
combination with the Pythia resonance decay switches, this also allows you to
set restrictions on flavours appearing in the final state.
I :is 1 for beam side of event and 2 for target side.
J :enumerates flavours according to the KF code; see section 5.1.
KFIN(I,J) = 0 : the parton/particle is forbidden.
KFIN(I,J) = 1 : the parton/particle is allowed.
Note: by default, the following are switched on: d, u, s, c, b, e− , νe , µ− , νµ , τ − ,ντ , g, γ, Z0 , W+ and their antiparticles. In particular, top is off, and has to188

be switched on explicitly if needed.
CKIN : kinematics cuts that can be set by you before the PYINIT call, and that affectthe region of phase space within which events are generated. Some cuts are‘hardwired’ while most are ‘softwired’. The hardwired ones are directly related
to the kinematical variables used in the event selection procedure, and therefore
have negligible effects on program efficiency. The most important of these are
CKIN(1) - CKIN(8), CKIN(27) - CKIN(28), and CKIN(31) - CKIN(32). The
softwired ones are most of the remaining ones, that cannot be fully taken into
account in the kinematical variable selection, so that generation in constrained
regions of phase space may be slow. In extreme cases the phase space may beso small that the maximization procedure fails to find any allowed points at all(although some small region might still exist somewhere), and therefore switches
off some subprocesses, or aborts altogether.
√
CKIN(1), CKIN(2) : (D = 2., −1. GeV) range of allowed m̂ = ŝ values. If CKIN(2)< 0., the upper limit is inactive.
CKIN(3), CKIN(4) : (D = 0., −1. GeV) range of allowed p̂⊥ values for hard 2 → 2
processes, with transverse momentum p̂⊥ defined in the rest frame of the hardinteraction. If CKIN(4) < 0., the upper limit is inactive. For processes thatare singular in the limit p̂⊥ → 0 (see CKIN(6)), CKIN(5) provides an additional
constraint. The CKIN(3) and CKIN(4) limits can also be used in 2 → 1 → 2
processes. Here, however, the product masses are not known and hence areassumed to be vanishing in the event selection. The actual p⊥ range for massive
products is thus shifted downwards with respect to the nominal one.
Note 1: for processes that are singular in the limit p̂⊥ → 0, a careful choice of
CKIN(3) value is not only a matter of technical convenience, but a requirement for obtaining sensible results. One example is the hadroproduction of
a W± or Z0 gauge boson together with a jet, discussed in section 8.4.2. Herethe point is that this is a first-order process (in αs ), correcting the zerothorder process of a W± or Z0 without any jet. A full first-order description
would also have to include virtual corrections in the low-p̂⊥ region.
Generalizing also to other processes, the simple-minded higher-order description breaks down when CKIN(3) is selected so small that the higher-orderprocess cross section corresponds to a non-negligible fraction of the lowerorder one. This number will vary depending on the process considered andthe c.m. energy used, but could easily be tens of GeV rather than the default 1 GeV provided as technical cut-off in CKIN(5). Processes singular inp̂⊥ → 0 should therefore only be used to describe the high-p⊥ behaviour,while the lowest-order process complemented with parton showers should
give the inclusive distribution and in particular the one at small p⊥ values.
Technically the case of QCD production of two jets is slightly more complicated, and involves eikonalization to multiple parton–parton scattering,
section 11.2, but again the conclusion is that the processes have to be handled with care at small p⊥ values.
Note 2: there are a few situations in which CKIN(3) may be overwritten; especiallywhen different subprocess classes are mixed in γp or γγ collisions, see section
8.3.
CKIN(5) : (D = 1. GeV) lower cut-off on p̂⊥ values, in addition to the CKIN(3) cut
above, for processes that are singular in the limit p̂⊥ → 0 (see CKIN(6)).
CKIN(6) : (D = 1. GeV) hard 2 → 2 processes, which do not proceed only via anintermediate resonance (i.e. are 2 → 1 → 2 processes), are classified as singularin the limit p̂⊥ → 0 if either or both of the two final-state products has a mass
m < CKIN(6).

189

CKIN(7), CKIN(8) : (D = −10., 10.) range of allowed scattering subsystem rapidities
y = y ∗ in the c.m. frame of the event, where y = (1/2) ln(x1 /x2 ). (Following thenotation of this section, the variable should be given as y ∗ , but because of its
frequent use, it was called y in section 7.2.)
CKIN(9), CKIN(10) : (D = −40., 40.) range of allowed (true) rapidities for the productwith largest rapidity in a 2 → 2 or a 2 → 1 → 2 process, defined in the c.m.
frame of the event, i.e. max(y3∗ , y4∗ ). Note that rapidities are counted with sign,i.e. if y3∗ = 1 and y4∗ = −2 then max(y3∗ , y4∗ ) = 1.
CKIN(11), CKIN(12) : (D = −40., 40.) range of allowed (true) rapidities for the productwith smallest rapidity in a 2 → 2 or a 2 → 1 → 2 process, defined in thec.m. frame of the event, i.e. min(y3∗ , y4∗ ). Consistency thus requires CKIN(11) ≤CKIN(9) and CKIN(12) ≤ CKIN(10).
CKIN(13), CKIN(14) : (D = −40., 40.) range of allowed pseudorapidities for the product with largest pseudorapidity in a 2 → 2 or a 2 → 1 → 2 process, definedin the c.m. frame of the event, i.e. max(η3∗ , η4∗ ). Note that pseudorapidities arecounted with sign, i.e. if η3∗ = 1 and η4∗ = −2 then max(η3∗ , η4∗ ) = 1.
CKIN(15), CKIN(16) : (D = −40., 40.) range of allowed pseudorapidities for the product with smallest pseudorapidity in a 2 → 2 or a 2 → 1 → 2 process, defined in
the c.m. frame of the event, i.e. min(η3∗ , η4∗ ). Consistency thus requires CKIN(15)≤ CKIN(13) and CKIN(16) ≤ CKIN(14).
CKIN(17), CKIN(18) : (D = −1., 1.) range of allowed cos θ∗ values for the product withlargest cos θ∗ value in a 2 → 2 or a 2 → 1 → 2 process, defined in the c.m. frameof the event, i.e. max(cos θ3∗ , cos θ4∗ ).
CKIN(19), CKIN(20) : (D = −1., 1.) range of allowed cos θ∗ values for the product withsmallest cos θ∗ value in a 2 → 2 or a 2 → 1 → 2 process, defined in the c.m.
frame of the event, i.e. min(cos θ3∗ , cos θ4∗ ). Consistency thus requires CKIN(19)≤ CKIN(17) and CKIN(20) ≤ CKIN(18).
CKIN(21), CKIN(22) : (D = 0., 1.) range of allowed x1 values for the parton on side 1
that enters the hard interaction.
CKIN(23), CKIN(24) : (D = 0., 1.) range of allowed x2 values for the parton on side 2that enters the hard interaction.
CKIN(25), CKIN(26) : (D = −1., 1.) range of allowed Feynman-x values, where xF =x1 − x2 .
CKIN(27), CKIN(28) : (D = −1., 1.) range of allowed cos θ̂ values in a hard 2 → 2
scattering, where θ̂ is the scattering angle in the rest frame of√the hard interaction.
CKIN(31), CKIN(32) : (D = 2., −1. GeV) range of allowed m̂0 = ŝ0 values, where m̂0
is the mass of the complete three- or four-body final state in 2 → 3 or 2 → 4
processes (while m̂, constrained in CKIN(1) and CKIN(2), here corresponds to theone- or two-body central system). If CKIN(32) < 0., the upper limit is inactive.
CKIN(35), CKIN(36) : (D = 0., −1. GeV2 ) range of allowed |t̂| = −t̂ values in 2 → 2
processes. Note that for Deeply Inelastic Scattering this is nothing but the Q2scale, in the limit that initial- and final-state radiation is neglected. If CKIN(36)< 0., the upper limit is inactive.
CKIN(37), CKIN(38) : (D = 0., −1. GeV2 ) range of allowed |û| = −û values in 2 → 2
processes. If CKIN(38) < 0., the upper limit is inactive.
CKIN(39), CKIN(40) : (D = 4., −1. GeV2 ) the W 2 range allowed in DIS processes, i.e.
subprocess number 10. If CKIN(40) < 0., the upper limit is inactive. Here W 2
is defined in terms of W 2 = Q2 (1 − x)/x. This formula is not quite correct, inthat (i) it neglects the target mass (for a proton), and (ii) it neglects initial-state
photon radiation off the incoming electron. It should be good enough for loose
cuts, however. These cuts are not checked if process 10 is called for two lepton
beams.

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CKIN(41) - CKIN(44) : (D = 12., −1., 12., −1. GeV) range of allowed mass values of
the two (or one) resonances produced in a ‘true’ 2 → 2 process, i.e. one not(only) proceeding through a single s-channel resonance (2 → 1 → 2). (These
are the ones listed as 2 → 2 in the tables in section 8.1.) Only particles with awidth above PARP(41) are considered as bona fide resonances and tested against
the CKIN limits; particles with a smaller width are put on the mass shell without
applying any cuts. The exact interpretation of the CKIN variables depends onthe flavours of the two produced resonances.
For two resonances like Z0 W+ (produced from fi f j → Z0 W+ ), which are not
identical and which are not each other’s antiparticles, one has
CKIN(41) < m1 < CKIN(42), andCKIN(43) < m2 < CKIN(44),where m1 and m2 are the actually generated masses of the two resonances, and 1
and 2 are defined by the order in which they are given in the production process
specification.
For two resonances like Z0 Z0 , which are identical, or W+ W− , which are eachother’s antiparticles, one instead has
CKIN(41) < min(m1 , m2 ) < CKIN(42), andCKIN(43) < max(m1 , m2 ) < CKIN(44).
In addition, whatever limits are set on CKIN(1) and, in particular, on CKIN(2)obviously affect the masses actually selected.
Note 1: if MSTP(42) = 0, so that no mass smearing is allowed, the CKIN values haveno effect (the same as for particles with too narrow a width).
Note 2: if CKIN(42) < CKIN(41) it means that the CKIN(42) limit is inactive; correspondingly, if CKIN(44) <CKIN(43) then CKIN(44) is inactive.
Note 3: if limits are active and the resonances are identical, it is up to you to ensurethat CKIN(41) ≤ CKIN(43) and CKIN(42) ≤ CKIN(44).
Note 4: for identical resonances, it is not possible to preselect which of the resonances is the lighter one; if, for instance, one Z0 is to decay to leptons andthe other to quarks, there is no mechanism to guarantee that the lepton pair
has a mass smaller than the quark one.
Note 5: the CKIN values are applied to all relevant 2 → 2 processes equally, which
may not be what one desires if several processes are generated simultaneously. Some caution is therefore urged in the use of the CKIN(41) CKIN(44) values. Also in other respects, you are recommended to takeproper care: if a Z0 is only allowed to decay into bb, for example, setting its
mass range to be 2–8 GeV is obviously not a good idea.
CKIN(45) - CKIN(48) : (D = 12., −1., 12., −1. GeV) range of allowed mass valuesof the two (or one) secondary resonances produced in a 2 → 1 → 2 process (like gg → h0 → Z0 Z0 ) or even a 2 → 2 → 4 (or 3) process (likeqq → Z0 h0 → Z0 W+ W− ). Note that these CKIN values only affect the secondary resonances; the primary ones are constrained by CKIN(1), CKIN(2) andCKIN(41) - CKIN(44) (indirectly, of course, the choice of primary resonancemasses affects the allowed mass range for the secondary ones). What is considered to be a resonance is defined by PARP(41); particles with a width smaller
than this are automatically put on the mass shell. The description closely parallels the one given for CKIN(41) - CKIN(44). Thus, for two resonances that are
not identical or each other’s antiparticles, one has
CKIN(45) < m1 < CKIN(46), andCKIN(47) < m2 < CKIN(48),where m1 and m2 are the actually generated masses of the two resonances, and
1 and 2 are defined by the order in which they are given in the decay channel
specification in the program (see e.g. output from PYSTAT(2) or PYLIST(12)).
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For two resonances that are identical or each other’s antiparticles, one insteadhas
CKIN(45) < min(m1 , m2 ) < CKIN(46), andCKIN(47) < max(m1 , m2 ) < CKIN(48).
Notes 1 - 5: as for CKIN(41) - CKIN(44), with trivial modifications.
Note 6: setting limits on secondary resonance masses is possible in any of the channels of the allowed types (see above). However, so far only h0 → Z0 Z0 and
h0 → W+ W− have been fully implemented, such that an arbitrary mass
range below the naı̈ve mass threshold may be picked. For other possible resonances, any restrictions made on the allowed mass range are not reflected in
the cross section; and further it is not recommendable to pick mass windowsthat make a decay on the mass shell impossible.
CKIN(49) - CKIN(50) : allow minimum mass limits to be passed from PYRESD to
PYOFSH. They are used for tertiary and higher resonances, i.e. those not controlled by CKIN(41) - CKIN(48). They should not be touched by the user.
CKIN(51) - CKIN(56) : (D = 0., −1., 0., −1., 0., −1. GeV) range of allowed transverse momenta in a true 2 → 3 process. This means subprocesses such as
121–124 for h0 production, and their H0 , A0 and H±± equivalents. CKIN(51) CKIN(54) corresponds to p⊥ ranges for scattered partons, in order of appearance,
i.e. CKIN(51) - CKIN(52) for the parton scattered off the beam and CKIN(53)- CKIN(54) for the one scattered off the target. CKIN(55) and CKIN(56) here
sets p⊥ limits for the third product, the h0 , i.e. the CKIN(3) and CKIN(4) values have no effect for this process. Since the p⊥ of the Higgs is not one of the
primary variables selected, any constraints here may mean reduced Monte Carloefficiency, while for these processes CKIN(51) - CKIN(54) are ‘hardwired’ andtherefore do not cost anything. As usual, a negative value implies that the upper
limit is inactive.
CKIN(61) - CKIN(78) : allows to restrict the range of kinematics for the photons generated off the lepton beams with the ’gamma/lepton’ option of PYINIT. In eachquartet of numbers, the first two corresponds to the range allowed on incomingside 1 (beam) and the last two to side 2 (target). The cuts are only applicable fora lepton beam. Note that the x and Q2 (P 2 ) variables are the basis for the generation, and so can be restricted with no loss of efficiency. For leptoproduction (i.e.
lepton on hadron) the W is uniquely given by the one x value of the problem, sohere also W cuts are fully efficient. The other cuts may imply a slowdown of the
program, but not as much as if equivalent cuts only are introduced after eventsare fully generated. See [Fri00] for details.
CKIN(61) - CKIN(64) : (D = 0.0001, 0.99, 0.0001, 0.99) allowed range for the energy
fractions x that the photon take of the respective incoming lepton energy. These
fractions are defined in the c.m. frame of the collision, and differ from energy
fractions as defined in another frame. (Watch out at HERA!) In order to avoid
some technical problems, absolute lower and upper limits are set internally at
0.0001 and 0.9999.
CKIN(65) - CKIN(68) : (D = 0., −1., 0., −1. GeV2 ) allowed range for the space-likevirtuality of the photon, conventionally called either Q2 or P 2 , depending on
process. A negative number means that the upper limit is inactive, i.e. purely
given by kinematics. A nonzero lower limit is implicitly given by kinematics
constraints.
CKIN(69) - CKIN(72) : (D = 0., −1., 0., −1.) allowed range of the scattering angle θof the lepton, defined in the c.m. frame of the event. (Watch out at HERA!) A
negative number means that the upper limit is inactive, i.e. equal to π.
CKIN(73) - CKIN(76) : (D = 0.0001, 0.99, 0.0001, 0.99) allowed range for the light-cone
fraction y that the photon take of the respective incoming lepton energy. The
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light-cone is defined by the four-momentum of the lepton or hadron on the otherside of the event (and thus deviates from true light-cone fraction by mass effects
that normally are negligible). The y value is related to the x and Q2 (P 2 ) values
by y = x + Q2 /s if mass terms are neglected.
CKIN(77), CKIN(78) : (D = 2., −1. GeV) allowed range for W , i.e. either the photon–
hadron or photon–photon invariant mass. A negative number means that theupper limit is inactive.

9.3

The General Switches and Parameters

The PYPARS common block contains the status code and parameters that regulate theperformance of the program. All of them are provided with sensible default values, so thata novice user can neglect them, and only gradually explore the full range of possibilities.
Some of the switches and parameters in PYPARS will be described later, in the shower andbeam-remnants sections.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
Purpose: to give access to status code and parameters that regulate the performanceof the program. If the default values, denoted below by (D = . . . ), are notsatisfactory, they must in general be changed before the PYINIT call. Exceptions,i.e. variables that can be changed for each new event, are denoted by (C).
MSTP(1) : (D = 3) maximum number of generations. Automatically set ≤ 4.
MSTP(2) : (D = 1) calculation of αs at hard interaction, in the routine PYALPS.
= 0 : αs is fixed at value PARU(111).
= 1 : first-order running αs .
= 2 : second-order running αs .
MSTP(3) : (D = 2) selection of Λ value in αs for MSTP(2) ≥ 1.
= 1 : Λ is given by PARP(1) for hard interactions, by PARP(61) for space-likeshowers, by PARP(72) for time-like showers not from a resonance decay,and by PARJ(81) for time-like ones from a resonance decay (including e.g.
γ/Z0 → qq decays, i.e. conventional e+ e− physics). This Λ is assumed to bevalid for 5 flavours; for the hard interaction the number of flavours assumed
can be changed by MSTU(112).
= 2 : Λ value is chosen according to the parton-distribution-function parameterizations. The choice is always based on the proton parton-distribution set
selected, i.e. is unaffected by pion and photon parton-distribution selection.
All the Λ values are assumed to refer to 4 flavours, and MSTU(112) is set
accordingly. This Λ value is used both for the hard scattering and the initialand final-state radiation. The ambiguity in the choice of the Q2 argument
still remains (see MSTP(32), MSTP(64) and MSTJ(44)). This Λ value is usedalso for MSTP(57) = 0, but the sensible choice here would be to use MSTP(2)= 0 and have no initial- or final-state radiation. This option does not change
the PARJ(81) value of time-like parton showers in resonance decays, so that
LEP experience on this specific parameter is not overwritten unwittingly.
Therefore PARJ(81) can be updated completely independently.
= 3 : as = 2, except that here also PARJ(81) is overwritten in accordance withthe Λ value of the proton parton-distribution-function set.
MSTP(4) : (D = 0) treatment of the Higgs sector, predominantly the neutral one.
= 0 : the h0 is given the Standard Model Higgs couplings, while H0 and A0 couplings should be set by you in PARU(171) - PARU(175) and PARU(181) PARU(185), respectively.

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you should set couplings for all three Higgs bosons, for the h0 in PARU(161)- PARU(165), and for the H0 and A0 as above.
= 2 : the mass of h0 in PMAS(25,1) and the tan β value in PARU(141) are used toderive H0 , A0 and H± masses, and h0 , H0 , A0 and H± couplings, using therelations of the Minimal Supersymmetric extension of the Standard Modelat Born level [Gun90]. Existing masses and couplings are overwritten by thederived values. See section 8.5.3 for discussion on parameter constraints.
= 3:
as = 2, but using relations at the one-loop level. This option is not yet
implemented as such. However, if you initialize the SUSY machinery with
IMSS(1) = 1, then the SUSY parameters will be used to calculate also Higgs
masses and couplings. These are stored in the appropriate slots, and thevalue of MSTP(4) is overwritten to 1.
MSTP(7) : (D = 0) choice of heavy flavour in subprocesses 81–85. Does not apply for
MSEL = 4 - 8, where the MSEL value always takes precedence.
= 0 : for processes 81–84 (85) the ‘heaviest’ flavour allowed for gluon (photon)
splitting into a quark–antiquark (fermion–antifermion) pair, as set in the
MDME array. Note that ‘heavy’ is defined as the one with largest KF code, sothat leptons take precedence if they are allowed.
= 1 - 8 : pick this particular quark flavour; e.g., MSTP(7) = 6 means that top will
be produced.
= 11 - 18 : pick this particular lepton flavour. Note that neutrinos are not possible, i.e. only 11, 13, 15 and 17 are meaningful alternatives. Lepton pair
production can only occur in process 85, so if any of the other processes
have been switched on they are generated with the same flavour as would
be obtained in the option MSTP(7) = 0.
MSTP(8) : (D = 0) choice of electroweak parameters to use in the decay widths of resonances (W, Z, h, . . . ) and cross sections (production of W’s, Z’s, h’s, . . . ).
= 0 : everything is expressed in terms of a running αem (Q2 ) and a fixed sin2 θW ,i.e. GF is nowhere used.
√= 1 : a replacement is made according to αem (Q2 ) → 2GF m2W sin2 θW /π in all
widths and cross sections. If GF and mZ are considered as given, this meansthat sin2 θW and mW are the only free electroweak parameter.
= 2 : a replacement is made as for = 1, but additionally sin2 θW is constrained by
the relation sin2 θW = 1 − m2W /m2Z . This means that mW remains as a freeparameter, but that the sin2 θW value in PARU(102) is never used, except inthe vector couplings in the combination v = a−4 sin2 θW e. This latter degreeof freedom enters e.g. for forward-backward asymmetries in Z0 decays.
Note: this option does not affect the emission of real photons in the initial andfinal state, where αem is always used. However, it does affect also purely
electromagnetic hard processes, such as qq → γγ.
MSTP(9) : (D = 0) inclusion of top (and fourth generation) as allowed remnant flavour
q0 in processes that involve q → q0 + W branchings as part of the overall process,and where the matrix elements have been calculated under the assumption thatq0 is massless.
= 0 : no.
= 1 : yes, but it is possible, as before, to switch off individual channels by thesetting of MDME switches. Mass effects are taken into account, in a crude
fashion, by rejecting events where kinematics becomes inconsistent whenthe q0 mass is included.
MSTP(11) : (D = 1) use of electron parton distribution in e+ e− and ep interactions.
= 0 : no, i.e. electron carries the whole beam energy.
= 1 : yes, i.e. electron carries only a fraction of beam energy in agreement with
next-to-leading electron parton-distribution function, thereby including the
= 1 :

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effects of initial-state bremsstrahlung.
MSTP(12) : (D = 0) use of e− (‘sea’, i.e. from e → γ → e), e+ , quark and gluon distribution functions inside an electron.
= 0 : off.
= 1 : on, provided that MSTP(11) ≥ 1. Quark and gluon distributions are obtained by numerical convolution of the photon content inside an electron (asgiven by the bremsstrahlung spectrum of MSTP(11) = 1) with the quarkand gluon content inside a photon. The required numerical precision is set
by PARP(14). Since the need for numerical integration makes this option
somewhat more time-consuming than ordinary parton-distribution evaluation, one should only use it when studying processes where it is needed.
Note: for all traditional photoproduction/DIS physics this option is superseded by
the ’gamma/lepton’ option for PYINIT calls, but can still be of use for someless standard processes.
MSTP(13) : (D = 1) choice of Q2 range over which electrons are assumed to radiate
photons; affects normalization of e− (sea), e+ , γ, quark and gluon distributions
inside an electron for MSTP(12) = 1.
= 1 : range set by Q2 argument of parton-distribution-function call, i.e. by Q2 scaleof the hard interaction. Therefore parton distributions are proportional toln(Q2 /m2e ).
= 2 : range set by the user-determined Q2max , given in PARP(13). Parton distributions are assumed to be proportional to ln((Q2max /m2e )(1 − x)/x2 ). This isnormally most appropriate for photoproduction, where the electron is supposed to go undetected, i.e. scatter less than Q2max .
Note: the choice of effective range is especially touchy for the quark and gluon
distributions. An (almost) on-the-mass-shell photon has a VMD piece that
dies away for a virtual photon. A simple convolution of distribution functions
does not take this into account properly. Therefore the contribution from Q
values above the ρ mass should be suppressed. A choice of Qmax ≈ 1 GeV
is then appropriate for a photoproduction limit description of physics. See
also note for MSTP(12).
MSTP(14) : (D = 30) structure of incoming photon beam or target. Historically, numbers
up to 10 were set up for real photons, and subsequent ones have been added
also to allow for virtual photon beams. The reason is that the earlier options
specify e.g. direct×VMD, summing over the possibilities of which photon is direct
and which VMD. This is allowed when the situation is symmetric, i.e. for two
incoming real photons, but not if one is virtual. Some of the new options agreewith previous ones, but are included to allow a more consistent pattern. Further
options above 25 have been added also to include DIS processes.
= 0 : a photon is assumed to be point-like (a direct photon), i.e. can only interact
in processes which explicitly contain the incoming photon, such as fi γ → fi g
for γp interactions. In γγ interactions both photons are direct, i.e the mainprocess is γγ → fi f i .
= 1 : a photon is assumed to be resolved, i.e. can only interact through its constituent quarks and gluons, giving either high-p⊥ parton–parton scatterings
or low-p⊥ events. Hard processes are calculated with the use of the full
photon parton distributions. In γγ interactions both photons are resolved.
= 2 : a photon is assumed resolved, but only the VMD piece is included in theparton distributions, which therefore mainly are scaled-down versions of theρ0 /π 0 ones. Both high-p⊥ parton–parton scatterings and low-p⊥ events areallowed. In γγ interactions both photons are VMD-like.
= 3 : a photon is assumed resolved, but only the anomalous piece of the photon
parton distributions is included. (This event class is called either anomalous195

= 4 :

= 5 :

= 6 :

= 7 :

= 10 := 11 := 12 := 13 := 14 := 15 := 16 := 17 := 18 := 19 := 20 := 21 := 22 := 23 := 24 := 25 := 26 := 27 := 28 := 29 := 30 :or GVMD; we will use both interchangeably, though the former is more
relevant for high-p⊥ phenomena and the latter for low-p⊥ ones.) In γγ
interactions both photons are anomalous.
in γγ interactions one photon is direct and the other resolved. A typical
process is thus fi γ → fi g. Hard processes are calculated with the use of thefull photon parton distributions for the resolved photon. Both possibilities ofwhich photon is direct are included, in event topologies and in cross sections.
This option cannot be used in configurations with only one incoming photon.
in γγ interactions one photon is direct and the other VMD-like. Both possibilities of which photon is direct are included, in event topologies and incross sections. This option cannot be used in configurations with only one
incoming photon.
in γγ interactions one photon is direct and the other anomalous. Bothpossibilities of which photon is direct are included, in event topologies andin cross sections. This option cannot be used in configurations with only
one incoming photon.
in γγ interactions one photon is VMD-like and the other anomalous. Only
high-p⊥ parton–parton scatterings are allowed. Both possibilities of whichphoton is VMD-like are included, in event topologies and in cross sections.
This option cannot be used in configurations with only one incoming photon.
the VMD, direct and anomalous/GVMD components of the photon are automatically mixed. For γp interactions, this means an automatic mixture of
the three classes 0, 2 and 3 above [Sch93, Sch93a], for γγ ones a mixture of
the six classes 0, 2, 3, 5, 6 and 7 above [Sch94a]. Various restrictions exist
for this option, as discussed in section 8.3.1.
direct×direct (see note 5); intended for virtual photons.
direct×VMD (i.e. first photon direct, second VMD); intended for virtualphotons.
direct×anomalous; intended for virtual photons.
VMD×direct; intended for virtual photons.
VMD×VMD; intended for virtual photons.
VMD×anomalous; intended for virtual photons.
anomalous×direct; intended for virtual photons.
anomalous×VDM; intended for virtual photons.
anomalous×anomalous; intended for virtual photons.
a mixture of the nine above components, 11–19, in the same spirit as = 10
provides a mixture for real gammas (or a virtual gamma on a hadron). For
gamma-hadron, this option coincides with = 10.
direct×direct (see note 5).
direct×resolved.
resolved×direct.
resolved×resolved.
a mixture of the four above components, offering a simpler alternative to =20 in cases where the parton distributions of the photon have not been split
into VMD and anomalous components. For γ-hadron, only two components
need be mixed.
DIS×VMD/p.
DIS×anomalous.
VMD/p×DIS.
anomalous×DIS.
a mixture of all the 4 (for γ ∗ p) or 13 (for γ ∗ γ ∗ ) components that are available,
i.e. (the relevant ones of) 11–19 and 26–29 above; is as = 20 with the DIS
processes mixed in.
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Note 1: the MSTP(14) options apply for a photon defined by a ’gamma’ or’gamma/lepton’ beam in the PYINIT call, but not to those photons implicitly
obtained in a ’lepton’ beam with the MSTP(12) = 1 option. This latter approach to resolved photons is more primitive and is no longer recommended
for QCD processes.
Note 2: for real photons our best understanding of how to mix event classes isprovided by the option 10 above, which also can be obtained by combining
three (for γp) or six (for γγ) separate runs. In a simpler alternative the
VMD and anomalous classes are joined into a single resolved class. Thenγp physics only requires two separate runs, with 0 and 1, and γγ physics
requires three, with 0, 1 and 4.
Note 3: most of the new options from 11 onwards are not needed and therefore notdefined for ep collisions. The recommended ’best’ value thus is MSTP(14) =30, which also is the new default value.
Note 4: as a consequence of the appearance of new event classes, the MINT(122) andMSTI(9) codes are not the same for γ ∗ γ ∗ events as for γp, γ ∗ p or γγ ones.
Instead the code is 3(i1 − 1) + i2 , where i is 1 for direct, 2 for VMD and 3 foranomalous/GVMD and indices refer to the two incoming photons. For γ ∗ pcode 4 is DIS, and for γ ∗ γ ∗ codes 10–13 corresponds to the MSTP(14) codes
26–29. As before, MINT(122) and MSTI(9) are only defined when several
processes are to be mixed, not when generating one at a time. Also the
MINT(123) code is modified (not shown here).
Note 5: the direct×direct event class excludes lepton pair production when run
with the default MSEL = 1 option (or MSEL = 2), in order not to confuse
users. You can obtain lepton pairs as well, e.g. by running with MSEL = 0
and switching on the desired processes by hand.
Note 6: for all non-QCD processes, a photon is assumed unresolved when MSTP(14)= 10, 20 or 25. In principle, both the resolved and direct possibilities ought
to be explored, but this mixing is not currently implemented, so picking
direct at least will explore one of the two main alternatives rather thannone. Resolved processes can be accessed by the more primitive machineryof having a lepton beam and MSTP(12) = 1.
MSTP(15) : (D = 0) possibility to modify the nature of the anomalous photon component(as used with the appropriate MSTP(14) options), in particular with respect to thescale choices and cut-offs of hard processes. These options are mainly intended
for comparative studies and should not normally be touched. Some of the issues
are discussed in [Sch93a], while others have only been used for internal studies
and are undocumented.
= 0 : none, i.e. the same treatment as for the VMD component.
= 1 : evaluate the anomalous parton distributions at a scale Q2 /PARP(17)2 .
= 2 : as = 1, but instead of PARP(17) use either PARP(81)/PARP(15) or
PARP(82)/PARP(15), depending on MSTP(82) value.
= 3 : evaluate the anomalous parton distribution functions of the photon as
f γ,anom (x, Q2 , p20 ) − f γ,anom (x, Q2 , r2 Q2 ) with r =PARP(17).
= 4 : as = 3, but instead of PARP(17) use either PARP(81)/PARP(15) or
PARP(82)/PARP(15), depending on MSTP(82) value.
= 5 : use larger p⊥min for the anomalous component than for the VMD one, butotherwise no difference.
MSTP(16) : (D = 1) choice of definition of the fractional momentum taken by a photon
radiated off a lepton. Enters in the flux factor for the photon rate, and therebyin cross sections.
= 0 : x, i.e. energy fraction in the rest frame of the event.
= 1 : y, i.e. light-cone fraction.
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MSTP(17) : (D = 4) possibility of a extra factor for processes involving resolved virtualphotons, to approximately take into account the effects of longitudinal photons.
Given on the form
R = 1 + PARP(165) r(Q2 , µ2 ) fL (y, Q2 )/fT (y, Q2 ).
Here the 1 represents the basic transverse contribution, PARP(165) is some arbitrary overall factor, and fL /fT the (known) ratio of longitudinal to transverse
photon flux factors. The arbitrary function r depends on the photon virtuality Q2 and the hard scale µ2 of the process. See [Fri00] for a discussion of theoptions.
= 0 : No contribution, i.e. r = 0.
= 1 : r = 4µ2 Q2 /(µ2 + Q2 )2 .
= 2 : r = 4Q2 /(µ2 + Q2 ).
= 3 : r = 4Q2 /(m2ρ + Q2 ).
= 4 : r = 4m2V Q2 /(m2V + Q2 )2 , where mV is the vector meson mass for VMD and
2k⊥ for GVMD states. Since there is no µ dependence here (as well as for= 3 and = 5) also minimum-bias cross sections are affected, where µ would
be vanishing. Currently the actual vector meson mass in the VMD case isreplaced by mρ , for simplicity.
= 5 : r = 4Q2 /(m2V + Q2 ), with mV and comments as above.
Note: for a photon given by the ’gamma/lepton’ option in the PYINIT call, the yspectrum is dynamically generated and y is thus known from event to event.
For a photon beam in the PYINIT call, y is unknown from the onset, and has
to be provided by you if any longitudinal factor is to be included. So longas these values, in PARP(167) and PARP(168), are at their default values, 0,
it is assumed they have not been set and thus the MSTP(17) and PARP(165)
values are inactive.
MSTP(18) : (D = 3) choice of p⊥min for direct processes.
= 1 : same as for VMD and GVMD states, i.e. the p⊥min (W 2 ) scale. Primarily
intended for real photons.
= 2 : p⊥min is chosen to be PARP(15), i.e. the original old behaviour proposed in[Sch93, Sch93a]. In that case, also parton distributions, jet cross sectionsand αs values were dampened for small p⊥ , so it may not be easy to obtainfull backwards compatibility with this option.
= 3 : as = 1, but if the Q scale of the virtual photon is above the VMD/GVMD
p⊥min (W 2 ), p⊥min is chosen equal to Q. This is part of the strategy to mix
in DIS processes at p⊥ below Q, e.g. in MSTP(14) = 30.
MSTP(19) : (D = 4) choice of partonicPcross section in the DIS process 99.
= 0 : QPM answer 4π 2 αem /Q2 q e2q (xq(x, Q2 ) + xq(x, Q2 )) (with parton distributions frozen below the lowest Q allowed in the parameterization). Notethat this answer is divergent for Q2 → 0 and thus violates gauge invariance.
= 1 : QPM answer is modified by a factor Q2 /(Q2 + m2ρ ) to provide a finite crosssection in the Q2 → 0 limit. A minimal regularization recipe.
= 2 : QPM answer is modified by a factor Q4 /(Q2 + m2ρ )2 to provide a vanishingcross section in the Q2 → 0 limit. Appropriate if one assumes that thenormal photoproduction description gives the total cross section for Q2 = 0,
without any DIS contribution.
= 3 : as = 2, but additionally suppression by a parameterized factor f (W 2 , Q2 )(different for γ ∗ p and γ ∗ γ ∗ ) that avoids double-counting the direct-process
region where p⊥ > Q. Shower evolution for DIS events is then also restricted
to be at scales below Q, whereas evolution all the way up to W is allowedin the other options above.
= 4 : as = 3, but additionally include factor 1/(1 − x) for conversion from F2 to198

σ. This is formally required, but is only relevant for small W 2 and therefore
often neglected.
MSTP(20) : (D = 3) suppression of resolved (VMD or GVMD) cross sections, introduced
to compensate for an overlap with DIS processes in the region of intermediate
Q2 and rather small W 2 .
= 0 : no; used as is.
> 1 : yes, by a factor (W 2 /(W 2 + Q21 + Q22 ))MSTP(20) (where Q2i = 0 for an
incoming hadron).
Note: the suppression factor is joined with the dipole suppression stored in
VINT(317) and VINT(318).
MSTP(21) : (D = 1) nature of fermion–fermion scatterings simulated in process 10 byt-channel exchange.
= 0 : all off (!).
= 1 : full mixture of γ ∗ /Z0 neutral current and W± charged current.
= 2 : γ neutral current only.
= 3 : Z0 neutral current only.
= 4 : γ ∗ /Z0 neutral current only.
= 5 : W± charged current only.
MSTP(22) : (D = 0) special override of normal Q2 definition used for maximum of partonshower evolution, intended for Deeply Inelastic Scattering in lepton–hadron
events, see section 10.4.
MSTP(23) : (D = 1) for Deeply Inelastic Scattering processes (10 and 83), this option
allows the x and Q2 of the original hard scattering to be retained by the final electron when showers are considered (with warnings as below; partly obsolete).
= 0 : no correction procedure, i.e. x and Q2 of the scattered electron differ fromthe originally generated x and Q2 .
= 1 : post facto correction, i.e. the change of electron momentum, by initial andfinal QCD radiation, primordial k⊥ and beam-remnant treatment, is corrected for by a shuffling of momentum between the electron and hadron sidein the final state. Only process 10 is corrected, while process 83 is not.
= 2 : as = 1, except that both process 10 and 83 are treated. This option isdangerous, especially for top, since it may well be impossible to ‘correct’in process 83: the standard DIS kinematics definitions are based on theassumption of massless quarks. Therefore infinite loops are not excluded.
Note 1: the correction procedure will fail for a fraction of the events, which are thus
rejected (and new ones generated in their place). The correction option is
not unambiguous, and should not be taken too seriously. For very small Q2
values, the x is not exactly preserved even after this procedure.
Note 2: this switch does not affect the recommended DIS description obtained witha ’gamma/lepton’ beam/target in PYINIT, where x and Q2 are always conserved.
MSTP(25) : (D = 0) angular decay correlations in Higgs decays to W+ W− or Z0 Z0 tofour fermions [Skj93].
= 0 : assuming the Higgs decay is pure scalar for h0 and H0 and pure pseudoscalarfor A0 .
= 1 : assuming the Higgs decay is always pure scalar (CP-even).
= 2 : assuming the Higgs decay is always pure pseudoscalar (CP-odd).
= 3 : assuming the Higgs decay is a mixture of the two (CP-even and CP-odd),
including the CP-violating interference term. The parameter η, PARP(25)sets the strength of the CP-odd admixture, with the interference term being
proportional to η and the CP-odd one to η 2 .
Note : since the decay of an A0 to W+ W− or Z0 Z0 is vanishing at the Born level,and no loop diagrams are included, currently this switch is only relevant for199

h0 and H0 . It is mainly intended to allow ‘straw man’ studies of the quantum
numbers of a Higgs state, decoupled from the issue of branching ratios.
MSTP(31) : (D = 1) parameterization of total, elastic and diffractive cross sections.
= 0 : everything is to be set by you yourself in the PYINT7 common block. For
photoproduction, additionally you need to set VINT(281). Normally you
would set these values once and for all before the PYINIT call, but if you run
with variable energies (see MSTP(171)) you can also set it before each new
PYEVNT call.
= 1 : Donnachie–Landshoff for total cross section [Don92], and Schuler–Sjöstrand
for elastic and diffractive cross sections [Sch94, Sch93a].
MSTP(32) : (D = 8) Q2 definition in hard scattering for 2 → 2 processes. For resonanceproduction Q2 is always chosen to be ŝ = m2R , where mR is the mass of the resonance. For gauge boson scattering processes V V → V V the W or Z0 squared
mass is used as scale in parton distributions. See PARP(34) for a possibility tomodify the choice below by a multiplicative factor.
The newer options 6–10 are specifically intended for processes with incoming
virtual photons. These are ordered from a ‘minimal’ dependence on the virtualities to a ‘maximal’ one, based on reasonable kinematics considerations. The
old default value MSTP(32) = 2 forms the starting point, with no dependence atall, and the new default is some intermediate choice. Notation is that P12 and P22
are the virtualities of the two incoming particles, p⊥ the transverse momentumof the scattering process, and m3 and m4 the masses of the two outgoing partons. For a direct photon, P 2 is the photon virtuality and x = 1. For a resolved
photon, P 2 still refers to the photon, rather than the unknown virtuality of thereacting parton in the photon, and x is the momentum fraction taken by thisparton.
= 1 : Q2 = 2ŝt̂û/(ŝ2 + t̂2 + û2 ).
= 2 : Q2 = (m2⊥3 + m2⊥4 )/2 = p2⊥ + (m23 + m24 )/2.
= 3 : Q2 = min(−t̂, −û).
= 4 : Q2 = ŝ.
= 5 : Q2 = −t̂.
= 6 : Q2 = (1 + x1 P12 /ŝ + x2 P22 /ŝ)(p2⊥ + m23 /2 + m24 /2).
= 7 : Q2 = (1 + P12 /ŝ + P22 /ŝ)(p2⊥ + m23 /2 + m24 /2).
= 8 : Q2 = p2⊥ + (P12 + P22 + m23 + m24 )/2.
= 9 : Q2 = p2⊥ + P12 + P22 + m23 + m24 .
= 10 : Q2 = s (the full energy-squared of the process).
= 11 : Q2 = (m3 + m4 )2 /4.
= 12 : Q2 is set by the user as fixed numbers, factorization scale in PARP(193) andrenormalization scale in PARP(194).
= 13 : Q2 = p2⊥ , i.e. without any dependence on masses.
Note: options 6 and 7 are motivated by assuming that one wants a scale thatinterpolates between t̂ for small t̂ and û for small û, such as Q2 = −t̂û/(t̂+û).
When kinematics for the 2 → 2 process is constructed as if an incoming
photon is massless when it is not, it gives rise to a mismatch factor 1 + P 2 /ŝ
(neglecting the other masses) in this Q2 definition, which is then what isused in option 7 (with the neglect of some small cross-terms when bothphotons are virtual). When a virtual photon is resolved, the virtuality of
the incoming parton can be anything from xP 2 and upwards. So option 6
uses the smallest kinematically possible value, while 7 is more representativeof the typical scale. Option 8 and 9 are more handwaving extensions of thedefault option, with 9 specially constructed to ensure that the Q2 scale isalways bigger than P 2 .
MSTP(33) : (D = 0) inclusion of K factors in hard cross sections for parton–parton
200interactions (i.e. for incoming quarks and gluons).
= 0 : none, i.e. K = 1.
= 1 : a common K factor is used, as stored in PARP(31).
= 2 : separate factors are used for ordinary (PARP(31)) and colour annihilationgraphs (PARP(32)).
= 3 : A K factor is introduced by a shift in the αs Q2 argument, αs =αs (PARP(33)Q2 ).
MSTP(34) : (D = 1) use of interference term in matrix elements for QCD processes, seesection 8.2.1.
= 0 : excluded (i.e. string-inspired matrix elements).
= 1 : included (i.e. conventional QCD matrix elements).
Note: for the option MSTP(34) = 1, i.e. interference terms included, these terms
are divided between the different possible colour configurations according to
the pole structure of the (string-inspired) matrix elements for the different
colour configurations.
MSTP(35) : (D = 0) threshold behaviour for heavy-flavour production, i.e. ISUB = 81,
82, 84, 85, and also for Z and Z0 decays. The non-standard options are mainly
intended for top, but can be used, with less theoretical reliability, also for charmand bottom (for Z and Z0 only top and heavier flavours are affected). The
threshold factors are given in eqs. (136) and (137).
= 0 : naı̈ve lowest-order matrix-element behaviour.
= 1 : enhancement or suppression close to threshold, according to the colour structure of the process. The αs value appearing in the threshold factor (which
is not the same as the αs of the lowest-order 2 → 2 process) is taken to be
fixed at the value given in PARP(35). The threshold factor used in an event
is stored in PARI(81).
= 2 : as = 1, but the αs value appearing
q in the threshold factor is taken to be run2
ning, with argument Q = mQ (m̂ − 2mQ )2 + Γ2Q . Here mQ is the nominal
heavy-quark mass, ΓQ is the width of the heavy-quark-mass distribution,and m̂ is the invariant mass of the heavy-quark pair. The ΓQ value has tobe stored by you in PARP(36). The regularization of αs at low Q2 is given
by MSTP(36).
MSTP(36) : (D = 2) regularization of αs in the limit Q2 → 0 for the threshold factor
obtainable in the MSTP(35) = 2 option; see MSTU(115) for a list of the possibilities.
MSTP(37) : (D = 1) inclusion of running quark masses in Higgs production (qq → h0 )and decay (h0 → qq) couplings, obtained by calls to the PYMRUN function. Also
included for charged Higgs and technipion production and decay.
= 0 : not included, i.e. fixed quark masses are used according to the values in the
PMAS array.
= 1 : included, with running starting from the value given in the PMAS array, at a
Q0 scale of PARP(37) times the quark mass itself, up to a Q scale given by
the Higgs mass. This option only works when αs is allowed to run (so one
can define a Λ value). Therefore it is only applied if additionally MSTP(2)≥ 1.
MSTP(38) : (D = 5) handling of quark loop masses in the box graphs gg → γγ and
gg → gγ, and in the Higgs production loop graphs qq → gh0 , qg → qh0 and
gg → gh0 , and their equivalents with H0 or A0 instead of h0 .
= 0 : for gg → γγ and gg → gγ the program will for each flavour automaticallychoose the massless approximation for light quarks and the full massive
formulae for heavy quarks, with a dividing line between light and heavy
quarks that depends on the actual ŝ scale. For Higgs production, all quark
loop contributions are included with the proper masses. This option is then201

correct only in the Standard Model Higgs scenario, and should not be used
e.g. in the MSSM.
≥1 : for gg → γγ and gg → gγ the program will use the massless approximation
throughout, assuming the presence of MSTP(38) effectively massless quark
species (however, at most 8). Normally one would use = 5 for the inclusionof all quarks up to bottom, and = 6 to include top as well. For Higgs
production, the approximate expressions derived in the mt → ∞ limit areused, rescaled to match the correct gg → h0 /H0 /A0 cross sections. This
procedure should work, approximately, also for non-standard Higgs particles.
Warning: for = 0, numerical instabilities may arise in gg → γγ and gg → gγ for
scattering at small angles. You are therefore recommended in this case to
set CKIN(27) and CKIN(28) so as to exclude the range of scattering angles
that is not of interest anyway. Numerical problems may also occur for Higgs
production with = 0, and additionally the lengthy expressions make the code
error-prone.
MSTP(39) : (D = 2) choice of Q2 scale for parton distributions and initial-state parton
showers in processes gg → QQh or qq → QQh.
= 1 : m2Q .
= 2 : max(m2⊥Q , m2⊥Q ) = m2Q + max(p2⊥Q , p2⊥Q ).
= 3 : m2h , where mh is the actual Higgs mass of the event, fluctuating from one
event to the next.
= 4 : ŝ = (ph + pQ + pQ )2 .
= 5 : m2h , where mh is the nominal, fixed Higgs mass.
= 6 : (m3 + m5 )2 /4.
= 7 : (m2⊥3 + m2⊥4 )/2.
= 8 : set by the user as fixed numbers, factorization scale in PARP(193) and renormalization scale in PARP(194).
MSTP(40) : (D = 0) option for Coulomb correction in process 25, W+ W− pair production, see [Kho96]. The value of the Coulomb correction factor for each event isstored in VINT(95).
= 0 : ‘no Coulomb’. Is the often-used reference point.
= 1 : ‘unstable Coulomb’, gives the correct first-order expression valid in the nonrelativistic limit. Is the reasonable option to use as a ‘best bet’ descriptionof LEP 2 physics.
= 2 : ‘second-order Coulomb’ gives the correct second-order expression valid in
the non-relativistic limit. In principle this is even better than = 1, but thedifferences are negligible and computer time does go up because of the needfor a numerical integration in the weight factor.
= 3 : ‘dampened Coulomb’, where the unstable Coulomb expression has been
modified by a (1 − β)2 factor in front of the arctan term. This is intended as
an alternative to = 1 within the band of our uncertainty in the relativistic
limit.
= 4 : ‘stable Coulomb’, i.e. effects are calculated as if the W’s were stable. Is
incorrect, and mainly intended for comparison purposes.
Note : unfortunately the W’s at LEP 2 are not in the non-relativistic limit, so theseparation of Coulomb effects from other radiative corrections is not gauge
invariant. The options above should therefore be viewed as indicative only,
not as the ultimate answer.
MSTP(41) : (D = 2) master switch for all resonance decays (Z0 , W± , t, h0 , Z00 , W0± , H0 ,A0 , H± , LQ , R0 , d∗ , u∗ , . . . ).
= 0 : all off.
= 1 : all on.

202

= 2 :
Note:on or off depending on their individual MDCY values.
also for MSTP(41) = 1 it is possible to switch off the decays of specific resonances by using the MDCY(KC,1) switches in Pythia. However, since the
MDCY values are overwritten in the PYINIT call when MSTP(41) = 1 (or 0),
individual resonances should then be switched off after the PYINIT call.
Warning: for top, leptoquark and other colour-carrying resonances it is dangerous
to switch off decays if one later on intends to let them decay (with PYEXEC);
see section 8.6.4.
MSTP(42) : (D = 1) mass treatment in 2 → 2 processes, where the final-state resonances
have finite width (see PARP(41)). (Does not apply for the production of a single
s-channel resonance, where the mass appears explicitly in the cross section of
the process, and thus is always selected with width.)= 0 : particles are put on the mass shell.
= 1 : mass generated according to a Breit–Wigner.
MSTP(43) : (D = 3) treatment of Z0 /γ ∗ interference in matrix elements. So far implemented in subprocesses 1, 15, 19, 22, 30 and 35; in other processes what is calleda Z0 is really a Z0 only, without the γ ∗ piece.
= 1 : only γ ∗ included.
= 2 : only Z0 included.
= 3 : complete Z0 /γ ∗ structure (with interference) included.
MSTP(44) : (D = 7) treatment of Z00 /Z0 /γ ∗ interference in matrix elements.
= 1 : only γ ∗ included.
= 2 : only Z0 included.
= 3 : only Z00 included.
= 4 : only Z0 /γ ∗ (with interference) included.
= 5 : only Z00 /γ ∗ (with interference) included.
= 6 : only Z00 /Z0 (with interference) included.
= 7 : complete Z00 /Z0 /γ ∗ structure (with interference) included.
MSTP(45) : (D = 3) treatment of WW → WW structure (ISUB = 77).
= 1 : only W+ W+ → W+ W+ and W− W− → W− W− included.
= 2 : only W+ W− → W+ W− included.
= 3 : all charge combinations WW → WW included.
MSTP(46) : (D = 1) treatment of V V → V 0 V 0 structures (ISUB = 71–77), where V
represents a longitudinal gauge boson.
= 0 : only s-channel Higgs exchange included (where existing). With this option,
subprocesses 71–72 and 76–77 will essentially be equivalent to subprocesses
5 and 8, respectively, with the proper decay channels (i.e. only Z0 Z0 or
W+ W− ) set via MDME. The description obtained for subprocesses 5 and 8 inthis case is more sophisticated, however; see section 8.5.2.
= 1 : all graphs contributing to V V → V 0 V 0 processes are included.
= 2 : only graphs not involving Higgs exchange (either in s, t or u channel) are
included; this option then gives the naı̈ve behaviour one would expect if noHiggs exists, including unphysical unitarity violations at high energies.
= 3 : the strongly interacting Higgs-like model of Dobado, Herrero and Terron[Dob91] with Padé unitarization. Note that to use this option it is necessary
to set the Higgs mass to a large number like 20 TeV (i.e. PMAS(25,1) =20000). The parameter ν is stored in PARP(44), but should not have to bechanged.
= 4 : as = 3, but with K-matrix unitarization [Dob91].
= 5 : the strongly interacting QCD-like model of Dobado, Herrero and Terron[Dob91] with Padé unitarization. The parameter ν is stored in PARP(44),but should not have to be changed. The effective techni-ρ mass in this modelis stored in PARP(45); by default it is 2054 GeV, which is the expected value
203

for three technicolors, based on scaling up the ordinary ρ mass appropriately.
as = 5, but with K-matrix unitarization [Dob91]. While PARP(45) still isa parameter of the model, this type of unitarization does not give rise to aresonance at a mass of PARP(45).
MSTP(47) : (D = 1) (C) angular orientation of decay products of resonances (Z0 , W± ,t, h0 , Z00 , W0± , etc.), either when produced singly or in pairs (also from an h0
decay), or in combination with a single quark, gluon or photon.
= 0 : independent decay of each resonance, isotropic in c.m. frame of the resonance.
= 1 : correlated decay angular distributions according to proper matrix elements,
to the extent these are implemented.
MSTP(48) : (D = 0) (C) switch for the treatment of γ ∗ /Z0 decay for process 1 in e+ e−
events.
= 0 : normal machinery.
= 1 : if the decay of the Z0 is to either of the five lighter quarks, d, u, s, c or b, thespecial treatment of Z0 decay is accessed, including matrix element options,according to section 6.1.
This option is based on the machinery of the PYEEVT and associated routines
when it comes to the description of QCD multijet structure and the angularorientation of jets, but relies on the normal PYEVNT machinery for everythingelse: cross section calculation, initial-state photon radiation, flavour composition of decays (i.e. information on channels allowed), etc.
The initial state has to be e+ e− ; forward-backward asymmetries would notcome out right for quark-annihilation production of the γ ∗ /Z0 and therefore
the machinery defaults to the standard one in such cases.
You can set the behaviour for the MSTP(48) option using the normal matrix
element related switches. This especially means MSTJ(101) for the selectionof first- or second-order matrix elements (= 1 and = 2, respectively). Further
selectivity is obtained with the switches and parameters MSTJ(102) (for the
angular orientation part only), MSTJ(103) (except the production thresholdfactor part), MSTJ(106), MSTJ(108) - MSTJ(111), PARJ(121), PARJ(122),
and PARJ(125) - PARJ(129). Information can be read from MSTJ(120),
MSTJ(121), PARJ(150), PARJ(152) - PARJ(156), PARJ(168), PARJ(169),
PARJ(171).
The other e+ e− switches and parameters should not be touched. In mostcases they are simply not accessed, since the related part is handled by the
PYEVNT machinery instead. In other cases they could give incorrect or misleading results. Beam polarization as set by PARJ(131) - PARJ(134), forinstance, is only included for the angular orientation, but is missing for the
cross section information. PARJ(123) and PARJ(124) for the Z0 mass andwidth are set in the PYINIT call based on the input mass and calculated
widths.
The cross section calculation is unaffected by the matrix element machinery.
Thus also for negative MSTJ(101) values, where only specific jet multiplicities are generated, the PYSTAT cross section is the full one.
MSTP(49) : (D = 1) assumed variation of the Higgs width to massive gauge boson
pairs,
√
+
−
0 0
± 0
i.e. W W , Z Z and W Z , as a function of the actual mass m̂ = ŝ and thenominal mass mh . The switch applies both to h0 , H0 , A0 and H± decays.
= 0 : the width is proportional to m̂3 ; thus the high-mass tail of the Breit–Wigner
is enhanced.
= 1 : the width is proportional to m2h m̂. For a fixed Higgs mass mh this meansa width variation across the Breit–Wigner more in accord with other resonances (such as the Z0 ). This alternative gives more emphasis to the low= 6 :204mass tail, where the parton distributions are peaked (for hadron colliders).
This option is favoured by resummation studies [Sey95a].
Note 1: the partial width of a Higgs to a fermion pair is always taken to be proportional to the actual Higgs mass m̂, irrespectively of MSTP(49). Also thewidth to a gluon or photon pair (via loops) is unaffected.
Note 2: this switch does not affect processes 71–77, where a fixed Higgs width isused in order to control cancellation of divergences.
MSTP(50) : (D = 0) Switch to allow or not longitudinally polarized incoming beams,
with the two polarizations stored in PARJ(131) and PARJ(132), respectively.
Most cross section expressions with polarization reduce to the unpolarized behaviour for the default PARJ(131) = PARJ(132) = 0., and then this switch issuperfluous and not implemented. Currently MSTP(50) is only used in process
25, ff → W+ W− , for reasons explained in section 8.8.
= 0 : no polarization effects, no matter what PARJ(131) and PARJ(132) values
are set.
= 1 : include polarization information in the cross section of the process and forangular correlations.
MSTP(51) : (D = 7) choice of proton parton-distribution set; see also MSTP(52).
= 1 : CTEQ 3L (leading order).
= 2 : CTEQ 3M (MS).
= 3 : CTEQ 3D (DIS).
= 4 : GRV 94L (leading order).
= 5 : GRV 94M (MS).
= 6 : GRV 94D (DIS).
= 7 : CTEQ 5L (leading order).
= 8 : CTEQ 5M1 (MS; slightly updated version of CTEQ 5M).
= 11 : GRV 92L (leading order).
= 12 : EHLQ set 1 (leading order; 1986 updated version).
= 13 : EHLQ set 2 (leading order; 1986 updated version).
= 14 : Duke–Owens set 1 (leading order).
= 15 : Duke–Owens set 2 (leading order).
= 16 : simple ansatz with all parton distributions of the form c/x, with c someconstant; intended for internal debug use only.
Note 1: distributions 11–15 are obsolete and should not be used for current physics
studies. They are only implemented to have some sets in common between
Pythia 5 and 6, for cross-checks.
Note 2: since all parameterizations have some region of applicability, the parton
distributions are assumed frozen below the lowest Q2 covered by the parameterizations. In some cases, evolution is also frozen above the maximum
Q2 .
MSTP(52) : (D = 1) choice of proton parton-distribution-function library.
= 1 : the internal Pythia one, with parton distributions according to theMSTP(51) above.
= 2 : the Pdflib one [Plo93], with the Pdflib (version 4) NGROUP and NSET
numbers to be given as MSTP(51) = 1000×NGROUP + NSET, or similarly for
the LHAPDF one [Gie02].
Note 1: to make use of option 2, it is necessary to link Pdflib/LHAPDF. Additionally, on most computers, the three dummy routines PDFSET, STRUCTM
and (for virtual photons) STRUCTP at the end of the Pythia file should be
removed or commented out.
Warning: for external parton distribution libraries, Pythia does not check whether
MSTP(51) corresponds to a valid code, or if special x and Q2 restrictions exist
for a given set, such that crazy values could be returned. This puts an extra
205responsibility on you.
Note 2: when Pdflib/LHAPDF is used, Pythia can initialize either with a fouror a five-flavour Λ, depending on how NFL in the /W50511/ commonblock isset, extracting either QCDL4 or QCDL5 from the /W50512/ commonblock.
MSTP(53) : (D = 3) choice of pion parton-distribution set; see also MSTP(54).
= 1 : Owens set 1.
= 2 : Owens set 2.
= 3 : GRV LO (updated version).
MSTP(54) : (D = 1) choice of pion parton-distribution-function library.
= 1 : the internal Pythia one, with parton distributions according to theMSTP(53) above.
= 2 : the Pdflib one [Plo93], with the Pdflib (version 4) NGROUP and NSET
numbers to be given as MSTP(53) = 1000×NGROUP + NSET, or similarly for
the LHAPDF one [Gie02].
Note: to make use of option 2, it is necessary to link Pdflib/LHAPDF. Additionally, on most computers, the three dummy routines PDFSET, STRUCTM and
STRUCTP at the end of the Pythia file should be removed or commented
out.
Warning: for external parton distribution libraries, Pythia does not check whether
MSTP(53) corresponds to a valid code, or if special x and Q2 restrictions exist
for a given set, such that crazy values could be returned. This puts an extra
responsibility on you.
MSTP(55) : (D = 5) choice of the parton-distribution set of the photon; see also MSTP(56)and MSTP(60).
= 1 : Drees–Grassie.
= 5 : SaS 1D (in DIS scheme, with Q0 = 0.6 GeV).
= 6 : SaS 1M (in ms scheme, with Q0 = 0.6 GeV).
= 7 : SaS 2D (in DIS scheme, with Q0 = 2 GeV).
= 8 : SaS 2M (in ms scheme, with Q0 = 2 GeV).
= 9 : SaS 1D (in DIS scheme, with Q0 = 0.6 GeV).
= 10 : SaS 1M (in ms scheme, with Q0 = 0.6 GeV).
= 11 : SaS 2D (in DIS scheme, with Q0 = 2 GeV).
= 12 : SaS 2M (in ms scheme, with Q0 = 2 GeV).
Note 1: sets 5–8 use the parton distributions of the respective set, and nothingelse. These are appropriate for most applications, e.g. jet production in
γp and γγ collisions. Sets 9–12 instead are appropriate for γ ∗ γ processes,
i.e. DIS scattering on a photon, as measured in F2γ . Here the anomalous
contribution for c and b quarks are handled by the Bethe-Heitler formulae,
and the direct term is artificially lumped with the anomalous one, so thatthe event simulation more closely agrees with what will be experimentallyobserved in these processes. The agreement with the F2γ parameterizationis still not perfect, e.g. in the treatment of heavy flavours close to threshold.
Note 2: sets 5–12 contain both VMD pieces and anomalous pieces, separately parameterized. Therefore the respective piece is automatically called, whatever
MSTP(14) value is used to select only a part of the allowed photon interactions. For other sets (set 1 above or Pdflib/LHAPDF sets), usually there
is no corresponding subdivision. Then an option like MSTP(14) = 2 (VMD
part of photon only) is based on a rescaling of the pion distributions, while
MSTP(14) = 3 gives the SaS anomalous parameterization.
Note 3: formally speaking, the k0 (or p0 ) cut-off in PARP(15) need not be set inany relation to the Q0 cut-off scales used by the various parameterizations.
Indeed, due to the familiar scale choice ambiguity problem, there could well
be some offset between the two. However, unless you know what you are206

doing, it is recommended that you let the two agree, i.e. set PARP(15) =0.6 for the SaS 1 sets and = 2. for the SaS 2 sets.
MSTP(56) : (D = 1) choice of photon parton-distribution-function library.
= 1 : the internal Pythia one, with parton distributions according to theMSTP(55) above.
= 2 : the Pdflib one [Plo93], with the Pdflib (version 4) NGROUP and NSET
numbers to be given as MSTP(55) = 1000×NGROUP + NSET, or similarly for
the LHAPDF one [Gie02]. When the VMD and anomalous parts of thephoton are split, like for MSTP(14) = 10, it is necessary to specify pion
set to be used for the VMD component, in MSTP(53) and MSTP(54), while
MSTP(55) here is irrelevant.
= 3 : when the parton distributions of the anomalous photon are requested, thehomogeneous solution is provided, evolved from a starting value PARP(15) tothe requested Q scale. The homogeneous solution is normalized so that the
net momentum is unity, i.e. any factors of αem /2π and charge have been leftout. The flavour of the original q is given in MSTP(55) (1, 2, 3, 4 or 5 for d, u,s, c or b); the value 0 gives a mixture according to squared charge, with the
exception that c and b are only allowed above the respective mass threshold(Q > mq ). The four-flavour Λ value is assumed given in PARP(1); it isautomatically recalculated for 3 or 5 flavours at thresholds. This option is
not intended for standard event generation, but is useful for some theoreticalstudies.
Note: to make use of option 2, it is necessary to link Pdflib/LHAPDF. Additionally, on most computers, the three dummy routines PDFSET, STRUCTM and
STRUCTP at the end of the Pythia file should be removed or commented
out.
Warning 1: for external parton-distribution libraries, Pythia does not check
whether MSTP(55) corresponds to a valid code, or if special x and Q2 restrictions exist for a given set, such that crazy values could be returned. Thisputs an extra responsibility on you.
Warning 2: so much of the machinery for virtual photons is based on a subdivision of
the photon according to the SaS prescription that a usage of Pdflib cannot
be recommended for such; in some cases unphysical results may arise frommismatches between what Pdflib delivers and what is assumed internally.
MSTP(57) : (D = 1) choice of Q2 dependence in parton-distribution functions.
= 0 : parton distributions are evaluated at nominal lower cut-off value Q20 , i.e. aremade Q2 -independent.
= 1 : the parameterized Q2 dependence is used.
= 2 : the parameterized parton-distribution behaviour is kept at large Q2 and x,
but modified at small Q2 and/or x, so that parton distributions vanish in
the limit Q2 → 0 and have a theoretically motivated small-x shape [Sch93a].
This option is only valid for the p and n. It is obsolete within the current’gamma/lepton’ framework.
= 3 : as = 2, except that also the π ± is modified in a corresponding manner.
A VMD photon is not mapped to a pion, but is treated with the same
photon parton distributions as for other MSTP(57) values, but with properly
modified behaviour for small x or Q2 . This option is obsolete within thecurrent ’gamma/lepton’ framework.
MSTP(58) : (D = min(5, 2×MSTP(1))) maximum number of quark flavours used in parton
distributions, and thus also for initial-state space-like showers. If some distributions (notably t) are absent in the parameterization selected in MSTP(51), theseare obviously automatically excluded.
MSTP(59) : (D = 1) choice of electron-inside-electron parton distribution.
207

= 1 :

the recommended standard for LEP 1, next-to-leading exponentiated, see[Kle89], p. 34.
= 2 : the recommended ‘β’ scheme for LEP 2, also next-to-leading exponentiated,see [Bee96], p. 130.
MSTP(60) : (D = 7) extension of the SaS real-photon distributions to off-shell photons,
especially for the anomalous component. See [Sch96] for an explanation of theoptions. The starting point is the expression in eq. (48), which requires a numerical integration of the anomalous component, however, and therefore is notconvenient. Approximately, the dipole damping factor can be removed and compensated by a suitably shifted lower integration limit, whereafter the integral
simplifies. Different ‘goodness’ criteria for the choice of the shifted lower limit isrepresented by the options 2–7 below.
= 1 : dipole dampening by integration; very time-consuming.
= 2 : P02 = max(Q20 , P 2 ).
= 3 : P 0 20 = Q20 + P 2 .
= 4 : Peff that preserves momentum sum.
= 5 : Pint that preserves momentum and average evolution range.
= 6 : Peff , matched to P0 in P 2 → Q2 limit.
= 7 : Pint , matched to P0 in P 2 → Q2 limit.
MSTP(61) : (D = 2) (C) master switch for initial-state QCD and QED radiation.
= 0 : off.
= 1 : on.
= 1 : on for QCD radiation in hadronic events and QED radiation in leptonic
ones.
= 2 : on for QCD and QED radiation in hadronic events and QED radiation in
leptonic ones.
MSTP(62) - MSTP(70) : (C) further switches for initial-state radiation, see section 10.4.
MSTP(71) : (D = 1) (C) master switch for final-state QCD and QED radiation.
= 0 : off.
= 1 : on.
Note: additional switches (e.g. for conventional/coherent showers) are available in
MSTJ(38) - MSTJ(50) and PARJ(80) - PARJ(90), see section 10.4.
MSTP(72): (C) further switch for initial-state radiation, see section 10.4.
MSTP(81) : (D = 1) master switch for multiple interactions.
= 0 : off.
= 1 : on.
MSTP(82) - MSTP(86) : further switches for multiple interactions, see section 11.6.
MSTP(91) - MSTP(95) : switches for beam-remnant treatment, see section 11.6.
MSTP(101) : (D = 3) (C) structure of diffractive system.
= 1 : forward moving diquark + interacting quark.
= 2 : forward moving diquark + quark joined via interacting gluon (‘hairpin’ configuration).
= 3 : a mixture of the two options above, with a fraction PARP(101) of the former
type.
MSTP(102) : (D = 1) (C) decay of a ρ0 meson produced by ‘elastic’ scattering of an incoming γ, as in γp → ρ0 p, or the same with the hadron diffractively excited.
= 0 : the ρ0 is allowed to decay isotropically, like any other ρ0 .
= 1 : the decay ρ0 → π + π − is done with an angular distribution proportional tosin2 θ in its rest frame, where the z axis is given by the direction of motionof the ρ0 . The ρ0 decay is then done as part of the hard process, i.e. also
when MSTP(111) = 0.
MSTP(110) : (D = 0) switch to allow some or all resonance widths to be modified by
the factor PARP(110). This is not intended for serious physics studies. The
208

main application is rather to generate events with an artificially narrow resonance width in order to study the detector-related smearing effects on the mass
resolution.
> 0 : rescale the particular resonance with KF = MSTP(110). If the resonance hasan antiparticle, this one is affected as well.
= -1 : rescale all resonances, except t, t, Z0 and W± .
= -2 : rescale all resonances.
Warning: only resonances with a width evaluated by PYWIDT are affected, and preferentially then those with MWID value 1 or 3. For other resonances theappearance of effects or not depends on how the cross sections have been
implemented. So it is important to check that indeed the mass distribution
is affected as expected. Also beware that, if a sequential decay chain is
involved, the scaling may become more complicated. Furthermore, depending on implementational details, a cross section may or may not scale with
PARP(110) (quite apart from differences related to the convolution with parton distributions etc.). All in all, it is then an option to be used only with
open eyes, and for very specific applications.
MSTP(111) : (D = 1) (C) master switch for fragmentation and decay, as obtained witha PYEXEC call.
= 0 : off.
= 1 : on.
= -1 : only choose kinematical variables for hard scattering, i.e. no jets are defined.
This is useful, for instance, to calculate cross sections (by Monte Carlo integration) without wanting to simulate events; information obtained with
PYSTAT(1) will be correct.
MSTP(112) : (D = 1) (C) cuts on partonic events; only affects an exceedingly tiny fractionof events. Normally this concerns what happens in the PYPREP routine, if a colour
singlet subsystem has a very small invariant mass and attempts to collapse it toa single particle fail, see section 12.4.1.
= 0 : no cuts (can be used only with independent fragmentation, at least in principle).
= 1 : string cuts (as normally required for fragmentation).
MSTP(113) : (D = 1) (C) recalculation of energies of partons from their momenta andmasses, to be done immediately before and after fragmentation, to partly compensate for some numerical problems appearing at high energies.
= 0 : not performed.
= 1 : performed.
MSTP(115) : (D = 0) (C) choice of colour rearrangement scenario for process 25, W+ W−
pair production, when both W’s decay hadronically. (Also works for process 22,
Z0 Z0 production, except when the Z’s are allowed to fluctuate to very small
masses.) See section 12.4.2 for details.
= 0 : no reconnection.
= 1 : scenario I, reconnection inspired by a type I superconductor, with the reconnection probability related to the overlap volume in space and time between
the W+ and W− strings. Related parameters are found in PARP(115) PARP(119), with PARP(117) of special interest.
= 2 : scenario II, reconnection inspired by a type II superconductor, with reconnection possible when two string cores cross. Related parameter in
PARP(115).
= 3 : scenario II’, as model II but with the additional requirement that a reconnection will only occur if the total string length is reduced by it.
= 5 : the GH scenario, where the reconnection can occur that reduces the total
string length (λ measure) most. PARP(120) gives the fraction of such event
209

where a reconnection is actually made; since almost all events could allow areconnection that would reduce the string length, PARP(120) is almost thesame as the reconnection probability.
= 11 : the intermediate scenario, where a reconnection is made at the ‘origin’ of
events, based on the subdivision of all radiation of a qq system as coming
either from the q or the q. PARP(120) gives the assumed probability thata reconnection will occur. A somewhat simpleminded model, but not quite
unrealistic.
= 12 : the instantaneous scenario, where a reconnection is allowed to occur before
the parton showers, and showering is performed inside the reconnected systems with maximum virtuality set by the mass of the reconnected systems.
PARP(120) gives the assumed probability that a reconnection will occur. Is
completely unrealistic, but useful as an extreme example with very large
effects.
MSTP(121) : (D = 0) calculation of kinematics selection coefficients and differential crosssection maxima for included (by you or default) subprocesses.
= 0 : not known; to be calculated at initialization.
= 1 : not known; to be calculated at initialization; however, the maximum value
then obtained is to be multiplied by PARP(121) (this may be useful if aviolation factor has been observed in a previous run of the same kind).
= 2 : known; kinematics selection coefficients stored by you in COEF(ISUB,J) (J= 1–20) in common block PYINT2 and maximum of the corresponding differential cross section times Jacobians in XSEC(ISUB,1) in common block
PYINT5. This is to be done for each included subprocess ISUB before initialization, with the sum of all XSEC(ISUB,1) values, except for ISUB = 95,
stored in XSEC(0,1).
MSTP(122) : (D = 1) initialization and differential cross section maximization print-out.
Also, less importantly, level of information on where in phase space a cross section
maximum has been violated during the run.
= 0 : none.
= 1 : short message at initialization; only when an error (i.e. not a warning) is
generated during the run.
= 2 : detailed message, including full maximization., at initialization; always during run.
MSTP(123) : (D = 2) reaction to violation of maximum differential cross section or to
occurence of negative differential cross sections (except when allowed for external
processes, i.e. when IDWTUP < 0).
= 0 : stop generation, print message.
= 1 : continue generation, print message for each subsequently larger violation.
= 2 : as = 1, but also increase value of maximum.
MSTP(124) : (D = 1) (C) frame for presentation of event.
= 1 : as specified in PYINIT.
= 2 : c.m. frame of incoming particles.
= 3 : hadronic c.m. frame for DIS events, with warnings as given for PYFRAM.
MSTP(125) : (D = 1) (C) documentation of partonic process, see section 5.3.2 for details.
= 0 : only list ultimate string/particle configuration.
= 1 : additionally list short summary of the hard process.
= 2 : list complete documentation of intermediate steps of parton-shower evolution.
MSTP(126) : (D = 100) number of lines at the beginning of event record that are reserved for event-history information; see section 5.3.2. This value should neverbe reduced, but may be increased at a later date if more complicated processes
210are included.
MSTP(127) : (D = 0) possibility to continue run even if none of the requested processes
have non-vanishing cross sections.
= 0 : no, the run will be stopped in the PYINIT call.
= 1 : yes, the PYINIT execution will finish normally, but with the flag MSTI(53)= 1 set to signal the problem. If nevertheless PYEVNT is called after this,the run will be stopped, since no events can be generated. If instead a new
PYINIT call is made, with changed conditions (e.g. modified supersymmetry parameters in a SUSY run), it may now become possible to initialize
normally and generate events.
MSTP(128) : (D = 0) storing of copy of resonance decay products in the documentation
section of the event record, and mother pointer (K(I,3)) relation of the actual
resonance decay products (stored in the main section of the event record) to the
documentation copy.
= 0 : products are stored also in the documentation section, and each productstored in the main section points back to the corresponding entry in the
documentation section.
= 1 : products are stored also in the documentation section, but the products
stored in the main section point back to the decaying resonance copy in themain section.
= 2 : products are not stored in the documentation section; the products stored
in the main section point back to the decaying resonance copy in the main
section.
MSTP(129) : (D = 10) for the maximization of 2 → 3 processes (ISET(ISUB) = 5) eachphase-space point in τ , y and τ 0 is tested MSTP(129) times in the other dimensions(at randomly selected points) to determine the effective maximum in the (τ , y,τ 0 ) point.
MSTP(131) : (D = 0) master switch for pile-up events, i.e. several independent hadron–hadron interactions generated in the same bunch–bunch crossing, with the events
following one after the other in the event record. See section 11.5 for details.
= 0 : off, i.e. only one event is generated at a time.
= 1 : on, i.e. several events are allowed in the same event record. Information on
the processes generated may be found in MSTI(41) - MSTI(50).
MSTP(132) - MSTP(134) : further switches for pile-up events, see section 11.6.
MSTP(141) : (D = 0) calling of PYKCUT in the event-generation chain, for inclusion ofuser-specified cuts.
= 0 : not called.
= 1 : called.
MSTP(142) : (D = 0) calling of PYEVWT in the event-generation chain, either to give
weighted events or to modify standard cross sections. See PYEVWT description insection 9.1 for further details.
= 0 : not called.
= 1 : called; the distribution of events among subprocesses and in kinematics variables is modified by the factor WTXS, set by you in the PYEVWT call, but
events come with a compensating weight PARI(10) = 1./WTXS, such that
total cross sections are unchanged.
= 2 : called; the cross section itself is modified by the factor WTXS, set by you in
the PYEVWT call.
MSTP(143) : (D = 0) calling of UPVETO in the event-generation chain, to give the possibly
to abort the generation of an event.
= 0 : not called, so no events are aborted (for this reason).
= 1 : yes, UPVETO is called, from inside the PYEVNT routine (but not from PYEVNW),
and a user can then decide whether to abort the current event or not.
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MSTP(145) : (D = 0) choice of polarization state for NRQCD production of charmonium
or bottomonium, processes in the ranges 421–439 and 461–479.
= 0 : unpolarized squared partonic amplitude.
= 1 : helicity or density matrix elements, as chosen by MSTP(146) and MSTP(147).
Only intended for experts.
MSTP(146) : (D = 1) choice of polarization reference frame when MSTP(145) = 1.
= 1 : recoil (recommended since it matches how Pythia defines particle directions, which the others do not obviously do).
= 2 : Gottfried–Jackson.
= 3 : target.
= 4 : Collins–Soper.
MSTP(147) : (D = 0) particular helicity or density matrix component when MSTP(145)= 1.
= 0 : helicity 0.
= 1 : helicity ±1.
= 2 : helicity ±2.
= 3 : density matrix element ρ0,0 .
= 4 : density matrix element ρ1,1 .
= 5 : density matrix element ρ1,0 .
= 6 : density matrix element ρ1,−1 .
(8)MSTP(148) : (D = 0) possibility to allow final-state shower evolution of the cc[3 S1 ] and(8)bb[3 S1 ] states produced in the NRQCD production of charmonium or bottomo(8)nium. Switching it on may exaggerate shower effects, since not all QQ[3 S1 ]comes from the fragmentation component where radiation is expected.
= 0 : off.
= 1 : on.
(8)
MSTP(149) : (D = 0) if the QQ[3 S1 ] states are allowed to radiate, MSTP(148) = 1, it(8)(8)
determines the kinematics of the QQ[3 S1 ] → QQ[3 S1 ] + g branching.
(8)= 0 : always pick the QQ[3 S1 ] to be the harder, i.e. z > 0.5.
= 1 : allow z < 0.5 and z > 0.5 equally.
MSTP(151) : (D = 0) introduce smeared position of primary vertex of events.
= 0 : no, i.e. the primary vertex of each event is at the origin.
= 1 : yes, with Gaussian distributions separately in x, y, z and t. The respective
widths of the Gaussians have to be given in PARP(151) - PARP(154). Alsopile-up events obtain separate primary vertices. No provisions are made formore complicated beam-spot shapes, e.g. with a spread in z that varies asa function of t. Note that a large beam spot combined with some of the
MSTJ(22) options may lead to many particles not being allowed to decay atall.
MSTP(161) : (D = 0) unit number of file on which PYUPIN should write its initialization
info, and from which UPINIT should read it back in, in cases where the Les
Houches Accord is used to store Pythia hard processes.
MSTP(162) : (D = 0) unit number of file on which PYUPEV should write its event info,
and from which UPEVNT should read it back in, in cases where the Les Houches
Accord is used to store Pythia hard processes.
MSTP(171) : (D = 0) possibility of variable energies from one event to the next. For
further details see section 9.8.
= 0 : no; i.e. the energy is fixed at the initialization call.
= 1 : yes; i.e. a new energy has to be given for each new event.
Warning: variable energies cannot be used in conjunction with the internal generation of a virtual photon flux obtained by a PYINIT call with ’gamma/lepton’
argument. The reason is that a variable-energy machinery is now used in212

ternally for the γ-hadron or γγ subsystem, with some information saved atinitialization for the full energy.
MSTP(172) : (D = 2) options for generation of events with variable energies, applicablewhen MSTP(171) = 1.
= 1 : an event is generated at the requested energy, i.e. internally a loop is performed over possible event configurations until one is accepted. If the requested c.m. energy of an event is below PARP(2) the run is aborted. Crosssection information can not be trusted with this option, since it depends on
how you decided to pick the requested energies.
= 2 : only one event configuration is tried. If that is accepted, the event is generated in full. If not, no event is generated, and the status code MSTI(61)= 1 is returned. You are then expected to give a new energy, looping until
an acceptable event is found. No event is generated if the requested c.m.
energy is below PARP(2), instead MSTI(61) = 1 is set to signal the failure.
In principle, cross sections should come out correctly with this option.
MSTP(173) : (D = 0) possibility for you to give in an event weight to compensate for abiased choice of beam spectrum.
= 0 : no, i.e. event weight is unity.
= 1 : yes; weight to be given for each event in PARP(173), with maximum weight
given at initialization in PARP(174).
MSTP(181) : (R) Pythia version number.
MSTP(182) : (R) Pythia subversion number.
MSTP(183) : (R) last year of change for Pythia.
MSTP(184) : (R) last month of change for Pythia.
MSTP(185) : (R) last day of change for Pythia.
PARP(1) : (D = 0.25 GeV) nominal ΛQCD used in running αs for hard scattering (seeMSTP(3)).
PARP(2) : (D = 10. GeV) lowest c.m. energy for the event as a whole that the program
will accept to simulate.
PARP(13) : (D = 1. GeV2 ) Q2max scale, to be set by you for defining maximum scale
allowed for photoproduction when using the option MSTP(13) = 2.
PARP(14) : (D = 0.01) in the numerical integration of quark and gluon parton distributions inside an electron, the successive halvings of evaluation-point spacingis interrupted when two values agree in relative size, |new−old|/(new+old), tobetter than PARP(14). There are hardwired lower and upper limits of 2 and 8
halvings, respectively.
PARP(15) : (D = 0.5 GeV) lower cut-off p0 used to define minimum transverse momentum in branchings γ → qq in the anomalous event class of γp interactions, i.e.
sets the dividing line between the VMD and GVMD event classes.
PARP(16) : (D = 1.) the anomalous parton-distribution functions of the photon
are taken to have the charm and bottom flavour thresholds at virtuality
PARP(16)×m2q .
PARP(17) : (D = 1.) rescaling factor used for the Q argument of the anomalous parton
distributions of the photon, see MSTP(15).
PARP(18) : (D = 0.4 GeV) scale kρ , such that the cross sections of a GVMD state of2
relative to those of a VMD state. Should
scale k⊥ is suppressed by a factor kρ2 /k⊥be of order mρ /2, with some finetuning to fit data.
PARP(25) : (D = 0.) parameter η describing the admixture of CP-odd Higgs decays for
MSTP(25) = 3.
PARP(31) : (D = 1.5) common K factor multiplying the differential cross section for
hard parton–parton processes when MSTP(33) = 1 or 2, with the exception ofcolour annihilation graphs in the latter case.

213

PARP(32) : (D = 2.0) special K factor multiplying the differential cross section in hardcolour annihilation graphs, including resonance production, when MSTP(33) =2.
PARP(33) : (D = 0.075) this factor is used to multiply the ordinary Q2 scale in αs at
the hard interaction for MSTP(33) = 3. With the default value, which is only to
be taken as an example, the effective K factor thus obtained for jet production
is in accordance with the NLO results in [Ell86], modulo the danger of doublecounting because of parton-shower corrections to jet rates.
PARP(34) : (D = 1.) the Q2 scale defined by MSTP(32) is multiplied by PARP(34) when
it is used as argument for parton distributions and αs at the hard interaction. It
does not affect αs when MSTP(33) = 3, nor does it change the Q2 argument ofparton showers.
PARP(35) : (D = 0.20) fix αs value that is used in the heavy-flavour threshold factor
when MSTP(35) = 1.
PARP(36) : (D = 0. GeV) the width ΓQ for the heavy flavour studied in processes ISUB
= 81 or 82; to be used for the threshold factor when MSTP(35) = 2.
PARP(37) : (D = 1.) for MSTP(37) = 1 this regulates the point at which the reference on-shell quark mass in Higgs and technicolor couplings is assumed defined in PYMRUN calls; specifically the running quark mass is assumed to coincide with the fix one at an energy scale PARP(37) times the fix quark mass,
i.e. mrunning (PARP(37)×mfix ) = mfix . See discussion at eq. (90) on ambiguity of
PARP(37) choice.
PARP(38) : (D = 0.70 GeV3 ) the squared wave function at the origin, |R(0)|2 , of the
J/ψ wave function. Used for processes 86 and 106–108. See ref. [Glo88].
PARP(39) : (D = 0.006 GeV3 ) the squared derivative of the wave function at the origin,|R0 (0)|2 /m2 , of the χc wave functions. Used for processes 87–89 and 104–105.
See ref. [Glo88].
PARP(41) : (D = 0.020 GeV) in the process of generating mass for resonances, andoptionally to force that mass to be in a given range, only resonances with a total
width in excess of PARP(41) are generated according to a Breit–Wigner shape(if allowed by MSTP(42)), while narrower resonances are put on the mass shell.
PARP(42) : (D = 2. GeV) minimum mass of resonances assumed to be allowed whenevaluating total width of h0 to Z0 Z0 or W+ W− for cases when the h0 is so lightthat (at least) one Z/W is forced to be off the mass shell. Also generally used assafety check on minimum mass of resonance. Note that some CKIN values may
provide additional constraints.
PARP(43) : (D = 0.10) precision parameter used in numerical integration of width for achannel with at least one daughter off the mass shell.
PARP(44) : (D = 1000.) the ν parameter of the strongly interacting Z/W model of
Dobado, Herrero and Terron [Dob91]; see MSTP(46) = 3.
PARP(45) : (D = 2054. GeV) the effective techni-ρ mass parameter of the stronglyinteracting model of Dobado, Herrero and Terron [Dob91]; see MSTP(46) = 5.
On physical grounds it should not be chosen smaller than about 1 TeV or larger
than about the default value.
PARP(46) : (D = 123. GeV) the Fπ decay constant that appears inversely quadraticallyin all techni-η partial decay widths [Eic84, App92].
PARP(47) : (D = 246. GeV) vacuum expectation value v used in the DHT scenario[Dob91] to define the width of the techni-ρ; this width is inversely proportionalv2.
PARP(48) : (D = 50.) the Breit–Wigner factor in the cross section is set to vanish formasses that deviate from the nominal one by more than PARP(48) times thenominal resonance width (i.e. the width evaluated at the nominal mass). Is
used in most processes with a single s-channel resonance, but there are some214

exceptions, notably γ ∗ /Z0 and W± . The reason for this option is that the conventional Breit–Wigner description is at times not really valid far away from theresonance position, e.g. because of interference with other graphs that should
then be included. The wings of the Breit–Wigner can therefore be removed.
PARP(50) : (D = 0.054) dimensionless coupling, which enters quadraticallyin all partial√∗
widths of the excited graviton G resonance, is κmG∗ = 2x1 k/M Pl , wherex1 ≈ 3.83 is the first zero of the J1 Bessel function and M Pl is the modified
Planck mass scale [Ran99, Bij01].
PARP(61) - PARP(65) : (C) parameters for initial-state radiation, see section 10.4.
PARP(71) - PARP(72) : (C) parameter for final-state radiation, see section 10.4.
PARP(78) - PARP(90) : parameters for multiple interactions, see section 11.6.
PARP(91) - PARP(100) : parameters for beam-remnant treatment, see section 11.6.
PARP(101) : (D = 0.50) fraction of diffractive systems in which a quark is assumed
kicked out by the pomeron rather than a gluon; applicable for option MSTP(101)= 3.
PARP(102) : (D = 0.28 GeV) the mass spectrum of diffractive states (in single anddouble diffractive scattering) is assumed to start PARP(102) above the mass ofthe particle that is diffractively excited. In this connection, an incoming γ istaken to have the selected VMD meson mass, i.e. mρ , mω , mφ or mJ/ψ .
PARP(103) : (D = 1.0 GeV) if the mass of a diffractive state is less than PARP(103)above the mass of the particle that is diffractively excited, the state is forced todecay isotropically into a two-body channel. In this connection, an incoming γ
is taken to have the selected VMD meson mass, i.e. mρ , mω , mφ or mJ/ψ . If themass is higher than this threshold, the standard string fragmentation machinery
is used. The forced two-body decay is always carried out, also when MSTP(111)= 0.
PARP(104) : (D = 0.8 GeV) minimum energy above threshold for which hadron–hadron
total, elastic and diffractive cross sections are defined. Below this energy, analternative description in terms of specific few-body channels would have beenrequired, and this is not modelled in Pythia.
PARP(110) : (D = 1.) a rescaling factor for resonance widths, applied when MSTP(110)is switched on.
PARP(111) : (D = 2. GeV) used to define the minimum invariant mass of the remnant
hadronic system (i.e. when interacting partons have been taken away), together
with original hadron masses and extra parton masses. For a hadron or resolved
photon beam, this also implies a further constraint that the x of an interacting
parton be below 1 − 2 × PARP(111)/Ecm .
PARP(115) : (D = 1.5 fm) (C) the average fragmentation time of a string, giving the
exponential suppression that a reconnection cannot occur if strings decayed before crossing. Is implicitly fixed by the string constant and the fragmentation
function parameters, and so a significant change is not recommended.
PARP(116) : (D = 0.5 fm) (C) width of the type I string in reconnection calculations,giving the radius of the Gaussian distribution in x and y separately.
PARP(117) : (D = 0.6) (C) kI , the main free parameter in the reconnection probability
for scenario I; the probability is given by PARP(117) times the overlap volume,
up to saturation effects.
PARP(118), PARP(119) : (D = 2.5, 2.0) (C) fr and ft , respectively, used in the Monte
Carlo sampling of the phase space volume in scenario I. There is no real reason
to change these numbers.
PARP(120) : (D = 1.0) (D) (C) fraction of events in the GH, intermediate and instantaneous scenarios where a reconnection is allowed to occur. For the GH one
a further suppression of the reconnection rate occurs from the requirement of
reduced string length in a reconnection.
215

PARP(121) : (D = 1.) the maxima obtained at initial maximization are multiplied bythis factor if MSTP(121) = 1; typically PARP(121) would be given as the productof the violation factors observed (i.e. the ratio of final maximum value to initialmaximum value) for the given process(es).
PARP(122) : (D = 0.4) fraction of total probability that is shared democratically between
the COEF coefficients open for the given variable, with the remaining fraction
distributed according to the optimization results of PYMAXI.
PARP(131) : parameter for pile-up events, see section 11.6.
PARP(141) - PARP(150) : (D = 10*1.) matrix elements for charmonium and bottomonium production in the non-relativistic QCD framework (NRQCD). Current values are dummy only, and will be updated soon. These values are used in processes
421–439 and 461–479.
(1)
PARP(141) : hOJ/ψ [3 S1 ]i.
(8)
PARP(142) : hOJ/ψ [3 S1 ]i.
(8)
PARP(143) : hOJ/ψ [1 S0 ]i.
(8)PARP(144) : hOJ/ψ [3 P0 ]i/m2c .
(1)
PARP(145) : hOχc0 [3 P0 ]i/m2c .
(1)
PARP(146) : hOΥ [3 S1 ]i.
(8)
PARP(147) : hOΥ [3 S1 ]i.
(8)
PARP(148) : hOΥ [1 S0 ]i.
(8)PARP(149) : hOΥ [3 P0 ]i/m2b .
(1)
PARP(150) : hOχb0 [3 P0 ]i/m2b .
PARP(151) - PARP(154) : (D = 4*0.) (C) regulate the assumed beam-spot size. For
MSTP(151) = 1 the x, y, z and t coordinates of the primary vertex of each
event are selected according to four independent Gaussians. The widths of these
Gaussians are given by the four parameters, where the first three are in units ofmm and the fourth in mm/c.
PARP(161) - PARP(164) : (D = 2.20, 23.6, 18.4, 11.5) couplings fV2 /4π of the photon
to the ρ0 , ω, φ and J/ψ vector mesons.
PARP(165) : (D = 0.5) a simple multiplicative factor applied to the cross section for
the transverse resolved photons to take into account the effects of longitudinal
resolved photons, see MSTP(17). No preferred value, but typically one could use
PARP(165) = 1 as main contrast to the no-effect = 0, with the default arbitrarily
chosen in the middle.
PARP(167), PARP(168) : (D = 2*0) the longitudinal energy fraction y of an incoming
photon, side 1 or 2, used in the R expression given for MSTP(17) to evaluatefL (y, Q2 )/fT (y, Q2 ). Need not be supplied when a photon spectrum is generated
inside a lepton beam, but only when a photon is directly given as argument in
the PYINIT call.
PARP(171) : to be set, event-by-event, when variable energies are allowed, i.e.
when MSTP(171) = 1. If PYINIT is called with FRAME = ’CMS’ (= ’FIXT’),PARP(171) multiplies the c.m. energy (beam energy) used at initialization. For
the options ’3MOM’, ’4MOM’ and ’5MOM’, PARP(171) is dummy, since there themomenta are set in the P array. It is also dummy for the ’USER’ option, where
the choice of variable energies is beyond the control of Pythia.
PARP(173) : event weight to be given by you when MSTP(173) = 1.
PARP(174) : (D = 1.) maximum event weight that will be encountered in PARP(173)
during the course of a run with MSTP(173) = 1; to be used to optimize theefficiency of the event generation. It is always allowed to use a larger boundthan the true one, but with a corresponding loss in efficiency.
PARP(181) - PARP(189) : (D = 0.1, 0.01, 0.01, 0.01, 0.1, 0.01, 0.01, 0.01, 0.3) Yukawa
216

couplings of leptons to H++ , assumed same for H++
and H++
R . Is a symmetricL
3 × 3 array, where PARP(177+3*i+j) gives the coupling to a lepton pair withgeneration indices i and j. Thus the default matrix is dominated by the diagonal
elements and especially by the τ τ one.
PARP(190) : (D = 0.64) gL = e/ sin θW .
PARP(191) : (D = 0.64) gR , assumed same as gL .
PARP(192) : (D = 5 GeV) vL vacuum expectation√ value of the left-triplet. The corresponding vR is assumed given by vR = 2MWR /gR and is not stored explicitly.
PARP(193) : (D = 1D4 GeV2 ) factorization scale Q2 for parton densities, to be set byuser when MSTP(32) = 12 for 2 → 2 processes or MSTP(39) = 8 for 2 → 3 ones.
PARP(194) : (D = 1D4 GeV2 ) renormalization scale Q2 , to be set by user when MSTP(32)= 12 for 2 → 2 processes or MSTP(39) = 8 for 2 → 3 ones. For process 161 it
also sets the scale of running quark masses.

9.4

Further Couplings

In this section we collect information on the two routines for running αs and αem , and onother couplings of standard and non-standard particles found in the PYDAT1 and PYTCSM
common blocks. Although originally begun for applications within the traditional particle
sector, this section of PYDAT1 has rapidly expanded towards the non-standard aspects, andis thus more of interest for applications to specific processes. It could therefore equallywell have been put somewhere else in this manual. Several other couplings indeed appear
in the PARP array in the PYPARS common block, see section 9.3, and the choice between
the two has largely been dictated by availability of space. The improved simulationof the TechniColor Strawman Model, described in [Lan02, Lan02a], and the resulting
proliferation of model parameters, has led to the introduction of the new PYTCSM common
block.
ALEM = PYALEM(Q2)
Purpose: to calculate the running electromagnetic coupling constant αem . Expressions
used are described in ref. [Kle89]. See MSTU(101), PARU(101), PARU(103) andPARU(104).
Q2 :the momentum transfer scale Q2 at which to evaluate αem .
ALPS = PYALPS(Q2)
Purpose: to calculate the running strong coupling constant αs , e.g. in matrix elements
and resonance decay widths. (The function is not used in parton showers, however, where formulae rather are written in terms of the relevant Λ values.) The
first- and second-order expressions are given by eqs. (27) and (32). See MSTU(111)- MSTU(118) and PARU(111) - PARU(118) for options.
Q2 :the momentum transfer scale Q2 at which to evaluate αs .
PM = PYMRUN(KF,Q2)
Purpose: to give running masses of d, u, s, c, b and t quarks according to eq. (90). For
all other particles, the PYMASS function is called by PYMRUN to give the normal
mass. Such running masses appear e.g. in couplings of fermions to Higgs andtechnipion states.
KF :flavour code.
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Q2 :
Note:

the momentum transfer scale Q2 at which to evaluate αs .
the nominal values, valid at a reference scale
Q2ref = max((PARP(37)mnominal )2 , 4Λ2 ),are stored in PARF(91) - PARF(96).

COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
Purpose: to give access to a number of status codes and parameters which regulate theperformance of the program as a whole. Here only those related to couplings aredescribed; the main description is found in section 14.3.
MSTU(101) : (D = 1) procedure for αem evaluation in the PYALEM function.
= 0 : αem is taken fixed at the value PARU(101).
= 1 : αem is running with the Q2 scale, taking into account corrections fromfermion loops (e, µ, τ , d, u, s, c, b).
= 2 : αem is fixed, but with separate values at low and high Q2 . For Q2 below(above) PARU(104) the value PARU(101) (PARU(103)) is used. The former
value is then intended for real photon emission, the latter for electroweak
physics, e.g. of the W/Z gauge bosons.
MSTU(111) : (I, D=1) order of αs evaluation in the PYALPS function. Is overwritten in
PYEEVT, PYONIA or PYINIT calls with the value desired for the process understudy.
= 0 : αs is fixed at the value PARU(111). As extra safety, Λ =PARU(117) is set
in PYALPS so that the first-order running αs agrees with the desired fixed αsfor the Q2 value used.
= 1 : first-order running αs is used.
= 2 : second-order running αs is used.
MSTU(112) : (D = 5) the nominal number of flavours assumed in the αs expression, withrespect to which Λ is defined.
MSTU(113) : (D = 3) minimum number of flavours that may be assumed in αs expression,see MSTU(112).
MSTU(114) : (D = 5) maximum number of flavours that may be assumed in αs expression,see MSTU(112).
MSTU(115) : (D = 0) treatment of αs singularity for Q2 → 0 in PYALPS calls. (Relevant
e.g. for QCD 2 → 2 matrix elements in the p⊥ → 0 limit, but not for showers,
where PYALPS is not called.)= 0 : allow it to diverge like 1/ ln(Q2 /Λ2 ).
= 1 : soften the divergence to 1/ ln(1 + Q2 /Λ2 ).
= 2 : freeze Q2 evolution below PARU(114), i.e. the effective argument ismax(Q2 ,PARU(114)).
MSTU(118) : (I) number of flavours nf found and used in latest PYALPS call.
PARU(101) : (D = 0.00729735=1/137.04) αem , the electromagnetic fine structure constant at vanishing momentum transfer.
PARU(102) : (D = 0.232) sin2 θW , the weak mixing angle of the standard electroweakmodel.
PARU(103) : (D = 0.007764=1/128.8) typical αem in electroweak processes; used forQ2 > PARU(104) in the option MSTU(101) = 2 of PYALEM. Although it can technically be used also at rather small Q2 , this αem value is mainly intended for highQ2 , primarily Z0 and W± physics.
PARU(104) : (D = 1 GeV2 ) dividing line between ‘low’ and ‘high’ Q2 values in the option
MSTU(101) = 2 of PYALEM.
PARU(105) : (D = 1.16639E-5 GeV−2 ) GF , the Fermi constant of weak interactions.

218

PARU(108) : (I) the αem value obtained in the latest call to the PYALEM function.
PARU(111) : (D = 0.20) fix αs value assumed in PYALPS when MSTU(111) = 0 (and alsoin parton showers when αs is assumed fix there).
PARU(112) : (I, D=0.25 GeV) Λ used in running αs expression in PYALPS. LikeMSTU(111), this value is overwritten by the calling physics routines, and is therefore purely nominal.
PARU(113) : (D = 1.) the flavour thresholds, for the effective number of flavours nf to
use in the αs expression, are assumed to sit at Q2 =PARU(113)×m2q , where mq
is the quark mass. May be overwritten from the calling physics routine.
PARU(114) : (D = 4 GeV2 ) Q2 value below which the αs value is assumed constant for
MSTU(115) = 2.
PARU(115) : (D = 10.) maximum αs value that PYALPS will ever return; is used as a lastresort to avoid singularities.
PARU(117) : (I) Λ value (associated with MSTU(118) effective flavours) obtained in latest
PYALPS call.
PARU(118) : (I) αs value obtained in latest PYALPS call.
PARU(121) - PARU(130) : couplings of a new Z00 ; for fermion default values are givenby the Standard Model Z0 values, assuming sin2 θW = 0.23. Since a generation dependence is now allowed for the Z00 couplings to fermions, the variables
PARU(121) - PARU(128) only refer to the first generation, with the second generation in PARJ(180) - PARJ(187) and the third in PARJ(188) - PARJ(195)
following exactly the same pattern. Note that e.g. the Z00 width contains squared
couplings, and thus depends quadratically on the values below.
PARU(121), PARU(122) : (D = −0.693, −1.) vector and axial couplings of down
type quarks to Z00 .
PARU(123), PARU(124) : (D = 0.387, 1.) vector and axial couplings of up type
quarks to Z00 .
PARU(125), PARU(126) : (D = −0.08, −1.) vector and axial couplings of leptons
to Z00 .
PARU(127), PARU(128) : (D = 1., 1.) vector and axial couplings of neutrinos toZ00 .
PARU(129) : (D = 1.) the coupling Z 00 → W+ W− is taken to be PARU(129)×(theStandard Model Z0 → W+ W− coupling)×(mW /mZ0 )2 . This gives a Z00 →W+ W− partial width that increases proportionately to the Z00 mass.
PARU(130) : (D = 0.) in the decay chain Z00 → W+ W− → 4 fermions, the angular
distribution in the W decays is supposed to be a mixture, with fraction 1. PARU(130) corresponding to the same angular distribution between the four
final fermions as in Z0 → W+ W− (mixture of transverse and longitudinalW’s), and fraction PARU(130) corresponding to h0 → W+ W− the same way(longitudinal W’s).
PARU(131) - PARU(136) : couplings of a new W0± ; for fermions default values are givenby the Standard Model W± values (i.e. V − A). Note that e.g. the W0± widthcontains squared couplings, and thus depends quadratically on the values below.
PARU(131), PARU(132) : (D = 1., −1.) vector and axial couplings of a quark–
antiquark pair to W0± ; is further multiplied by the ordinary CKM factors.
PARU(133), PARU(134) : (D = 1., −1.) vector and axial couplings of a leptonneutrino pair to W0± .
PARU(135) : (D = 1.) the coupling W0± → Z0 W± is taken to be PARU(135)×(theStandard Model W± → Z0 W± coupling)×(mW /mW 0 )2 . This gives a W0± →Z0 W± partial width that increases proportionately to the W0 mass.
PARU(136) : (D = 0.) in the decay chain W0± → Z0 W± → 4 fermions, the angular
distribution in the W/Z decays is supposed to be a mixture, with fraction219

1-PARU(136) corresponding to the same angular distribution between thefour final fermions as in W± → Z0 W± (mixture of transverse and longitudinal W/Z’s), and fraction PARU(136) corresponding to H± → Z0 W± thesame way (longitudinal W/Z’s).
PARU(141) : (D = 5.) tan β parameter of a two Higgs doublet scenario, i.e. the ratio of
vacuum expectation values. This affects mass relations and couplings in the Higgs
sector. If the Supersymmetry simulation is switched on, IMSS(1) nonvanishing,
PARU(141) will be overwritten by RMSS(5) at initialization, so it is the latter
variable that should be set.
PARU(142) : (D = 1.) the Z0 → H+ H− coupling is taken to be PARU(142)×(the MSSM
Z0 → H+ H− coupling).
PARU(143) : (D = 1.) the Z00 → H+ H− coupling is taken to be PARU(143)×(the MSSM
Z0 → H+ H− coupling).
PARU(145) : (D = 1.) quadratically multiplicative factor in the Z00 → Z0 h0 partial widthin left–right-symmetric models, expected to be unity (see [Coc91]).
PARU(146) : (D = 1.) sin(2α) parameter, enters quadratically as multiplicative factor
in the W0± → W± h0 partial width in left–right-symmetric models (see [Coc91]).
PARU(151) : (D = 1.) multiplicative factor in the LQ → q` squared Yukawa coupling,
and thereby in the LQ partial width and the q` → LQ and other cross sections.
Specifically, λ2 /(4π) =PARU(151)×αem , i.e. it corresponds to the k factor of[Hew88].
PARU(161) - PARU(168) : (D = 5*1., 3*0.) multiplicative factors that can be used
to modify the default couplings of the h0 particle in Pythia. Note that thefactors enter quadratically in the partial widths. The default values correspondto the couplings given in the minimal one-Higgs-doublet Standard Model, andare therefore not realistic in a two-Higgs-doublet scenario. The default values
should be changed appropriately by you. Also the last two default values should
be changed; for these the expressions of the minimal supersymmetric Standard
Model (MSSM) are given to show parameter normalization. Alternatively, the
SUSY machinery can generate all the couplings for IMSS(1), see MSTP(4).
PARU(161) : h0 coupling to down type quarks.
PARU(162) : h0 coupling to up type quarks.
PARU(163) : h0 coupling to leptons.
PARU(164) : h0 coupling to Z0 .
PARU(165) : h0 coupling to W± .
PARU(168) : h0 coupling to H± in γγ → h0 loops, in MSSM sin(β − α) +cos(2β) sin(β + α)/(2 cos2 θW ).
PARU(171) - PARU(178) : (D = 7*1., 0.) multiplicative factors that can be used tomodify the default couplings of the H0 particle in Pythia. Note that the factors enter quadratically in partial widths. The default values for PARU(171) PARU(175) correspond to the couplings given to h0 in the minimal one-Higgsdoublet Standard Model, and are therefore not realistic in a two-Higgs-doublet
scenario. The default values should be changed appropriately by you. Also
the last two default values should be changed; for these the expressions of theminimal supersymmetric Standard Model (MSSM) are given to show parameternormalization. Alternatively, the SUSY machinery can generate all the couplings
for IMSS(1), see MSTP(4).
PARU(171) : H0 coupling to down type quarks.
PARU(172) : H0 coupling to up type quarks.
PARU(173) : H0 coupling to leptons.
PARU(174) : H0 coupling to Z0 .
PARU(175) : H0 coupling to W ± .
PARU(176) : H0 coupling to h0 h0 , in MSSM cos(2α) cos(β +α)−2 sin(2α) sin(β +α).
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PARU(177) : H0 coupling to A0 A0 , in MSSM cos(2β) cos(β + α).
PARU(178) : H0 coupling to H± in γγ → H0 loops, in MSSM cos(β − α) −cos(2β) cos(β + α)/(2 cos2 θW ).
PARU(181) - PARU(190) : (D = 3*1., 2*0., 2*1., 3*0.) multiplicative factors that canbe used to modify the default couplings of the A0 particle in Pythia. Notethat the factors enter quadratically in partial widths. The default values for
PARU(181) - PARU(183) correspond to the couplings given to h0 in the minimal
one-Higgs-doublet Standard Model, and are therefore not realistic in a two-Higgsdoublet scenario. The default values should be changed appropriately by you.
PARU(184) and PARU(185) should be vanishing at the tree level, in the absenceof CP-violating phases in the Higgs sector, and are so set; normalization of these
couplings agrees with what is used for h0 and H0 . Also the other default values
should be changed; for these the expressions of the Minimal SupersymmetricStandard Model (MSSM) are given to show parameter normalization. Alternatively, the SUSY machinery can generate all the couplings for IMSS(1), see
MSTP(4).
PARU(181) : A0 coupling to down type quarks.
PARU(182) : A0 coupling to up type quarks.
PARU(183) : A0 coupling to leptons.
PARU(184) : A0 coupling to Z0 .
PARU(185) : A0 coupling to W± .
PARU(186) : A0 coupling to Z0 h0 (or Z∗ to A0 h0 ), in MSSM cos(β − α).
PARU(187) : A0 coupling to Z0 H0 (or Z∗ to A0 H0 ), in MSSM sin(β − α).
PARU(188) : As PARU(186), but coupling to Z00 rather than Z0 .
PARU(189) : As PARU(187), but coupling to Z00 rather than Z0 .
PARU(190) : A0 coupling to H± in γγ → A0 loops, 0 in MSSM.
PARU(191) - PARU(195) : (D = 4*0., 1.) multiplicative factors that can be used tomodify the couplings of the H± particle in Pythia. Currently only PARU(195)is in use. See above for related comments.
PARU(195) : H± coupling to W± h0 (or W∗± to H± h0 ), in MSSM cos(β − α).
PARU(197): (D = 0.) H0 coupling to W± H∓ within a two-Higgs-doublet model.
PARU(198): (D = 0.) A0 coupling to W± H∓ within a two-Higgs-doublet model.
PARJ(180) - PARJ(187) : couplings of the second generation fermions to the Z 00 , following the same pattern and with the same default values as the first one in
PARU(121) - PARU(128).
PARJ(188) - PARJ(195) : couplings of the third generation fermions to the Z 00 , following the same pattern and with the same default values as the first one in
PARU(121) - PARU(128).
COMMON/PYTCSM/ITCM(0:99),RTCM(0:99)
Purpose: to give access to a number of switches and parameters which regulate thesimulation of the TechniColor Strawman Model [Lan02, Lan02a], plus a few
further parameters related to the simulation of compositeness, mainly in earlier
incarnations of TechniColor.
ITCM(1) : (D = 4) NT C , number of technicolors; fixes the relative values of gem and getc .
ITCM(2) : (D = 0) Topcolor model.
= 0 : Standard Topcolor. Third generation quark couplings to the coloron areproportional to cot θ3 , see RTCM(21) below; first two generations are proportional to − tan θ3 .
= 1 : Flavor Universal Topcolor. All quarks couple with strength proportional to221cot θ3 .
ITCM(5) : (D = 0) presence of anomalous couplings in Standard Model processes, seesection 8.6.5 for further details.
= 0 : absent.
= 1 : left–left isoscalar model, with only u and d quarks composite (at the probed
scale).
= 2 : left–left isoscalar model, with all quarks composite.
= 3 : helicity-non-conserving model, with only u and d quarks composite (at theprobed scale).
= 4 : helicity-non-conserving model, with all quarks composite.
= 5 : coloured technihadrons, affecting the standard QCD 2 → 2 cross sections by
the exchange of Coloron or Colored Technirho, see section 8.6.7.
RTCM(1) : (D = 82 GeV) FT , the Technicolor decay constant.
RTCM(2) : (D = 4/3) QU , charge of up-type technifermion; the down-type technifermion
has a charge QD = QU − 1.
RTCM(3) : (D = 1/3) sin χ, where χ is the mixing angle between isotriplet technipioninteraction and√ mass eigenstates.
RTCM(4) : (D = 1/ 6) sin χ0 , where χ0 is the mixing angle between the isosinglet π 0 0tc
interaction and mass eigenstates.
RTCM(5) : (D = 1) Clebsch for technipi decays to charm. Appears squared in decay
rates.
RTCM(6) : (D = 1) Clebsch for technipi decays to bottom. Appears squared in decay
rates.
RTCM(7) : (D = 0.0182) Clebsch for technipi decays to top, estimated to be mb /mt .
Appears squared in decay rates.
RTCM(8) : (D = 1) Clebsch for technipi decays to τ . Appears squared in decay rates.
RTCM(9) : (D = 0) squared Clebsch for isotriplet technipi decays to gluons.
RTCM(10) : (D = 4/3) squared Clebsch for isosinglet technipi decays to gluons.
RTCM(11) : (D = 0.05) technirho–techniomega mixing parameters. Allows for isospinviolating decays of the techniomega.
RTCM(12) : (D = 200 GeV) vector technimeson decay parameter. Affects the decay rates
of vector technimesons into technipi plus transverse gauge boson.
RTCM(13) : (D = 200 GeV) axial mass parameter for technivector decays to transverse
gauge bosons√ and technipions.
RTCM(21) : (D = 0.08) tangent of Topcolor mixing angle, in the scenario with coloured
technihadrons described in section 8.6.7 and switched on with ITCM(5) = 5. For
ITCM(2) = 0, the coupling of the V8 to light quarks is suppressed by RTCM(21)2whereas the coupling to heavy (b and t) quarks is enhanced by 1/RTCM(21)2 . For
ITCM(21) = √1, the coupling to quarks is universal, and given by 1/RTCM(21)2 .
RTCM(22) : (D = 1/ 2) sine of isosinglet technipi mixing with Topcolor currents.
RTCM(23) : (D = 0) squared Clebsch for colour-octet technipi decays to charm.
RTCM(24) : (D = 0) squared Clebsch for colour-octet technipi decays to bottom.
RTCM(25) : (D = 0) squared Clebsch for colour-octet technipi decays to top.
RTCM(26) : (D = 5/3) squared Clebsch for colour-octet technipi decays to gluons.
RTCM(27) : (D = 250 GeV) colour-octet technirho decay parameter for decays to technipi
plus gluon.
RTCM(28) : (D = 250√GeV) hard mixing parameter between colour-octet technirhos.
RTCM(29) : (D = 1/ 2) magnitude of (1, 1) element of the U(2) matrices that diagonalize U-type technifermion condensates.
RTCM(30) : (D = 0 Radians)
phase for the element described above, RTCM(29).
√
RTCM(31) : (D = 1/ 2) Magnitude of (1, 1) element of the U(2) matrices that diagonalize D-type technifermion condensates.
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RTCM(32) : (D = 0 Radians) phase for the√ element described above, RTCM(31).
√
RTCM(33) : (D = 1) if ΓV8 (ŝ) > RTCM(33) ŝ, then ΓV8 (ŝ) is redefined to be RTCM(33) ŝ.
It thus prevents the coloron from becoming wider than its mass.
RTCM(41) : (D = 1000 GeV) compositeness scale Λ, used in processes involving excited
fermions, and for Standard Model processes when ITCM(5) is between 1 and 4.
RTCM(42) : (D = 1.) sign of the interference term between the standard cross section
and the compositeness term (η parameter); should be ±1; used for Standard
Model processes when ITCM(5) is between 1 and 4.
RTCM(43) - RTCM(45) : (D = 3*1.) strength of the SU(2), U(1) and SU(3) couplings,
respectively, in an excited fermion scenario; cf. f , f 0 and fs of [Bau90].
RTCM(46) : (D = 0.) anomalous magnetic moment of the W± in process 20; η = κ − 1,
where η = 0 (κ = 1) is the Standard Model value.

9.5

Supersymmetry Common-Blocks and Routines

The parameters available to the SUSY user are stored in the common block PYMSSM. In
general, options are set by the IMSS array, while real valued parameters are set by RMSS.
The entries IMSS(0) and RMSS(0) are not used, but are available for compatibility with
the C programming language. Note also that most options are only used by Pythia’sinternal SUSY machinery and are ineffective when external spectrum calculations areused, see section 8.7.5.
COMMON/PYMSSM/IMSS(0:99),RMSS(0:99)
Purpose: to give access to parameters that allow the simulation of the MSSM.
IMSS(1) : (D = 0) level of MSSM simulation.
= 0 : No MSSM simulation.
= 1 : A general MSSM simulation. The parameters of the model are set by thearray RMSS.
= 2 : An approximate SUGRA simulation using the analytic formulae of [Dre95]
to reduce the number of free parameters. In this case, only five input parameters are used. RMSS(1) is the common gaugino mass m1/2 , RMSS(8) is
the common scalar mass m0 , RMSS(4) fixes the sign of the higgsino mass µ,
RMSS(16) is the common trilinear coupling A, and RMSS(5) is tan β = v2 /v1 .
= 11 : Read spectrum from a SUSY Les Houches Accord (SLHA) conformant file.
The Logical Unit Number on which the file is opened should be put in
IMSS(21). If a decay table should also be read in, the corresponding Unit
Number (normally the same as the spectrum file) should be put in IMSS(22).
Cross sections are still calculated by Pythia, as are decays for those sparticles and higgs bosons for which a decay table is not found on the file.
= 12 : Invoke a runtime interface to Isasusy [Bae93] for determining SUSY mass
spectrum and mixing parameters. This provides a more precise solution of the renormalization group equations than is offered by the option = 2 above. The interface automatically asks the SUGRA routine (partof Isasusy) to solve the RGE’s for the weak scale mass spectrum andmixing parameters. The mSUGRA input parameters should be given in
RMSS as usual, i.e.: RMSS(1) = m1/2 , RMSS(4) = sign(µ), RMSS(5) = tan β,RMSS(8) = m0 , and RMSS(16) = A. As before, we are using the conventions of [Hab85, Gun86a] everywhere. Cross sections and decay widths arestill calculated by Pythia, using the output provided by Isasusy. Notethat since Pythia cannot always be expected to be linked with the Isajet
library, two dummy routines and a dummy function are included. These223

are SUGRA, SSMSSM and VISAJE, located towards the very bottom of the
Pythia source code. These routines must be removed and Pythia recompiled before a proper linking with Isajet can be achieved. Furthermore,
the common-block sizes and variable positions accessed in the SUGRA routine
have to match those of the Isajet version used, see section 8.7.5.
= 13 : File-based run-time Isasusy interface, i.e. using an Isajet input file. The
contents of the input file should be identical to what would normally be typed
when using the Isajet RGE executable stand-alone (normally isasugra.x).
The input file should be opened by the user in his/her main program andthe Logical Unit Number should be stored in IMSS(20), where Pythia
will look for it during initialization. Pythia will then pass the parameters
to the SUGRA subroutine in Isajet for RGE evolution and will afterwards
extract the electroweak scale mass and coupling spectra to its own common
blocks. For example, the first line of the input file should contain the model
code: 1 for mSUGRA, 2 for mGMSB, 3 for non-universal SUGRA, 4 for
SUGRA with truly unified gauge couplings, 5 for non-minimal GMSB, 6 for
SUGRA+right-handed neutrino, 7 for anomaly-mediated SUSY breaking.
The ensuing lines should contain the input parameters.
While option IMSS(1) = 12 above can only be used for mSUGRA scenarios,
but then is easy to use, the current option allows the full range of Isajet
models to be accessed.
IMSS(2) : (D = 0) treatment of U(1), SU(2), and SU(3) gaugino mass parameters.
= 0 : The gaugino parameters M1 , M2 and M3 are set by RMSS(1), RMSS(2), andRMSS(3), i.e. there is no forced relation between them.
= 1 : The gaugino parameters are fixed by the relation (3/5) M1 /α1 = M2 /α2 =M3 /α3 = X and the parameter RMSS(1). If IMSS(1) = 2, then RMSS(1) istreated as the common gaugino mass m1/2 and RMSS(20) is the GUT scale
coupling constant αGU T , so that X = m1/2 /αGU T .
= 2 : M1 is set by RMSS(1), M2 by RMSS(2) and M3 = M2 α3 /α2 . In such a
scenario, the U(1) gaugino mass behaves anomalously.
IMSS(3) : (D = 0) treatment of the gluino mass parameter.
= 0 : The gluino mass parameter M3 is used to calculate the gluino pole mass
with the formulae of [Kol96]. The effects of squark loops can significantly
shift the mass.
= 1 : M3 is the gluino pole mass. The effects of squark loops are assumed to havebeen included in this value.
IMSS(4) : (D = 1) treatment of the Higgs sector.
= 0 : The Higgs sector is determined by the approximate formulae of [Car95] andthe pseudoscalar mass MA set by RMSS(19).
= 1 : The Higgs sector is determined by the exact formulae of [Car95] and thepseudoscalar mass MA set by RMSS(19). The pole mass for MA is not thesame as the input parameter.
= 2 : The Higgs sector is fixed by the mixing angle α set by RMSS(18) and themass values PMAS(I,1), where I = 25, 35, 36, and 37.
= 3 : Call FeynHiggs [Hei99] for a precise calculation of Higgs masses. For thetime being, it can be invoked either when using an SLHA SUSY spectrum,
i.e. for IMSS(1) = 11, or when using the run-time interface to Isasusy,
i.e. for IMSS(1) = 12, 13. When FeynHiggs is to be linked, the threedummy routines FHSETFLAGS, FHSETPARA and FHHIGGSCORR need first be
removed from the Pythia library.
IMSS(5) : (D = 0) allows you to set the t̃, b̃ and τ̃ masses and mixing by hand.
= 0 : no, the program calculates itself.
224

= 1 :

yes, calculate from given input. The parameters RMSS(26) - RMSS(28)specify the mixing angle (in radians) for the sbottom, stop, and stau. The parameters RMSS(10) - RMSS(14) specify the two stop masses, the one sbottom mass (the other being fixed by the other parameters) and the two staumasses. Note that the masses RMSS(10), RMSS(11) and RMSS(13) correspond to the left-left entries of the diagonalized matrices, while RMSS(12)and RMSS(14) correspond to the right-right entries. Note that these entries
need not be ordered in mass.
IMSS(7) : (D = 0) treatment of the scalar masses in an extension of SUGRA models.
The presence of additional U(1) symmetries at high energy scales can modify
the boundary conditions for the scalar masses at the unification scale.
= 0 : No additional D-terms are included. In SUGRA models, all scalars have themass m0 at the unification scale.
= 1 : RMSS(23) - RMSS(25) are the values of DX , DY and DS at the unification
scale in the model of [Mar94]. The boundary conditions for the scalar masses
are shifted based on their quantum numbers under the additional U(1)symmetries.
IMSS(8) : (D = 0) treatment of the τ̃ mass eigenstates.
= 0 : The τ̃ mass eigenstates are calculated using the parameters
RMSS(13, 14, 17).
= 1 : The τ̃ mass eigenstates are identical to the interaction eigenstates, so they
are treated identically to ẽ and µ̃ .
IMSS(9) : (D = 0) treatment of the right-handed squark mass eigenstates for the firsttwo generations.
= 0 : The q̃R masses are fixed by RMSS(9). d̃R and ũR are identical except forElectroweak D-term contributions.
= 1 : The masses of d̃R and ũR are fixed by RMSS(9) and RMSS(22) respectively.
IMSS(10) : (D = 0) allowed decays for χ̃2 .
= 0 : The second lightest neutralino χ̃2 decays with a branching ratio calculated
from the MSSM parameters.
= 1 : χ̃2 is forced to decay only to χ̃1 γ, regardless of the actual branching ratio.
This can be used for detailed studies of this particular final state.
IMSS(11) : (D = 0) choice of the lightest superpartner (LSP).
= 0 : χ̃1 is the LSP.
= 1 : χ̃1 is the next to lightest superparter (NLSP) and the gravitino is the LSP.
The χ̃1 decay length is calculated from the gravitino mass set by RMSS(21)and the χ̃1 mass and mixing.
IMSS(13) : (D = 0) possibility to extend the particle content recognized by Pythia tothat of the Next-to-Minimal Supersymmetric Standard Model (NMSSM).
= 0 : MSSM particle content.
= 1 : NMSSM particle content
Note: at present, = 1 merely allows Pythia to recognize the NMSSM particles.
Pythia does not contain any internal machinery for doing calculations in
the NMSSM. Thus, the basic scattering processes should be generated by an
external program and handed to Pythia via the LHA interface for partonlevel events. This should then be combined with either setting the NMSSM
resonance decays by hand, or by reading in an SLHA decay table preparedby an external decay package.
IMSS(20) : (D = 0) Logical Unit Number on which the SUSY model parameter file isopened, for file-based run-time interface to the Isajet SUSY RGE machinery,see IMSS(1) = 13.
IMSS(21) : (D = 0) Logical Unit Number for SLHA spectrum read-in. Only used if
IMSS(1) = 11.
225

IMSS(22) : (D = 0) Read-in of SLHA decay table.
= 0 : No decays are read in. The internal Pythia machinery is used to calculate
decay rates.
> 0 : Read decays from SLHA file on unit number IMSS(22). During initialization, decay tables in the file will replace the values calculated by Pythia.
Particles for which the file does not contain a decay table will thus still havetheir decays calculated by Pythia. Any decay lines associated with a zero
width mother are ignored, giving a fast way of switching off decays without
having to comment out all the decay lines. In normal usage one would expect IMSS(22) to be equal to IMSS(21), to ensure that the spectrum anddecays are consistent with each other, but this is not a strict requirement.
IMSS(23) : (D = 0) writing of MSSM spectrum data.
= 0 : Don’t write out spectrum.
> 0 : Write out spectrum in SLHA format (calculated by Pythia or otherwise)
to file on unit number IMSS(23).
IMSS(24) : (D = 0) writing of MSSM particle decay table.
= 0 : Don’t write out decay table.
> 0 : Write out decay table in SLHA format to file on unit number IMSS(24). Not
implemented in the code yet. In normal usage one would expect IMSS(24) tobe equal to IMSS(23), to ensure that the spectrum and decays are consistent
with each other, but this is not a strict requirement.
IMSS(51) : (D = 0) Lepton number violation on/off (LLE type couplings).
= 0 : All LLE couplings off. LLE decay channels off.
= 1 : All LLE couplings set to common value given by 10-RMSS(51) .
= 2 : LLE couplings set to generation-hierarchical ‘natural’ values with common
normalization RMSS(51) (see section 8.7.7).
= 3 : All LLE couplings set to zero, but LLE decay channels not switched off. Nonzero couplings should be entered individually into the array RVLAM(I,J,K).
Because of the antisymmetry in I and J, only entries with I < J need beentered.
IMSS(52) : (D = 0) Lepton number violation on/off (LQD type couplings).
= 0 : All LQD couplings off. LQD decay channels off.
= 1 : All LQD couplings set to common value given by 10-RMSS(52) .
= 2 : LQD couplings set to generation-hierarchical ‘natural’ values with common
normalization RMSS(52) (see section 8.7.7).
= 3 : All LQD couplings set to zero, but LQD decay channels not switched
off. Non-zero couplings should be entered individually into the array
RVLAMP(I,J,K).
IMSS(53) : (D = 0) Baryon number violation on/off= 0 : All UDD couplings off. UDD decay channels off.
= 1 : All UDD couplings set to common value given by 10-RMSS(53) .
= 2 : UDD couplings set to generation-hierarchical ‘natural’ values with common
normalization RMSS(53) (see section 8.7.7).
= 3 : All UDD couplings set to zero, but UDD decay channels not switched
off. Non-zero couplings should be entered individually into the array
RVLAMB(I,J,K). Because of the antisymmetry in J and K, only entries withJ < K need be entered.
RMSS(1) : (D = 80. GeV) If IMSS(1) = 1 M1 , then U(1) gaugino mass. If IMSS(1) =2, then the common gaugino mass m1/2 .
RMSS(2) : (D = 160. GeV) M2 , the SU(2) gaugino mass.
RMSS(3) : (D = 500. GeV) M3 , the SU(3) (gluino) mass parameter.
RMSS(4) : (D = 800. GeV) µ, the higgsino mass parameter. If IMSS(1) = 2, only the226sign of µ is used.
RMSS(5) : (D = 2.) tan β, the ratio of Higgs expectation values.
RMSS(6) : (D = 250. GeV) Left slepton mass M`˜L . The sneutrino mass is fixed by asum rule.
RMSS(7) : (D = 200. GeV) Right slepton mass M`˜R .
RMSS(8) : (D = 800. GeV) Left squark mass Mq̃L . If IMSS(1) = 2, the common scalarmass m0 .
RMSS(9) : (D = 700. GeV) Right squark mass Mq̃R . Md̃R when IMSS(9) = 1.
RMSS(10) : (D = 800. GeV) Left squark mass for the third generation Mq̃L . When
IMSS(5) = 1, it is instead the t̃2 mass, and Mq̃L is a derived quantity.
RMSS(11) : (D = 700. GeV) Right sbottom mass Mb̃R . When IMSS(5) = 1, it is insteadthe b̃1 mass.
RMSS(12) : (D = 500. GeV) Right stop mass Mt̃R If negative, then it is assumed that
Mt̃2R < 0. When IMSS(5) = 1 , it is instead the t̃1 mass.
RMSS(13) : (D = 250. GeV) Left stau mass Mτ̃L .
RMSS(14) : (D = 200. GeV) Right stau mass Mτ̃R .
RMSS(15) : (D = 800. GeV) Bottom trilinear coupling Ab . When IMSS(5) = 1, it is a
derived quantity.
RMSS(16) : (D = 400. GeV) Top trilinear coupling At . If IMSS(1) = 2, the common
trilinear coupling A. When IMSS(5) = 1, it is a derived quantity.
RMSS(17) : (D = 0.) Tau trilinear coupling Aτ . When IMSS(5) = 1, it is a derived
quantity.
RMSS(18) : (D = 0.1) Higgs mixing angle α. This is only used when all of the Higgs
parameters are set by you, i.e IMSS(4) = 2.
RMSS(19) : (D = 850. GeV) Pseudoscalar Higgs mass parameter MA .
RMSS(20) : (D = 0.041) GUT scale coupling constant αGUT .
RMSS(21) : (D = 1.0 eV) The gravitino mass. Note nonconventional choice of units forthis particular mass.
RMSS(22) : (D = 800. GeV) ũR mass when IMSS(9) = 1.
RMSS(23) : (D = 104 GeV2 ) DX contribution to scalar masses when IMSS(7) = 1.
RMSS(24) : (D = 104 GeV2 ) DY contribution to scalar masses when IMSS(7) = 1.
RMSS(25) : (D = 104 GeV2 ) DS contribution to scalar masses when IMSS(7) = 1.
RMSS(26) : (D = 0.0 radians) when IMSS(5) = 1 it is the sbottom mixing angle.
RMSS(27) : (D = 0.0 radians) when IMSS(5) = 1 it is the stop mixing angle.
RMSS(28) : (D = 0.0 radians) when IMSS(5) = 1 it is the stau mixing angle.
RMSS(29) : (D = 2.4 × 1018 GeV) The Planck mass, used for calculating decays to lightgravitinos.
RMSS(30) - RMSS(33) : (D = 0.0, 0.0, 0.0, 0.0) complex phases for the mass parameters in RMSS(1) - RMSS(4), where the latter represent the moduli of the mass
parameters for the case of nonvanishing phases.
RMSS(40), RMSS(41) : used for temporary storage of the corrections ∆mt and ∆mb ,respectively, in the calculation of Higgs properties.
RMSS(51) : (D = 0.0) when IMSS(51) = 1 it is the negative logarithm of the common
value for all lepton-number-violating λ couplings (LLE). When IMSS(51) = 2
it is the constant of proportionality for generation-hierarchical λ couplings. See
section 8.7.7.
RMSS(52) : (D = 0.0) when IMSS(52) = 1 it is the negative logarithm of the common
value for all lepton-number-violating λ0 couplings (LQD). When IMSS(52) = 2
it is the constant of proportionality for generation-hierarchical λ0 couplings. See
section 8.7.7.
RMSS(53) : (D = 0.0) when IMSS(53) = 1 it is the negative logarithm of the common
value for all baryon-number-violating λ00 couplings (UDD). When IMSS(53) = 2
227

it is the constant of proportionality for generation-hierarchical λ00 couplings. See
section 8.7.7.
COMMON/PYSSMT/ZMIX(4,4),UMIX(2,2),VMIX(2,2),SMZ(4),SMW(2),&SFMIX(16,4),ZMIXI(4,4),UMIXI(2,2),VMIXI(2,2)
Purpose: to provide information on the neutralino, chargino, and sfermion mixing parameters. The variables should not be changed by you.
ZMIX(4,4) : the real part of the neutralino mixing matrix in the Bino–neutral Wino–Up
higgsino–Down higgsino basis.
UMIX(2,2) : the real part of the chargino mixing matrix in the charged Wino–charged
higgsino basis.
VMIX(2,2) : the real part of the charged conjugate chargino mixing matrix in the wino–
charged higgsino basis.
SMZ(4) : the signed masses of the neutralinos.
SMW(2) : the signed masses of the charginos.
SFMIX(16,4) : the sfermion mixing matrices T in the L–R basis, identified by the corresponding fermion, i.e. SFMIX(6,I) is the stop mixing matrix. The four entries
for each sfermion are T11 , T12 , T21 , and T22 .
ZMIXI(4,4) : the imaginary part of the neutralino mixing matrix in the Bino–neutral
Wino–Up higgsino–Down higgsino basis.
UMIXI(2,2) : the imaginary part of the chargino mixing matrix in the charged Wino–charged higgsino basis.
VMIXI(2,2) : the imaginary part of the charged conjugate chargino mixing matrix inthe wino–charged higgsino basis.
COMMON/PYMSRV/RVLAM(3,3,3), RVLAMP(3,3,3), RVLAMB(3,3,3)
Purpose: to provide information on lepton- and baryon-number-violating couplings.
RVLAM(3,3,3) : the lepton-number-violating λijk couplings. See IMSS(51), RMSS(51).
RVLAMP(3,3,3) : the lepton-number-violating λ0ijk couplings. See IMSS(52), RMSS(52).
RVLAMB(3,3,3) : the baryon-number-violating λ00ijk couplings. See IMSS(53), RMSS(53).
The following subroutines and functions need not be accessed by the user, but aredescribed for completeness.
SUBROUTINE PYAPPS : uses approximate analytic formulae to determine the full set of
MSSM parameters from SUGRA inputs.
SUBROUTINE PYGLUI : calculates gluino decay modes.
SUBROUTINE PYGQQB : calculates three-body decays of gluinos into neutralinos or charginos and third generation fermions. These routines are valid for large values oftan β.
SUBROUTINE PYCJDC : calculates the chargino decay modes.
SUBROUTINE PYHEXT : calculates the non–Standard Model decay modes of the Higgs
bosons.
SUBROUTINE PYHGGM : determines the Higgs boson mass spectrum using several inputs.
SUBROUTINE PYINOM : finds the mass eigenstates and mixing matrices for the charginos
and neutralinos.
SUBROUTINE PYMSIN : initializes the MSSM simulation.
SUBROUTINE PYSLHA : to read in or write out SUSY Les Houches Accord spectra anddecay tables. Can also be used stand-alone, before the call to PYINIT, to read in
SLHA decay tables for specific particles. See section 14.4 for how to do this.
SUBROUTINE PYNJDC : calculates neutralino decay modes.
228

SUBROUTINE PYPOLE : computes the Higgs boson masses using a renormalization group
improved leading-log approximation and two-loop leading-log corrections.
SUBROUTINE PYSFDC : calculates sfermion decay modes.
SUBROUTINE PYSUBH : computes the Higgs boson masses using only renormalizationgroup improved formulae.
SUBROUTINE PYTBDY : samples the phase space for three-body decays of neutralinos,
charginos, and the gluino.
SUBROUTINE PYTHRG : computes the masses and mixing matrices of the third generation
sfermions.
SUBROUTINE PYRVSF : R-violating sfermion decay widths.
SUBROUTINE PYRVNE : R-violating neutralino decay widths.
SUBROUTINE PYRVCH : R-violating chargino decay widths.
SUBROUTINE PYRVGW : calculates R-violating 3-body widths using PYRVI1, PYRVI2,PYRVI3, PYRVG1, PYRVG2, PYRVG3, PYRVG4, PYRVR, and PYRVS.
FUNCTION PYRVSB : calculates R-violating 2-body widths.
SUBROUTINE SUGRA : dummy routine, to avoid linking problems when Isajet is not
linked; see IMSS(1) = 12.
SUBROUTINE SSMSSM : dummy routine, to avoid linking problems when Isajet is not
linked; see IMSS(1) = 12.
FUNCTION VISAJE : dummy routine, to avoid linking problems when Isajet is not
linked; see IMSS(1) = 12.

9.6

General Event Information

When an event is generated with PYEVNT, some information on it is stored in the MSTI
and PARI arrays of the PYPARS common block (often copied directly from the internal
MINT and VINT variables). Further information is stored in the complete event record; seesection 5.2.
Part of the information is only relevant for some subprocesses; by default everythingirrelevant is set to 0. Kindly note that, like the CKIN constraints described in section 9.2,
kinematical variables normally (i.e. where it is not explicitly stated otherwise) refer to thenaı̈ve hard scattering, before initial- and final-state radiation effects have been included.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
Purpose: to provide information on latest event generated or, in a few cases, on statistics
accumulated during the run.
MSTI(1) : specifies the general type of subprocess that has occurred, according to the
ISUB code given in section 8.1.
MSTI(2) : whenever MSTI(1) (together with MSTI(15) and MSTI(16)) are not enoughto specify the type of process uniquely, MSTI(2) provides an ordering of thedifferent possibilities. This is particularly relevant for the different colour-flow
topologies possible in QCD 2 → 2 processes, but easily generalizes e.g. if a quarkis replaced by a squark. With i =MSTI(15), j =MSTI(16) and k =MSTI(2),the QCD possibilities are, in the classification scheme of [Ben84] (cf. section
8.2.1):ISUB = 11, i = j, qi qi → qi qi ;k = 1 : colour configuration A.
k = 2 : colour configuration B.
ISUB = 11, i 6= j, qi qj → qi qj ;k = 1 : only possibility.
ISUB = 12, qi qi → ql ql ;229

k = 1 : only possibility.
ISUB = 13, qi qi → gg;
k = 1 : colour configuration A.
k = 2 : colour configuration B.
ISUB = 28, qi g → qi g;
k = 1 : colour configuration A.
k = 2 : colour configuration B.
ISUB = 53, gg → ql ql ;k = 1 : colour configuration A.
k = 2 : colour configuration B.
ISUB = 68, gg → gg;
k = 1 : colour configuration A.
k = 2 : colour configuration B.
k = 3 : colour configuration C.
ISUB = 83, fq → f 0 Q (by t-channel W exchange; does not distinguish colour flows
but result of user selection);k = 1 : heavy flavour Q is produced on side 1.
k = 2 : heavy flavour Q is produced on side 2.
MSTI(3) : the number of partons produced in the hard interactions, i.e. the number
n of the 2 → n matrix elements used; it is sometimes 3 or 4 when a basic
2 → 1 or 2 → 2 process has been folded with two 1 → 2 initial branchings (likeqi qj → qk ql h0 ).
MSTI(4) : number of documentation lines at the beginning of the common block PYJETS
that are given with K(I,1) = 21; 0 for MSTP(125) = 0.
MSTI(5) : number of events generated to date in current run. In runs with the variableenergy option, MSTP(171) = 1 and MSTP(172) = 2, only those events that survive (i.e. that do not have MSTI(61) = 1) are counted in this number. That is,
MSTI(5) may be less than the total number of PYEVNT calls.
MSTI(6) : current frame of event, cf. MSTP(124).
MSTI(7), MSTI(8) : line number for documentation of outgoing partons/particles fromhard scattering for 2 → 2 or 2 → 1 → 2 processes (else = 0).
MSTI(9) : event class used in current event for γp or γγ events. The code depends onwhich process is being studied.
= 0 : for other processes than the ones listed below.
For γp or γ ∗ p events, generated with the MSTP(14) = 10 or MSTP(14) = 30 options:= 1 : VMD.
= 2 : direct.
= 3 : anomalous.
= 4 : DIS (only for γ ∗ p, i.e. MSTP(14) = 30).
For real γγ events, i.e. MSTP(14) = 10:= 1 : VMD×VMD.
= 2 : VMD×direct.
= 3 : VMD×anomalous .
= 4 : direct×direct.
= 5 : direct×anomalous.
= 6 : anomalous×anomalous.
For virtual γ ∗ γ ∗ events, i.e. MSTP(14) = 30, where the two incoming photons are notequivalent and the order therefore matters:= 1 : direct×direct.
= 2 : direct×VMD.
= 3 : direct×anomalous.
= 4 : VMD×direct.
= 5 : VMD×VMD.
230

= 6 : VMD×anomalous.
= 7 : anomalous×direct.
= 8 : anomalous×VMD.
= 9 : anomalous×anomalous.
= 10 : DIS×VMD.
= 11 : DIS×anomalous.
= 12 : VMD×DIS.
= 13 : anomalous×DIS.
MSTI(10) : is 1 if cross section maximum was violated in current event, and 0 if not.
MSTI(11) : KF flavour code for beam (side 1) particle.
MSTI(12) : KF flavour code for target (side 2) particle.
MSTI(13), MSTI(14) : KF flavour codes for side 1 and side 2 initial-state shower initiators.
MSTI(15), MSTI(16) : KF flavour codes for side 1 and side 2 incoming partons to thehard interaction.
MSTI(17), MSTI(18) : flag to signal if particle on side 1 or side 2 has been scattereddiffractively; 0 if no, 1 if yes.
MSTI(21) - MSTI(24) : KF flavour codes for outgoing partons from the hard interaction.
The number of positions actually used is process-dependent, see MSTI(3); trailing
positions not used are set = 0. For events with many outgoing partons, e.g. inexternal processes, also MSTI(25) and MSTI(26) could be used.
MSTI(25), MSTI(26) : KF flavour codes of the products in the decay of a single s-channel
resonance formed in the hard interaction. Are thus only used when MSTI(3) =1 and the resonance is allowed to decay.
MSTI(31) : number of hard or semi-hard scatterings that occurred in the current eventin the multiple-interaction scenario; is = 0 for a low-p⊥ event.
MSTI(32) : information on whether a reconnection occurred in the current event; is 0
normally but 1 in case of reconnection.
MSTI(41) : the number of pile-up events generated in the latest PYEVNT call (including
the first, ‘hard’ event).
MSTI(42) - MSTI(50) : ISUB codes for the events 2–10 generated in the pile-up-events
scenario. The first event ISUB code is stored in MSTI(1). If MSTI(41) is less than10, only as many positions are filled as there are pile-up events. If MSTI(41) isabove 10, some ISUB codes will not appear anywhere.
MSTI(51) : normally 0 but set to 1 if a UPEVNT call did not return an event, such thatPYEVNT could not generate an event. For further details, see section 9.9.
MSTI(52) : counter for the number of times the current event configuration failed in thegeneration machinery. For accepted events this is always 0, but the counter canbe used inside UPEVNT to check on anomalous occurrences. For further details,
see section 9.9.
MSTI(53) : normally 0, but 1 if no processes with non-vanishing cross sections were
found in a PYINIT call, for the case that MSTP(127) = 1.
MSTI(61) : status flag set when events are generated. It is only of interest for runs withvariable energies, MSTP(171) = 1, with the option MSTP(172) = 2.
= 0 : an event has been generated.
= 1 : no event was generated, either because the c.m. energy was too low or because the Monte Carlo phase space point selection machinery rejected thetrial point. A new energy is to be picked by you.
MSTI(71), MSTI(72) : KF code for incoming lepton beam or target particles, when aflux of virtual photons are generated internally for ’gamma/lepton’ beams, while
MSTI(11) and MSTI(12) is then the photon code.
PARI(1) : total integrated cross section for the processes under study, in mb. This231

number is obtained as a by-product of the selection of hard-process kinematics,
and is thus known with better accuracy when more events have been generated.
The value stored here is based on all events until the latest one generated.
PARI(2) : for unweighted events, MSTP(142) = 0 or = 2, it is the ratio
PARI(1)/MSTI(5), i.e. the ratio of total integrated cross section and number of events generated. Histograms should then be filled with unit event weight
and, at the end of the run, multiplied by PARI(2) and divided by the bin
width to convert results to mb/(dimension of the horizontal axis). For weighted
events, MSTP(142) = 1, MSTI(5) is replaced by the sum of PARI(10) values.
Histograms should then be filled with event weight PARI(10) and, as before, be
multiplied by PARI(2) and divided by the bin width at the end of the run. In
runs with the variable-energy option, MSTP(171) = 1 and MSTP(172) = 2, only
those events that survive (i.e. that do not have MSTI(61) = 1) are counted.
PARI(7) : an event weight, normally 1 and thus uninteresting, but for external processes
with IDWTUP = -1, -2 or -3 it can be −1 for events with negative cross section,with IDWTUP = 4 it can be an arbitrary non-negative weight of dimension mb,
and with IDWTUP = -4 it can be an arbitrary weight of dimension mb. (The difference being that in most cases a rejection step is involved to bring the accepted
events to a common weight normalization, up to a sign, while no rejection need
be involved in the last two cases.)
PARI(9) : is weight WTXS returned from PYEVWT call when MSTP(142) ≥ 1, otherwise is1.
PARI(10) : is compensating weight 1./WTXS that should be associated to events when
MSTP(142) = 1, else is 1.
PARI(11) : Ecm , i.e. total c.m. energy (except when using the ’gamma/lepton’ machinery, see PARI(101).
PARI(12) : s, i.e. squared total c.m. energy (except when using the ’gamma/lepton’machinery,√ see PARI(102).
PARI(13) : m̂ = ŝ, i.e. mass of the hard-scattering subsystem.
PARI(14) : ŝ of the hard subprocess (2 → 2 or 2 → 1).
PARI(15) : t̂ of the hard subprocess (2 → 2 or 2 → 1 → 2).
PARI(16) : û of the hard subprocess (2 → 2 or 2 → 1 → 2).
PARI(17) : p̂⊥ of the hard subprocess (2 → 2 or 2 → 1 → 2), evaluated in the rest frameof the hard interaction.
PARI(18) : p̂2⊥ of the hard subprocess; see PARI(17).
PARI(19) : m̂0 , the mass of the complete three- or four-body final state in 2 → 3 or
2 → 4 processes (while m̂, given in PARI(13), here corresponds to the one- or
two-body central system). Kinematically m̂ ≤ m̂0 ≤ Ecm .
PARI(20) : ŝ0 = m̂02 ; see PARI(19).
PARI(21) : Q of the hard-scattering subprocess. The exact definition is processdependent, see MSTP(32).
PARI(22) : Q2 of the hard-scattering subprocess; see PARI(21).
PARI(23) : Q of the outer hard-scattering subprocess. Agrees with PARI(21) for a 2 → 1
or 2 → 2 process. For a 2 → 3 or 2 → 4 W/Z fusion process, it is set by theW/Z mass scale, and for subprocesses 121 and 122 by the heavy-quark mass.
PARI(24) : Q2 of the outer hard-scattering subprocess; see PARI(23).
PARI(25) : Q scale used as maximum virtuality in parton showers. Is equal to PARI(23),except for Deeply Inelastic Scattering processes when MSTP(22) ≥ 1.
PARI(26) : Q2 scale in parton showers; see PARI(25).
PARI(31), PARI(32) : the momentum fractions x of the initial-state parton-shower initiators on side 1 and 2, respectively.
PARI(33), PARI(34) : the momentum fractions x taken by the partons at the hardinteraction, as used e.g. in the parton-distribution functions.
232

PARI(35) : Feynman-x, xF = x1 − x2 = PARI(33)−PARI(34).
PARI(36) : τ = ŝ/s = x1 x2 = PARI(33)×PARI(34).
PARI(37) : y = (1/2) ln(x1 /x2 ), i.e. rapidity of the hard-interaction subsystem in thec.m. frame of the event as a whole.
PARI(38) : τ 0 = ŝ0 /s = PARI(20)/PARI(12).
PARI(39), PARI(40) : the primordial k⊥ values selected in the two beam remnants.
PARI(41) : cos θ̂, where θ̂ is the scattering angle of a 2 → 2 (or 2 → 1 → 2) interaction,
defined in the rest frame of the hard-scattering subsystem.
PARI(42) : x⊥ , i.e. scaled transverse momentum of the hard-scattering subprocess, x⊥ =2p̂⊥ /Ecm = 2 PARI(17)/PARI(11).
PARI(43), PARI(44) : xL3 and xL4 , i.e. longitudinal momentum fractions of the two
scattered partons, in the range −1 < xL < 1, in the c.m. frame of the event as awhole.
PARI(45), PARI(46) : x3 and x4 , i.e. scaled energy fractions of the two scattered partons, in the c.m. frame of the event as a whole.
PARI(47), PARI(48) : y3∗ and y4∗ , i.e. rapidities of the two scattered partons in the c.m.
frame of the event as a whole.
PARI(49), PARI(50) : η3∗ and η4∗ , i.e. pseudorapidities of the two scattered partons in
the c.m. frame of the event as a whole.
PARI(51), PARI(52) : cos θ3∗ and cos θ4∗ , i.e. cosines of the polar angles of the two scattered partons in the c.m. frame of the event as a whole.
PARI(53), PARI(54) : θ3∗ and θ4∗ , i.e. polar angles of the two scattered partons, definedin the range 0 < θ∗ < π, in the c.m. frame of the event as a whole.
PARI(55), PARI(56) : azimuthal angles φ∗3 and φ∗4 of the two scattered partons, definedin the range −π < φ∗ < π, in the c.m. frame of the event as a whole.
PARI(61) : multiple interaction enhancement factor for current event. A large value
corresponds to a central collision and a small value to a peripheral one.
PARI(65) : sum of the transverse momenta of partons generated at the hardest interaction of the event, excluding initial- and final-state radiation, i.e. 2×PARI(17).
Only intended for 2 → 2 or 2 → 1 → 2 processes, i.e. not implemented for 2 → 3
ones.
PARI(66) : sum of the transverse momenta of all partons generated at the hardest interaction, including initial- and final-state radiation, resonance decay products,
and primordial k⊥ .
PARI(67) : scalar sum of transverse momenta of partons generated at hard interactions,excluding the hardest one, along with its initial- and final-state radiation (seePARI(66)). Is non-vanishing only in the multiple-interaction scenarios. In the
new scenario the initial- and final-state radiation associated with further interactions is included.
PARI(68) : currently equal to PARI(67).
PARI(69) : sum of transverse momenta of all partons generated in hard interactions(PARI(66) + PARI(68)) and, additionally, of all beam-remnant partons.
PARI(71), PARI(72) : sum of the momentum fractions x taken by initial-state partonshower initiators on side 1 and and side 2, excluding those of the hardest interaction. Is non-vanishing only in the multiple-interaction scenario.
PARI(73), PARI(74) : sum of the momentum fractions x taken by the partons at thehard interaction on side 1 and side 2, excluding those of the hardest interaction.
Is non-vanishing only in the multiple-interaction scenario.
PARI(75), PARI(76) : the x value of a photon that branches into quarks or gluons,i.e. x at interface between initial-state QED and QCD cascades, for the oldphotoproduction machinery.
PARI(77), PARI(78) : the χ values selected for beam remnants that are split into two
objects, describing how the energy is shared (see MSTP(92) and MSTP(94)); is233

vanishing if no splitting is needed.
PARI(81) : size of the threshold factor (enhancement or suppression) in the latest eventwith heavy-flavour production; see MSTP(35).
PARI(91) : average multiplicity n of pile-up events, see MSTP(133). Only relevant for
MSTP(133) = 1 or 2.
PARI(92) : average multiplicity hni of pile-up events as actually simulated, i.e. withmultiplicity = 0 events removed and the high-end tail truncated. Only relevantfor MSTP(133) = 1 or 2.
PARI(93) : for MSTP(133) = 1 it is the probability that a beam crossing will produce apile-up event at all, i.e. that there will be at least one hadron–hadron interaction;
for MSTP(133) = 2 the probability that a beam crossing will produce a pile-up
event with one hadron–hadron interaction of the desired rare type. See section
11.5.
PARI(101) : c.m. energy for the full collision, while PARI(11) gives the γ-hadron orγγ subsystem energy; used for virtual photons generated internally with the’gamma/lepton’ option.
PARI(102) : full squared c.m. energy, while PARI(12) gives the subsystem squared energy; used for virtual photons generated internally with the ’gamma/lepton’ option.
PARI(103), PARI(104) : x values, i.e. respective photon energy fractions of the incoming lepton in the c.m. frame of the event; used for virtual photons generated
internally with the ’gamma/lepton’ option.
PARI(105), PARI(106) : Q2 or P 2 , virtuality of the respective photon (thus the squareof VINT(3), VINT(4)); used for virtual photons generated internally with the’gamma/lepton’ option.
PARI(107), PARI(108) : y values, i.e. respective photon light-cone energy fraction of
the incoming lepton; used for virtual photons generated internally with the’gamma/lepton’ option.
PARI(109), PARI(110) : θ, scattering angle of the respective lepton in the c.m. frame ofthe event; used for virtual photons generated internally with the ’gamma/lepton’
option.
PARI(111), PARI(112) : φ, azimuthal angle of the respective scattered lepton in thec.m. frame of the event; used for virtual photons generated internally with the’gamma/lepton’ option.
PARI(113), PARI(114): the R factor defined at MSTP(17), giving a cross section enhancement from the contribution of resolved longitudinal photons.

9.7

How to Generate Weighted Events

By default Pythia generates unweighted events, i.e. all events in a run are on an equal
footing. This means that corners of phase space with low cross sections are poorly populated, as it should be. However, sometimes one is interested in also exploring such corners,in order to gain a better understanding of physics. A typical example would be the jetcross section in hadron collisions, which is dropping rapidly with increasing jet p⊥ , andwhere it is interesting to trace this drop over several orders of magnitude. Experimentally
this may be solved by prescaling event rates already at the trigger level, so that all highp⊥ events are saved but only a fraction of the lower-p⊥ ones. In this section we outline
procedures to generate events in a similar manner.
Basically two approaches can be used. One is to piece together results from different
subruns, where each subrun is restricted to some specific region of phase space. Withineach subrun all events then have the same weight, but subruns have to be combined according to their relative cross sections. The other approach is to let each event come withan associated weight, that can vary smoothly as a function of p⊥ . These two alterna234

tives correspond to stepwise or smoothly varying prescaling factors in the experimental
analogue. We describe them one after the other.
The phase space can be sliced in many different ways. However, for the jet rate andmany other processes, the most natural variable would be p⊥ itself. (For production of
a lepton pair by s-channel resonances, the invariant mass would be a better choice.) It
is not possible to specify beforehand the jet p⊥ ’s an event will contain, since this is acombination of the p̂⊥ of the hard scattering process with additional showering activity,with hadronization, with underlying event and with the jet clustering approach actuallyused. However, one would expect a strong correlation between the p̂⊥ scale and the jetp⊥ ’s. Therefore the full p̂⊥ range can be subdivided into a set of ranges by using the
CKIN(3) and CKIN(4) variables as lower and upper limits. This could be done e.g. foradjacent non-overlapping bins 10–20,20–40,40–70, etc.
Only if one would like to cover also very small p⊥ is there a problem with this strategy: since the naı̈ve jet cross section is divergent for p̂⊥ → 0, a unitarization procedure is
implied by setting CKIN(3) = 0 (or some other low value). This unitarization then disregards the actual CKIN(3) and CKIN(4) values and generates events over the full phasespace. In order not to double-count, then events above the intended upper limit of thefirst bin have to be removed by brute force.
A simple but complete example of a code performing this task (with some primitive
histogramming) is the following:
C...All real arithmetic in double precision.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)
C...Three Pythia functions return integers, so need declaring.
INTEGER PYK,PYCHGE,PYCOMPC...EXTERNAL statement links PYDATA on most platforms.
EXTERNAL PYDATA
C...The event record.
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
C...Selection of hard-scattering subprocesses.
COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)
C...Parameters.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
C...Bins of pT.
DIMENSION PTBIN(10)DATA PTBIN/0D0,10D0,20D0,40D0,70D0,110D0,170D0,250D0,350D0,1000D0/
C...Main parameters of run: c.m. energy and number of events per bin.
ECM=2000D0
NEV=1000
C...Histograms.
CALL PYBOOK(1,’dn_ev/dpThat’,100,0D0,500D0)
CALL PYBOOK(2,’dsigma/dpThat’,100,0D0,500D0)
CALL PYBOOK(3,’log10(dsigma/dpThat)’,100,0D0,500D0)
CALL PYBOOK(4,’dsigma/dpTjet’,100,0D0,500D0)
CALL PYBOOK(5,’log10(dsigma/dpTjet)’,100,0D0,500D0)
CALL PYBOOK(11,’dn_ev/dpThat, dummy’,100,0D0,500D0)
CALL PYBOOK(12,’dn/dpTjet, dummy’,100,0D0,500D0)
C...Loop over pT bins and initialize.
DO 300 IBIN=1,9
CKIN(3)=PTBIN(IBIN)
CKIN(4)=PTBIN(IBIN+1)
235

CALL PYINIT(’CMS’,’p’,’pbar’,ECM)
C...Loop over events. Remove unwanted ones in first pT bin.
DO 200 IEV=1,NEV
CALL PYEVNTPTHAT=PARI(17)IF(IBIN.EQ.1.AND.PTHAT.GT.PTBIN(IBIN+1)) GOTO 200
C...Store pThat. Cluster jets and store variable number of pTjet.
CALL PYFILL(1,PTHAT,1D0)
CALL PYFILL(11,PTHAT,1D0)
CALL PYCELL(NJET)
DO 100 IJET=1,NJET
CALL PYFILL(12,P(N+IJET,5),1D0)100
CONTINUE
C...End of event loop.
200
CONTINUE
C...Normalize cross section to pb/GeV and add up.
FAC=1D9*PARI(1)/(DBLE(NEV)*5D0)
CALL PYOPER(2,’+’,11,2,1D0,FAC)
CALL PYOPER(4,’+’,12,4,1D0,FAC)
C...End of loop over pT bins.
300 CONTINUE
C...Take logarithm and plot.
CALL PYOPER(2,’L’,2,3,1D0,0D0)
CALL PYOPER(4,’L’,4,5,1D0,0D0)
CALL PYNULL(11)CALL PYNULL(12)CALL PYHIST
END
The alternative to slicing the phase space is to used weighted events. This is possible
by making use of the PYEVWT routine:
CALL PYEVWT(WTXS)
Purpose: to allow you to reweight event cross sections, by process type and kinematicsof the hard scattering. There exists two separate modes of usage, described in
the following.
For MSTP(142) = 1, it is assumed that the cross section of the process is correctly
given by default in Pythia, but that one wishes to generate events biased toa specific region of phase space. While the WTXS factor therefore multiplies thenaı̈ve cross section in the choice of subprocess type and kinematics, the produced
event comes with a compensating weight PARI(10) = 1./WTXS, which should be
used when filling histograms etc. In the PYSTAT(1) table, the cross sections areunchanged (up to statistical errors) compared with the standard cross sections,but the relative composition of events may be changed and need no longer be inproportion to relative cross sections. A typical example of this usage is if one
236

wishes to enhance the production of high-p⊥ events; then a weight like WTXS=(p⊥ /p⊥0 )2 (with p⊥0 some fixed number) might be appropriate. See PARI(2) fora discussion of overall normalization issues.
For MSTP(142) = 2, on the other hand, it is assumed that the true cross section
is really to be modified by the multiplicative factor WTXS. The generated events
therefore come with unit weight, just as usual. This option is really equivalent toreplacing the basic cross sections coded in Pythia, but allows more flexibility:
no need to recompile the whole of Pythia.
The routine will not be called unless MSTP(142) ≥ 1, and never if ‘minimumbias’-type events (including elastic and diffractive scattering) are to be generated
as well. Further, cross sections for additional multiple interactions or pile-up
events are never affected. A dummy routine PYEVWT is included in the program
file, so as to avoid unresolved external references when the routine is not used.
WTXS: multiplication factor to ordinary event cross section; to be set (by you) in PYEVWT
call.
Remark : at the time of selection, several variables in the MINT and VINT arrays in
the PYINT1 common block contain information that can be used to make thedecision. The routine provided in the program file explicitly reads the variables
that have been defined at the time PYEVWT is called, and also calculates somederived quantities. The given list of information includes subprocess type ISUB,
Ecm , ŝ, t̂, û, p̂⊥ , x1 , x2 , xF , τ , y, τ 0 , cos θ̂, and a few more. Some of these may
not be relevant for the process under study, and are then set to zero.
Warning: the weights only apply to the hard-scattering subprocesses. There is no way
to reweight the shape of initial- and final-state showers, fragmentation, or otheraspects of the event.
There are some limitations to the facility. PYEVWT is called at an early stage of thegeneration process, when the hard kinematics is selected, well before the full event isconstructed. It then cannot be used for low-p⊥ , elastic or diffractive events, for which no
hard kinematics has been defined. If such processes are included, the event weighting isswitched off. Therefore it is no longer an option to run with CKIN(3) = 0.
Which weight expression to use may take some trial and error. In the above case, areasonable ansatz seems to be a weight behaving like p̂6⊥ , where four powers of p̂⊥ aremotivated by the partonic cross section behaving like 1/p̂4⊥ , and the remaining two by thefall-off of parton densities. An example for the same task as above one would then be:
C...All real arithmetic in double precision.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)
C...Three Pythia functions return integers, so need declaring.
INTEGER PYK,PYCHGE,PYCOMPC...EXTERNAL statement links PYDATA on most platforms.
EXTERNAL PYDATA
C...The event record.
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
C...Selection of hard-scattering subprocesses.
COMMON/PYSUBS/MSEL,MSELPD,MSUB(500),KFIN(2,-40:40),CKIN(200)
C...Parameters.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
C...Main parameters of run: c.m. energy, pTmin and number of events.
ECM=2000D0
CKIN(3)=5D0
NEV=10000

237

C...Histograms.
CALL PYBOOK(1,’dn_ev/dpThat’,100,0D0,500D0)
CALL PYBOOK(2,’dsigma/dpThat’,100,0D0,500D0)
CALL PYBOOK(3,’log10(dsigma/dpThat)’,100,0D0,500D0)
CALL PYBOOK(4,’dsigma/dpTjet’,100,0D0,500D0)
CALL PYBOOK(5,’log10(dsigma/dpTjet)’,100,0D0,500D0)
C...Initialize with weighted events.
MSTP(142)=1
CALL PYINIT(’CMS’,’p’,’pbar’,ECM)
C...Loop over events; read out pThat and event weight.
DO 200 IEV=1,NEV
CALL PYEVNTPTHAT=PARI(17)WT=PARI(10)C...Store pThat. Cluster jets and store variable number of pTjet.
CALL PYFILL(1,PTHAT,1D0)
CALL PYFILL(2,PTHAT,WT)
CALL PYCELL(NJET)
DO 100 IJET=1,NJET
CALL PYFILL(4,P(N+IJET,5),WT)
100
CONTINUE
C...End of event loop.
200
CONTINUE
C...Normalize cross section to pb/GeV, take logarithm and plot.
FAC=1D9*PARI(2)/5D0
CALL PYFACT(2,FAC)
CALL PYFACT(4,FAC)
CALL PYOPER(2,’L’,2,3,1D0,0D0)
CALL PYOPER(4,’L’,4,5,1D0,0D0)
CALL PYHIST
END
C*************************************************************SUBROUTINE PYEVWT(WTXS)C...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
INTEGER PYK,PYCHGE,PYCOMPC...Common block.
COMMON/PYINT1/MINT(400),VINT(400)
C...Read out pThat^2 and set weight.
PT2=VINT(48)WTXS=PT2**3

238

RETURN
END
Note that, in PYEVWT one cannot look for p̂⊥ in PARI(17), since this variable is only set
at the end of the event generation. Instead the internal VINT(48) is used. The dummy
copy of the PYEVWT routine found in the Pythia code shows what is available and how
to access this.
The above solutions do not work if the weighting should also depend on the shower
evolution, i.e. not only on the hard process. Such a rejection/acceptance ‘weight’ is particularly convenient when trying to combine matrix-element-generated events of different
jet multiplicities (in addition to whatever is the basic process) without doublecounting,e.g. using the L–CKKW or MLM matching prescriptions [Cat01]. To handle such situations, a routine UPVETO has been introduced. The name is intended to emphasize thelogical place along with the UPINIT and UPEVNT routines for the handling of external user
processes (see section 9.9), but it can also be used for internal Pythia processes.
CALL UPVETO(IVETO)
Purpose: to allow the user the possibility to abort the generation of an event immediately after parton showers but before underlying event and handronization isconsidered. The routine is called only if MSTP(143) = 1, and only for the old,
virtuality-ordered showers in PYEVNT, i.e. has not been implemented for the p⊥ ordered showers in PYEVNW.
IVETO : specifies choice made by user.
= 0 : retain current event and generate it in full.
= 1 : abort generation of current event and move to next.
Note 1: if resonances like W± , Z0 , t, Higgs and SUSY particles are handed undecayedfrom UPEVNT, or are generated internally in Pythia, they will also be undecayedat this stage; if decayed their decay products will have been allowed to shower.
Note 2: all partons at the end of the shower phase are stored in the HEPEVT commonblock,see section 5.4. The interesting information is:
NHEP : the number of such partons, in entries 1 through NHEP,ISTHEP(I) : where all intermediate products have code 2 and all the final ones (atthe time UPVETO is called) code 1,
IDHEP(I) : the particle identity code according to PDG conventions,
JMOHEP(1,I) : the mother position,
PHEP(J,I) : the (px , py , pz , E, m) of the particle.
The rest of the HEPEVT variables are zeroed.
Note 3: the cross section of processes, as shown with CALL PYSTAT(1), is reduced when
events are aborted. Such events are also counted in the ”fraction of events thatfail fragmentation cuts” in the last line of this table.
In the HEPEVT event record, all intermediate and ‘final’ particles of the hard process itself, i.e. of the matrix-element calculation, are listed as documentation lines, with
ISTHEP(I) = 2. The ‘final’-state particles that actually are defined when UPVETO is called,
after shower evolution but before multiple interactions have been added, have ISTHEP(I)= 1. These point back to the one of the ISTHEP(I) = 2 partons they originate from,as first mother. If a system does not radiate, the same set of partons will be repeated
twice, once with ISTHEP(I) = 2 and once with ISTHEP(I) = 1. A more typical example
would be that a set of partons with ISTHEP(I) = 1 point back to the same ‘mother’ with
ISTHEP(I) = 2. Note that all the intermediate stages of the shower evolution are notshown, only the original mother and all the final daughters. If the mother index is zerofor an ISTHEP(I) = 1 parton, it comes from initial-state radiation.
239

Of course, the separation of which radiation comes from where is often gaugedependent, i.e. model-dependent, and so should not be over-stressed. For instance, in
a t → bW+ → bud decay sequence, the gluon radiation off the b is rather unambiguous,
while u and d form part of the same radiating system, although some sensible separation
is provided by the program.

9.8

How to Run with Varying Energies

It is possible to use Pythia in a mode where the energy can be varied from one event to
the next, without the need to re-initialize with a new PYINIT call. This allows a significant
speed-up of execution, although it is not as fast as running at a fixed energy. It can not
be used for everything — we will come to the fine print at the end — but it should beapplicable for most tasks.
The master switch to access this possibility is in MSTP(171). By default it is off, so
you must set MSTP(171) = 1 before initialization. There are two submodes of running,with MSTP(172) being 1 or 2. In the former mode, Pythia will generate an event at therequested energy. This means that you have to know which energy you want beforehand.
In the latter mode, Pythia will often return without having generated an event — withflag MSTI(61) = 1 to signal that — and you are then requested to give a new energy. Theenergy spectrum of accepted events will then, in the end, be your naı̈ve input spectrum
weighted with the cross-section of the processes you study. We will come back to this.
The energy can be varied, whichever frame is given in the PYINIT call. (Except for’USER’, where such information is fed in via the HEPEUP common block and thus beyond
the control of Pythia.) When the frame is ’CMS’, PARP(171) should be filled with thefractional energy of each event, i.e. Ecm =PARP(171)×WIN, where WIN is the nominalc.m. energy of the PYINIT call. Here PARP(171) should normally be smaller than unity,
i.e. initialization should be done at the maximum energy to be encountered. For the’FIXT’ frame, PARP(171) should be filled by the fractional beam energy of that one,i.e. Ebeam =PARP(171)×WIN. For the ’3MOM’, ’4MOM’ and ’5MOM’ options, the two fourmomenta are given in for each event in the same format as used for the PYINIT call.
Note that there is a minimum c.m. energy allowed, PARP(2). If you give in values below
this, the program will stop for MSTP(172) = 1, and will return with MSTI(61) = 1 forMSTP(172) = 1.
To illustrate the use of the MSTP(172) = 2 facility, consider the case of beamstrahlungin e+ e− linear colliders. This is just for convenience; what is said here can be translated
easily into other situations. Assume that the beam spectrum is given by D(z), where z
is the fraction retained by the original e after beamstrahlung. Therefore 0 ≤ z ≤ 1 andthe integral of D(z) is unity. This is not perfectly general; one could imagine branchingse− → e− γ → e− e+ e− , which gives a multiplication in the number of beam particles. This
could either be expressed in terms of a D(z) with integral larger than unity or in termsof an increased luminosity. We will assume the latter, and use D(z) properly normalized.
2, the actual s0 after beamstrahlung is given by s0 = z1 z2 s.
Given a nominal s = 4Ebeam
For a process with a cross section σ(s) the total cross section is thenσtot =

Z 1Z 10

0

D(z1 ) D(z2 )σ(z1 z2 s) dz1 dz2 .

(157)

The cross section σ may in itself be an integral over a number of additional phase spacevariables. If the maximum of the differential cross section is known, a correct procedure
to generate events is
1. pick z1 and z2 according to D(z1 ) dz1 and D(z2 ) dz2 , respectively;
2. pick a set of phase space variables of the process, for the given s0 of the event;3. evaluate σ(s0 ) and compare with σmax ;240

4. if event is rejected, then return to step 1 to generate new variables;
5. else continue the generation to give a complete event.
You as a user are assumed to take care of step 1, and present the resulting kinematics
with incoming e+ and e− of varying energy. Thereafter Pythia will do steps 2–5, andeither return an event or put MSTI(61) = 1 to signal failure in step 4.
The maximization procedure does search in phase space to find σmax , but it does notvary the s0 energy in this process. Therefore the maximum search in the PYINIT callshould be performed where the cross section is largest. For processes with increasing
cross section as a function of energy this means at the largest energy that will ever be
encountered, i.e. s0 = s in the case above. This is the ‘standard’ case, but often one
encounters other behaviours, where more complicated procedures are needed. One suchcase would be the process e+ e− → Z∗0 → Z0 h0 , which is known to have a cross sectionthat increases near the threshold but is decreasing asymptotically. If one already knowsthat the maximum, for a given Higgs mass, appears at 300 GeV, say, then the PYINIT
call should be made with that energy, even if subsequently one will be generating events
for a 500 GeV collider.
In general, it may be necessary to modify the selection of z1 and z2 and assign acompensating event weight. For instance, consider a process with a cross section behaving
roughly like 1/s. Then the σtot expression above may be rewritten as
σtot =

Z 1Z 1D(z1 ) D(z2 )z1 z2 σ(z1 z2 s) dz1 dz2 .
z1
z2
0

0

(158)

The expression z1 z2 σ(s0 ) is now essentially flat in s0 , i.e. not only can σmax be found at
a convenient energy such as the maximum one, but additionally the Pythia generation
efficiency (the likelihood of surviving step 4) is greatly enhanced. The price to be paid isthat z has to be selected according to D(z)/z rather than according to D(z). Note that
D(z)/z is not normalized to unity. One therefore needs to define
ID =

Z 1

D(z)
dz ,
z
0

(159)

1 D(z)
.
ID z

(160)

and a properly normalized
D0 (z) =Then
σtot =

Z 1Z 100

2D0 (z1 ) D0 (z2 ) ID
z1 z2 σ(z1 z2 s) dz1 dz2 .

(161)

2
Therefore the proper event weight is IDz1 z2 . This weight should be stored by you, for each
event, in PARP(173). The maximum weight that will be encountered should be storedin PARP(174) before the PYINIT call, and not changed afterwards. It is not necessary
to know the precise maximum; any value larger than the true maximum will do, but
the inefficiency will be larger the cruder the approximation. Additionally you must put
MSTP(173) = 1 for the program to make use of weights at all. Often D(z) is not known
analytically; therefore ID is also not known beforehand, but may have to be evaluated (byyou) during the course of the run. Then you should just use the weight z1 z2 in PARP(173)and do the overall normalization yourself in the end. Since PARP(174) = 1 by default, in
this case you need not set this variable specially. Only the cross sections are affected by
the procedure selected for overall normalization, the events themselves still are properly
distributed in s0 and internal phase space.
Above it has been assumed tacitly that D(z) → 0 for z → 0. If not, D(z)/z isdivergent, and it is not possible to define a properly normalized D0 (z) = D(z)/z. If thecross section is truly diverging like 1/s, then a D(z) which is nonvanishing for z → 0

241does imply an infinite total cross section, whichever way things are considered. In cases
like that, it is necessary to impose a lower cut on z, based on some physics or detector
consideration. Some such cut is anyway needed to keep away from the minimum c.m.
energy required for Pythia events, see above.
The most difficult cases are those with a very narrow and high peak, such as theZ0 . One could initialize at the energy of maximum cross section and use D(z) as is,but efficiency might turn out to be very low. One might then be tempted to do more
complicated transforms of the kind illustrated above. As a rule it is then convenient towork in the variables τz = z1 z2 and yz = (1/2) ln(z1 /z2 ), cf. section 7.2.
Clearly, the better the behaviour of the cross section can be modelled in the choiceof z1 and z2 , the better the overall event generation efficiency. Even under the best of
circumstances, the efficiency will still be lower than for runs with fix energy. There is also a
non-negligible time overhead for using variable energies in the first place, from kinematics
reconstruction and (in part) from the phase space selection. One should therefore
not use√0
variable energies when not needed, and not use a large range of energies s if in the end
only a smaller range is of experimental interest.
This facility may be combined with most other aspects of the program. For instance,
it is possible to simulate beamstrahlung as above and still include bremsstrahlung with
MSTP(11) = 1. Further, one may multiply the overall event weight of PARP(173) with akinematics-dependent weight given by PYEVWT, although it is not recommended (since thechances of making a mistake are also multiplied). However, a few things do not work.
• It is not possible to use pile-up events, i.e. you must have MSTP(131) = 0.
• The possibility of giving in your own cross-section optimization coefficients, option
MSTP(121) = 2, would require more input than with fixed energies, and this option
should therefore not be used. You can still use MSTP(121) = 1, however.
• The multiple interactions scenario with MSTP(82) ≥ 2 only works approximately forenergies different from the initialization one. If the c.m. energy spread is smaller
than a factor 2, say, the approximation should be reasonable, but if the spread islarger one may have to subdivide into subruns of different energy bins. The initialization should be made at the largest energy to be encountered — whenevermultiple interactions are possible (i.e. for incoming hadrons and resolved photons)
this is where the cross sections are largest anyway, and so this is no further constraint. There is no simple possibility to change PARP(82) during the course of therun, i.e. an energy-independent p⊥0 must be assumed. By contrast, for MSTP(82) =1 p⊥min =PARP(81) can be set differently for each event, as a function of c.m. energy.
Initialization should then be done with PARP(81) as low as it is ever supposed to
become.

9.9

How to Include External Processes

Despite a large repertory of processes in Pythia, the number of interesting missing ones
clearly is even larger, and with time this discrepancy is likely to increase. There areseveral reasons why it is not practicable to imagine a Pythia which has ‘everything’.
One is the amount of time it takes to implement a process for the few Pythia authors,compared with the rate of new cross section results produced by the rather larger matrixelement calculations community. Another is the length of currently produced matrixelement expressions, which would make the program very bulky. A third argument isthat, whereas the phase space of 2 → 1 and 2 → 2 processes can be set up once and for
all according to a reasonably flexible machinery, processes with more final-state particles
are less easy to generate. To achieve a reasonable efficiency, it is necessary to tailor thephase-space selection procedure to the dynamics of the given process, and to the desired
experimental cuts.

242

At times, simple solutions may be found. Some processes may be seen just as trivial
modifications of already existing ones. For instance, you might want to add some extra
term, corresponding to contact interactions, to the matrix elements of a Pythia 2 → 2
process. In that case it is not necessary to go through the machinery below, but instead
you can use the PYEVWT routine (section 9.7) to introduce an additional weight for the
event, defined as the ratio of the modified to the unmodified differential cross sections. If
you use the option MSTP(142) = 2, this weight is considered as part of the ‘true’ crosssection of the process, and the generation is changed accordingly.
A Pythia expert could also consider implementing a new process along the lines of theexisting ones, hardwired in the code. Such a modification would have to be ported anytime
the Pythia program is upgraded, however (unless it is made available to the Pythia
authors and incorporated into the public distribution). For this and other reasons, we will
not consider this option in detail, but only provide a few generic remarks. The first step isto pick a process number ISUB among ones not in use. The process type needs to be set in
ISET(ISUB) and, if the final state consists of massive particles, these should be specified
in KFPR(ISUB,1) and KFPR(ISUB,2). Output is improved if a process name is set in
PROC(ISUB). The second and main step is to code the cross section of the hard-scattering
subprocess in the PYSIGH routine. Usually the best starting point is to use the code ofan existing similar process as a template for the new code required. The third step is toprogram the selection of the final state in PYSCAT, normally a simple task, especially ifagain a similar process (especially with respect to colour flow) can be used as template. In
many cases the steps above are enough, in others additional modifications are required to
PYRESD to handle process-specific non-isotropic decays of resonances. Further code may
also be required e.g. if a process can proceed via an intermediate resonance that can beon the mass shell.
The recommended solution, if a desired process is missing, is instead to include it
into Pythia as an ‘external’ process. In this section we will describe how it is possible
to specify the parton-level state of some hard-scattering process in a common block.
(‘Parton-level’ is not intended to imply a restriction to quarks and gluons as interacting
particles, but only that quarks and gluons are given rather than the hadrons they willproduce in the observable final state.) Pythia will read this common block, and add
initial- and final-state showers, beam remnants and underlying events, fragmentation anddecays, to build up an event in as much detail as an ordinary Pythia one. Anothercommon block is to be filled with information relevant for the run as a whole, wherebeams and processes are specified.
Such a facility has been available since long ago, and has been used e.g. together
with the CompHEP package. CompHEP [Puk99] is mainly intended for the automaticcomputation of matrix elements, but also allows the sampling of phase space according
to these matrix elements and thereby the generation of weighted or unweighted events.
These events can be saved on disk and thereafter read back in to Pythia for subsequentconsideration [Bel00].
At the Les Houches 2001 workshop it was decided to develop a common standard,that could be used by all matrix-elements-based generators to feed information into any
complete event generator — the Les Houches Accord (LHA) [Boo01]. It is similar to, but inits details different from, the approach previously implemented in Pythia. Furthermore,
it uses the same naming convention: all names in common blocks end with UP, short forUser(-defined) Process. This produces some clashes. Therefore the old facility, existing
up to and including Pythia 6.1, has been completely removed and replaced by the newone. Currently not every last detail of the standard has yet been implemented. In thedescription below we will emphasize such restrictions, as well as the solutions to aspects
not specified by the standard.
In particular, even with the common-block contents defined, it is not clear where they
are to be filled, i.e. how the external supplier of parton-level events should synchronize with243

Pythia. The solution adopted here — recommended in the standard — is to introduce
two subroutines, UPINIT and UPEVNT. The first is called by PYINIT at initialization to
obtain information about the run itself, and the other called by PYEVNT each time a
new event configuration is to be fed in. We begin by describing these two steps andtheir related common blocks, before proceeding with further details and examples. The
description is cast in a Pythia-oriented language, but for the common-block contents it
closely matches the generator-neutral standard in [Boo01]. Restrictions to or extensionsof the standard should be easily recognized, but in case you are vitally dependent onfollowing the standard exactly, you should of course check [Boo01].
Note that the UPVETO routine, introduced above in section 9.7, is a third member of the
UP.... family of routines. While not part of the LHA, it offers extra functionality thatallows the user to combine parton configuration input at different orders of perturbation
theory, in such a way that the matrix-elements description and the shower activity do not
doublecount emissions.
9.9.1

Run information

When PYINIT is called in the main program, with ’USER’ as first argument (which makes
the other arguments dummy), it signals that external processes are to be implemented.
Then PYINIT, as part of its initialization tasks, will call the routine UPINIT.
CALL UPINIT
Purpose: routine to be provided by you when you want to implement external processes,
wherein the contents of the HEPRUP common block are set. This information
specifies the character of the run, both beams and processes, see further below.
Note 1: alternatively, the HEPRUP common block could be filled already before PYINIT
is called, in which case UPINIT could be empty. We recommend UPINIT as thelogical place to collect the relevant information, however.
Note 2: a dummy copy of UPINIT is distributed with the program, in order to avoid
potential problems with unresolved external references. This dummy should not
be linked when you supply your own UPINIT routine. The code can be used toread initialization information previously written by PYUPIN, however.
INTEGER MAXPUP
PARAMETER (MAXPUP=100)
INTEGER IDBMUP,PDFGUP,PDFSUP,IDWTUP,NPRUP,LPRUPDOUBLE PRECISION EBMUP,XSECUP,XERRUP,XMAXUP
COMMON/HEPRUP/IDBMUP(2),EBMUP(2),PDFGUP(2),PDFSUP(2),&IDWTUP,NPRUP,XSECUP(MAXPUP),XERRUP(MAXPUP),XMAXUP(MAXPUP),&LPRUP(MAXPUP)
Purpose: to contain the initial information necessary for the subsequent generation ofcomplete events from externally provided parton configurations. The IDBMUP,
EBMUP, PDFGUP and PDFSUP variables specify the nature of the two incoming
beams. IDWTUP is a master switch, selecting the strategy to be used to mix
different processes. NPRUP gives the number of different external processes tomix, and XSECUP, XERRUP, XMAXUP and LPRUP information on each of these. The
contents in this common block must remain unchanged by the user during thecourse of the run, once set in the initialization stage.
This common block should be filled in the UPINIT routine or, alternatively, before
the PYINIT call. During the run, Pythia may update the XMAXUP values as244

required.
MAXPUP : the maximum number of distinguishable processes that can be defined. (Eachprocess in itself could consist of several subprocesses that have been distinguishedin the parton-level generator, but where this distinction is not carried along.)
IDBMUP : the PDG codes of the two incoming beam particles (or, in alternative terminology, the beam and target particles).
In Pythia, this replaces the information normally provided by the BEAM and
TARGET arguments of the PYINIT call. Only particles which are acceptable BEAM
or TARGET arguments may also be used in IDBMUP. The ’gamma/lepton’ options
are not available.
EBMUP : the energies, in GeV, of the two incoming beam particles. The first (second)particle is taken to travel in the +z (−z) direction.
The standard also allows non-collinear and varying-energy beams to be specified,see ISTUP = -9 below, but this is not yet implemented in Pythia.
PDFGUP, PDFSUP : the author group (PDFGUP) and set (PDFSUP) of the parton distributions of the two incoming beams, as used in the generation of the parton-level
events. Numbers are based on the Pdflib [Plo93] lists, and should extend to
LHAPDF [Gie02]. This enumeration may not always be up to date, but it
provides the only unique integer labels for parton distributions that we have.
Where no codes are yet assigned to the parton distribution sets used, one should
do as best as one can, and be prepared for more extensive user interventions
to interpret the information. For lepton beams, or when the information is notprovided for other reasons, one should put PDFGUP = PDFSUP = -1.
By knowing which set has been used, it is possible to reweight cross sections
event by event, to correspond to another set.
Note that Pythia does not access the PDFGUP or PDFSUP values in its description of internal processes or initial-state showers. If you want this to happen, youhave to manipulate the MSTP(51) - MSTP(56) switches. For instance, to accessPdflib for protons, put MSTP(51) = 1000*PDFGUP + PDFSUP and MSTP(52)= 2 in UPINIT. (And remove the dummy Pdflib routines, as described for
MSTP(52).) Also note that PDFGUP and PDFSUP allow an independent choiceof parton distributions on the two sides of the event, whereas Pythia only allows one single choice for all protons, another for all pions and a third for all
photons.
Pythia implements one extension not specified in the LHA: If you set PDFGUP(i)= -9 for either of the two incoming beams, i = 1 or = 2, this signals that beam
remnants are already included in the specified final state and should not be provided by Pythia. As a consequence, neither initial-state radiation nor multiple
interactions are applied to the given particle configuration, since these would
redefine the beam remnants. What remains is resonance decays, final-state radiation, hadronization and ordinary decays (unless explicitly switched off, ofcourse). One application could be to plug in parton-level configurations alreadygenerated by some other initial-state shower algorithm. A more typical example would be a generator for diffractive Higgs production, pp → ppH, where
Pythia would be used to address the Higgs decay with associated showers andhandronization. Note that, in accordance with the general rules, it is necessary
to provide the two incoming protons as the first two particles of the /HEPEUP/
event record, with status code −1. (Although this here happens to be redundant,
given the beam information provided at initialization.)IDWTUP : master switch dictating how event weights and cross sections should be interpreted. Several different models are presented in detail below. There will be
tradeoffs between these, e.g. a larger flexibility to mix and re-mix several different processes could require a larger administrative machinery. Therefore the245best strategy would vary, depending on the format of the input provided andthe output desired. In some cases, parton-level configurations have already been
generated with one specific model in mind, and then there may be no choice.
IDWTUP significantly affects the interpretation of XWGTUP, XMAXUP and XSECUP,
as described below, but the basic nomenclature is the following. XWGTUP is theevent weight for the current parton-level event, stored in the HEPEUP common
block. For each allowed external process i, XMAXUP(i) gives the maximum event
weight that could be encountered, while XSECUP(i) is the cross section of theprocess. Here i is an integer in the range between 1 and NPRUP; see the LPRUP
description below for comments on alternative process labels.
= 1 : parton-level events come with a weight when input to Pythia, but arethen accepted or rejected, so that fully generated events at output have acommon weight, customarily defined as +1. The event weight XWGTUP is a
non-negative dimensional quantity, in pb (converted to mb in Pythia), witha mean value converging to the total cross section of the respective process.
For each process i, the XMAXUP(i) value provides an upper estimate of how
large XWGTUP numbers can be encountered. There is no need to supply an
XSECUP(i) value; the cross sections printed with PYSTAT(1) are based entirely on the averages of the XWGTUP numbers (with a small correction for
the fraction of events that PYEVNT fails to generate in full for some reason).
The strategy is that PYEVNT selects which process i should be generated
next, based on the relative size of the XMAXUP(i) values. The UPEVNT routine has to fill the HEPEUP common block with a parton-level event of therequested type, and give its XWGTUP event weight. The event is accepted by
PYEVNT with probability XWGTUP/XMAXUP(i). In case of rejection, PYEVNT
selects a new process i and asks for a new event. This ensures that processes are mixed in proportion to their average XWGTUP values.
This model presumes that UPEVNT is able to return a parton-level event of
the process type requested by PYEVNT. It works well if each process is associated with an input stream of its own, either a subroutine generating events‘on the fly’ or a file of already generated events. It works less well if partonlevel events from different processes already are mixed in a single file, andtherefore cannot easily be returned in the order wanted by PYEVNT. In thelatter case one should either use another model or else consider reducing thelevel of ambition: even if you have mixed several different subprocesses ona file, maybe there is no need for Pythia to know this finer classification,in which case we may get back to a situation with one ‘process’ per external
file. Thus the subdivision into processes should be a matter of convenience,
not a strait-jacket. Specifically, the shower and hadronization treatment of
a parton-level event is independent of the process label assigned to it.
If the events of some process are already available unweighted, then a correct mixing of this process with others is ensured by putting XWGTUP =XMAXUP(i), where both of these numbers now is the total cross section of
the process.
Each XMAXUP(i) value must be known from the very beginning, e.g. froman earlier exploratory run. If a larger value is encountered during the courseof the run, a warning message will be issued and the XMAXUP(i) value (andits copy in XSEC(ISUB,1)) increased. Events generated before this time will
have been incorrectly distributed, both in the process composition and in
the phase space of the affected process, so that a bad estimate of XMAXUP(i)may require a new run with a better starting value.
The model described here agrees with the one used for internal Pythia
processes, and these can therefore freely be mixed with the external ones.
246

Internal processes are switched on with MSUB(ISUB) = 1, as usual, either
before the PYINIT call or in the UPINIT routine. One cannot use MSEL toselect a predefined set of processes, for technical reasons, wherefore MSEL =0 is hardcoded when external processes are included.
A reweighting of events is feasible, e.g. by including a kinematics-dependent
K factor into XWGTUP, so long as XMAXUP(i) is also properly modified totake this into account. Optionally it is also possible to produce events with
non-unit weight, making use the PYEVWT facility, see section 9.7. This works
exactly the same way as for internal Pythia processes, except that the event
information available inside PYEVWT would be different for external processes.
You may therefore wish to access the HEPEUP common block inside your own
copy of PYEVWT, where you calculate the event weight.
In summary, this option provides maximal flexibility, but at the price ofpotentially requiring the administration of several separate input streams ofparton-level events.
= -1 : same as = 1 above, except that event weights may be either positive or negative on input, and therefore can come with an output weight of +1 or −1.
This weight is uniquely defined by the sign of XWGTUP. It is also stored in
PARI(7). The need for negative-weight events arises in some next-to-leadingorder calculations, but there are inherent dangers, discussed in section 9.9.5
below.
In order to allow a correct mixing between processes, a process of indeterminate cross section sign has to be split up in two, where one always
gives a positive or vanishing XWGTUP, and the other always gives it negative
or vanishing. The XMAXUP(i) value for the latter process should give thenegative XWGTUP of largest magnitude that will be encountered. PYEVNT selects which process i that should be generated next, based on the relative
size of the |XMAXUP(i)| values. A given event is accepted with probability|XWGTUP|/|XMAXUP(i)|.
= 2 : parton-level events come with a weight when input to Pythia, but arethen accepted or rejected, so that events at output have a common weight,
customarily defined as +1. The non-negative event weight XWGTUP and its
maximum value XMAXUP(i) may or may not be dimensional quantities; itdoes not matter since only the ratio XWGTUP/XMAXUP(i) will be used. Instead XSECUP(i) contains the process cross section in pb (converted to mb
in Pythia). It is this cross section that appears in the PYSTAT(1) table,
only modified by the small fraction of events that PYEVNT fails to generatein full for some reason.
The strategy is that PYEVNT selects which process i should be generated
next, based on the relative size of the XSECUP(i) values. The UPEVNT routine has to fill the HEPEUP common block with a parton-level event of therequested type, and give its XWGTUP event weight. The event is accepted by
PYEVNT with probability XWGTUP/XMAXUP(i). In case of rejection, the process number i is retained and PYEVNT asks for a new event of this kind. This
ensures that processes are mixed in proportion to their XSECUP(i) values.
This model presumes that UPEVNT is able to return a parton-level event of
the process type requested by PYEVNT, with comments exactly as for the =1 option.
If the events of some process are already available unweighted, then a correct mixing of this process with others is ensured by putting XWGTUP =XMAXUP(i).
Each XMAXUP(i) and XSECUP(i) value must be known from the very beginning, e.g. from an earlier integration run. If a larger value is encoun247

tered during the course of the run, a warning message will be issued and the
XMAXUP(i) value increased. This will not affect the process composition, but
events generated before this time will have been incorrectly distributed in
the phase space of the affected process, so that a bad estimate of XMAXUP(i)may require a new run with a better starting value.
While the generation model is different from the normal internal Pythia
one, it is sufficiently close that internal processes can be freely mixed withthe external ones, exactly as described for the = 1 option. In such a mix,
internal processes are selected according to their equivalents of XMAXUP(i)and at rejection a new i is selected, whereas external ones are selected according to XSECUP(i) with i retained when an event is rejected.
A reweighting of individual events is no longer simple, since this wouldchange the XSECUP(i) value nontrivially. Thus a new integration run withthe modified event weights would be necessary to obtain new XSECUP(i)and XMAXUP(i) values. An overall rescaling of each process separately canbe obtained by modifying the XSECUP(i) values accordingly, however, e.g.
by a relevant K factor.
In summary, this option is similar to the = 1 one. The input of XSECUP(i)allows good cross section knowledge also in short test runs, but at the priceof a reduced flexibility to reweight events.
= -2 : same as = 2 above, except that event weights may be either positive or negative on input, and therefore can come with an output weight of +1 or −1.
This weight is uniquely defined by the sign of XWGTUP. It is also stored in
PARI(7). The need for negative-weight events arises in some next-to-leadingorder calculations, but there are inherent dangers, discussed in section 9.9.5
below.
In order to allow a correct mixing between processes, a process of indeterminate cross section sign has to be split up in two, where one always gives a
positive or vanishing XWGTUP, and the other always gives it negative or vanishing. The XMAXUP(i) value for the latter process should give the negative
XWGTUP of largest magnitude that will be encountered, and XSECUP(i) should
give the integrated negative cross section. PYEVNT selects which process ithat should be generated next, based on the relative size of the |XSECUP(i)|values. A given event is accepted with probability |XWGTUP|/|XMAXUP(i)|.
= 3 : parton-level events come with unit weight when input to Pythia, XWGTUP= 1, and are thus always accepted. This makes the XMAXUP(i) superfluous,
while XSECUP(i) should give the cross section of each process.
The strategy is that that the next process type i is selected by the user
inside UPEVNT, at the same time as the HEPEUP common block is filled withinformation about the parton-level event. This event is then unconditionallyaccepted by PYEVNT, except for the small fraction of events that PYEVNT fails
to generate in full for some reason.
This model allows UPEVNT to read events from a file where different processes
already appear mixed. Alternatively, you are free to devise and implement
your own mixing strategy inside UPEVNT, e.g. to mimic the ones alreadyoutlined for PYEVNT in = 1 and = 2 above.
The XSECUP(i) values should be known from the beginning, in order for
PYSTAT(1) to produce a sensible cross section table. This is the only place
where it matters, however. That is, the processing of events inside Pythia
is independent of this information.
In this model it is not possible to mix with internal Pythia processes, since
not enough information is available to perform such a mixing.
A reweighting of events is completely in the hands of the UPEVNT author. In
248the case that all events are stored in a single file, and all are to be handed
on to PYEVNT, only a common K factor applied to all processes would bepossible.
In summary, this option puts more power — and responsibility — in thehands of the author of the parton-level generator. It is very convenient forthe processing of unweighted parton-level events stored in a single file. The
price to be paid is a reduced flexibility in the reweighting of events, or in
combining processes at will.
= -3 : same as = 3 above, except that event weights may be either +1 or −1.
This weight is uniquely defined by the sign of XWGTUP. It is also stored in
PARI(7). The need for negative-weight events arises in some next-to-leadingorder calculations, but there are inherent dangers, discussed in section 9.9.5
below.
Unlike the = -1 and = -2 options, there is no need to split a process in two,each with a definite XWGTUP sign, since PYEVNT is not responsible for the
mixing of processes. It may well be that the parton-level-generator author
has enforced such a split, however, to solve a corresponding mixing problem
inside UPEVNT. Information on the relative cross section in the negative- andpositive-weight regions may also be useful to understand the character andvalidity of the calculation (large cancellations means trouble!).
= 4 : parton-level events come with a weight when input to Pythia, and thisweight is to be retained unchanged at output. The event weight XWGTUP is a
non-negative dimensional quantity, in pb (converted to mb in Pythia, andas such also stored in PARI(7)), with a mean value converging to the total
cross section of the respective process. When histogramming results, one ofthese event weights would have to be used.
The strategy is exactly the same as = 3 above, except that the event weightis carried along from UPEVNT to the PYEVNT output. Thus again all control
is in the hands of the UPEVNT author.
A cross section can be calculated from the average of the XWGTUP values, asin the = 1 option, and is displayed by PYSTAT(1). Here it is of purely informative character, however, and does not influence the generation procedure.
Neither XSECUP(i) or XMAXUP(i) needs to be known or supplied.
In this model it is not possible to mix with internal Pythia processes, since
not enough information is available to perform such a mixing.
A reweighting of events is completely in the hands of the UPEVNT author,
and is always simple, also when events appear sequentially stored in a single
file.
In summary, this option allows maximum flexibility for the parton-levelgenerator author, but potentially at the price of spending a significant
amount of time processing events of very small weight. Then again, insome cases it may be an advantage to have more events in the tails of a
distribution in order to understand those tails better.
= -4 : same as = 4 above, except that event weights in XWGTUP may be either
positive or negative. In particular, the mean value of XWGTUP is converging
to the total cross section of the respective process. The need for negativeweight events arises in some next-to-leading-order calculations, but there are
inherent dangers, discussed in section 9.9.5 below.
Unlike the = -1 and = -2 options, there is no need to split a process in
two, each with a definite XWGTUP sign, since PYEVNT does not have to mix
processes. However, as for option = -3, such a split may offer advantages inunderstanding the character and validity of the calculation.
NPRUP : the number of different external processes, with information stored in the first249

NPRUP entries of the XSECUP, XERRUP, XMAXUP and LPRUP arrays.
XSECUP : cross section for each external process, in pb. This information is mandatory
for IDWTUP = ±2, helpful for ±3, and not used for the other options.
XERRUP : the statistical error on the cross section for each external process, in pb.
Pythia will never make use of this information, but if it is available anyway it
provides a helpful service to the user of parton-level generators.
Note that, if a small number nacc of events pass the experimental√selection cuts,
the statistical error on this cross section is limited by δσ/σ ≈ 1/ nacc , irrespectively of the quality of the original integration. Furthermore, at least in hadronic
physics, systematic errors from parton distributions and higher orders typicallyare much larger than the statistical errors.
XMAXUP : the maximum event weight XWGTUP that is likely to be encountered for eachexternal process. For IDWTUP = ±1 it has dimensions pb, while the dimensionality
need not be specified for ±2. For the other IDWTUP options it is not used.
LPRUP : a unique integer identifier of each external process, free to be picked by you foryour convenience. This code is used in the IDPRUP identifier of which process
occured.
In Pythia, an external process is thus identified by three different integers. The
first is the Pythia process number, ISUB. This number is assigned by PYINIT
at the beginning of each run, by scanning the ISET array for unused process
numbers, and reclaiming such in the order they are found. The second is thesequence number i, running from 1 through NPRUP, used to find informationin the cross section arrays. The third is the LPRUP(i) number, which can beanything that the user wants to have as a unique identifier, e.g. in a larger
database of processes. For Pythia to handle conversions, the two KFPR numbersof a given process ISUB are overwritten with the second and third numbers above.
Thus the first external process will land in ISUB = 4 (currently), and could have
LPRUP(1) = 13579. In a PYSTAT(1) call, it would be listed as User process
13579.
9.9.2

Event information

Inside the event loop of the main program, PYEVNT will be called to generate the next event,as usual. When this is to be an external process, the parton-level event configuration andthe event weight is found by a call from PYEVNT to UPEVNT.
CALL UPEVNT
Purpose: routine to be provided by you when you want to implement external processes,
wherein the contents of the HEPEUP common block are set. This information
specifies the next parton-level event, and some additional event information, see
further below. How UPEVNT is expected to solve its task depends on the model
selected in IDWTUP, see above. Specifically, note that the process type IDPRUP has
already been selected for some IDWTUP options (and then cannot be overwritten),while it remains to be chosen for others.
Note : a dummy copy of UPEVNT is distributed with the program, in order to avoid
potential problems with unresolved external references. This dummy should not
be linked when you supply your own UPEVNT routine. The code can be used toread event information previously written by PYUPEV, however.

250

INTEGER MAXNUPPARAMETER (MAXNUP=500)
INTEGER NUP,IDPRUP,IDUP,ISTUP,MOTHUP,ICOLUP
DOUBLE PRECISION XWGTUP,SCALUP,AQEDUP,AQCDUP,PUP,VTIMUP,SPINUP
COMMON/HEPEUP/NUP,IDPRUP,XWGTUP,SCALUP,AQEDUP,AQCDUP,IDUP(MAXNUP),&ISTUP(MAXNUP),MOTHUP(2,MAXNUP),ICOLUP(2,MAXNUP),PUP(5,MAXNUP),&VTIMUP(MAXNUP),SPINUP(MAXNUP)
Purpose : to contain information on the latest external process generated in UPEVNT.
A part is one-of-a-kind numbers, like the event weight, but the bulk of the information is a listing of incoming and outgoing particles, with history, colour,
momentum, lifetime and spin information.
MAXNUP : the maximum number of particles that can be specified by the external process.
The maximum of 500 is more than Pythia is set up to handle. By default,
MSTP(126) = 100, at most 96 particles could be specified, since 4 additional
entries are needed in Pythia for the two beam particles and the two initiatorsof initial-state radiation. If this default is not sufficient, MSTP(126) would haveto be increased at the beginning of the run.
NUP : the number of particle entries in the current parton-level event, stored in the
NUP first entries of the IDUP, ISTUP, MOTHUP, ICOLUP, PUP, VTIMUP and SPINUP
arrays.
The special value NUP = 0 is used to denote the case where UPEVNT is unable
to provide an event, at least of the type requested by PYEVNT, e.g. because all
events available in a file have already been read. For such an event also the error
flag MSTI(51) = 1 instead of the normal = 0.
IDPRUP : the identity of the current process, as given by the LPRUP codes.
When IDWTUP = ±1 or ±2, IDPRUP is selected by PYEVNT and already set whenentering UPEVNT. Then UPEVNT has to provide an event of the specified process
type, but cannot change IDPRUP. When IDWTUP = ±3 or ±4, UPEVNT is free toselect the next process, and then should set IDPRUP accordingly.
XWGTUP : the event weight. The precise definition of XWGTUP depends on the value ofthe IDWTUP master switch. For IDWTUP = 1 or = 4 it is a dimensional quantity,
in pb, with a mean value converging to the total cross section of the respective
process. For IDWTUP = 2 the overall normalization is irrelevant. For IDWTUP =3 only the value +1 is allowed. For negative IDWTUP also negative weights areallowed, although positive and negative weights cannot appear mixed in the sameprocess for IDWTUP = -1 or = -2.
SCALUP : scale Q of the event, as used in the calculation of parton distributions (factorization scale). If the scale has not been defined, this should be denoted by using
the value -1.
In Pythia, this is input to PARI(21) - PARI(26) (and internally VINT(51) VINT(56)) When SCALUP is non-positive, the invariant mass of the parton-level
event is instead used as scale. Either of these comes to set the maximum virtuality in the initial-state parton showers. The same scale is also used for the first
final-state shower, i.e. the one associated with the hard scattering. As in internal
events, PARP(67) and PARP(71) offer multiplicative factors, whereby the respective initial- or final-state showering Q2max scale can be modified relative to thescale above. Any subsequent final-state showers are assumed to come from resonance decays, where the resonance mass always sets the scale. Since SCALUP is
not directly used inside Pythia to evaluate parton densities, its role as regulatorof parton-shower activity may be the more important one.
AQEDUP : the QED coupling αem used for this event. If αem has not been defined, this251should be denoted by using the value -1.
In Pythia, this value is stored in VINT(57). It is not used anywhere, however.
AQCDUP : the QCD coupling αs used for this event. If αs has not been defined, this should
be denoted by using the value -1.
In Pythia, this value is stored in VINT(58). It is not used anywhere, however.
IDUP(i) : particle identity code, according to the PDG convention, for particle i. As an
extension to this standard, IDUP(i) = 0 can be used to designate an intermediate state of undefined (and possible non-physical) character, e.g. a subsystemwith a mass to be preserved by parton showers.
In the Pythia event record, this corresponds to the KF = K(I,2) code. But
note that, here and in the following, the positions i in HEPEUP and I in PYJETS
are likely to be different, since Pythia normally stores more information in the
beginning of the event record. Since K(I,2) = 0 is forbidden, the IDUP(i) = 0
code is mapped to K(I,2) = 90.
ISTUP(i) : status code of particle i.
= -1 : an incoming particle of the hard-scattering process.
In Pythia, currently it is presumed that the first two particles, i = 1 and= 2, are of this character, and none of the others. If this is not the case,the HEPEUP record will be rearranged to put such entries first. If the listingis still not acceptable after this, the program execution will stop. This isa restriction relative to the standard, which allows more possibilities. It isalso presumed that these two particles are given with vanishing masses andparallel to the respective incoming beam direction, i.e. E = pz for the firstand E = −pz for the second. Should the particles not be massless at input,
E and pz is shuffled between the two incoming partons to assure this, while
preserving the total quantities. The assignment of space-like virtualities
and nonvanishing p⊥ ’s from initial-state radiation and primordial k⊥ ’s is theprerogative of Pythia.
= 1 : an outgoing final-state particle.
Such a particle can, of course, be processed further by Pythia, to add
showers and hadronization, or perform decays of any remaining resonances.
= 2 : an intermediate resonance, whose mass should be preserved by parton showers. For instance, in a process such as e+ e− → Z0 h0 → qq bb, the Z0 andh0 should both be flagged this way, to denote that the qq and bb systems
should have their individual masses preserved. In a more complex example,
du → W− Z0 g → `− ν ` qq g, both the W− and Z0 particles and the W− Z0
pseudoparticle (with IDUP(i) = 0) could be given with status 2.
Often mass preservation is correlated with colour singlet subsystems, but
this need not be the case. In e+ e− → tt → bW+ bW− , the b and b would
be in a colour singlet state, but not with a preserved mass. Instead thet = bW+ and t = bW− masses would be preserved, i.e. when b radiatesb → bg the recoil is taken by the W+ . Exact mass preservation also by thehadronization stage is only guaranteed for colour singlet subsystems, however, at least for string fragmentation, since it is not possible to define asubset of hadrons that uniquely belong only with a single coloured particle.
The assignment of intermediate states is not always quantum mechanicallywell-defined. For instance, e+ e− → µ− µ+ νµ ν µ can proceed both througha W+ W− and a Z0 Z0 intermediate state, as well as through other graphs,
which can interfere with each other. It is here the responsibility of thematrix-element-generator author to pick one of the alternatives, according
to some convenient recipe. One option might be to perform two calculations,
one complete to select the event kinematics and calculate the event weight,
and a second with all interference terms neglected to pick the event history252

according to the relative weight of each graph. Often one particular graphwould dominate, because a certain pairing of the final-state fermions would
give invariant masses on or close to some resonance peaks.
In Pythia, the identification of an intermediate resonance is not only a matter of preserving a mass, but also of improving the modelling of the finalstate shower evolution, since matrix-element-correction factors have been
calculated for a variety of possible resonance decays and implemented in therespective parton-shower description, see section 10.2.6.
= 3 : an intermediate resonance, given for documentation only, without any demand that the mass should be preserved in subsequent showers.
In Pythia, currently particles defined with this option are not treated any
differently from the ones with = 2.
= -2 : an intermediate space-like propagator, defining an x and a Q2 , in the Deeply
Inelastic Scattering terminology, which should be preserved.
In Pythia, currently this option is not defined and should not be used. If
it is, the program execution will stop.
= -9 : an incoming beam particle at time t = −∞. Such beams are not required inmost cases, since the HEPRUP common block normally contains the information. The exceptions are studies with non-collinear beams and with varyingenergy beams (e.g. from beamstrahlung, section 7.1.3), where HEPRUP does
not supply sufficient flexibility. Information given with = -9 overwrites the
one given in HEPRUP.
This is an optional part of the standard, since it may be difficult to combinewith some of the IDWTUP options.
Currently it is not recognized by Pythia. If it is used, the program execution will stop.
MOTHUP(1,i), MOTHUP(2,i) : position of the first and last mother of particle i. Decay
products will normally have only one mother. Then either MOTHUP(2,i) = 0 or
MOTHUP(2,i) = MOTHUP(1,i). Particles in the outgoing state of a 2 → n process
have two mothers. This scheme does not limit the number of mothers, so long as
these appear consecutively in the listing, but in practice there will likely never
be more than two mothers per particle.
As has already been mentioned for ISTUP(i) = 2, the definition of history is
not always unique. Thus, in a case like e+ e− → µ+ µ− γ, proceeding via anintermediate γ ∗ /Z0 , the squared matrix element contains an interference term
between initial- and final-state emission of the photon. This ambiguity has to beresolved by the matrix-elements-based generator.
In Pythia, only information on the first mother survives into K(I,3). This isadequate for resonance decays, while particles produced in the primary 2 → nprocess are given mother code 0, as is customary for internal processes. It implies
that two particles are deemed to have the same mothers if the first one agrees; it isdifficult to conceive of situations where this would not be the case. Furthermore,
it is assumed that the MOTHUP(1,i) < i, i.e. that mothers are stored ahead of
their daughters, and that all daughters of a mother are listed consecutively, i.e.
without other particles interspersed. If this is not the case, the HEPEUP record
will be rearranged so as to adhere to these principles.
Pythia has a limit of at most 80 particles coming from the same mother, for the
final-state parton-shower algorithm to work. In fact, the shower is optimized fora primary 2 → 2 process followed by some sequence of 1 → 2 resonance decays.
Then colour coherence with the initial state, matrix-element matching to gluon
emission in resonance decays, and other sophisticated features are automatically
included. By contrast, the description of emission in systems with three or morepartons is less sophisticated. Apart from problems with the algorithm itself,253

more information than is provided by the standard would be needed to do agood job. Specifically, there is a significant danger of double-counting or gaps
between the radiation already covered by matrix elements and the one added by
the shower. The omission from HEPEUP of intermediate resonances known to be
there, so that e.g. two consecutive 1 → 2 decays are bookkept as a single 1 → 3
branching, is a simple way to reduce the reliability of your studies!
ICOLUP(1,i), ICOLUP(2,i) : integer tags for the colour flow lines passing through thecolour and anticolour, respectively, of the particle. Any particle with colour(anticolour), such as a quark (antiquark) or gluon, will have the first (second)
number nonvanishing.
The tags can be viewed as a numbering of different colours in the NC → ∞ limit
of QCD. Any nonzero integer can be used to represent a different colour, but
the standard recommends to stay with positive numbers larger than MAXNUP to
avoid confusion between colour tags and the position labels i of particles.
The colour and anticolour of a particle is defined with the respect to the physicaltime ordering of the process, so as to allow a unique definition of colour flow
also through intermediate particles. That is, a quark always has a nonvanishingcolour tag ICOLUP(1,i), whether it is in the initial, intermediate or final state.
A simple example would be qq → tt → bW+ bW− , where the same colour label
is to be used for the q, the t and the b. Correspondingly, the q, t and b shareanother colour label, now stored in the anticolour position ICOLUP(2,i).
The colour label in itself does not distinguish between the colour or the anticolourof a given kind; that information appears in the usage either of the ICOLUP(1,i)
or of the ICOLUP(2,i) position for the colour or anticolour, respectively. Thus,
in a W+ → ud decay, the u and d would share the same colour label, but stored
in ICOLUP(1,i) for the u and in ICOLUP(2,i) for the d.
In general, several colour flows are possible in a given subprocess. This leads toambiguities, of a character similar to the ones for the history above, and as is discussed in section 8.2.1. Again it is up to the author of the matrix-elements-based
generator to find a sensible solution. It is useful to note that all interference terms
between different colour flow topologies vanish in the NC → ∞ limit of QCD.
One solution would then be to use a first calculation in standard QCD to selectthe momenta and find the weight of the process, and a second with NC → ∞to pick one specific colour topology according to the relative probabilities in thislimit.
The above colour scheme also allows for baryon-number-violating processes. Sucha vertex would appear as ‘dangling’ colour lines, when the ICOLUP and MOTHUP
information is correlated. For instance, in ũ → dd the ũ inherits an existing
colour label, while the two d’s are produced with two different new labels.
Several examples of colour assignments, both with and without baryon number
violation, are given in [Boo01].
In Pythia, baryon number violation is not yet implemented as part of theexternal-process machinery (but exists for internal processes, including internallyhandled decays of resonances provided externally). It will require substantial extra work to lift this restriction, and this is not imminent.
PUP(1,i), PUP(2,i), PUP(3,i), PUP(4,i), PUP(5,i) : the particle momentum vector (px , py , pz , E, m), with units of GeV. A space-like virtuality is denoted by anegative sign on the mass.
Apart from the index order, this exactly matches the P momentum conventionsin PYJETS.
Pythia is forgiving when it comes to using other masses than its own, e.g. forquarks. Thus the external process can be given directly with the mb used in
the calculation, without any worry how this matches the Pythia default. How254

ever, remember that the two incoming particles with ISTUP(i) = -1 have to bemassless, and will be modified correspondingly if this is not the case at input.
VTIMUP(i) : invariant lifetime cτ in mm, i.e. distance from production to decay. Oncethe primary vertex has been selected, the subsequent decay vertex positionsin space and time can be constructed step by step, by also making use of the
momentum information. Propagation in vacuum, without any bending e.g. bymagnetic fields, has to be assumed.
This exactly corresponds to the V(I,5) component in PYJETS. Note that it isused in Pythia to track colour singlet particles through distances that might beobservable in a detector. It is not used to trace the motion of coloured partonsat fm scales, through the hadronization process. Also note that Pythia will
only use this information for intermediate resonances, not for the initial- andfinal-state particles. For instance, for an undecayed τ − , the lifetime is selected
as part of the τ − decay process, not based on the VTIMUP(i) value.
SPINUP(i) : cosine of the angle between the spin vector of a particle and its threemomentum, specified in the lab frame, i.e. the frame where the event as a wholeis defined. This scheme is neither general nor complete, but it is chosen as asensible compromise.
The main foreseen application is τ ’s with a specific helicity. Typically a relativistic τ − (τ + ) coming from a W− (W+ ) decay would have helicity and SPINUP(i)= −1 (+1). This could be changed by the boost from the W rest frame to thelab frame, however. The use of a real number, rather than an integer, allows foran extension to the non-relativistic case.
Particles which are unpolarized or have unknown polarization should be given
SPINUP(i) = 9.
Explicit spin information is not used anywhere in Pythia. It is implicit in many
production and decay matrix elements, which often contain more correlation information than could be conveyed by the simple spin numbers discussed here.
Correspondingly, it is to be expected that the external generator already performed the decays of the W’s, the Z’s and the other resonances, so as to include
the full spin correlations. If this is not the case, such resonances will normally
be decayed isotropically. Some correlations could appear in decay chains: thePythia decay t → bW+ is isotropic, but the subsequent W+ → q1 q2 decay
contains implicit W helicity information from the t decay.
Also τ decays performed by Pythia would be isotropic. An interface routine
PYTAUD (see section 14.2) can be used to link to external τ decay generators, but
is based on defining the τ in the rest frame of the decay that produces it, and so is
not directly applicable here. It could be rewritten to make use of the SPINUP(i)information, however. In the meantime, and of course also afterwards, a valid
option is to perform the τ decays yourself before passing ‘parton-level’ events to
Pythia.
One auxiliary routine exists, that formally is part of the Pythia package, but could
be used by any generator:
SUBROUTINE PYUPRE : called immediately after UPEVNT has been called to provide a userprocess event. It will rearrange the contents of the HEPEUP common block so thatafterwards the two incoming partons appear in lines 1 and 2, so that all mothers
appear ahead of their daughters, and so that the daughters of a decay are listedconsecutively. Such an order can thereby be presumed to exist in the subsequentparsing of the event. If the rules already are obeyed, the routine does not change
the order. Further, the routine can shuffle energy and momentum between thetwo incoming partons to ensure that they are both massless.

2559.9.3

An example

To exemplify the above discussion, consider the explicit case of qq or gg → tt →bW+ bW− → bq1 q2 bq3 q4 . These two processes are already available in Pythia, but
without full spin correlations. One might therefore wish to include them from some external generator. A physics analysis would then most likely involve angular correlations
intended to set limits on (or reveal traces of) anomalous couplings. However, so as togive a simple self-contained example, instead consider the analysis of the charged multiplicity distribution. This actually offers a simple first cross-check between the internaland external implementations of the same process. The main program might then looksomething like
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
C...User process event common block.
INTEGER MAXNUPPARAMETER (MAXNUP=500)
INTEGER NUP,IDPRUP,IDUP,ISTUP,MOTHUP,ICOLUP
DOUBLE PRECISION XWGTUP,SCALUP,AQEDUP,AQCDUP,PUP,VTIMUP,SPINUP
COMMON/HEPEUP/NUP,IDPRUP,XWGTUP,SCALUP,AQEDUP,AQCDUP,IDUP(MAXNUP),&ISTUP(MAXNUP),MOTHUP(2,MAXNUP),ICOLUP(2,MAXNUP),PUP(5,MAXNUP),&VTIMUP(MAXNUP),SPINUP(MAXNUP)
SAVE /HEPEUP/
C...PYTHIA common block.
COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5)
SAVE /PYJETS/
C...Initialize.
CALL PYINIT(’USER’,’ ’,’ ’,0D0)
C...Book histogram. Reset event counter.
CALL PYBOOK(1,’Charged multiplicity’,100,-1D0,199D0)
NACC=0
C...Event loop; check that not at end of run; list first events.
DO 100 IEV=1,1000
CALL PYEVNTIF(NUP.EQ.0) GOTO 110
NACC=NACC+1
IF(IEV.LE.3) CALL PYLIST(7)IF(IEV.LE.3) CALL PYLIST(2)C...Analyse event; end event loop.
CALL PYEDIT(3)CALL PYFILL(1,DBLE(N),1D0)
100 CONTINUE
C...Statistics and histograms.
110 CALL PYSTAT(1)CALL PYFACT(1,1D0/DBLE(NACC))CALL PYHIST

256

END
There PYINIT is called with ’USER’ as first argument, implying that the rest is dummy.
The event loop itself looks fairly familiar, but with two additions. One is that NUP ischecked after each event, since NUP = 0 would signal a premature end of the run, withthe external generator unable to return more events. This would be the case e.g. if events
have been stored on file, and the end of this file is reached. The other is that CALLPYLIST(7) can be used to list the particle content of the HEPEUP common block (withsome information omitted, on vertices and spin), so that one can easily compare this input
with the output after Pythia processing, CALL PYLIST(2). An example of a PYLIST(7)listing would be
Event listing of user process at input (simplified)
I IST ID Mothers
1 -1
21
0
0
2 -1
21
0
0
3 2
6
1
2
4 2
-6
1
2
5 1
5
3
0
6 2
24
3
0
7 1
-5
4
0
8 2 -24
4
0
9 1
2
6
0
10 1
-1
6
0
11 1
-2
8
0
12 1
1
8
0

Coloursp_x
p_y
p_z
101 1090.000
0.000 269.223
109 1020.000
0.000 -225.566
101
072.569 153.924 -10.554
0 102 -72.569 -153.924
54.211101
056.51933.34353.9100016.050 120.581 -64.464
0 102
44.127 -60.882
25.50700 -116.696 -93.042
28.705103
0-8.667
11.85916.0630 103
24.717 108.722 -80.527
0 104 -33.709 -22.471 -26.877
104
0 -82.988 -70.571
55.582E
269.223225.566244.347250.44185.045159.30279.527170.91421.766137.53648.617122.297

m
0.000
0.000
175.030
175.5654.500
80.1504.500
78.184
0.000
0.000
0.000
0.000

Note the reverse listing of ID(UP) and IST(UP) relative to the HEPEUP order, to havebetter agreement with the PYJETS one. (The ID column is wider in real life, to allow for
longer codes, but has here been reduced to fit the listing onto the page.)
The corresponding PYLIST(2) listing of course would be considerably longer, containing a complete event as it does. Also the particles above would there appear boosted by
the effects of initial-state radiation and primordial k⊥ ; copies of them further down in
the event record would also include the effects of final-state radiation. The full story isavailable with MSTP(125) = 2, while the default listing omits some of the intermediate
steps.
The PYINIT call will generate a call to the user-supplied routine UPINIT. It is herethat we need to specify the details of the generation model. Assume, for instance, that
qq- and gg-initiated events have been generated in two separate runs for Tevatron Run
II, with weighted events stored in two separate files. By the end of each run, cross section
and maximum weight information has also been obtained, and stored on separate files.
Then UPINIT could look like
SUBROUTINE UPINIT
C...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
C...User process initialization common block.
INTEGER MAXPUP
PARAMETER (MAXPUP=100)
INTEGER IDBMUP,PDFGUP,PDFSUP,IDWTUP,NPRUP,LPRUP
257

DOUBLE PRECISION EBMUP,XSECUP,XERRUP,XMAXUP
COMMON/HEPRUP/IDBMUP(2),EBMUP(2),PDFGUP(2),PDFSUP(2),&IDWTUP,NPRUP,XSECUP(MAXPUP),XERRUP(MAXPUP),XMAXUP(MAXPUP),&LPRUP(MAXPUP)SAVE /HEPRUP/C....Pythia common block - needed for setting PDF’s; see below.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)SAVE /PYPARS/
C...Set incoming beams: Tevatron Run II.
IDBMUP(1)=2212
IDBMUP(2)=-2212
EBMUP(1)=1000D0
EBMUP(2)=1000D0
C...Set PDF’s of incoming beams: CTEQ 5L.
C...Note that Pythia will not look at PDFGUP and PDFSUP.
PDFGUP(1)=4PDFSUP(1)=46PDFGUP(2)=PDFGUP(1)PDFSUP(2)=PDFSUP(1)C...Set use of CTEQ 5L in internal Pythia code.
MSTP(51)=7
C...If you want Pythia to use PDFLIB, you have to set it by hand.
C...(You also have to ensure that the dummy routines
C...PDFSET, STRUCTM and STRUCTP in Pythia are not linked.)C
MSTP(52)=2
C
MSTP(51)=1000*PDFGUP(1)+PDFSUP(1)C...Decide on weighting strategy: weighted on input, cross section known.
IDWTUP=2
C...Number of external processes.
NPRUP=2
C...Set up q qbar -> t tbar.
OPEN(21,FILE=’qqtt.info’,FORM=’unformatted’,ERR=100)
READ(21,ERR=100) XSECUP(1),XERRUP(1),XMAXUP(1)LPRUP(1)=661
OPEN(22,FILE=’qqtt.events’,FORM=’unformatted’,ERR=100)
C...Set up g g -> t tbar.
OPEN(23,FILE=’ggtt.info’,FORM=’unformatted’,ERR=100)
READ(23,ERR=100) XSECUP(2),XERRUP(2),XMAXUP(2)LPRUP(2)=662
OPEN(24,FILE=’ggtt.events’,FORM=’unformatted’,ERR=100)
RETURN
C...Stop run if file operations fail.
100 WRITE(*,*) ’Error! File open or read failed. Program stopped.’
258

STOP
END
Here unformatted read/write is used to reduce the size of the event files, but at the price ofa platform dependence. Formatted files are preferred if they are to be shipped elsewhere.
The rest should be self-explanatory.
Inside the event loop of the main program, PYEVNT will call UPEVNT to obtain the next
parton-level event. In its simplest form, only a single READ statement would be necessary
to read information on the next event, e.g. what is shown in the event listing earlier inthis section, with a few additions. Then the routine could look like
SUBROUTINE UPEVNTC...Double precision and integer declarations.
IMPLICIT DOUBLE PRECISION(A-H, O-Z)IMPLICIT INTEGER(I-N)
C...User process event common block.
INTEGER MAXNUPPARAMETER (MAXNUP=500)
INTEGER NUP,IDPRUP,IDUP,ISTUP,MOTHUP,ICOLUP
DOUBLE PRECISION XWGTUP,SCALUP,AQEDUP,AQCDUP,PUP,VTIMUP,SPINUP
COMMON/HEPEUP/NUP,IDPRUP,XWGTUP,SCALUP,AQEDUP,AQCDUP,IDUP(MAXNUP),&ISTUP(MAXNUP),MOTHUP(2,MAXNUP),ICOLUP(2,MAXNUP),PUP(5,MAXNUP),&VTIMUP(MAXNUP),SPINUP(MAXNUP)
SAVE /HEPEUP/
C...Pick file to read from, based on requested event type.
IUNIT=22
IF(IDPRUP.EQ.662) IUNIT=24
C...Read event from this file. (Except that NUP and IDPRUP are known.)
NUP=12READ(IUNIT,ERR=100,END=100) XWGTUP,SCALUP,AQEDUP,AQCDUP,&(IDUP(I),ISTUP(I),MOTHUP(1,I),MOTHUP(2,I),ICOLUP(1,I),&ICOLUP(2,I),(PUP(J,I),J=1,5),VTIMUP(I),SPINUP(I),I=1,NUP)
C...Return, with NUP=0 if read failed.
RETURN
100 NUP=0
RETURN
END
However, in reality one might wish to save disk space by not storing redundant information. The XWGTUP and SCALUP numbers are vital, while AQEDUP and AQCDUP are purely
informational and can be omitted. In a gg → tt → bW+ bW− → bq1 q2 bq3 q4 event,
only the q1 , q2 , q3 and q4 flavours need be given, assuming that the particles are always
stored in the same order. For a qq initial state, the q flavour should be added to the list.
The ISTUP, MOTHUP and ICOLUP information is the same in all events of a given process,except for a twofold ambiguity in the colour flow for gg initial states. All VTIMUP vanish
and the SPINUP are uninteresting since spins have already been taken into account by thekinematics of the final fermions. (It would be different if one of the W’s decayed leptonically to a τ .) Only the PUP values of the six final fermions need be given, since the other259

momenta and masses can be reconstructed from this, remembering that the two initial
partons are massless and along the beam pipe. The final fermions are on the mass shell,so their masses need not be stored event by event, but can be read from a small table.
The energy of a particle can be reconstructed from its momentum and mass. Overall
transverse momentum conservation removes two further numbers. What remains is thus
5 integers and 18 real numbers, where the reals could well be stored in single precision.
Of course, the code needed to unpack information stored this way would be lengthy but
trivial. Even more compact storage strategies could be envisaged, e.g. only to save theweight and the seed of a dedicated random-number generator, to be used to generate thenext parton-level event. It is up to you to find the optimal balance between disk spaceand coding effort.
9.9.4

PYTHIA as a generator of external processes

It is possible to write Pythia-generated hard processes to disk, and then read them backin for the generation of a complete event, making use of the LHA conventions. This facility
may be of limited practical usefulness, and is mainly intended for debug purposes. Thus
there are some limitations to what it can do, especially no pileup and no photon beams(including the ’gamma/l’ options), and no weighted events. Basically, it is intended forhard processes in pp/pp/e+ e− collisions. Events are stored using the IDWTUP = 3 strategy,
i.e. already mixed and reweighted, so no rejection step is needed.
To generate the appropriate files, you have to:
1. Open one file where the initial run information in HEPRUP can be stored, and giveits file number in MSTP(161).
2. Open one file where events in the HEPEUP format can be stored, and give its filenumber in MSTP(162).
3. Define hard processes as normally, and initialize with PYINIT.
4. In the event loop, replace the call to PYEVNT (or PYEVNW) by a call to PYUPEV, which
will generate the hard process as usual, and write this process onto file number
MSTP(162).
5. After all events have been generated, call PYUPIN to store the run information.
(The name of this routine may seem inappropriate, since it is not called at theinitialization but at the end. However, the name refers to this information being
read in first in the next step, see below).
Once you have these files, you can use them in a later run to generate complete events.
This run follows the normal pattern for user-defined processes, with only minor additions.
1. Open the file where the initial run information in HEPRUP was stored, and give its
file number in MSTP(161).
2. Open the file where events in the HEPEUP format were stored, and give its file number
in MSTP(162).
3. Initialize as usual with CALL PYINIT(’USER’,’ ’,’ ’,0D0). The ’dummy’ UPINIT
routine in the Pythia library contains the code for reading the initial run information
written by PYUPIN (but not in any other format), so in this case you need not supply
any routine of your own.
4. In the event loop, call PYEVNT (or PYEVNW) as usual to generate the next event. The’dummy’ UPEVNT routine in the Pythia library contains the code for reading theevent information written by PYUPEV (but not in any other format), so in this caseyou need not supply any routine of your own.
9.9.5

Further comments

This section contains additional information on a few different topics: cross section interpretation, negative-weight events, relations with other Pythia switches and routines,
260

and error conditions.
In several IDWTUP options, the XWGTUP variable is supposed to give the differential
cross section of the current event, times the phase-space volume within which events are
generated, expressed in picobarns. (Converted to millibarns inside Pythia.) This means
that, in the limit that many events are generated, the average value of XWGTUP gives
the total cross section of the simulated process. Of course, the tricky part is that thedifferential cross section usually is strongly peaked in a few regions of the phase space,such that the average probability to accept an event, hXWGTUPi/XMAXUP(i) is small. It
may then be necessary to find a suitable set of transformed phase-space coordinates, forwhich the correspondingly transformed differential cross section is better behaved.
To avoid confusion, here is a more formal version of the above paragraph. Call dX
the differential phase space, e.g. for a 2 → 2 process dX = dx1 dx2 dt̂, where x1 and x2
are the momentum fractions carried by the two incoming partons and t̂ the Mandelstam
variable of the scattering (see section 7.2). Call dσ/dX the differential cross section ofP
the process, e.g. for 2 → 2: dσ/dX = ij fi (x1 , Q2 ) fj (x2 , Q2 ) dσ̂ij /dt̂, i.e. the productof parton distributions and hard-scattering matrix elements, summed over all allowed
incoming
R flavours i and j. The physical cross section that one then wants to generate
is σ = (dσ/dX) dX, where the integral is over the allowed phase-space volume. The
event generation procedure consists of selecting an X uniformly in dX and thenR evaluating the weight dσ/dX at this point. XWGTUP is now simply XWGTUP= dσ/dX dX, i.e.
the differential cross section times the considered volume of phase space. Clearly, when
averaged over many events, XWGTUP will correctly estimate the desired cross section. If
XWGTUP fluctuates too much, one may try to transform to newvariables X 0 , where eventsR00
are now picked accordingly to dX and XWGTUP= dσ/dX dX 0 .
A warning. It is important that X is indeed uniformly picked within the allowed phasespace, alternatively that any Jacobians are properly taken into account. For instance, inthe case above, one approach would be to pick x1 , x2 and t̂ uniformlyR in the ranges
0 < x1 < 1, 0 < x2 < 1, and −s < t̂ < 0, with full phase space volume dX = s. The
cross section would only be non-vanishing inside the physical region given by −sx1 x2 < t̂
(in the massless case), i.e. Monte Carlo efficiency is likely to be low. However, if one wereto choose t̂ values only in the range −ŝ < t̂ < 0, small ŝ values would be favoured, since
the density of selected t̂ values would be larger there. Without the use of a compensating
Jacobian ŝ/s, an incorrect answer would be obtained. Alternatively, one could start out
with a phase space like dX = dx1 dx2 d(cos θ̂), where the limits decouple. Of course, thecos θ̂ variable can be translated back into a t̂, which will then always be in the desired
range −ŝ < t̂ < 0. The transformation itself here gives the necessary Jacobian.
At times, it is convenient to split a process into a discrete set of subprocesses for theparton-level generation, without retaining these in the IDPRUP classification. For instance,
the cross section above contains a summation over incoming partons. An alternative
would then have been to let each subprocess correspond to one unique combination ofincoming flavours. When an event of process type iPis to be generated, first a specificsubprocess ik is selected with probability f ik , where k f ik = 1. For this subprocess an
XWGTUPk is generated as above, except that there is no longer a summation over incoming
flavours. Since only a fraction f ik of all events now contain this part of the cross section,ka compensating factor 1/f ik is introduced, i.e. XWGTUP=XWGTUP/f ik . Further, one has toPikik
define XMAXUP(i)= maxk XMAXUP /f and XSECUP(i)= k XSECUPik . The generation
efficiency will be maximized for the f ik coefficients selected proportional to XMAXUPik , but
this is no requirement.
The standard allows external parton-level events to come with negative weights, unlike
the case for internal Pythia processes. In order to avoid indiscriminate use of this option,some words of caution are in place. In next-to-leading-order calculations, events withnegative weights may occur as part of the virtual corrections. In any physical observable261

quantity, the effect of such events should cancel against the effect of real events with
one more parton in the final state. For instance, the next-to-leading order calculation ofgluon scattering contains the real process gg → ggg, with a positive divergence in the softand collinear regions, while the virtual corrections to gg → gg are negatively divergent.
Neglecting the problems of outgoing gluons collinear with the beams, and those of soft
gluons, two nearby outgoing gluons in the gg → ggg process can be combined into one
effective one, such that the divergences can be cancelled.
If rather widely separated gluons can be combined, the remaining negative contributions are not particularly large.
Different separation criteria could be considered; oneq2
example would be ∆R = (∆η) + (∆ϕ)2 ≈ 1. The recombination of well separated
partons is at the price of an arbitrariness in the choice of clustering algorithm, when
two gluons of nonvanishing invariant mass are to be combined into one massless one, as
required to be able to compare with the kinematics of the massless gg → gg process when
performing the divergence cancellations. Alternatively, if a smaller ∆R cut is used, where
the combining procedure is less critical, there will be more events with large positive andnegative weights that are to cancel.
Without proper care, this cancellation could easily be destroyed by the subsequent
showering description, as follows. The standard for external processes does not provide
any way to pass information on the clustering algorithm used, so the showering routine
will have to make its own choice what region of phase space to populate with radiation.
One choice could be to allow a cone defined by the nearest colour-connected parton (seesection 10.1.3 for a discussion). There could then arise a significant mismatch in shower
description between events where two gluons are just below or just above the ∆R cut forbeing recombined, equivalently between gg → gg and gg → ggg events. Most of the phasespace may be open for the former, while only the region below ∆R may be it for thelatter. Thus the average ‘two-parton’ events may end up containing significantly more jet
activity than the corresponding ‘three-parton’ ones. The smaller the ∆R cut, the more
severe the mismatch, both on an event-by-event basis and in terms of the event rates
involved.
One solution would be to extend the standard also to specify which clustering algorithm has been used in the matrix-element calculation, and with what parameter values.
Any shower emission that would give rise to an extra jet, according to this algorithm,
would be vetoed. If a small ∆R cut is used, this is almost equivalent to allowing noshower activity at all. (That would still leave us with potential mismatch problems in the
hadronization description. Fortunately the string fragmentation approach is very powerful in this respect, with a smooth transition between two almost parallel gluons and asingle one with the full energy [Sjö84].) But we know that the unassisted matrix-element
description cannot do a good job of the internal structure of jets on an event-by-event
basis, since multiple-gluon emission is the norm in this region. Therefore a ∆R ∼ 1 will
be required, to let the matrix elements describe the wide-angle emission and the showers
the small-angle one. This again suggests a picture with only a small contribution fromnegative-weight events. In summary, the appearance of a large fraction of negative-weight
events should be a sure warning sign that physics is likely to be incorrectly described.
The above example illustrates that it may, at times, be desirable to sidestep the standard and provide further information directly in the Pythia common blocks. (Currently
there is no exact match to the clustering task mentioned above, although the alreadydescribed UPVETO routine, section 9.7, could be constructed to make use of such information. Here we concentrate on a few simpler ways to intervene, possibly to be used inconjunction with UPVETO.) Then it is useful to note that, apart from the hard-process generation machinery itself, the external processes are handled almost exactly as the internal
ones. Thus essentially all switches and parameter values related to showers, underlying
events and hadronization can be modified at will. This even applies to alternative listingmodes of events and history pointers, as set by MSTP(128). Also some of the informa262

tion on the hard scattering is available, such as MSTI(3), MSTI(21) - MSTI(26), andPARI(33) - PARI(38). Before using them, however, it is prudent to check that yourvariables of interest do work as intended for the particular process you study. Several
differences do remain between internal and external processes, in particular related to
final-state showers and resonance decays. For internal processes, the PYRESD routine will
perform a shower (if relevant) directly after each decay. A typical example would be thata t → bW decay is immediately followed by a shower, which could change the momentumof the W before it decays in its turn. For an external process, this decay chain would
presumably already have been carried out. When the equivalent shower to the aboveis performed, it is therefore now necessary also to boost the decay products of the W.
The special sequence of showers and boosts for external processes is administrated by the
PYADSH routine. Should the decay chain not have been carried out, e.g if HEPEUP event
record contains an undecayed Z0 , then PYRESD will be called to let it decay. The decay
products will be visible also in the documentation section, as for internal processes.
You are free to make use of whatever tools you want in your UPINIT and UPEVNT
routines, and normally there would be little or no contact with the rest of Pythia, except
as described above. However, several Pythia tools can be used, if you so wish. One
attractive possibility is to use PYPDFU for parton-distribution-function evaluation. Other
possible tools could be PYR for random-number generation, PYALPS for αs evaluation,
PYALEM for evaluation of a running αem , and maybe a few more.
We end with a few comments on anomalous situations. As already described, you
may put NUP = 0 inside UPEVNT, e.g. to signal the end of the file from which events are
read. If the program encounters this value at a return from UPEVNT, then it will also exitfrom PYEVNT, without incrementing the counters for the number of events generated. It
is then up to you to have a check on this condition in your main event-generation loop.
This you do either by looking at NUP or at MSTI(51); the latter is set to 1 if no event was
generated.
It may also happen that a parton-level configuration fails elsewhere in the PYEVNTcall. For instance, the beam-remnant treatment occasionally encounters situations it
cannot handle, wherefore the parton-level event is rejected and a new one generated.
This happens also with ordinary (not user-defined) events, and usually comes about asa consequence of the initial-state radiation description leaving too little energy for theremnant. If the same hard scattering were to be used as input for a new initial-stateradiation and beam-remnant attempt, it could then work fine. There is a possibility togive events that chance, as follows. MSTI(52) counts the number of times a hard-scattering
configuration has failed to date. If you come in to UPEVNT with MSTI(52) non-vanishing,
this means that the latest configuration failed. So long as the contents of the HEPEUP
common block are not changed, such an event may be given another try. For instance, aline
IF(MSTI(52).GE.1.AND.MSTI(52).LE.4) RETURNat the beginning of UPEVNT will give each event up to five tries; thereafter a new one
would be generated as usual. Note that the counter for the number of events is updated
at each new try. The fraction of failed configurations is given in the bottom line of the
PYSTAT(1) table.
The above comment only refers to very rare occurrences (less than one in a hundred),which are not errors in a strict sense; for instance, they do not produce any error messages
on output. If you get warnings and error messages that the program does not understandthe flavour codes or cannot reconstruct the colour flows, it is due to faults of yours, andgiving such events more tries is not going to help.

263

9.10

Interfaces to Other Generators

In the previous section an approach to including external processes in Pythia was explained. While general enough, it may not always be the optimal choice. In particular, fore+ e− annihilation events one may envisage some standard cases where simpler approaches
could be pursued. A few such standard interfaces are described in this section.
In e+ e− annihilation events, a convenient classification of electroweak physics is by the
number of fermions in the final state. Two fermions from Z0 decay is LEP1 physics, four
fermions can come e.g. from W+ W− or Z0 Z0 events at LEP2, and at higher energies six
fermions are produced by three-gauge-boson production or top-antitop. Often interferenceterms are non-negligible, requiring much more complex matrix-element expressions thanare normally provided in Pythia. Dedicated electroweak generators often exist, however,and the task is therefore to interface them to the generic parton showering and hadronization machinery available in Pythia. In the LEP2 workshop [Kno96] one possible strategy
was outlined to allow reasonably standardized interfaces between the electroweak and the
QCD generators. The LU4FRM routine was provided for the key four-fermion case. Thisroutine is now included here, in slightly modified form, together with two siblings for twoand six fermions. The former is trivial and included mainly for completeness, while thelatter is rather more delicate.
In final states with two or three quark–antiquark pairs, the colour connection is notunique. For instance, a udud final state could either stem from a W+ W− or a Z0 Z0intermediate state, or even from interference terms between the two. In order to shower
and fragment the system, it is then necessary to pick one of the two alternatives, e.g.
according to the relative matrix element weight of each alternative, with the interferenceterm dropped. Some different such strategies are proposed as options below.
Note that here we discuss purely perturbative ambiguities. One can imagine colour
reconnection at later stages of the process, e.g. if the intermediate state indeed is W+ W− ,a soft-gluon exchange could still result in colour singlets uu and dd. We are then no longer
speaking of ambiguities related to the hard process itself but rather to the possibility of
nonperturbative effects. This is an interesting topic in itself, addressed in section 12.4.2
but not here.
The fermion-pair routines are not set up to handle QCD four-jet events, i.e. events ofthe types qqgg and qqq0 q0 (with q0 q0 coming from a gluon branching). Such events are
generated in normal parton showers, but not necessarily at the right rate (a problem that
may be especially interesting for massive quarks like b). Therefore one would like to starta QCD final-state parton shower from a given four-parton configuration. Already sometime ago, a machinery was developed to handle this kind of occurrences [And98a]. Thisapproach has now been adapted to Pythia, in a somewhat modified form, see section
10.2.7. The main change is that, in the original work, the colour flow was picked in a
separate first step (not discussed in the publication, since it is part of the standard 4parton configuration machinery of PYEEVT), which reduces the number of allowed qqgg
parton-shower histories. In the current implementation, more geared towards completely
external generators, no colour flow assumptions are made, meaning a few more possible
shower histories to pick between. Another change is that mass effects are better respectedby the z definition. The code contains one new user routine, PY4JET, two new auxiliary
ones, PY4JTW and PY4JTS, and significant additions to the PYSHOW showering routine.
CALL PY2FRM(IRAD,ITAU,ICOM)
Purpose: to allow a parton shower to develop and partons to hadronize from a twofermion starting point. The initial list is supposed to be ordered such that thefermion precedes the antifermion. In addition, an arbitrary number of photons
may be included, e.g. from initial-state radiation; these will not be affected by264the operation and can be put anywhere. The scale for QCD (and QED) finalstate radiation is automatically set to be the mass of the fermion-antifermion
pair. (It is thus not suited for Bhabha scattering.)
IRAD : final-state QED radiation.
= 0 : no final-state photon radiation, only QCD showers.
= 1 : photon radiation inside each final fermion pair, also leptons, in addition to
the QCD one for quarks.
ITAU : handling of τ lepton decay (where Pythia does not include spin effects, althoughsome generators provide the helicity information that would allow a more sophisticated modelling).
= 0 : τ ’s are considered stable (and can therefore be decayed afterwards).
= 1 : τ ’s are allowed to decay.
ICOM : place where information about the event (flavours, momenta etc.) is stored at
input and output.
= 0 : in the HEPEVT common block (meaning that information is automaticallytranslated to PYJETS before treatment and back afterwards).
= 1 : in the PYJETS common block. All fermions and photons can be given withstatus code K(I,1) = 1, flavour code in K(I,2) and five-momentum (momentum, energy, mass) in P(I,J). The V vector and remaining components
in the K one are best put to zero. Also remember to set the total number of
entries N.
CALL PY4FRM(ATOTSQ,A1SQ,A2SQ,ISTRAT,IRAD,ITAU,ICOM)
Purpose: to allow a parton shower to develop and partons to hadronize from a fourfermion starting point. The initial list of fermions is supposed to be orderedin the sequence fermion (1) – antifermion (2) – fermion (3) – antifermion (4).
The flavour pairs should be arranged so that, if possible, the first two could
come from a W+ and the second two from a W− ; else each pair should haveflavours consistent with a Z0 . In addition, an arbitrary number of photons may
be included, e.g. from initial-state radiation; these will not be affected by theoperation and can be put anywhere. Since the colour flow need not be unique,three real and one integer numbers are providing further input. Once the colour
pairing is determined, the scale for final-state QCD (and QED) radiation is
automatically set to be the mass of the respective fermion–antifermion pair.
(This is the relevant choice for normal fermion pair production from resonancedecay, but is not suited e.g. for γγ processes dominated by small-t propagators.)
The pairing is also meaningful for QED radiation, in the sense that a four-lepton
final state is subdivided into two radiating subsystems in the same way. Only
if the event consists of one lepton pair and one quark pair is the information
superfluous.
ATOTSQ : total squared amplitude for the event, irrespective of colour flow.
A1SQ : squared amplitude for the configuration with fermions 1 + 2 and 3 + 4 as the twocolour singlets.
A2SQ : squared amplitude for the configuration with fermions 1 + 4 and 3 + 2 as the twocolour singlets.
ISTRAT : the choice of strategy to select either of the two possible colour configurations.
Here 0 is supposed to represent a reasonable compromise, while 1 and 2 areselected so as to give the largest reasonable spread one could imagine.
= 0 : pick configurations according to relative probabilities A1SQ : A2SQ.
= 1 : assign the interference contribution to maximize the 1 + 2 and 3 + 4 pairingof fermions.

265

= 2 :

assign the interference contribution to maximize the 1 + 4 and 3 + 2 pairingof fermions.
IRAD : final-state QED radiation.
= 0 : no final-state photon radiation, only QCD showers.
= 1 : photon radiation inside each final fermion pair, also leptons, in addition to
the QCD one for quarks.
ITAU : handling of τ lepton decay (where Pythia does not include spin effects, althoughsome generators provide the helicity information that would allow a more sophisticated modelling).
= 0 : τ ’s are considered stable (and can therefore be decayed afterwards).
= 1 : τ ’s are allowed to decay.
ICOM : place where information about the event (flavours, momenta etc.) is stored at
input and output.
= 0 : in the HEPEVT common block (meaning that information is automaticallytranslated to PYJETS before treatment and back afterwards).
= 1 : in the PYJETS common block. All fermions and photons can be given withstatus code K(I,1) = 1, flavour code in K(I,2) and five-momentum (momentum, energy, mass) in P(I,J). The V vector and remaining components
in the K one are best put to zero. Also remember to set the total number of
entries N.
Comment : Also colour reconnection phenomena can be studied with the PY4FRM routine. MSTP(115) can be used to switch between the scenarios, with default being
no reconnection. Other reconnection parameters also work as normally, includingthat MSTI(32) can be used to find out whether a reconnection occured or not. In
order for the reconnection machinery to work, the event record is automaticallycomplemented with information on the W+ W− or Z0 Z0 pair that produced thefour fermions, based on the rules described above.
We remind that the four first parameters of the PY4FRM call are supposed toparameterize an ambiguity on the perturbative level of the process, which has
to be resolved before parton showers are performed. The colour reconnection
discussed here is (in most scenarios) occuring on the nonperturbative level, after
the parton showers.
CALL PY6FRM(P12,P13,P21,P23,P31,P32,PTOP,IRAD,ITAU,ICOM)
Purpose: to allow a parton shower to develop and partons to hadronize from a sixfermion starting point. The initial list of fermions is supposed to be orderedin the sequence fermion (1) – antifermion (2) – fermion (3) – antifermion (4) –fermion (5) – antifermion (6). The flavour pairs should be arranged so that, if
possible, the first two could come from a Z0 , the middle two from a W+ andthe last two from a W− ; else each pair should have flavours consistent with a
Z0 . Specifically, this means that in a tt event, the t decay products would befound in 1 (b) and 3 and 4 (from the W+ decay) and the t ones in 2 (b) and 5
and 6 (from the W− decay). In addition, an arbitrary number of photons may
be included, e.g. from initial-state radiation; these will not be affected by theoperation and can be put anywhere. Since the colour flow need not be unique,further input is needed to specify this. The number of possible interference
contributions being much larger than for the four-fermion case, we have not tried
to implement different strategies. Instead six probabilities may be input for thedifferent pairings, that you e.g. could pick as the six possible squared amplitudes,
or according to some more complicated scheme for how to handle the interferenceterms. The treatment of final-state cascades must be quite different for top events266

and the rest. For a normal three-boson event, each fermion pair would form one
radiating system, with scale set equal to the fermion-antifermion invariant mass.
(This is the relevant choice for normal fermion pair production from resonancedecay, but is not suited e.g. for γγ processes dominated by small-t propagators.)
In the top case, on the other hand, the b (b) would be radiating with a recoiltaken by the W+ (W− ) in such a way that the t (t) mass is preserved, while theW dipoles would radiate as normal. Therefore you need also supply a probability
for the event to be a top one, again e.g. based on some squared amplitude.
P12, P13, P21, P23, P31, P32 : relative probabilities for the six possible pairings offermions with antifermions. The first (second) digit tells which antifermion thefirst (second) fermion is paired with, with the third pairing given by elimination.
Thus e.g. P23 means the first fermion is paired with the second antifermion, thesecond fermion with the third antifermion and the third fermion with the first
antifermion. Pairings are only possible between quarks and leptons separately.
The sum of probabilities for allowed pairings is automatically normalized to unity.
PTOP : the probability that the configuration is a top one; a number between 0 and 1.
In this case, it is important that the order described above is respected, withthe b and b coming first. No colour ambiguity exists if the top interpretation is
selected, so then the P12 - P32 numbers are not used.
IRAD : final-state QED radiation.
= 0 : no final-state photon radiation, only QCD showers.
= 1 : photon radiation inside each final fermion pair, also leptons, in addition to
the QCD one for quarks.
ITAU : handling of τ lepton decay (where Pythia does not include spin effects, althoughsome generators provide the helicity information that would allow a more sophisticated modelling).
= 0 : τ ’s are considered stable (and can therefore be decayed afterwards).
= 1 : τ ’s are allowed to decay.
ICOM : place where information about the event (flavours, momenta etc.) is stored at
input and output.
= 0 : in the HEPEVT common block (meaning that information is automaticallytranslated to PYJETS before treatment and back afterwards).
= 1 : in the PYJETS common block. All fermions and photons can be given withstatus code K(I,1) = 1, flavour code in K(I,2) and five-momentum (momentum, energy, mass) in P(I,J). The V vector and remaining components
in the K one are best put to zero. Also remember to set the total number of
entries N.
CALL PY4JET(PMAX,IRAD,ICOM)
Purpose: to allow a parton shower to develop and partons to hadronize from a qqgg
or qqq0 q0 original configuration. The partons should be ordered exactly as indicated above, with the primary qq pair first and thereafter the two gluons or
the secondary q0 q0 pair. (Strictly speaking, the definition of primary and secondary fermion pair is ambiguous. In practice, however, differences in topological variables like the pair mass should make it feasible to have some sensible
criterion on an event-by-event basis.) Within each pair, fermion should precede
antifermion. In addition, an arbitrary number of photons may be included, e.g.
from initial-state radiation; these will not be affected by the operation and canbe put anywhere. The program will select a possible parton-shower history fromthe given parton configuration, and then continue the shower from there on. The
history selected is displayed in lines NOLD + 1 to NOLD + 6, where NOLD is the N267

value before the routine is called. Here the masses and energies of intermediate
partons are clearly displayed. The lines NOLD + 7 and NOLD + 8 contain theequivalent on-mass-shell parton pair from which the shower is started.
PMAX : the maximum mass scale (in GeV) from which the shower is started in thosebranches that are not already fixed by the matrix-element history. If PMAX is
set zero (actually below PARJ(82), the shower cutoff scale), the shower starting
scale is instead set to be equal to the smallest mass of the virtual partons in
the reconstructed shower history. A fixed PMAX can thus be used to obtain areasonably exclusive set of four-jet events (to that PMAX scale), with little five-jet
contamination, while the PMAX = 0 option gives a more inclusive interpretation,with five- or more-jet events possible. Note that the shower is based on evolution
in mass, meaning the cut is really one of mass, not of p⊥ , and that it may therefore
be advantageous to set up the matrix elements cuts accordingly if one wishes tomix different event classes. This is not a requirement, however.
IRAD : final-state QED radiation.
= 0 : no final-state photon radiation, only QCD showers.
= 1 : photon radiation inside each final fermion pair, also leptons, in addition to
the QCD one for quarks.
ICOM : place where information about the event (flavours, momenta etc.) is stored at
input and output.
= 0 : in the HEPEVT common block (meaning that information is automaticallytranslated to PYJETS before treatment and back afterwards).
= 1 : in the PYJETS common block. All fermions and photons can be given withstatus code K(I,1) = 1, flavour code in K(I,2) and five-momentum (momentum, energy, mass) in P(I,J). The V vector and remaining components
in the K one are best put to zero. Also remember to set the total number of
entries N.

9.11

Other Routines and Common Blocks

The subroutines and common blocks that you will come in direct contact with havealready been described. A number of other routines and common blocks exist, and those
not described elsewhere are here briefly listed for the sake of completeness. The PYG***
routines are slightly modified versions of the SAS*** ones of the SaSgam library. The
common block SASCOM is renamed PYINT8. If you want to use the parton distributions
for standalone purposes, you are encouraged to use the original SaSgam routines rather
than going the way via the Pythia adaptations.
SUBROUTINE PYINRE : to initialize the widths and effective widths of resonances.
SUBROUTINE PYINBM(CHFRAM,CHBEAM,CHTARG,WIN) : to read in and identify the beam
(CHBEAM) and target (CHTARG) particles and the frame (CHFRAM) as given in the
PYINIT call; also to save the original energy (WIN).
SUBROUTINE PYINKI(MODKI) : to set up the event kinematics, either at initialization(MODKI = 0) or for each separate event, the latter when the program is run withvarying kinematics (MODKI = 1).
SUBROUTINE PYINPR : to set up the partonic subprocesses selected with MSEL. For γpand γγ, also the MSTP(14) value affects the choice of processes. In particular,
options such as MSTP(14) = 10 and = 30 sets up the several different kinds ofprocesses that need to be mixed, with separate cuts for each.
SUBROUTINE PYXTOT : to give the parameterized total, double diffractive, single diffractive and elastic cross sections for different energies and colliding hadrons or photons.
SUBROUTINE PYMAXI : to find optimal coefficients COEF for the selection of kinematicalvariables, and to find the related maxima for the differential cross section times
268

Jacobian factors, for each of the subprocesses included.
SUBROUTINE PYPILE(MPILE) : to determine the number of pile-up events, i.e. events
appearing in the same beam–beam crossing.
SUBROUTINE PYSAVE(ISAVE,IGA) : saves and restores parameters and cross section values between the several γp and γγ components of mixing options such as
MSTP(14) = 10 and = 30. The options for ISAVE are (1) a complete save of
all parameters specific to a given component, (2) a partial save of cross-section
information, (3) a restoration of all parameters specific to a given component,(4) as 3 but preceded by a random selection of component, and (5) a summationof component cross sections (for PYSTAT). The subprocess code in IGA is the one
described for MSTI(9); it is input for options 1, 2 and 3 above, output for 4 anddummy for 5.
SUBROUTINE PYGAGA(IGA) : to generate photons according to the virtual photon flux
around a lepton beam, for the ’gamma/lepton’ option in PYINIT.
IGA = 1 : call at initialization to set up x and Q2 limits etc.
IGA = 2 : call at maximization step to give estimate of maximal photon flux factor.
IGA = 3 : call at the beginning of the event generation to select the kinematics ofthe photon emission and to give the flux factor.
IGA = 4 : call at the end of the event generation to set up the full kinematics of thephoton emission.
SUBROUTINE PYRAND : to generate the quantities characterizing a hard scattering on theparton level, according to the relevant matrix elements.
SUBROUTINE PYSCAT : to find outgoing flavours and to set up the kinematics and colour
flow of the hard scattering.
SUBROUTINE PYRESD(IRES) : to allow resonances to decay, including chains of successive
decays and parton showers. Normally only two-body decays of each resonance,
but a few three-body decays are also implemented.
IRES : The standard call from PYEVNT, for the hard process, has IRES = 0, andthen finds resonances to be treated based on the subprocess number ISUB.
In case of a nonzero IRES only the resonance in position IRES of the event
record is considered. This is used by PYEVNT and PYEXEC to decay leftover
resonances. (Example: a b → W + t branching may give a t quark as beam
remnant.)
SUBROUTINE PYDIFF : to handle diffractive and elastic scattering events.
SUBROUTINE PYDISG : to set up kinematics, beam remnants and showers in the 2 → 1
DIS process γ ∗ f → f. Currently initial-state radiation is not yet implemented,
while final-state is.
SUBROUTINE PYDOCU : to compute cross sections of processes, based on current Monte
Carlo statistics, and to store event information in the MSTI and PARI arrays.
SUBROUTINE PYWIDT(KFLR,SH,WDTP,WDTE) : to calculate widths and effective widths ofresonances. Everything is given in dimensions of GeV.
SUBROUTINE PYOFSH(MOFSH,KFMO,KFD1,KFD2,PMMO,RET1,RET2) : to calculate partial
widths into channels off the mass shell, and to select correlated masses of resonance pairs.
SUBROUTINE PYKLIM(ILIM) : to calculate allowed kinematical limits.
SUBROUTINE PYKMAP(IVAR,MVAR,VVAR) : to calculate the value of a kinematical variable
when this is selected according to one of the simple pieces.
SUBROUTINE PYSIGH(NCHN,SIGS) : to give the differential cross section (multiplied by
the relevant Jacobians) for a given subprocess and kinematical setup. With time,
the PYSIGH increased to a size of over 7000 lines, which gave some compilers
problems. Therefore, currently, all the phase-space and parton-density genericweights remain in PYSIGH itself, whereas the process-specific matrix elements
have been grouped according to:269

• PYSGQC : normal QCD processes, plus some similar instead with photons;• PYSGHF : heavy flavour production, open and closed;• PYSGWZ : W/Z processes, except as below;• PYSGHG : Higgs processes (2 doublets), except Higgs pairs in PYSGSU, but
including longitudinal WW scattering for a heavy Higgs;• PYSGSU : SUSY processes, including Higgs pair production;• PYSGTC : Technicolor processes, including some related compositeness ones;• PYSGEX : assorted exotic processes, including new gauge bosons (Z0 /W0 ),leptoquarks (LQ ), horizontal gauge bosons (R0 ), a generation of excited
fermions (d∗ /u∗ /e∗− /νe∗ ), left–right-symmetric scenarios (H++ /ZR /WR ),and extra dimensions (G∗ ).
SUBROUTINE PYPDFL(KF,X,Q2,XPQ) : to give parton distributions for p and n in theoption with modified behaviour at small Q2 and x, see MSTP(57).
SUBROUTINE PYPDFU(KF,X,Q2,XPQ) : to give parton-distribution functions (multipliedby x, i.e. xfi (x, Q2 )) for an arbitrary particle (of those recognized by Pythia).
Generic driver routine for the following, specialized ones.
KF :flavour of probed particle, according to KF code.
X :x value at which to evaluate parton distributions.
Q2 :Q2 scale at which to evaluate parton distributions.
XPQ : array of dimensions XPQ(-25:25), which contains the evaluated parton distributions xfi (x, Q2 ). Components i ordered according to standard KF code;
additionally the gluon is found in position 0 as well as 21 (for historicalreasons).
Note: the above set of calling arguments is enough for a real photon, but has tobe complemented for a virtual one. This is done by VINT(120).
SUBROUTINE PYPDEL(KFA,X,Q2,XPEL) : to give e/µ/τ parton distributions.
SUBROUTINE PYPDGA(X,Q2,XPGA) : to give the photon parton distributions for sets otherthan the SaS ones.
SUBROUTINE PYGGAM(ISET,X,Q2,P2,IP2,F2GM,XPDFGM) : to construct the SaS F2 andparton distributions of the photon by summing homogeneous (VMD) and inhomogeneous (anomalous) terms. For F2 , c and b are included by the Bethe-Heitler
formula; in the ‘ms’ scheme additionally a C γ term is added. IP2 sets treatmentof virtual photons, with same code as MSTP(60). Calls PYGVMD, PYGANO, PYGBEH,
and PYGDIR.
SUBROUTINE PYGVMD(ISET,KF,X,Q2,P2,ALAM,XPGA,VXPGA) : to evaluate the parton
distributions of a VMD photon, evolved homogeneously from an initial scale P 2
to Q2 .
SUBROUTINE PYGANO(KF,X,Q2,P2,ALAM,XPGA,VXPGA) : to evaluate the parton distributions of the anomalous photon, inhomogeneously evolved from a scale P 2 (where
it vanishes) to Q2 .
SUBROUTINE PYGBEH(KF,X,Q2,P2,PM2,XPBH) : to evaluate the Bethe-Heitler cross section for heavy flavour production.
SUBROUTINE PYGDIR(X,Q2,P2,AK0,XPGA) : to evaluate the direct contribution, i.e. theC γ term, as needed in ‘ms’ parameterizations.
SUBROUTINE PYPDPI(X,Q2,XPPI) : to give pion parton distributions.
SUBROUTINE PYPDPR(X,Q2,XPPR) : to give proton parton distributions. Calls several
auxiliary routines: PYCTEQ, PYGRVL, PYGRVM, PYGRVD, PYGRVV, PYGRVW, PYGRVS,PYCT5L, PYCT5M and PYPDPO.
FUNCTION PYHFTH(SH,SQM,FRATT) : to give heavy-flavour threshold factor in matrix elements.
SUBROUTINE PYSPLI(KF,KFLIN,KFLCH,KFLSP) : to give hadron remnant or remnants
left behind when the reacting parton is kicked out.

270

FUNCTION PYGAMM(X) : to give the value of the ordinary Γ(x) function (used in someparton-distribution parameterizations).
SUBROUTINE PYWAUX(IAUX,EPS,WRE,WIM) : to evaluate the two auxiliary functions W1
and W2 appearing in some cross section expressions in PYSIGH.
SUBROUTINE PYI3AU(EPS,RAT,Y3RE,Y3IM) : to evaluate the auxiliary function I3 appearing in some cross section expressions in PYSIGH.
FUNCTION PYSPEN(XREIN,XIMIN,IREIM) : to calculate the real and imaginary part of
the Spence function [Hoo79].
SUBROUTINE PYQQBH(WTQQBH) : to calculate matrix elements for the two processes gg →QQh0 and qq → QQh0 .
SUBROUTINE PYRECO(IW1,IW2,NSD1,NAFT1)) : to perform nonperturbative reconnection among strings in W+ W− and Z0 Z0 events. The physics of this routine
is described as part of the fragmentation story, section 12.4.2, but for technicalreasons the code is called directly in the event generation sequence.
BLOCK DATA PYDATA : to give sensible default values to all status codes and parameters.
SUBROUTINE PYCKBD : to check that a few different parameters in BLOCK DATA PYDATA
have been set. This addresses a compiler bug, where it has happened that only apart of PYDATA is loaded if libraries are linked in some order (while it may work
fine with another order).
COMMON/PYINT1/MINT(400),VINT(400)
Purpose: to collect a host of integer- and real-valued variables used internally in the
program during the initialization and/or event generation stage. These variables
must not be changed by you.
MINT(1) : specifies the general type of subprocess that has occurred, according to the
ISUB code given in section 8.1.
MINT(2) : whenever MINT(1) (together with MINT(15) and MINT(16)) are not sufficientto specify the type of process uniquely, MINT(2) provides an ordering of thedifferent possibilities, see MSTI(2). Internally and temporarily, in process 53
MINT(2) is increased by 2 or 4 for c or b, respectively.
MINT(3) : number of partons produced in the hard interactions, i.e. the number n of
the 2 → n matrix elements used; is sometimes 3 or 4 when a basic 2 → 1 or
2 → 2 process has been convoluted with two 1 → 2 initial branchings (likeqq0 → q00 q000 h0 ).
MINT(4) : number of documentation lines at the beginning of the common block PYJETS
that are given with K(I,1) = 21; 0 for MSTP(125) = 0.
MINT(5) : number of events generated to date in current run. In runs with the variableenergy option, MSTP(171) = 1 and MSTP(172) = 2, only those events that survive (i.e. that do not have MSTI(61) = 1) are counted in this number. That is,
MINT(5) may be less than the total number of PYEVNT calls.
MINT(6) : current frame of event (see MSTP(124) for possible values).
MINT(7), MINT(8) : line number for documentation of outgoing partons/particles fromhard scattering for 2 → 2 or 2 → 1 → 2 processes (else = 0).
MINT(10) : is 1 if cross section maximum was violated in current event, and 0 if not.
MINT(11) : KF flavour code for beam (side 1) particle.
MINT(12) : KF flavour code for target (side 2) particle.
MINT(13), MINT(14) : KF flavour codes for side 1 and side 2 initial-state shower initiators.
MINT(15), MINT(16) : KF flavour codes for side 1 and side 2 incoming partons to thehard interaction. (For use in PYWIDT calls, occasionally MINT(15) = 1 signals
the presence of an as yet unspecified quark, but the original value is then restored271

afterwards.)
MINT(17), MINT(18) : flag to signal if particle on side 1 or side 2 has been scattereddiffractively; 0 if no, 1 if yes.
MINT(19), MINT(20) : flag to signal initial-state structure with parton inside photon
inside electron on side 1 or side 2; 0 if no, 1 if yes.
MINT(21) - MINT(24) : KF flavour codes for outgoing partons from the hard interaction.
The number of positions actually used is process-dependent, see MINT(3); trailing
positions not used are set = 0. For events with many outgoing partons, e.g. inexternal processes, also MINT(25) and MINT(26) could be used.
MINT(25), MINT(26) : KF flavour codes of the products in the decay of a single s-channel
resonance formed in the hard interaction. Are thus only used when MINT(3) =1 and the resonance is allowed to decay.
MINT(30) : normally 0, but when 1 or 2 it is a signal for PYPDFU that PDF’s are tobe evaluated taking into account the partons already removed from the beam
remnant on side 1 or 2 of the event.
MINT(31) : number of hard or semi-hard scatterings that occurred in the current eventin the multiple-interaction scenario; is = 0 for a low-p⊥ event.
MINT(32) : information on whether a nonperturbative colour reconnection occurred in
the current event; is 0 normally but 1 in case of reconnection.
MINT(33) : Switch to select how to trace colour connections in PYPREP.
= 0 : Old method, using K(I,4) and K(I,5) in /PYJETS/.
= 1 : New method, using Les Houches Accord style colour tags in /PYCTAG/.
MINT(34) : counter for the number of final-state colour reconnections in the new multiple
interactions scenario.
MINT(35) : internal switch to tell which routine generates the event, so that some details(partly physics, partly administrative) can be processed differently in the calledroutines.
= 1 : PYEVNT generates event, using the ‘old’ shower, multiple interactions andbeam-remnants machinery.
= 2 : PYEVNT generates event, using the ‘intermediate’ model with the new handling of beam remnants but old showers.
= 3 : PYEVNW generates event, using the ‘new’ model.
MINT(36) : the currently considered interaction number for (trial) showers and a (trial)
additional interaction, i.e. = 1 for the hardest interaction and > 1 for additional
interactions. MINT(36) ≤ MINT(31) for showering, while MINT(36) = MINT(31)+ 1 for an additional (trial) interaction.
MINT(41), MINT(42) : type of incoming beam or target particle; 1 for lepton and 2 forhadron. A photon counts as a lepton if it is not resolved (direct or DIS) and as
a hadron if it is resolved (VMD or GVMD).
MINT(43) : combination of incoming beam and target particles. A photon counts as a
hadron.
= 1 : lepton on lepton.
= 2 : lepton on hadron.
= 3 : hadron on lepton.
= 4 : hadron on hadron.
MINT(44) : as MINT(43), but a photon counts as a lepton.
MINT(45), MINT(46) : structure of incoming beam and target particles.
= 1 : no internal structure, i.e. a lepton or photon carrying the full beam energy.
= 2 : defined with parton distributions that are not peaked at x = 1, i.e. a hadron
or a resolved (VMD or GVMD) photon.
= 3 : defined with parton distributions that are peaked at x = 1, i.e. a resolved
lepton.
MINT(47) : combination of incoming beam- and target-particle parton-distribution func272

tion types.
= 1 : no parton distribution either for beam or target.
= 2 : parton distributions for target but not for beam.
= 3 : parton distributions for beam but not for target.
= 4 : parton distributions for both beam and target, but not both peaked at x = 1.
= 5 : parton distributions for both beam and target, with both peaked at x = 1.
= 6 : parton distribution is peaked at x = 1 for target and no distribution at all
for beam.
= 7 : parton distribution is peaked at x = 1 for beam and no distribution at allfor target.
MINT(48) : total number of subprocesses switched on.
MINT(49) : number of subprocesses that are switched on, apart from elastic scatteringand single, double and central diffractive.
MINT(50) : combination of incoming particles from a multiple interactions point ofview.
= 0 : the total cross section is not known; therefore no multiple interactions are
possible.
= 1 : the total cross section is known; therefore multiple interactions are possible if
switched on. Requires beams of hadrons, VMD photons or GVMD photons.
MINT(51) : internal flag that event failed cuts.
= 0 : no problem.
= 1 : event failed; new one to be generated.
= 2 : event failed; no new event is to be generated but instead control is to begiven back to the user. Is intended for user-defined processes, when NUP =0.
MINT(52) : internal counter for number of lines used (in PYJETS) before multiple interactions are considered.
MINT(53) : internal counter for number of lines used (in PYJETS) before beam remnants
are considered.
MINT(54) : internal counter for the number of lines used (in PYJETS) after beam remnants are considered.
MINT(55) : the heaviest new flavour switched on for QCD processes, specifically theflavour to be generated for ISUB = 81, 82, 83 or 84.
MINT(56) : the heaviest new flavour switched on for QED processes, specifically for ISUB
= 85. Note that, unlike MINT(55), the heaviest flavour may here be a lepton,
and that heavy means the one with largest KF code.
MINT(57) : number of times the beam-remnant treatment has failed, and the same basic
kinematical setup is used to produce a new parton-shower evolution and beamremnant set. Mainly used in leptoproduction, for the MSTP(23) options when x
and Q2 are to be preserved.
MINT(61) : internal switch for the mode of operation of resonance width calculations in
PYWIDT for γ ∗ /Z0 or γ ∗ /Z0 /Z00 .
= 0 : without reference to initial-state flavours.
= 1 : with reference to given initial-state flavours.
= 2 : for given final-state flavours.
MINT(62) : internal switch for use at initialization of h0 width.
= 0 : use widths into ZZ∗ or WW∗ calculated before.
= 1 : evaluate widths into ZZ∗ or WW∗ for current Higgs mass.
MINT(63) : internal switch for use at initialization of the width of a resonance definedwith MWID(KC) = 3.
= 0 : use existing widths, optionally with simple energy rescaling.
= 1 : evaluate widths at initialization, to be used subsequently.
MINT(65) : internal switch to indicate initialization without specified reaction.
273

= 0 : normal initialization.
= 1 : initialization with argument ’none’ in PYINIT call.
MINT(71) : switch to tell whether current process is singular for p⊥ → 0 or not.
= 0 : non-singular process, i.e. proceeding via an s-channel resonance or with bothproducts having a mass above CKIN(6).
= 1 : singular process.
MINT(72) : number of s-channel resonances that may contribute to the cross section.
MINT(73) : KF code of first s-channel resonance; 0 if there is none.
MINT(74) : KF code of second s-channel resonance; 0 if there is none.
MINT(81) : number of selected pile-up events.
MINT(82) : sequence number of currently considered pile-up event.
MINT(83) : number of lines in the event record already filled by previously consideredpile-up events.
MINT(84) : MINT(83) + MSTP(126), i.e. number of lines already filled by previously considered events plus number of lines to be kept free for event documentation.
MINT(91) : is 1 for a lepton–hadron event and 0 else. Used to determine whether a
PYFRAM(3) call is possible.
MINT(92) : is used to denote region in (x, Q2 ) plane when MSTP(57) = 2, according to
numbering in [Sch93a]. Simply put, 0 means that the modified proton parton
distributions were not used, 1 large x and Q2 , 2 small Q2 but large x, 3 small x
but large Q2 and 4 small x and Q2 .
MINT(93) : is used to keep track of parton distribution set used in the latest STRUCTM callto Pdflib or LHAPDF. The code for this set is stored in the form MINT(93) =1000000×NPTYPE + 1000×NGROUP + NSET. The stored previous value is compared with the current new value to decide whether a PDFSET call is needed toswitch to another set.
MINT(101), MINT(102) : is normally 1, but is 4 when a resolved photon (appearing onside 1 or 2) can be represented by either of the four vector mesons ρ0 , ω, φ andJ/ψ.
MINT(103), MINT(104) : KF flavour code for the two incoming particles, i.e. the same as
MINT(11) and MINT(12). The exception is when a resolved photon is represented
by a vector meson (a ρ0 , ω, φ or J/ψ). Then the code of the vector meson is
given.
MINT(105) : is either MINT(103) or MINT(104), depending on which side of the event
currently is being studied.
MINT(107), MINT(108) : if either or both of the two incoming particles is a photon,then the respective value gives the nature assumed for that photon. The code
follows the one used for MSTP(14):= 0 : direct photon.
= 1 : resolved photon.
= 2 : VMD-like photon.
= 3 : anomalous photon.
= 4 : DIS photon.
MINT(109) : is either MINT(107) or MINT(108), depending on which side of the event
currently is being studied.
MINT(111) : the frame given in PYINIT call, 0–5 for ’NONE’, ’CMS’, ’FIXT’, ’3MOM’,’4MOM’ and ’5MOM’, respectively, and 11 for ’USER’. Option 12 signals input of
Les Houches Accord events without beam-remnant processing, see PDFGUP(i) =-9 in section 9.9.
MINT(121) : number of separate event classes to initialize and mix.
= 1 : the normal value.
= 2 - 13 : for γp/γ ∗ p/γγ/γ ∗ γ/γ ∗ γ ∗ interaction when MSTP(14) is set to mix different photon components.
274

MINT(122) : event class used in current event for γp or γγ events generated with one of
the MSTP(14) options mixing several event classes; code as described for MSTI(9).
MINT(123) : event class used in the current event, with the same list of possibilities
as for MSTP(14), except that options 1, 4 or 10 do not appear. = 8 denotes
DIS×VMD/p or vice verse, = 9 DIS*GVMD or vice versa. Apart from a different
coding, this is exactly the same information as is available in MINT(122).
MINT(141), MINT(142) : for ’gamma/lepton’ beams, KF code for incoming lepton beamor target particles, while MINT(11) and MINT(12) is then the photon code. A
nonzero value is the main check whether the photon emission machinery should
be called at all.
MINT(143) : the number of tries before a successful kinematics configuration is found in
PYGAGA, used for ’gamma/lepton’ beams. Used for the cross section updating in
PYRAND.
MINT(351) : Current number of multiple interactions, excluding the hardest in an event;is normally equal to MINT(31) - 1 = MSTI(31) - 1, but is = 0 when MINT(31)= MSTI(31) = 0.
MINT(352) : Current number of initial-state-radiation branchings.
MINT(353) : Current number of final-state-radiation branchings
MINT(354) : Current number of multiple interactions joinings (not fully implemented.)
VINT(1) : Ecm , c.m. energy.
2
VINT(2) : s (= Ecm) squared mass of complete system.
VINT(3) : mass of beam particle. Can be negative to denote a space-like particle, e.g. aγ ∗.
VINT(4) : mass of target particle. Can be negative to denote a space-like particle, e.g.
a γ∗.
VINT(5) : absolute value of momentum of beam (and target) particle in c.m. frame.
VINT(6) - VINT(10) : θ, ϕ and β for rotation and boost from c.m. frame to userspecified frame.
VINT(11) : τmin .
VINT(12) : ymin .
VINT(13) : cos θ̂min for cos θ̂ ≤ 0.
VINT(14) : cos θ̂min for cos θ̂ ≥ 0.
VINT(15) : x2⊥min .
0
VINT(16) : τmin.
VINT(17) : (D = 0.) absolute lower p2⊥ scale, as used to determine exit of multiple
interactions and and initial-state radiation routines.
VINT(18) : soft destructive colour interference scale used for regularising the multiple
interactions and initial-state radiation matrix elements.
VINT(21) : τ .
VINT(22) : y.
VINT(23) : cos θ̂.
VINT(24) : ϕ (azimuthal angle).
VINT(25) : x2⊥ .
VINT(26) : τ 0 .
VINT(31) : τmax .
VINT(32) : ymax .
VINT(33) : cos θ̂max for cos θ̂ ≤ 0.
VINT(34) : cos θ̂max for cos θ̂ ≥ 0.
VINT(35) : x2⊥max .
0.
VINT(36) : τmax
VINT(41), VINT(42) : the momentum fractions x taken by the partons at the hardinteraction, as used e.g. in the parton-distribution functions. For process 99 this275

agrees with the Bjorken definition, including target mass corrections, i.e., for a
proton target, x = Q2 /(W 2 + Q2 − m2p ).
√VINT(43) : m̂ = ŝ, mass of hard-scattering subsystem.
VINT(44) : ŝ of the hard subprocess (2 → 2 or 2 → 1).
VINT(45) : t̂ of the hard subprocess (2 → 2 or 2 → 1 → 2).
VINT(46) : û of the hard subprocess (2 → 2 or 2 → 1 → 2).
VINT(47) : p̂⊥ of the hard subprocess (2 → 2 or 2 → 1 → 2), i.e. transverse momentum
evaluated in the rest frame of the scattering.
VINT(48) : p̂2⊥ of the hard subprocess; see VINT(47).
VINT(49) : m̂0 , the mass of the complete three- or four-body final state in 2 → 3 or
2 → 4 processes.
VINT(50) : ŝ0 = m̂02 ; see VINT(49).
VINT(51) : Q of the hard subprocess. The exact definition is process-dependent, see
MSTP(32).
VINT(52) : Q2 of the hard subprocess; see VINT(51).
VINT(53) : Q of the outer hard-scattering subprocess, used as scale for parton distribution function evaluation. Agrees with VINT(51) for a 2 → 1 or 2 → 2 process.
For a 2 → 3 or 2 → 4 W/Z fusion process, it is set by the W/Z mass scale, andfor subprocesses 121 and 122 by the heavy-quark mass.
VINT(54) : Q2 of the outer hard-scattering subprocess; see VINT(53).
VINT(55) : Q scale used as maximum virtuality in parton showers. Is equal to VINT(53),except for DIS processes when MSTP(22) > 0.
VINT(56) : Q2 scale in parton showers; see VINT(55).
VINT(57) : αem value of hard process.
VINT(58) : αs value of hard process.
VINT(59) : sin θ̂ (cf. VINT(23)); used for improved numerical precision in elastic anddiffractive scattering.
VINT(61) : p⊥ scale used as maximum transverse momentum for multiple interactions.
See MSTP(86).
VINT(62) : p⊥ 2 maximum scale for multiple interactions, see VINT(61).
VINT(63), VINT(64) : nominal m2 values, i.e. without final-state radiation effects, forthe two (or one) partons/particles leaving the hard interaction. For elastic VMD
and GVMD events, this equals VINT(69)2 or VINT(70)2 , and for diffractive events
it is above that.
VINT(65) : p̂init , i.e. common nominal absolute momentum of the two partons entering
the hard interaction, in their rest frame.
VINT(66) : p̂fin , i.e. common nominal absolute momentum of the two partons leaving
the hard interaction, in their rest frame.
VINT(67), VINT(68) : mass of beam and target particle, as VINT(3) and VINT(4),except that an incoming γ is assigned the ρ0 , ω or φ mass. (This also applies
for a GVMD photon, where the mass of the VMD state with the equivalentflavour content is chosen.) Used for elastic scattering γp → ρ0 p and other similar
processes.
VINT(69), VINT(70) : the actual mass of a VMD or GVMD state; agrees with the above
for VMD but is selected as a larger number for GVMD, using the approximate
association m = 2k⊥ . Thus the mass selection for a GVMD state is according
to dm2 /(m2 + Q2 )2 between limits 2k0 < m < 2k1 = 2p⊥min (W 2 ). Required forelastic and diffractive events.
VINT(71) : initially it is p⊥min of process, but this is replaced by the actual p⊥ once theprocess has actually been selected. For a normal hard process p⊥min is either
CKIN(3) or CKIN(5), depending on which is larger, and whether the process is
singular in p⊥ → 0 or not. For multiple interactions it is either the energy-276dependent p⊥min derived from PARP(81) or the fraction 0.08 × p⊥0 derived from
PARP(82), depending on MSTP(82) setting. (In the latter scenario, formallyp⊥ = 0 is allowed but is numerically unstable; hence the small nonvanishingvalue.)
VINT(73) : τ = m2 /s value of first resonance, if any; see MINT(73).
VINT(74) : mΓ/s value of first resonance, if any; see MINT(73).
VINT(75) : τ = m2 /s value of second resonance, if any; see MINT(74).
VINT(76) : mΓ/s value of second resonance, if any; see MINT(74).
VINT(80) : correction factor (evaluated in PYOFSH) for the cross section of resonances
produced in 2 → 2 processes, if only some mass range of the full Breit–Wigner
shape is allowed by user-set mass cuts (CKIN(2), CKIN(45) - CKIN(48)).
VINT(95) : the value of the Coulomb factor in the current event, see MSTP(40). For
MSTP(40) = 0 it is = 1, else it is > 1.
VINT(97) : an event weight, normally 1 and thus uninteresting, but for external processes with IDWTUP = -1, -2 or -3 it can be −1 for events with negative crosssection, with IDWTUP = 4 it can be an arbitrary non-negative weight of dimension mb, and with IDWTUP = -4 it can be an arbitrary weight of dimension mb.
(The difference being that in most cases a rejection step is involved to bringthe accepted events to a common weight normalization, up to a sign, while no
rejection need be involved in the last two cases.)
VINT(98) : is sum of VINT(100) values for current run.
VINT(99) : is weight WTXS returned from PYEVWT call when MSTP(142) ≥ 1, otherwise is1.
VINT(100) : is compensating weight 1./WTXS that should be associated with events when
MSTP(142) = 1, otherwise is 1.
VINT(108) : ratio of maximum differential cross section observed to maximum differential cross section assumed for the generation; cf. MSTP(123).
VINT(109) : ratio of minimal (negative!) cross section observed to maximum differential
cross section assumed for the generation; could only become negative if crosssections are incorrectly included.
VINT(111) - VINT(116) : for MINT(61) = 1 gives kinematical factors for the different
pieces contributing to γ ∗ /Z0 or γ ∗ /Z0 /Z00 production, for MINT(61) = 2 gives
sum of final-state weights for the same; coefficients are given in the order pureγ ∗ , γ ∗ –Z0 interference, γ ∗ –Z00 interference, pure Z0 , Z0 –Z00 interference and pureZ00 .
VINT(117) : width of Z0 ; needed in γ ∗ /Z0 /Z00 production.
VINT(120) : mass of beam or target particle, i.e. coincides with VINT(3) or VINT(4),depending on which side of the event is considered. Is used to bring informationon the user-defined virtuality of a photon beam to the parton distributions of
the photon.
VINT(131) : total cross section (in mb) for subprocesses allowed in the pile-up events
scenario according to the MSTP(132) value.
VINT(132) : n =VINT(131)×PARP(131)of pile-up events, cf. PARI(91).
P
P
VINT(133) : hni = i i Pi / i Pi of pile-up events as actually simulated, i.e. 1 ≤ i ≤ 200(or smaller), see PARI(92).
VINT(134) : number
related to probability to have an eventPin a beam–beam crossing; isP iexp(−n) i n /i! for MSTP(133) = 1 and exp(−n) i ni /(i − 1)! for MSTP(133)= 2, cf. PARI(93).
VINT(138) : size of the threshold factor (enhancement or suppression) in the latest eventwith heavy-flavour production; see MSTP(35).
VINT(140) : extra rescaling factor when sum of companion momentum distributions
exceed the total amount available. Is normally = 1.
VINT(141), VINT(142) : x values for the parton-shower initiators of the hardest inter277

action; used to find what
is left for multiple interactions.
P
VINT(143), VINT(144) : 1− i xi for all scatterings; used for rescaling each new x-value
in the multiple-interaction parton-distribution-function evaluation.
VINT(145) : estimate of total parton–parton cross section for multiple interactions; used
for MSTP(82) ≥ 2.
VINT(146) : common correction factor fc in the multiple-interaction probability; used
for MSTP(82) ≥ 2 (part of e(b), see eq. (218)).
VINT(147) : average hadronic matter overlap; used for MSTP(82) ≥ 2 (needed in evaluation of e(b), see eq. (218)).
VINT(148) : enhancement factor for current event in the multiple-interaction probability,
defined as the actual overlap divided by the average one; used for MSTP(82) ≥ 2(is e(b) of eq. (218)).
VINT(149) : x2⊥ cut-off or turn-off for multiple interactions. For MSTP(82) ≤ 1 it is
4p2⊥min /W 2 , for MSTP(82) ≥ 2 it is 4p2⊥0 /W 2 . For hadronic collisions, W 2 = s, but
in photoproduction or γγ physics the W 2 scale refers to the hadronic subsystem
squared energy. This may vary from event to event, so VINT(149) needs to berecalculated.
VINT(150) : probability to keep the given event in the multiple-interaction scenario withvarying impact parameter, as given by the exponential factor in eq. (220).
VINT(151), VINT(152) : sum of x values for all the multiple-interaction partons.
VINT(153) : current differential cross section value obtained from PYSIGH; used in multiple interactions only.
VINT(154) : current p⊥min (s) or p⊥min (W 2 ), used for multiple interactions and also as
upper cut-off k1 if the GVMD k⊥ spectrum. See comments at VINT(149).
VINT(155), VINT(156) : the x value of a photon that branches into quarks or gluons,i.e. x at interface between initial-state QED and QCD cascades, in the old photoproduction machinery.
VINT(157), VINT(158) : the primordial k⊥ values selected in the two beam remnants.
VINT(159), VINT(160) : the χ values selected for beam remnants that are split into
two objects, describing how the energy is shared (see MSTP(92) and MSTP(94));
is 0 if no splitting is needed.
VINT(161) - VINT(200) : sum of Cabibbo–Kobayashi–Maskawa squared matrix elements that a given flavour is allowed to couple to. Results are stored in format
VINT(180+KF) for quark and lepton flavours and antiflavours (which need not be
the same; see MDME(IDC,2)). For leptons, these factors are normally unity.
VINT(201) - VINT(220) : additional variables needed in phase-space selection for 2 →3 processes with ISET(ISUB) = 5. Below indices 1, 2 and 3 refer to scatteredpartons 1, 2 and 3, except that the q four-momentum variables are q1 + q2 →q10 + q20 + q30 . All kinematical variables refer to the internal kinematics of the
3-body final state — the kinematics of the system as a whole is described by τ 0
and y, and the mass distribution of particle 3 (a resonance) by τ .
VINT(201) : m1 .
VINT(202) : p2⊥1 .
VINT(203) : ϕ1 .
VINT(204) : M1 (mass of propagator particle).
VINT(205) : weight for the p2⊥1 choice.
VINT(206) : m2 .
VINT(207) : p2⊥2 .
VINT(208) : ϕ2 .
VINT(209) : M2 (mass of propagator particle).
VINT(210) : weight for the p2⊥2 choice.
VINT(211) : y3 .
VINT(212) : y3max .
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VINT(213) : ² = ±1; choice between two mirror solutions 1 ↔ 2.
VINT(214) : weight associated to ²-choice.
VINT(215) : t1 = (q1 − q10 )2 .
VINT(216) : t2 = (q2 − q20 )2 .
VINT(217) : q1 q20 four-product.
VINT(218) : q2 q10 four-product.
VINT(219) : qq10 q20 four-product.
VINT(220) : (m2⊥12 − m2⊥1 − m2⊥2 )2 − 4m2⊥1 m2⊥2 , where m⊥12 is the transverse
mass of the q10 q20 system.
VINT(221) - VINT(225) : θ, ϕ and β for rotation and boost from c.m. frame to hadronicc.m. frame of a lepton–hadron event.
VINT(231) : Q2min scale for current parton-distribution function set.
VINT(232) : valence quark distribution of a VMD photon; set in PYPDFU and used in
PYPDFL.
VINT(281) : for resolved photon events, it gives the ratio between the total γX crosssection and the total π 0 X cross section, where X represents the target particle.
VINT(283), VINT(284) : virtuality scale at which a GVMD/anomalous photon on the
beam or target side of the event is being resolved. More precisely, it gives the2
k⊥of the γ → qq vertex. For elastic and diffractive scatterings, m2 /4 is stored,
where m is the mass of the state being diffracted. For clarity, we point out
that elastic and diffractive events are characterized by the mass of the diffractive
states but without any primordial k⊥ , while jet production involves a primordialk⊥ but no mass selection. Both are thus not used at the same time, but for
GVMD/anomalous photons, the standard (though approximate) identification2k⊥= m2 /4 ensures agreement between the two applications.
VINT(285) : the CKIN(3) value provided by you at initialization; subsequently CKIN(3)may be overwritten (for MSTP(14) = 10) but VINT(285) stays.
VINT(289) : squared c.m. energy found in PYINIT call.
VINT(290) : the WIN argument of a PYINIT call.
VINT(291) - VINT(300) : the two five-vectors of the two incoming particles, as reconstructed in PYINKI. These may vary from one event to the next.
VINT(301) - VINT(320) : used when a flux of virtual photons is being generated by
the PYGAGA routine, for ’gamma/lepton’ beams.
VINT(301) : c.m. energy for the full collision, while VINT(1) gives the γ-hadron orγγ subsystem energy.
VINT(302) : full squared c.m. energy, while VINT(2) gives the subsystem squaredenergy.
VINT(303), VINT(304) : mass of the beam or target lepton, while VINT(3) or
VINT(4) give the mass of a photon emitted off it.
VINT(305), VINT(306) : x values, i.e. respective photon energy fractions of theincoming lepton in the c.m. frame of the event.
VINT(307), VINT(308) : Q2 or P 2 , virtuality of the respective photon (thus thesquare of VINT(3), VINT(4)).
VINT(309), VINT(310) : y values, i.e. respective photon light-cone energy fractionof the lepton.
VINT(311), VINT(312) : θ, scattering angle of the respective lepton in the c.m.
frame of the event.
VINT(313), VINT(314) : φ, azimuthal angle of the respective scattered lepton in
the c.m. frame of the event.
VINT(315), VINT(316): the R factor defined at MSTP(17), giving a cross section
enhancement from the contribution of resolved longitudinal photons.
VINT(317) : dipole suppression factor in PYXTOT for current event.
VINT(318) : dipole suppression factor in PYXTOT at initialization.
279

VINT(319) : photon flux factor in PYGAGA for current event.
VINT(320) : photonPflux factor in PYGAGA at initialization.
VINT(351) : scalarp⊥ of multiple interactions, excluding the hardest process.
(Note that the total E⊥ is twice this, since each interaction produces two
opposite-pT P
partons.)
VINT(352) : Scalar p⊥ of initial-state radiation branchings, with p⊥ defined withrespect to the direction of the branching parton. (Note that each branchingproduces twoPopposite-p⊥ daughters, resulting in a factor two more activity.)
VINT(353) : Scalar P p⊥ of final-state radiation branchings, with comment as above.
VINT(354) : Scalar p⊥ of multiple interactions joinings (not fully implemented).
VINT(356) : p⊥ of hardest multiple interaction (i.e. excluding the very first andhardest interaction).
VINT(357) : p⊥ of hardest initial-state radiation branching.
VINT(358) : p⊥ of hardest final-state radiation branching.
VINT(359) : p⊥ of hardest multiple interactions joining.
COMMON/PYINT2/ISET(500),KFPR(500,2),COEF(500,20),ICOL(40,4,2)
Purpose: to store information necessary for efficient generation of the different subprocesses, specifically type of generation scheme and coefficients of the Jacobian.
Also to store allowed colour-flow configurations. These variables must not bechanged by you.
ISET(ISUB) : gives the type of kinematical-variable selection scheme used for subprocess
ISUB.
= 0 : elastic, diffractive and low-p⊥ processes.
= 1 : 2 → 1 processes (irrespective of subsequent decays).
= 2 : 2 → 2 processes (i.e. the bulk of processes).
= 3 : 2 → 3 processes (like qq0 → q00 q000 h0 ).
= 4 : 2 → 4 processes (like qq0 → q00 q000 W+ W− ).
= 5 : ‘true’ 2 → 3 processes, one method.
= 8 : 2 → 1 process γ ∗ fi → fi where, unlike the 2 → 1 processes above, ŝ = 0.
= 9 : 2 → 2 in multiple interactions (p⊥ as kinematics variable).
= 11 : a user-defined process.
= -1 : legitimate process which has not yet been implemented.
= -2 : ISUB is an undefined process code.
KFPR(ISUB,J) : give the KF flavour codes for the products produced in subprocess ISUB.
If there is only one product, the J = 2 position is left blank. Also, quarks
and leptons assumed massless in the matrix elements are denoted by 0. The
main application is thus to identify resonances produced (Z0 , W± , h0 , etc.). For
external processes, KFPR instead stores information on process numbers in thetwo external classifications, see section 9.9.
COEF(ISUB,J) : factors used in the Jacobians in order to speed up the selection of kinematical variables. More precisely, the shape of the cross section is given as thesum of terms with different behaviour, where the integral over the allowed phasespace is unity for each term. COEF gives the relative strength of these terms,
normalized so that the sum of coefficients for each variable used is unity. Notethat which coefficients are indeed used is process-dependent.
ISUB : standard subprocess code.
J = 1 : τ selected according 1/τ .
J = 2 : τ selected according to 1/τ 2 .
J = 3 : τ selected according to 1/(τ (τ + τR )), where τR = m2R /s is τ value ofresonance; only used for resonance production.

280

J = 4 : τ selected according to Breit–Wigner of form 1/((τ − τR )2 + γR2 ), whereτR = m2R /s is τ value of resonance and γR = mR ΓR /s is its scaled mass
times width; only used for resonance production.
J = 5 : τ selected according to 1/(τ (τ + τR0 )), where τR0 = m2R0 /s is τ value of
second resonance; only used for simultaneous production of two resonances.
J = 6 : τ selected according to second Breit–Wigner of form 1/((τ − τR0 )2 + γR2 0 ),where τR0 = m2R0 /s is τ value of second resonance and γR0 = mR0 ΓR0 /s is
its scaled mass times width; is used only for simultaneous production of two
resonances, like γ ∗ /Z0 /Z00 .
J = 7 : τ selected according to 1/(1 − τ ); only used when both parton distributions
are peaked at x = 1.
J = 8 : y selected according to y − ymin .
J = 9 : y selected according to ymax − y.
J = 10 : y selected according to 1/ cosh(y).
J = 11 : y selected according to 1/(1 − exp(y − ymax )); only used when beam parton
distribution is peaked close to x = 1.
J = 12 : y selected according to 1/(1−exp(ymin −y)); only used when target parton
distribution is peaked close to x = 1.
J = 13 : z = cos θ̂ selected evenly between limits.
J = 14 : z = cos θ̂ selected according to 1/(a − z), where a = 1 + 2m23 m24 /ŝ2 , m3
and m4 being the masses of the two final-state particles.
J = 15 : z = cos θ̂ selected according to 1/(a + z), with a as above.
J = 16 : z = cos θ̂ selected according to 1/(a − z)2 , with a as above.
J = 17 : z = cos θ̂ selected according to 1/(a + z)2 , with a as above.
J = 18 : τ 0 selected according to 1/τ 0 .
J = 19 : τ 0 selected according to (1 − τ /τ 0 )3 /τ 02 .
J = 20 : τ 0 selected according to 1/(1 − τ 0 ); only used when both parton distributions are peaked close to x = 1.
ICOL : contains information on different colour-flow topologies in hard 2 → 2 processes.
COMMON/PYINT3/XSFX(2,-40:40),ISIG(1000,3),SIGH(1000)
Purpose: to store information on parton distributions, subprocess cross sections anddifferent final-state relative weights. These variables must not be changed byyou.
XSFX : current values of parton-distribution functions xf (x) on beam and target side.
ISIG(ICHN,1) : incoming parton/particle on the beam side to the hard interaction forallowed channel number ICHN. The number of channels filled with relevant information is given by NCHN, one of the arguments returned in a PYSIGH call. Thus
only 1 ≤ICHN≤NCHN is filled with relevant information.
ISIG(ICHN,2) : incoming parton/particle on the target side to the hard interaction forallowed channel number ICHN. See also comment above.
ISIG(ICHN,3) : colour-flow type for allowed channel number ICHN; see MSTI(2) list. See
also above comment. For ‘subprocess’ 96 uniquely, ISIG(ICHN,3) is also used totranslate information on what is the correct subprocess number (11, 12, 13, 28,
53 or 68); this is used for reassigning subprocess 96 to either of these.
SIGH(ICHN) : evaluated differential cross section for allowed channel number ICHN, i.e.
matrix-element value times parton distributions, for current kinematical setup(in addition, Jacobian factors are included in the numbers, as used to speed up
generation). See also comment for ISIG(ICHN,1).

281

COMMON/PYINT4/MWID(500),WIDS(500,5)
Purpose: to store character of resonance width treatment and partial and effective decay
widths for the different resonances. These variables should normally not bechanged by you.
MWID(KC) : gives the character of particle with compressed code KC, mainly as usedin PYWIDT to calculate widths of resonances (not necessarily at the nominalmass).
= 0 : an ordinary particle; not to be treated as resonance.
= 1 : a resonance for which the partial and total widths (and hence branchingratios) are dynamically calculated in PYWIDT calls; i.e. special code has toexist for each such particle. The effects of allowed/disallowed secondary
decays are included, both in the relative composition of decays and in theprocess cross section.
= 2 : The total width is taken to be the one stored in PMAS(KC,2) and the relative
branching ratios the ones in BRAT(IDC) for decay channels IDC. There is thenno need for any special code in PYWIDT to handle a resonance. During therun, the stored PMAS(KC,2) and BRAT(IDC) values are used to calculate thetotal and partial widths of the decay channels. Some extra information andassumptions are then used. Firstly, the stored BRAT values are assumed to be
the full branching ratios, including all possible channels and all secondary
decays. The actual relative branching fractions are modified to take into
account that the simulation of some channels may be switched off (evenselectively for a particle and an antiparticle), as given by MDME(IDC,1), andthat some secondary channels may not be allowed, as expressed by the WIDS
factors. This also goes into
√ process cross sections. Secondly, it is assumed
that all widths scale like ŝ/m, the ratio of the actual to the nominal mass.
A further nontrivial change as a function of the actual mass can be set foreach channel by the MDME(IDC,2) value, see section 14.4.
= 3 : a hybrid version of options 1 and 2 above. At initialization the PYWIDT
code is used to calculate PMAS(KC,2) and BRAT(IDC) at the nominal massof the resonance. Special code must then exist in PYWIDT for the particle.
The PMAS(KC,2) and BRAT(IDC) values overwrite the default ones. In thesubsequent generation of events, the simpler scheme of option 2 is used, thus
saving some execution time.
Note: the Z and Z0 cannot be used with options 2 and 3, since the more complicated interference structure implemented for those particles is only handledcorrectly for option 1.
WIDS(KC,J) : gives the dimensionless suppression factor to cross sections caused by theclosing of some secondary decays, as calculated in PYWIDT. It is defined as theratio of the total width of channels switched on to the total width of all possible
channels (replace width by squared width for a pair of resonances). The on/off
status of channels is set by the MDME switches; see section 14.4. The information
in WIDS is used e.g. in cross-section calculations. Values are built up recursively
from the lightest particle to the heaviest one at initialization, with the exceptionthat W and Z are done already from the beginning (since these often are forced
off the mass shell). WIDS can go wrong in case you have perverse situations where
the branching ratios vary rapidly as a function of energy, across the resonanceshape. This then influences process cross sections.
KC :standard KC code for resonance considered.
J = 1 : suppression when a pair of resonances of type KC are produced together.
When an antiparticle exists, the particle–antiparticle pair (such as W+ W− )282is the relevant combination, else the particle–particle one (such as Z0 Z0 ).
J = 2 : suppression for a particle of type KF when produced on its own, or together
with a particle of another type.
J = 3 : suppression for an antiparticle of type KF when produced on its own, or
together with a particle of another type.
J = 4 : suppression when a pair of two identical particles are produced, for a particle
which has a nonidentical antiparticle (e.g. W+ W+ ).
J = 5 : suppression when a pair of two identical antiparticles are produced, for aparticle which has a nonidentical antiparticle (e.g. W− W− ).
COMMON/PYINT5/NGENPD,NGEN(0:500,3),XSEC(0:500,3)
Purpose: to store information necessary for cross-section calculation and differential
cross-section maximum violation. These variables must not be changed by you.
NGEN(ISUB,1) : gives the number of times that the differential cross section (times Jacobian factors) has been evaluated for subprocess ISUB, with NGEN(0,1) the sumof these.
NGEN(ISUB,2) : gives the number of times that a kinematical setup for subprocess ISUB
is accepted in the generation procedure, with NGEN(0,2) the sum of these.
NGEN(ISUB,3) : gives the number of times an event of subprocess type ISUB is generated,with NGEN(0,3) the sum of these. Usually NGEN(ISUB,3) = NGEN(ISUB,2), i.e.
an accepted kinematical configuration can normally be used to produce an event.
XSEC(ISUB,1) : estimated maximum differential cross section (times the Jacobian factors used to speed up the generation process) for the different subprocesses inuse, with XSEC(0,1) the sum of these (except low-p⊥ , i.e. ISUB = 95). For external processes special rules may apply, see section 9.9. In particular, negative
cross sections and maxima may be allowed. In this case, XSEC(ISUB,1) stores
the absolute value of the maximum, since this is the number that allows theappropriate mixing of subprocesses.
XSEC(ISUB,2) : gives the sum of differential cross sections (times Jacobian factors) for
the NGEN(ISUB,1) phase-space points evaluated so far.
XSEC(ISUB,3) : gives the estimated integrated cross section for subprocess ISUB, basedon the statistics accumulated so far, with XSEC(0,3) the estimated total crosssection for all subprocesses included (all in mb). This is exactly the information
obtainable by a PYSTAT(1) call.
Warning : for γp and γγ events, when several photon components are mixed (seeMSTP(14)), a master copy of these numbers for each component is stored in
the PYSAVE routine. What is then visible after each event is only the numbers
for the last component considered, not the full statistics. A special PYSAVE call,performed e.g. in PYSTAT, is required to obtain the sum of all the components.
COMMON/PYINT6/PROC(0:500)
CHARACTER PROC*28
Purpose: to store character strings for the different possible subprocesses; used whenprinting tables.
PROC(ISUB) : name for the different subprocesses, according to ISUB code. PROC(0)denotes all processes.
COMMON/PYINT7/SIGT(0:6,0:6,0:5)

283

Purpose: to store information on total, elastic and diffractive cross sections. Thesevariables should only be set by you for the option MSTP(31) = 0; else theyshould not be touched. All numbers are given in mb.
SIGT(I1,I2,J) : the cross section, both total and subdivided by class (elastic, diffractiveetc.). For a photon to be considered as a VMD meson the cross sections are
additionally split into the contributions from the various meson states.
I1, I2 : allowed states for the incoming particle on side 1 and 2, respectively.
= 0 : sum of all allowed states. Except for a photon to be considered as a
VMD meson this is the only nonvanishing entry.
= 1 : the contribution from the ρ0 VMD state.
= 2 : the contribution from the ω VMD state.
= 3 : the contribution from the φ VMD state.
= 4 : the contribution from the J/ψ VMD state.
= 5, 6 : reserved for future use.
J :the total and partial cross sections.
= 0 : the total cross section.
= 1 : the elastic cross section.
= 2 : the single diffractive cross section AB → XB.
= 3 : the single diffractive cross section AB → AX.
= 4 : the double diffractive cross section.
= 5 : the inelastic, non-diffractive cross section.
Warning: if you set these values yourself, it is important that they are internallyconsistent, since this is not explicitly checked by the program. Thus thecontributions J = 1 - 5 should add up to the J = 0 one and, for VMD
photons, the contributions I = 1 - 4 should add up to the I = 0 one.
COMMON/PYINT8/XPVMD(-6:6),XPANL(-6:6),XPANH(-6:6),XPBEH(-6:6),&XPDIR(-6:6)
Purpose: to store the various components of the photon parton distributions when the
PYGGAM routine is called.
XPVMD(KFL) : gives distributions of the VMD part (ρ0 , ω and φ).
XPANL(KFL) : gives distributions of the anomalous part of light quarks (d, u and s).
XPANH(KFL) : gives distributions of the anomalous part of heavy quarks (c and b).
XPBEH(KFL) : gives Bethe-Heitler distributions of heavy quarks (c and b). This provides
an alternative to XPANH, i.e. both should not be used at the same time.
XPDIR(KFL) : gives direct correction to the production of light quarks (d, u and s). This
term is nonvanishing only in the ms scheme, and is applicable for F2γ rather thanfor the parton distributions themselves.
COMMON/PYINT9/VXPVMD(-6:6),VXPANL(-6:6),VXPANH(-6:6),VXPDGM(-6:6)
Purpose: to give the valence parts of the photon parton distributions (x-weighted, as
usual) when the PYGGAM routine is called. Companion to PYINT8, which gives
the total parton distributions.
VXPVMD(KFL) : valence distributions of the VMD part; matches XPVMD in PYINT8.
VXPANL(KFL) : valence distributions of the anomalous part of light quarks; matches
XPANL in PYINT8.
VXPANH(KFL) : valence distributions of the anomalous part of heavy quarks; matches
XPANH in PYINT8.
VXPDGM(KFL) : gives the sum of valence distributions parts; matches XPDFGM in the284

PYGGAM call.
Note 1: the Bethe-Heitler and direct contributions in XPBEH(KFL) and XPDIR(KFL) in
PYINT8 are pure valence-like, and therefore are not duplicated here.
Note 2: the sea parts of the distributions can be obtained by taking the appropriate
differences between the total distributions and the valence distributions.

285

10

Initial- and Final-State Radiation

Starting from the hard interaction, initial- and final-state radiation corrections may be
added. This is normally done by making use of the parton-shower language — only for thee+ e− → qq process does Pythia offer a matrix-element option (described in section 6.1).
The algorithms used to generate initial- and final-state showers are rather different, andare therefore described separately below, starting with the conceptually easier final-stateone. Before that, some common elements are introduced.
As a further doubling-up, recently new transverse-momentum-ordered showers wereintroduced as an alternative to the older virtuality-ordered ones. The p⊥ -ordering offers
several advantages, on its own and especially in combination with the new, more sophisticated multiple interactions scenarios described in section 11.4. In the long run, the
new algorithms may be the only ones to survive, but they are not yet sufficiently wellestablished that the older can be removed; in addition, comparisons between different orderings are helpful for a better understanding of the powers and limitations of the shower
approach [Ple05, Ska05]. While the newer routines are quite different in many respects,
they still share a lot of the philosophy of the older ones. Therefore it is feasible to give areasonably detailed presentation of the old formalisms and only provide a brief summaryof the main differences introduced by the p⊥ -ordering.
The main references for virtuality-ordered final-state showers are [Ben87a, Nor01] andfor ditto initial-state ones [Sjö85, Miu99], while the transverse-momentum-ordered showersof both kinds are described in [Sjö04a].

10.1

Shower Evolution

In the leading-logarithmic picture, a shower may be viewed as a sequence of 1 → 2
branchings a → bc. Here a is called the mother and b and c the two daughters. Eachdaughter is free to branch in its turn, so that a tree-like structure can evolve. We will use
the word ‘parton’ for all the objects a, b and c involved in the branching process, i.e. notonly for quarks and gluons but also for leptons and photons. The branchings included in
the program are q → qg, g → gg, g → qq, q → qγ and ` → `γ. Photon branchings, i.e.
γ → qq and γ → ``, have not been included so far, since they are reasonably rare andsince no urgent need for them has been perceived. Furthermore, the γ → qq branchingis intimately related to the issue of the hadronic nature of the photon, which requires amuch more sophisticated machinery to handle, see section 7.7.2.
A word on terminology may be in order. The algorithms described here are customarilyreferred to as leading-log showers. This is correct insofar as no explicit corrections fromhigher orders are included, i.e. there are no O(αs2 ) terms in the splitting kernels, neither
by new 1 → 3 processes nor by corrections to the 1 → 2 ones. However, it would begrossly misleading to equate leading-log showers with leading-log analytical calculations.
In particular, the latter contain no constraints from energy–momentum conservation: theradiation off a quark is described in the approximation that the quark does not lose
any energy when a gluon is radiated, so that the effects of multiple emissions factorize.
Therefore energy–momentum conservation is classified as a next-to-leading-log correction.
In a Monte Carlo shower, on the other hand, energy–momentum conservation is explicit
branching by branching. By including coherence phenomena and optimized choices of αs
scales, further information on higher orders is inserted. While the final product is still
not certified fully to comply with a NLO/NLL standard, it is well above the level of an
unsophisticated LO/LL analytic calculation.

286

10.1.1

The evolution equations

In the shower formulation, the kinematics of each branching is given in terms of two
variables, Q2 and z. Somewhat different interpretations may be given to these variables,
and indeed this is one main area where the various programs on the market differ. Q2
has dimensions of squared mass, and is related to the mass or transverse momentum scaleof the branching. z gives the sharing of the a energy and momentum between the two
daughters, with parton b taking a fraction z and parton c a fraction 1 − z. To specify thekinematics, an azimuthal angle ϕ of the b around the a direction is needed in addition;in the simple discussions ϕ is chosen to be isotropically distributed, although options for
non-isotropic distributions currently are the defaults.
The probability for a parton to branch is given by the evolution equations (also called
DGLAP or Altarelli–Parisi [Gri72, Alt77]). It is convenient to introduce
t = ln(Q2 /Λ2 )dt = d ln(Q2 ) =

⇒

dQ2
,
Q2

(162)

where Λ is the QCD Λ scale in αs . Of course, this choice is more directed towards the
QCD parts of the shower, but it can be used just as well for the QED ones. In terms ofthe two variables t and z, the differential probability dP for parton a to branch is nowdPa =X αabcb,c2π

Pa→bc (z) dt dz .

(163)

Here the sum is supposed to run over all allowed branchings, for a quark q → qg andq → qγ, and so on. The αabc factor is αem for QED branchings and αs for QCD ones (tobe evaluated at some suitable scale, see below).
The splitting kernels Pa→bc (z) are
1 + z2,1−z(1 − z(1 − z))2NC,
z(1 − z)TR (z 2 + (1 − z)2 ) ,1 + z2e2q,
1−z
1 + z2e2`,
1−z

Pq→qg (z) = CF
Pg→gg (z) =Pg→qq (z) =Pq→qγ (z) =P`→`γ (z) =(164)

with CF = 4/3, NC = 3, TR = nf /2 (i.e. TR receives a contribution of 1/2 for eachallowed qq flavour), and e2q and e2` the squared electric charge (4/9 for u-type quarks, 1/9
for d-type ones, and 1 for leptons).
Persons familiar with analytical calculations may wonder why the ‘+ prescriptions’and δ(1 − z) terms of the splitting kernels in eq. (164) are missing. These complications
fulfil the task of ensuring flavour and energy conservation in the analytical equations. The
corresponding problem is solved trivially in Monte Carlo programs, where the shower evolution is traced in detail, and flavour and four-momentum are conserved at each branching.
The legacy left is the need to introduce a cut-off on the allowed range of z in splittings, so
as to avoid the singular regions corresponding to excessive production of very soft gluons.
Also note that Pg→gg (z) is given here with a factor NC in front, while it is sometimes
shown with 2NC . The confusion arises because the final state contains two identical partons. With the normalization above, Pa→bc (z) is interpreted as the branching probability
287

for the original parton a. On the other hand, one could also write down the probability
that a parton b is produced with a fractional energy z. Almost all the above kernels can beused unchanged also for this purpose, with the obvious symmetry Pa→bc (z) = Pa→cb (1−z).
For g → gg, however, the total probability to find a gluon with energy fraction z is thesum of the probability to find either the first or the second daughter there, and that gives
the factor of 2 enhancement.
10.1.2The Sudakov form factor

The t variable fills the function of a kind of time for the shower evolution. In finalstate showers, t is constrained to be gradually decreasing away from the hard scattering,in initial-state ones to be gradually increasing towards the hard scattering. This does
not mean that an individual parton runs through a range of t values: in the end, eachbranching is associated with a fixed t value, and the evolution procedure is just a wayof picking that value. It is only the ensemble of partons in many events that evolves
continuously with t, cf. the concept of parton distributions.
For a given t value we define the integral of the branching probability over all allowed
z values,
Z z+ (t)αabc
Ia→bc (t) =dz
Pa→bc (z) .
(165)
2πz− (t)
ThePnaı̈ve probability that a branching occurs during a small range oft values, δt, is given
Pby b,c Ia→bc (t) δt, and thus the probability for no emission by 1 − b,c Ia→bc (t) δt.
If the evolution of parton a starts at a ‘time’ t0 , the probability that the parton has
not yet branched at a ‘later time’ t > t0 is given by the product of the probabilities thatit did not branch in any of the small intervals δt between t0 and t. In other words, lettingδt → 0, the no-branching probability exponentiates:



Z t



t0

Pno−branching (t0 , t) = exp −dt0

XIa→bc (t0 )

b,c





(166)

= Sa (t) .

Thus the actual probability that a branching of a occurs at t is given by









 Z t
X
dPa
dPno−branching (t0 , t) X
Ia→bc (t0 ) .
=−=Ia→bc (t) exp − dt0


dt
dt
t0
b,c
b,c

(167)

The first factor is the naı̈ve branching probability, the second the suppression due
to the conservation of total probability: if a parton has already branched at a ‘time’t0 < t, it can no longer branch at t. This is nothing but the exponential factor thatis familiar from radioactive decay. In parton-shower language the exponential factor
Sa (t) = Pno−branching (t0 , t) is referred to as the Sudakov form factor [Sud56].
The ordering in terms of increasing t above is the appropriate one for initial-state
showers. In final-state showers the evolution is from an initial tmax (set by the hard
scattering) and towards smaller t. In that case the integral from t0 to t in eqs. (166) and(167) is replaced by an integral from t to tmax . Since, by convention, the Sudakov factor
is still defined from the lower cut-off t0 , i.e. gives the probability that a parton startingat scale t will not have branched by the lower cut-off scale t0 , the no-branching factor isactually Pno−branching (tmax , t) = Sa (tmax )/Sa (t).
We note that the above structure is exactly of the kind discussed in section 4.2. The
veto algorithm is therefore extensively used in the Monte Carlo simulation of parton
showers.

288

10.1.3

Matching to the hard scattering

The evolution in Q2 is begun from some maximum scale Q2max for final-state parton
showers, and is terminated at (a possibly different) Q2max for initial-state showers. In
general there is some ambiguity associated with the choice of Q2max . Indeed, since theparton-shower language does not guarantee agreement with higher-order matrix-element
results, neither in absolute shape nor normalization, there is no unique prescription for a‘best’ choice. Generically Qmax should be of the order of the hard-scattering scale, i.e. thelargest virtuality should be associated with the hard scattering, and initial- and final-stateparton showers should only involve virtualities smaller than that. This may be viewed justas a matter of sound book-keeping: in a 2 → n graph, a 2 → 2 hard-scattering subgraphcould be chosen several different ways, but if all the possibilities were to be generated
then the cross section would be double-counted (or, rather, multiple-counted). Therefore
one should define the 2 → 2 ‘hard’ piece of a 2 → n graph as the one that involves thelargest virtuality.
Of course, the issue of double-counting depends on what processes are actually generated in the program. If one considers a qqg final state at hadron colliders, it could comeabout either as a qq pair with a gluon emission q → qg, or as a gg pair with a gluon
splitting g → qq, or in many other ways, so that the danger of double-counting is very
real. In general, this applies to any hard scattering process that already contains one ormore QCD jets at the matrix-element level. On the other hand, consider the productionof a low-p⊥ , low-mass Drell–Yan pair of leptons, together with two quark jets. Such a
process in principle could proceed by having a γ ∗ emitted off a quark leg, with a quark–
quark scattering as hard interaction. However, since this process is not included in the
program, there is no actual danger of (this particular) double-counting, and so the scaleof evolution could be picked larger than the mass of the Drell–Yan pair, as we shall see.
For most 2 → 2 scattering processes in Pythia, the Q2 scale of the hard scattering
is chosen to be Q2hard = p2⊥ (when the final-state particles are massless, otherwise masses
are added). In final-state showers, when Q is associated with the mass of the branching
parton, transverse momenta generated in the shower are constrained by p⊥ < Q/2. Anordering that the shower p⊥ should be smaller than the hard-scattering p⊥ therefore
corresponds roughly to Q2max = 4Q2hard , which is the default assumption. The constraints
are slightly different for initial-state showers, where the space-like virtuality Q2 attaches
better to p2⊥ , and therefore different considerations suggest anything between Q2max =Q2hard and Q2max = 4Q2hard as a sensible default.
We iterate that these limits, set by PARP(71) and PARP(67), respectively, are imagined
sensible when there is a danger of double-counting; if not, large values could well be
relevant to cover a wider range of topologies (see e.g. the study of ‘power’ vs. ‘wimpy’showers in [Ple05]), but always with some caution. (See also MSTP(68).)
The situation is rather better for the final-state showers in the decay of any coloursinglet particles, or coloured but reasonably long-lived ones, such as the Z0 or the h0 ,either as part of a hard 2 → 1 → 2 process, or anywhere else in the final state. Then weknow that Qmax has to be put equal to the particle mass. It is also possible to match theparton-shower evolution to the first-order matrix-element results.
QCD processes such as qg → qg pose a special problem when the scattering angle issmall. Coherence effects (see below) may then restrict the emission further than what
is just given by the Qmax scale introduced above. This is most easily viewed in the rest
frame of the 2 → 2 hard-scattering subprocess. Some colours flow from the initial to
the final state. The bulk of the radiation associated with such a colour flow should be
restricted to a cone with opening angle given by the difference between the original andthe final colour directions; there is one such cone around the incoming parton for initialstate radiation and one around the outgoing parton for final-state radiation. Colours thatare annihilated or created in the process effectively correspond to an opening angle of289

180◦ and therefore the emission is not constrained for these. For a gluon, which have twocolours and therefore two different cones, a random choice is made between the two for
the first branching. Further, coherence effects also imply azimuthal anisotropies of theemission inside the allowed cones.
Finally, it is important to note that several different descriptions of the ‘same’ process
may coexist within the program. For the most part, these descriptions differ simply bywhich parts of the given process are treated as being collinear (i.e. with corresponding
leading collinear logarithms resummed to all orders) and which as being high-p⊥ (i.e. withcorresponding fixed-order diagrams calculated in perturbation theory). Section 8.4.2 gives
two classic examples. One is the correspondence between the description of a single W
or Z with additional jet production by showering, or the same picture obtained by using
explicit matrix elements to generate at least one jet in association with the W/Z. The
other is the generation of Z0 bb final states either starting from bb → Z0 , or from bg → Z0 b
or from gg → bbZ0 . As a rule of thumb, to be used with common sense, one would startfrom as low an order as possible for an inclusive description, where the low-p⊥ region
is likely to generate most of the cross section, whereas higher-order topologies are more
relevant for studies of exclusive event samples at high p⊥ .

10.2

Final-State Showers

Final-state showers are time-like, i.e. all virtualities m2 = E 2 − p2 ≥ 0. The maximum allowed virtuality scale Q2max is set by the hard-scattering process, and thereafter
the virtuality is decreased in each subsequent branching, down to the cut-off scale Q20 .
This cut-off scale is used to regulate both soft and collinear divergences in the emission
probabilities.
Many different approaches can be chosen for the parton-shower algorithm, e.g. in termsof evolution variables, kinematics reconstruction and matrix-element corrections. The
traditional approach in Pythia is the mass-ordered PYSHOW algorithm. As an alternative,a p⊥ -ordered PYPTFS algorithm has recently been introduced. It is described only briefly
at the end of this section.
The main points of the PYSHOW showering algorithm are as follows.
• It is a leading-log algorithm, of the improved, coherent kind, i.e. with angular ordering.
• It can be used for an arbitrary initial pair of partons or, in fact, for any number
between one and eighty given entities (including hadrons and gauge bosons) although
only quarks, gluons, leptons, squarks and gluinos can initiate a shower.
• The set of showering partons may be given in any frame, but the evolution is carriedout in the c.m. frame of the showering partons (except if only one parton is input).
• Energy and momentum are conserved exactly at each step of the showering process.
• If an initial pair comes from the decay of a known resonance (also a coloured one
such as top), an additional rejection technique is used in the gluon emission off aparton of the pair, so as to reproduce the lowest-order differential 3-jet cross section.
• In subsequent branchings, angular ordering (coherence effects) is imposed.
• Gluon helicity effects, i.e. correlations between the production plane and the decay
plane of a gluon, can be included.
• The first-order running αs expression is used, with the Q2 scale given by (an approximation to) the squared transverse momentum of a branching. The default 5-flavour
ΛQCD , which should not be regarded as a proper ΛMS , is 0.29 GeV.
• The parton shower is by default cut off at a mass scale of 1 GeV.
Let us now proceed with a more detailed description.

29010.2.1

The choice of evolution variable

In the PYSHOW algorithm, the evolution variable Q2 is associated with the squared massof the branching parton, Q2 = m2a for a branching a → bc. As a consequence, t =ln(Q2 /Λ2 ) = ln(m2a /Λ2 ). This Q2 choice is not unique, and indeed other programs haveother definitions: Herwig uses Q2 ≈ m2 /(2z(1−z)) [Mar88] and Ariadne (and PYPTFS)
Q2 = p2⊥ ≈ z(1 − z)m2 [Pet88]. Below we will also modify the Q2 choice to give a betteraccount of mass effects, e.g. for b quarks.
With Q a mass scale, the lower cut-off Q0 is one in mass. To be more precise, in a
QCD shower, the Q0 parameter is used to derive effective masses
meff,g =meff,q =

1
Q0 ,
2
s

1
m2q + Q20 ,
4

(168)

where the mq have been chosen as typical kinematical quark masses, see section 13.2.1.
A parton cannot branch unless its mass is at least the sum of the lightest pair of allowed
decay products, i.e. the minimum mass scale at which a branching is possible ismmin,g = 2 meff,g = Q0 ,mmin,q = meff,q + meff,g ≥ Q0 .

(169)

The above masses are used to constrain the allowed range of Q2 and z values. However,once it has been decided that a parton cannot branch any further, that parton is put on
the mass shell, i.e. ‘final-state’ gluons are massless.
When also photon emission is included, a separate Q0 scale is introduced for the QEDpart of the shower, and used to calculate cut-off masses by analogy with eqs. (168) and(169) above [Sjö92c]. By default the two Q0 scales are chosen equal, and have the value
1 GeV. If anything, one would be inclined to allow a cut-off lower for photon emission
than for gluon one. In that case the allowed z range of photon emission would be larger
than that of gluon emission, and at the end of the shower evolution only photon emission
would be allowed.
Photon and gluon emission differ fundamentally in that photons appear as physicalparticles in the final state, while gluons are confined. For photon emission off quarks,
however, the confinement forces acting on the quark may provide an effective photon
emission cut-off at larger scales than the bare quark mass. Soft and collinear photons
could also be emitted by the final-state charged hadrons [Bar94a]; the matching between
emission off quarks and off hadrons is a delicate issue, and we therefore do not attempt
to address the soft-photon region.
For photon emission off leptons, there is no need to introduce any collinear emission
cut-off beyond what is given by the lepton mass, but we keep the same cut-off approachas for quarks, although at a smaller scale. However, note that, firstly, the program is
not aimed at high-precision studies of lepton pairs (where interference terms between
initial- and final-state radiation also would have to be included), and, secondly, mostexperimental procedures would include the energy of collinear photons into the effectiveenergy of a final-state lepton.
10.2.2The choice of energy splitting variable

The final-state radiation machinery is always applied in the c.m. frame of the hard scattering, from which normally emerges a pair of evolving partons. Occasionally there may
be one evolving parton recoiling against a non-evolving one, as in qq → gγ, where only
the gluon evolves in the final state, but where the energy of the photon is modified by the291

branching activity of the gluon. (With only one evolving parton and nothing else, it would
not be possible to conserve energy and momentum when the parton is assigned a mass.)
Thus, before the evolution is performed, the parton pair is boosted to their common c.m.
frame, and rotated to sit along the z axis. After the evolution, the full parton shower isrotated and boosted back to the original frame of the parton pair.
The interpretation of the energy and momentum splitting variable z is not unique,and in fact the program allows the possibility to switch between four different alternatives[Ben87a], ‘local’ and ‘global’ z definition combined with ‘constrained’ or ‘unconstrained’
evolution. In all four of them, the z variable is interpreted as an energy fraction, i.e.
Eb = zEa and Ec = (1 − z)Ea . In the ‘local’ choice of z definition, energy fractions aredefined in the rest frame of the grandmother, i.e. the mother of parton a. The preferredchoice is the ‘global’ one, in which energies are always evaluated in the c.m. frame ofthe hard scattering. The two definitions agree for the branchings of the partons thatemerge directly from the hard scattering, since the hard scattering itself is considered tobe the ‘mother’ of the first generation of partons. For instance, in Z0 → qq the Z0 is
considered the mother of the q and q, even though the branching is not handled by theparton-showering machinery. The ‘local’ and ‘global’ definitions diverge for subsequent
branchings, where the ‘global’ tends to allow more shower evolution.
In a branching a → bc the kinematically allowed range of z = za values, z− < z < z+ ,is given by
q


1



(m2a − m2b − m2c )2 − 4m2b m2c 

m2 − m2 |pa |z± = 1 + b 2 c ±
2
ma
Ea



m2a

.

(170)

With ‘constrained’ evolution, these bounds are respected in the evolution. The cut-off
masses meff,b and meff,c are used to define the maximum allowed z range, within which
za is chosen, together with the ma value. In the subsequent evolution of b and c, only
pairs of mb and mc are allowed for which the already selected za fulfils the constraints in
eq. (170).
For ‘unconstrained’ evolution, which is the preferred alternative, one may start off byassuming the daughters to be massless, so that the allowed z range is
(

)

1
|pa |
z± =
1±
θ(ma − mmin,a )
2
Ea

,

(171)

where θ(x) is the step function, θ(x) = 1 for x > 0 and θ(x) = 0 for x < 0. The decay(0)kinematics into two massless four-vectors pb and p(0)is now straightforward. Once mb
cand mc have been found from the subsequent evolution, subject only to the constraints
mb < za Ea , mc < (1 − za )Ea and mb + mc < ma , the actual massive four-vectors may be
defined as(0)(0)
pb,c = pb,c ± (rc p(0)(172)
c − rb pb ) ,where
rb,c =

m2a ± (m2c − m2b ) −q(m2a − m2b − m2c )2 − 4m2b m2c

.
(173)
2m2a
In other words, the meaning of za is somewhat reinterpreted post facto. Needless to say,the ‘unconstrained’ option allows more branchings to take place than the ‘constrained’
one. In the following discussion we will only refer to the ‘global, unconstrained’ z choice.
10.2.3First branchings and matrix-element matching

The final-state evolution is normally started from some initial parton pair 1 + 2, at a
Q2max scale determined by deliberations already discussed. When the evolution of parton292

1 is considered, it is assumed that parton 2 is on-shell, so that the parton 1 energy andmomentum are simple functions of its mass (and of the c.m. energy of the pair, which
is fixed), and hence also the allowed z1 range for splittings is a function of this mass,eq. (171). Correspondingly, parton 2 is evolved under the assumption that parton 1 is
on-shell. After both partons have been assigned masses, their correct energies may befound, which are smaller than originally assumed. Therefore the allowed z ranges haveshrunk, and it may happen that a branching has been assigned a z value outside thisrange. If so, the parton is evolved downwards in mass from the rejected mass value; ifboth z values are rejected, the parton with largest mass is evolved further. It may also
happen that the sum of m1 and m2 is larger than the c.m. energy, in which case the one
with the larger mass is evolved downwards. The checking and evolution steps are iterated
until an acceptable set of m1 , m2 , z1 and z2 has been found.
The procedure is an extension of the veto algorithm, where an initial overestimationof the allowed z range is compensated by rejection of some branchings. One should note,however, that the veto algorithm is not strictly applicable for the coupled evolution in two
variables (m1 and m2 ), and that therefore some arbitrariness is involved. This is manifest
in the choice of which parton will be evolved further if both z values are unacceptable, orif the mass sum is too large.
For a pair of particles which comes from the decay of a resonance within the Standard
Model or its MSSM supersymmetric extension, the first branchings are matched to theexplicit first-order matrix elements for decays with one additional gluon in the final state,
see subsection 10.2.6 below. Here we begin by considering in detail how γ ∗ /Z0 → qq ismatched to the matrix element for γ ∗ /Z0 → qqg [Ben87a].
The matching is based on a mapping of the parton-shower variables on to the 3-jetphase space. To produce a 3-jet event, γ ∗ /Z0 → q(p1 )q(p2 )g(p3 ), in the shower language,
one will pass through an intermediate state, where either the q or the q is off the mass
shell. If the former is the case then2m2 = (p1 + p3 )2 = Ecm(1 − x2 ) ,
E1
x1
x1
z =
=
=
,
E1 + E3x1 + x3
2 − x2(174)

where xi = 2Ei /Ecm . The q emission case is obtained with 1 ↔ 2. The parton-shower
splitting expression in terms of m2 and z, eq. (163), can therefore be translated into thefollowing differential 3-jet rate:
1 dσPSαs1=
CF×σ dx1 dx22π
(1 − x1 )(1 − x2 )
(
Ã
Ã
µ
¶2 !
µ
¶2 !)
x1
x2
1 − x1
1 − x2
1+
+
1+
,
×
x3
2 − x2x3
2 − x1(175)

where the first term inside the curly bracket comes from emission off the quark and thesecond term from emission off the antiquark. The corresponding expression in matrixelement language isno
1 dσMEαs1=
CFx21 + x22 .
σ dx1 dx22π
(1 − x1 )(1 − x2 )(176)

With the kinematics choice of Pythia, the matrix-element expression is always smaller
than the parton-shower one. It is therefore possible to run the shower as usual, but toimpose an extra weight factor dσME /dσPS , which is just the ratio of the expressions incurly brackets. If a branching is rejected, the evolution is continued from the rejected
Q2 value onwards (the veto algorithm). The weighting procedure is applied to the first293

branching of both the q and the q, in each case with the (nominal) assumption that noneof the other partons branch (neither the sister nor the daughters), so that the relationsof eq. (174) are applicable.
If a photon is emitted instead of a gluon, the emission rate in parton showers is givenby
1 dσPS1
αem
×=σ dx1 dx2
2π (1 − x1 )(1 − x2 )
(
Ã
Ã
µ
¶2 !
µ
¶2 !)
x1
x2
2 1 − x1
2 1 − x2
×
eq
1+
+ eq
1+
, (177)x3
2 − x2x3
2 − x1and in matrix elements by [Gro81]
αem
1 dσME1=
σ dx1 dx2
2π (1 − x1 )(1 − x2 )(µ

1 − x1
1 − x2
− eq
eq
x3
x3

¶2 ³

x21 + x22

´

)

.

(178)

As in the gluon emission case, a weighting factor dσME /dσPS can therefore be applied wheneither the original q (`) or the original q (`) emits a photon. For a neutral resonance,such as Z0 , where eq = −eq , the above expressions simplify and one recovers exactly thesame ratio dσME /dσPS as for gluon emission.
Compared with the standard matrix-element treatment, a few differences remain. The
shower one automatically contains the Sudakov form factor and an αs running as a functionof the p2⊥ scale of the branching. The shower also allows all partons to evolve further,which means that the naı̈ve kinematics assumed for a comparison with matrix elements
is modified by subsequent branchings, e.g. that the energy of parton 1 is reduced when
parton 2 is assigned a mass. All these effects are formally of higher order, and so do
not affect a first-order comparison. This does not mean that the corrections need besmall, but experimental results are encouraging: the approach outlined does as good as
explicit second-order matrix elements for the description of 4-jet production, better insome respects (like overall rate) and worse in others (like some angular distributions).
10.2.4

Subsequent branches and angular ordering

The shower evolution is (almost) always done on a pair of partons, so that energy andmomentum can be conserved. In the first step of the evolution, the two original partons
thus undergo branchings 1 → 3 + 4 and 2 → 5 + 6. As described above, the allowed m1 ,m2 , z1 and z2 ranges are coupled by kinematical constraints. In the second step, the pair
3 + 4 is evolved and, separately, the pair 5 + 6. Considering only the former (the latter(0)
is trivially obtained by symmetry), the partons thus have nominal initial energies E3 =(0)(0)
z1 E1 and E4 = (1−z1 )E1 , and maximum allowed virtualities mmax,3 = min(m1 , E3 ) and(0)mmax,4 = min(m1 , E4 ). Initially partons 3 and 4 are evolved separately, giving masses
m3 and m4 and splitting variables z3 and z4 . If m3 + m4 > m1 , the parton of 3 and 4
that has the largest ratio of mi /mmax,i is evolved further. Thereafter eq. (172) is used toconstruct corrected energies E3 and E4 , and the z values are checked for consistency. If
a branching has to be rejected because the change of parton energy puts z outside theallowed range, the parton is evolved further.
This procedure can then be iterated for the evolution of the two daughters of parton
3 and for the two of parton 4, etc., until each parton reaches the cut-off mass mmin . Thenthe parton is put on the mass shell.
The model, as described so far, produces so-called conventional showers, whereinmasses are strictly decreasing in the shower evolution. Emission angles are decreasing
only in an average sense, however, which means that also fairly ‘late’ branchings can give294

partons at large angles. Theoretical studies beyond the leading-log level show that thisis not correct [Mue81], but that destructive interference effects are large in the region of
non-ordered emission angles. To a good first approximation, these so-called coherence
effects can be taken into account in parton-shower programs by requiring a strict ordering in terms of decreasing emission angles. (Actually, the fact that the shower describedhere is already ordered in mass implies that the additional cut on angle will be a bit too
restrictive. While effects from this should be small at current energies, some deviations
become visible at very high energies.)
The coherence phenomenon is known already from QED. One manifestation is the
Chudakov effect [Chu55], discovered in the study of high-energy cosmic γ rays impinging
on a nuclear target. If a γ is converted into a highly collinear e+ e− pair inside theemulsion, the e+ and e− in their travel through the emulsion ionize atoms and thereby
produce blackening. However, near the conversion point the blackening is small: the e+
and e− then are still close together, so that an atom traversed by the pair does not resolve
the individual charges of the e+ and the e− , but only feels a net charge close to zero. Only
later, when the e+ and e− are separated by more than a typical atomic radius, are thetwo able to ionize independently of each other.
The situation is similar in QCD, but is further extended, since now also gluons carrycolour. For example, in a branching q0 → qg the q and g share the newly created pairof opposite colour–anticolour charges, and therefore the q and g cannot emit subsequentgluons incoherently. Again the net effect is to reduce the amount of soft gluon emission:
since a soft gluon (emitted at large angles) corresponds to a large (transverse) wavelength,
the soft gluon is unable to resolve the separate colour charges of the q and the g, andonly feels the net charge carried by the q0 . Such a soft gluon g0 (in the region θq0 g0 > θqg )could therefore be thought of as being emitted by the q0 rather than by the q–g system.
If one considers only emission that should be associated with the q or the g, to a good
approximation (in the soft region), there is a complete destructive interference in theregions of non-decreasing opening angles, while partons radiate independently of eachother inside the regions of decreasing opening angles (θqg0 < θqg and θgg0 < θqg ), once
azimuthal angles are averaged over. The details of the colour interference pattern arereflected in non-uniform azimuthal emission probabilities.
The first branchings of the shower are not affected by the angular-ordering requirement— since the evolution is performed in the c.m. frame of the original parton pair, where theoriginal opening angle is 180◦ , any angle would anyway be smaller than this — but here
instead the matrix-element matching procedure is used, where applicable. Subsequently,
each opening angle is compared with that of the preceding branching in the shower.
For a branching a → bc the kinematical approximation
Ãp⊥b p⊥c q
1
1
θa ≈
+
≈ za (1 − za )ma+EbEcza Ea (1 − za )Ea!

maza (1 − za ) Ea

=q

1

(179)

is used to derive the opening angle (this is anyway to the same level of approximation
as the one in which angular ordering is derived). With θb of the b branching calculated
similarly, the requirement θb < θa can be reduced to
1 − zazb (1 − zb )>.
2
mb
za m2a(180)

Since photons do not obey angular ordering, the check on angular ordering is not
performed when a photon is emitted. When a gluon is emitted in the branching after a
photon, its emission angle is restricted by that of the preceding QCD branching in theshower, i.e. the photon emission angle does not enter.

295

10.2.5

Other final-state shower aspects

The electromagnetic coupling constant for the emission of photons on the mass shell is
αem = αem (Q2 = 0) ≈ 1/137. For the strong coupling constant several alternatives areavailable, the default being the first-order expression αs (p2⊥ ), where p2⊥ is defined by theapproximate expression p2⊥ ≈ z(1 − z)m2 . Studies of next-to-leading-order corrections
favour this choice [Ama80]. The other alternatives are a fixed αs and an αs (m2 ).
With the default choice of p2⊥ as scale in αs , a further cut-off is introduced on theallowed phase space of gluon emission, not present in the options with fixed αs or withαs (m2 ), nor in the QED shower. A minimum requirement, to ensure a well-defined αs ,is that p⊥ /Λ > 1.1, but additionally Pythia requires that p⊥ > Q0 /2. This latter
requirement is not a necessity, but it makes sense when p⊥ is taken to be the preferred scaleof the branching process, rather than e.g. m. It reduces the allowed z range, compared withthe purely kinematical constraints. Since the p⊥ cut is not present for photon emission,
the relative ratio of photon to gluon emission off a quark is enhanced at small virtualities
compared with naı̈ve expectations; in actual fact this enhancement is largely compensatedby the running of αs , which acts in the opposite direction. The main consequence, however,is that the gluon energy spectrum is peaked at around Q0 and rapidly vanishes for energies
below that, whilst the photon spectrum extends almost all the way to zero energy.
Previously it was said that azimuthal angles in branchings are chosen isotropically.
In fact, it is possible to include some effects of gluon polarization, which correlate theproduction and the decay planes of a gluon, such that a g → gg branching tends to takeplace in the production plane of the gluon, while a decay out of the plane is favoured for
g → qq. The formulae are given e.g. in ref. [Web86], as simple functions of the z value
at the vertex where the gluon is produced and of the z value when it branches. Also
coherence phenomena lead to non-isotropic azimuthal distributions [Web86]. In eithercase the ϕ azimuthal variable is first chosen isotropically, then the weight factor due topolarization times coherence is evaluated, and the ϕ value is accepted or rejected. In caseof rejection, a new ϕ is generated, and so on.
While the normal case is to have an initial pair of partons, there are a few examples
where one or three partons have to be allowed to shower. If only one parton is given,
it is not possible to conserve both energy and momentum. The choice has been madeto conserve energy and jet direction, but the momentum vector is scaled down when
the radiating parton acquires a mass. The ‘rest frame of the system’, used e.g. in the z
definition, is taken to be whatever frame the jet is given in.
In Υ → ggg decays and other configurations (e.g. from external processes) with threeor more primary parton, one is left with the issue how the kinematics from the on-shell
matrix elements should be reinterpreted for an off-shell multi-parton configuration. We
have made the arbitrary choice of preserving the direction of motion of each parton in
the rest frame of the system, which means that all three-momenta are scaled down by
the same amount, and that some particles gain energy at the expense of others. Mass
multiplets outside the allowed phase space are rejected and the evolution continued.
Finally, it should be noted that two toy shower models are included as options. One
is a scalar-gluon model, in which the q → qg branching kernel is replaced by Pq→qg (z) =2(1−z). The couplings of the gluon, g → gg and g → qq, have been left as free parameters,
3since they depend on the colour structure assumed in the model. The spectra are flat inz for a spin 0 gluon. Higher-order couplings of the type g → ggg could well contributesignificantly, but are not included. The second toy model is an Abelian vector one. In thisoption g → gg branchings are absent, and g → qq ones enhanced. More precisely, in the
splitting kernels, eq. (164), the Casimir factors are changed as follows: CF = 4/3 → 1,
NC = 3 → 0, TR = nf /2 → 3nf . When using either of these options, one should beaware that also a number of other components in principle should be changed, from therunning of αs to the whole concept of fragmentation. One should therefore not take them296

Table 26: Processes for which matching to matrix elements with one extra gluon in the
final state has been calculated. Colour quantum numbers are denoted with 1 for singlet,
3 for triplet and 8 for octet. See the text for an explanation of the γ5 column and further
comments.
colourspin

γ5

example

codes

1→3+3

—

—

(eikonal)

6–9

1→3+3

1, γ5 , 1 ± γ5

Z → qq

11 – 141, γ5 , 1 ± γ5

t → bW+

16 – 191, γ5 , 1 ± γ5h0 → qq

21 – 24

3→3+1

1 → 12 + 12
1
→ 21 + 1
2
0 → 12 + 12
1
→ 21 + 02

1, γ5 , 1 ± γ5

t → bH+

26 – 29

1→3+3

1→0+0

1

Z0 → q̃q̃

31 – 34

3→3+1

0→0+1

1

q̃ → q̃0 W+

36 – 39

1→3+3

0→0+0

1

h0 → q̃q̃

41 – 44

3→3+1

0→0+0

1

q̃ → q̃0 H+

46 – 49

1→3+3

1, γ5 , 1 ± γ5χ̃ → qq̃

51 – 541, γ5 , 1 ± γ5q̃ → qχ̃

56 – 591, γ5 , 1 ± γ5t → t̃χ̃

61 – 641, γ5 , 1 ± γ5g̃ → qq̃

66 – 691, γ5 , 1 ± γ5q̃ → qg̃

71 – 74

3→3+8

1→ 21 + 02
0 → 12 + 12
1
→ 0 + 122
1
→ 21 + 02
0 → 12 + 12
1
→ 0 + 122

1, γ5 , 1 ± γ5t → t̃g̃

76 – 79

1→8+8

—

—

(eikonal)

81 – 84

3→3+1
1→3+3

3→3+1
3→3+1
8→3+3
3→3+8

0

too seriously.
10.2.6

Merging with massive matrix elements

The matching to first-order matrix-elements is well-defined for massless quarks, and was
originally used unchanged for massive ones. A first attempt to include massive matrix
elements did not compensate for mass effects in the shower kinematics, and therefore
came to exaggerate the suppression of radiation off heavy quarks [Nor01, Bam00]. Now
the shower has been modified to solve this issue, and also improved and extended to cover
better a host of different reactions [Nor01].
The starting point is the calculation of the processes a → bc and a → bcg, each atleading order, where the ratio
WME (x1 , x2 ) =1
dσ(a → bcg)
σ(a → bc) dx1 dx2(181)

gives the process-dependent differential gluon-emission rate. Here the phase space variables are x1 = 2Eb /ma and x2 = 2Ec /ma , expressed in the rest frame of particle a. Taking
the Standard Model and the minimal supersymmetric extension thereof as templates, a297

wide selection of generic colour and spin structures have been addressed, as shown in Table 26. When allowed, processes have been calculated for an arbitrary mixture of ‘parities’,
i.e. with or without a γ5 factor, like in the vector/axial vector structure of γ ∗ /Z0 . Various
combinations of 1 and γ5 may also arise e.g. from the wave functions of the sfermion partners to the left- and right-handed fermion states. In cases where the correct combination
is not provided, an equal mixture of the two is assumed as a reasonable compromise. All
the matrix elements are encoded in the new function PYMAEL(NI,X1,X2,R1,R2,ALPHA),
where NI distinguishes the matrix elements, ALPHA is related to the γ5 admixture and themass ratios r1 = mb /ma and r2 = mc /ma are free parameters. This routine is called by
PYSHOW, but might also have an interest on its own.
In order to match to the singularity structure of the massive matrix elements, theevolution variable Q2 is changed from m2 to m2 − m2on−shell , i.e. 1/Q2 is the propagatorof a massive particle [Nor01]. For the shower history b → bg this gives a differential
probability
WPS,1 (x1 , x2 ) =αsαs1
dQ2 2 dz
2
,
=
CF 2
CF
2
2π
Q 1 − z dx1 dx2
2πx3 (1 + r2 − r12 − x2 )(182)

where the numerator 1 + z 2 of the splitting kernel for q → qg has been replaced by a
2 in the shower algorithm. For a process with only one radiating parton in the final
state, such as t → bW+ , the ratio WME /WPS,1 gives the acceptance probability for anemission in the shower. The singularity structure exactly agrees between ME and PS,
giving a well-behaved ratio always below unity. If both b and c can radiate, there is asecond possible shower history that has to be considered. The matrix element is here
split in two parts, one arbitrarily associated with b → bg branchings and the other withc → cg ones. A convenient choice is WME,1 = WME (1 + r12 − r22 − x1 )/x3 and WME,2 =WME (1+r22 −r12 −x2 )/x3 , which again gives matching singularity structures in WME,i /WPS,iand thus a well-behaved Monte Carlo procedure.
Top, squarks and gluinos can radiate gluons, as shown in Table 26 for the case ofresonance decays. Radiation is included also in a primary production process such as
qg → q̃g̃, but then without a perfect match to the respective first-order emission matrix
elements, which here also would contain interference with initial-state radiation. Insteada close analogue is found in the Table, with the same final-state colour and spin structure,
to ensure that at least the limit of collinear radiation is handled correctly. Furthermore, inthis case, the maximum scale of emission is regulated by the standard shower parameters,
and not simply set by the decaying resonance mass.
Also subsequent emissions of gluons off the primary particles are corrected to WME .
To this end, a reduced-energy system is constructed, which retains the kinematics of the
branching under consideration but omits the gluons already emitted, so that an effective
three-body shower state can be mapped to an (x1 , x2 , r1 , r2 ) set of variables. For lightquarks this procedure is almost equivalent with the original one of using the simple universal splitting kernels after the first branching. For heavy quarks it offers an improved
modelling of mass effects also in the collinear region.
Some related further changes have been introduced, a few minor as default and somemore significant ones as non-default options [Nor01]. This includes the description ofcoherence effects and αs arguments, in general and more specifically for secondary heavy
flavour production by gluon splittings. The problem in the latter area is that data at
LEP1 show a larger rate of secondary charm and bottom production than predicted
in most shower descriptions [Bam00, Man00], or in analytical studies [Sey95]. This isbased on applying the same kind of coherence considerations to g → qq branchings as tog → gg, which is not fully motivated by theory. In the lack of an unambiguous answer, itis therefore helpful to allow options that can explore the range of uncertainty.
Further issues remain to be addressed, e.g. radiation off particles with non-negligible
width, where interference between radiation before and after the decay is not considered.
298

In general, however, the new description of mass effects in the shower should allow an
improved description of gluon radiation in many different processes.
10.2.7

Matching to four-parton events

The shower routine, as described above, is optimized for two objects forming the showeringsystem, within which energy and momentum should be conserved. However, occasionallymore than two initial objects are given, e.g. if one would like to consider the subclassof e+ e− → qqgg events in order to study angular correlations as a test of the coupling
structure of QCD. Such events are generated in the showering of normal e+ e− → qq
events, but not with high efficiency within desired cuts, and not with the full angularstructure included in the shower. Therefore four-parton matrix elements may be therequired starting point but, in order to ‘dress up’ these partons, one nevertheless wishes to
add shower emission. A possibility to start from three partons has existed since long, but
only with [And98a] was an approach for four parton introduced, and with the possibility
to generalize to more partons, although this latter work has not yet been done.
The basic idea is to cast the output of matrix element generators in the form of aparton-shower history, that then can be used as input for a complete parton shower. In
the shower, that normally would be allowed to develop at random, some branchings are
now fixed to their matrix-element values while the others are still allowed to evolve in
the normal shower fashion. The preceding history of the event is also in these random
branchings then reflected e.g. in terms of kinematical or dynamical (e.g. angular ordering)
constraints.
Consider e.g. the qqgg case. The matrix-element expression contains contributions
from five graphs, and from interferences between them. The five graphs can also be readas five possible parton-shower histories for arriving at the same four-parton state, buthere without the possibility of including interferences. The relative probability for eachof these possible shower histories can be obtained from the rules of shower branchings.
For example, the relative probability for the history where e+ e− → q(1)q(2), followed byq(1) → q(3)g(4) and g(4) → g(5)g(6), is given by:
P = P1→34 P4→56 =2
1 4 1 + z34
1 (1 − z56 (1 − z56 ))2
×3
m21 3 1 − z34 m24
z56 (1 − z56 )(183)

where the probability for each branching contains the mass singularity, the colour factor
and the momentum splitting kernel. The masses are given by
m21 = p21 = (p3 + p5 + p6 )2 ,m24 = p24 = (p5 + p6 )2 ,(184)

and the z values by
zbc = za→bc =m2a Ebm2 − λ + m2b − m2c
− a
λ Ea
2λ

q

with λ =(185)(m2a − m2b − m2c )2 − 4m2b m2c .

We here assume that the on-shell mass of quarks can be neglected. The form of theprobability then matches the expression used in the parton-shower algorithm.
Variants on the above probabilities are imaginable. For instance, in the spirit of thematrix-element approach we have assumed a common αs for all graphs, which thus neednot be shown, whereas the parton-shower language normally assumes αs = αs (p2⊥ ) to299

be a function of the transverse momentum of each branching, One could also include
information on azimuthal anisotropies.
The relative probability P for each of the five possible parton-shower histories canbe used to select one of the possibilities at random. (A less appealing alternative would
be a ‘winner takes all’ strategy, i.e. selecting the configuration with the largest P.) The
selection fixes the values of the m, z and ϕ at two vertices. The azimuthal angle ϕis defined by the daughter parton orientation around the mother direction. When theconventional parton-shower algorithm is executed, these values are then forced on theotherwise random evolution. This forcing cannot be exact for the z values, since thefinal partons given by the matrix elements are on the mass shell, while the corresponding
partons in the parton shower might be virtual and branch further. The shift between
the wanted and the obtained z values are rather small, very seldom above 10−6 . Moresignificant are the changes of the opening angle between two daughters: when daughters
originally assumed massless are given a mass the angle between them tends to be reduced.
This shift has a non-negligible tail even above 0.1 radians. The ‘narrowing’ of jets bythis mechanism is compensated by the broadening caused by the decay of the massive
daughters, and thus overall effects are not so dramatic.
All other branchings of the parton shower are selected at random according to thestandard evolution scheme. There is an upper limit on the non-forced masses from internal
logic, however. For instance, for four-parton matrix elements, the singular regions are
typically avoided with a cut y > 0.01, where y is the square of the minimal scaled invariant
mass between any pair of partons. Larger y values could be used for some purposes, whilesmaller ones give so large four-jet rates that the need to include Sudakov form factors canno longer be neglected. The y > 0.01 cut roughly corresponds to m > 9 GeV at LEP 1
energies, so the hybrid approach must allow branchings at least below 9 GeV in order toaccount for the emission missing from the matrix-element part. Since no 5-parton emission
is generated by the second-order matrix elements, one could also allow a threshold higher
than 9 GeV in order to account for this potential emission. However, if any such massis larger than one of the forced masses, the result would be a different history than thechosen one, and one would risk some double-counting issues. So, as an alternative, one
could set the minimum invariant mass between any of the four original partons as the
maximum scale of the subsequent shower evolution.
10.2.8A new p⊥ -ordered final-state shower

The traditional PYSHOW routine described above gives a mass-ordered time-like cascade,with angular ordering by veto. It offers an alternative to the Herwig angular-orderedshower [Mar88] and Ariadne p⊥ -ordered dipole emission [Gus88, Pet88]. For most properties, comparably good descriptions can be obtained of LEP data by all three [Kno96],
although Ariadne seems to do slightly better than the others.
Recently, the possibility to combine separately generated n-parton configurations from
Born-level matrix element expressions has attracted attention, see [Cat01]. This requires
the implementation of vetoed parton showers, to forbid emissions that would lead to
double-counting, and trial parton showers, to generate the appropriate Sudakovs lacking
in the matrix elements [Lön02]. The PYSHOW algorithm is not well suited for this kindof applications, since the full evolution process cannot easily be factored into a set of
evolution steps in well-defined mass ranges — the kinematics is closely tied both to themother and the daughter virtualities of a branching. Further, as we will see in section 11,
the multiple-interactions scenarios are most appropriately defined in terms of transverse
momenta, also for showers.
As an alternative, the new PYPTFS routine [Sjö04a] offers a shower algorithm borrowing
several of the dipole ideas, combined with many of the old PYSHOW elements. It is a hybrid
between the traditional parton shower and the dipole emission approaches, in the sense300

that the branching process is associated with the evolution of a single parton, like ina shower, but recoil effects occur inside dipoles. That is, the daughter partons of abranching are put on-shell. Instead a recoiling partner is assigned for each branching, andenergy and momentum is ‘borrowed’ from this partner to give mass to the parton about
to branch. In this sense, the branching and recoiling partons form a dipole. Often the
two are colour-connected, i.e. the colour of one matches the anticolour of the other, but
this need not be the case. For instance, in t → bW+ the W+ takes the recoil when theb radiates a gluon. Furthermore, the radiation of a gluon is split into two dipoles, again
normally by colour.
The evolution variable is approximately the p2⊥ of a branching, where p⊥ is the transverse momentum for each of the two daughters with respect to the direction of the mother,in the rest frame of the dipole. (The recoiling parton does not obtain any p⊥ kick in thisframe; only its longitudinal momentum is affected.) For the simple case of massless radiating partons and small virtualities relative to the kinematically possible ones, and in
the limit that recoil effects from further emissions can be neglected, it agrees with the dijp⊥ -clustering distance defined in the original LUCLUS (now PYCLUS) algorithm, see section
15.3.1.
All emissions are ordered in a single sequence p⊥max > p⊥1 > p⊥2 > . . . > p⊥min .
That is, all initial partons are evolved from the input p⊥max scale, and the one with thelargest p⊥ is chosen to undergo the first branching. Thereafter, all partons now existing
are evolved downwards from p⊥1 , and a p⊥2 is chosen, and so on, until p⊥min is reached.
(Technically, the p⊥ values for partons not directly or indirectly affected by a branchingneed not be reselected.) As already noted above, the evolution of a gluon is split inevolution on two separate sides, with half the branching kernel each, but with different
kinematical constraints since the two dipoles have different masses. The evolution of aquark is also split, into one p⊥ scale for gluon emission and one for photon one, in general
corresponding to different dipoles.
With the choices above, the evolution factorizes. That is, a set of successive calls,
where the p⊥min of one call becomes the p⊥max of the next, gives the same result (on the
average) as one single call for the full p⊥ range. This is the key element to allow Sudakovs
to be conveniently obtained from trial showers, and to veto emissions above some p⊥
scale, as required to combine different n-parton configurations. (Not yet implemented asa standard facility, however.)The formal p⊥ definition is
p2⊥evol = z(1 − z)(m2 − m20 ) ,(186)

where p⊥evol is the evolution variable, z gives the energy sharing in the branching, as
selected from the branching kernels, m is the off-shell mass of the branching parton and
m0 its on-shell value. This p⊥evol is also used as scale for the running αs .
When a p⊥evol has been selected, this is translated to a m2 = m20 + p2⊥evol /(z(1 − z))(the same formula as above, rewritten). From there on, the three-body kinematics of abranching is constructed as in PYSHOW [Nor01]. This includes the interpretation of z andthe handling of nonzero on-shell masses for branching and recoiling partons, which leads
to the physical p⊥ not identical to the p⊥evol defined here. In this sense, p⊥evol becomes aformal variable, while m really is a well-defined mass of a parton.
Also the handling of matrix-element matching closely follows the machinery of [Nor01],
once the p⊥evol has been converted to a mass of the branching parton. In general, the‘other’ parton used to define the matrix element need not be the same as the recoilingpartner. To illustrate, consider a Z0 → qq decay. In the first branching, say gluon emission
off the q, obviously the q takes the recoil, and the new q, g and q momenta are used tomatch to the qqg matrix element. The next time q branches, the recoil is now taken by
the current colour-connected gluon, but the matrix element corrections are based on the
newly created q and g momenta together with the q (not the recoiling g!) momentum.
301

That way we hope to achieve the most realistic description of mass effects in the collinear
and soft regions.
The shower inherits some further elements from PYSHOW, such as azimuthal anisotropies
in gluon branchings from polarization effects.
The relevant parameters will have to be retuned, since the shower is quite different
from the mass-ordered one of PYSHOW. In particular, it appears that the five-flavour ΛQCD
value in PARJ(81) has to be reduced relative to the current default, roughly by a factorof two (from 0.29 to 0.14 GeV). After such a retuning, PYPTFS (combined with string
fragmentation) appears to give an even better description of LEP1 data than does PYSHOW[Rud04].

10.3

Initial-State Showers

The initial-state shower algorithms in Pythia are not quite as sophisticated as the finalstate ones. This is partly because initial-state radiation is less well understood theoretically, and partly because the programming task is more complicated and ambiguous. Still,
the program at disposal is known to do a reasonably good job of describing existing data,such as Z0 production properties at hadron colliders [Sjö85]. It can be used both for QCD
showers and for photon emission off leptons (e, µ or τ ; relative to earlier versions, thedescription of incoming µ and τ are better geared to represent the differences in lepton
mass, and the lepton-inside-lepton parton distributions are properly defined).
Again we begin with a fairly model-independent overview before zooming in on theold virtuality-ordered algorithm implemented in PYSSPA. The new transverse-momentumordered formalism in PYPTIS, described at the end, shares much of the same philosophy,
apart from the quite important choice of evolution variable, of course.
10.3.1The shower structure

A fast hadron may be viewed as a cloud of quasi-real partons. Similarly a fast lepton
may be viewed as surrounded by a cloud of photons and partons; in the program the twosituations are on an equal footing, but here we choose the hadron as example. At each
instant, each individual parton initiates a virtual cascade, branching into a number ofpartons. This cascade of quantum fluctuations can be described in terms of a tree-likestructure, composed of many subsequent branchings a → bc. Each branching involves
some relative transverse momentum between the two daughters. In a language wherefour-momentum is conserved at each vertex, this implies that at least one of the b andc partons must have a space-like virtuality, m2 < 0. Since the partons are not on themass shell, the cascade only lives a finite time before reassembling, with those parts ofthe cascade that are most off the mass shell living the shortest time.
A hard scattering, e.g. in deeply inelastic leptoproduction, will probe the hadron at agiven instant. The probe, i.e. the virtual photon in the leptoproduction case, is able toresolve fluctuations in the hadron up to the Q2 scale of the hard scattering. Thus probes
at different Q2 values will seem to see different parton compositions in the hadron. Thechange in parton composition with t = ln(Q2 /Λ2 ) is given by the evolution equations
µ

αabc
x
dfb (x, t) X Z dx0=
fa (x0 , t)
Pa→bc 0
0
dt
x
2π
x
a,c

¶

.

(187)

Here the fi (x, t) are the parton-distribution functions, expressing the probability of findinga parton i carrying a fraction x of the total momentum if the hadron is probed at virtuality
Q2 . The Pa→bc (z) are given in eq. (164). As before, αabc is αs for QCD shower and αem
for QED ones.

302

Eq. (187) is closely related to eq. (163): dPa describes the probability that a given
parton a will branch (into partons b and c), dfb the influx of partons b from the branchingsof partons a. (The expression dfb in principle also should contain a loss term for partonsb that branch; this term is important for parton-distribution evolution, but does notappear explicitly in what we shall be using eq. (187) for.) The absolute form of parton
distributions for a hadron cannot be predicted in perturbative QCD, but rather have tobe parameterized at some Q0 scale, with the Q2 dependence thereafter given by eq. (187).
Available parameterizations are discussed in section 7.1. The lepton and photon parton
distributions inside a lepton can be fully predicted, but here for simplicity are treated onequal footing with hadron parton distributions.
If a hard interaction scatters a parton out of the incoming hadron, the ‘coherence’[Gri83] of the cascade is broken: the partons can no longer reassemble completely backto the cascade-initiating parton. In this semiclassical picture, the partons on the ‘main
chain’ of consecutive branchings that lead directly from the initiating parton to the scattered parton can no longer reassemble, whereas fluctuations on the ‘side branches’ to thischain may still disappear. A convenient description is obtained by assigning a space-likevirtuality to the partons on the main chain, in such a way that the partons on the side
branches may still be on the mass shell. Since the momentum transfer of the hard process
can put the scattered parton on the mass shell (or even give it a time-like virtuality, sothat it can initiate a final-state shower), one is then guaranteed that no partons have aspace-like virtuality in the final state. (In real life, confinement effects obviously imply
that partons need not be quite on the mass shell.) If no hard scattering had taken place,the virtuality of the space-like parton line would still force the complete cascade to reassemble. Since the virtuality of the cascade probed is carried by one single parton, it ispossible to equate the space-like virtuality of this parton with the Q2 scale of the cascade,to be used e.g. in the evolution equations. Coherence effects [Gri83, Bas83] guarantee that
the Q2 values of the partons along the main chain are strictly ordered, with the largest
Q2 values close to the hard scattering.
Further coherence effects have been studied [Cia87], with particular implications forthe structure of parton showers at small x. None of these additional complications are
implemented in the current algorithm, with the exception of a few rather primitive options
that do not address the full complexity of the problem.
Instead of having a tree-like structure, where all legs are treated democratically, thecascade is reduced to a single sequence of branchings a → bc, where the a and b partonsare on the main chain of space-like virtuality, m2a,b < 0, while the c partons are on themass shell and do not branch. (Later we will include the possibility that the c partons
may have positive virtualities, m2c > 0, which leads to the appearance of time-like ‘finalstate’ parton showers on the side branches.) This truncation of the cascade is only possible
when it is known which parton actually partakes in the hard scattering: of all the possible
cascades that exist virtually in the incoming hadron, the hard scattering will select one.
To obtain the correct Q2 evolution of parton distributions, e.g., it is essential that all
branches of the cascade be treated democratically. In Monte Carlo simulation of space-likeshowers this is a major problem. If indeed the evolution of the complete cascade is to befollowed from some small Q20 up to the Q2 scale of the hard scattering, it is not possibleat the same time to handle kinematics exactly, since the virtuality of the various partonscannot be found until after the hard scattering has been selected. This kind of ‘forward
evolution’ scheme therefore requires a number of extra tricks to be made to work. Further,in this approach it is not known e.g. what the ŝ of the hard scattering subsystem will be
until the evolution has been carried out, which means that the initial-state evolution andthe hard scattering have to be selected jointly, a not so trivial task.
Instead we use the ‘backwards evolution’ approach [Sjö85], in which the hard scattering
is first selected, and the parton shower that preceded it is subsequently reconstructed.
This reconstruction is started at the hard interaction, at the Q2max scale, and thereafter303

step by step one moves ‘backwards’ in ‘time’, towards smaller Q2 , all the way back to theparton-shower initiator at the cut-off scale Q20 . This procedure is possible if evolved parton
distributions are used to select the hard scattering, since the fi (x, Q2 ) contain the inclusive
summation of all initial-state parton-shower histories that can lead to the appearance ofan interacting parton i at the hard scale. What remains is thus to select an exclusive
history from the set of inclusive ones. In this way, backwards evolution furnishes a veryclear and intuitive picture of the relationship between the inclusive (parton distributions)and exclusive (initial-state showers) description of the same physics.
10.3.2Longitudinal evolution

The evolution equations, eq. (187), express that, during a small increase dt, there is a
probability for parton a with momentum fraction x0 to become resolved into parton b
at x = zx0 and another parton c at x0 − x = (1 − z)x0 . Correspondingly, in backwards
evolution, during a decrease dt a parton b may be ‘unresolved’ into parton a. The relative
probability dPb for this to happen is given by the ratio dfb /fb . Using eq. (187) one obtainsµ

X Z dx0 fa (x0 , t) αabc
dfb (x, t)
xdPb == |dt|
Pa→bc 00fb (x, t)
x fb (x, t) 2π
x
a,c

¶

.

(188)

Summing up the cumulative effect of many small changes dt, the probability for no radiation exponentiates. Therefore one may define a form factor(

Sb (x, tmax , t) = exp −(= exp −

Z tmaxt

Z tmaxt

X Z dx0 fa (x0 , t0 ) αabc (t0 )
0

dt

a,c
0

dt

XZ
a,c

x0 fb (x, t0 )

2π

µ

Pa→bc

x
x0

x0 fa (x0 , t0 )αabc (t0 )Pa→bc (z)dz
2πxfb (x, t0 )

¶)

)

,

(189)

giving the probability that a parton b remains at x from tmax to a t < tmax .
It may be useful to compare this with the corresponding expression for forward evolution, i.e. with Sa (t) in eq. (166). The most obvious difference is the appearance of parton
distributions in Sb . Parton distributions are absent in Sa : the probability for a given
parton a to branch, once it exists, is independent of the density of partons a or b. The
parton distributions in Sb , on the other hand, express the fact that the probability fora parton b to come from the branching of a parton a is proportional to the number ofpartons a there are in the hadron, and inversely proportional to the number of partons b.
Thus the numerator fa in the exponential of Sb ensures that the parton composition of
the hadron is properly reflected. As an example, when a gluon is chosen at the hard scattering and evolved backwards, this gluon is more likely to have been emitted by a u thanby a d if the incoming hadron is a proton. Similarly, if a heavy flavour is chosen at thehard scattering, the denominator fb will vanish at the Q2 threshold of the heavy-flavour
production, which means that the integrand diverges and Sb itself vanishes, so that no
heavy flavour remain below threshold.
Another difference between Sb and Sa , already touched upon, is that the Pg→gg (z)splitting kernel appears with a normalization 2NC in Sb but only with NC in Sa , since
two gluons are produced but only one decays in a branching.
A knowledge of Sb is enough to reconstruct the parton shower backwards. At eachbranching a → bc, three quantities have to be found: the t value of the branching (which
defines the space-like virtuality Q2b of parton b), the parton flavour a and the splitting
variable z. This information may be extracted as follows:304

1. If parton b partook in the hard scattering or branched into other partons at a scale
tmax , the probability that b was produced in a branching a → bc at a lower scale t is
Ã

!

XZ
dPb
αabc (t0 )dSb (x, tmax , t)
x0 fa (x0 , t0 )=−=dz
Pa→bc (z)Sb (x, tmax , t) .
dt
dt
2π
xfb (x, t0 )
a,c
(190)
If no branching is found above the cut-off scale t0 the iteration is stopped and partonb is assumed to be massless.
2. Given the t of a branching, the relative probabilities for the different allowed branchings a → bc are given by the z integrals above, i.e. byZdz

αabc (t)x0 fa (x0 , t)
Pa→bc (z).
2πxfb (x, t)(191)

3. Finally, with t and a known, the probability distribution in the splitting variablez = x/x0 = xb /xa is given by the integrand in eq. (191).
In addition, the azimuthal angle ϕ of the branching is selected isotropically, i.e. no spin
or coherence effects are included in this distribution.
The selection of t, a and z is then a standard task of the kind than can be performed
with the help of the veto algorithm. Specifically, upper and lower bounds for parton distributions are used to find simple functions that are everywhere larger than the integrands
in eq. (191). Based on these simple expressions, the integration over z may be carried out,
and t, a and z values selected. This set is then accepted with a weight given by a ratio of
the correct integrand in eq. (191) to the simple approximation used, both evaluated forthe given set. Since parton distributions, as a rule, are not in a simple analytical form,
it may be tricky to find reasonably good bounds to parton distributions. It is necessary
to make different assumptions for valence and sea quarks, and be especially attentive
close to a flavour threshold ([Sjö85]). An electron distribution inside an electron behavesdifferently from parton distributions encountered in hadrons, and has to be consideredseparately.
A comment on soft-gluon emission. Nominally the range of the z integral in Sb isx ≤ z ≤ 1. The lower limit corresponds to x0 = x/z = 1, where the fa parton distributionsin the numerator vanish and the splitting kernels are finite, wherefore no problems areencountered here. At the upper cut-off z = 1 the splitting kernels Pq→qg (z) and Pg→gg
diverge. This is the soft-gluon singularity: the energy carried by the emitted gluon isvanishing, xg = x0 − x = (1 − z)x0 = (1 − z)x/z → 0 for z → 1. In order to calculate theintegral over z in Sb , an upper cut-off zmax = x/(x + x² ) is introduced, i.e. only branchings
with z ≤ zmax are included in Sb . Here x² is a small number, typically chosen so that thegluon energy is above 2 GeV√when calculated
in the rest frame of the hard scattering.
√That is, the gluon energy xg s/2 ≥ x² s/2 = 2 GeV/γ, where γ is the boost factor of
the hard scattering. The average amount of energy carried away by gluons in the range
xg < x² , over the given range of t values from ta to tb , may be estimated [Sjö85]. The
finally selected z value may thus be picked as z = zhard hzsoft (ta , tb )i, where zhard is theoriginally selected z value and zsoft is the correction factor for soft gluon emission.
In QED showers, the smallness of αem means that one can use rather smaller cut-off
values without obtaining large amounts of emission. A fixed small cut-off xγ > 10−10 istherefore used to avoid the region of very soft photons. As has been discussed in section
7.1.3, the electron distribution inside the electron is cut off at xe < 1−10−10 , for numericalreasons, so the two cuts are closely matched.
The cut-off scale Q0 may be chosen separately for QCD and QED showers, just as infinal-state radiation. The defaults are 1 GeV and 0.001 GeV, respectively. The former
is the typical hadronic mass scale, below which radiation is not expected resolvable; thelatter is of the order of the electron mass. Photon emission is also allowed off quarks
305in hadronic interactions, with the same cut-off as for gluon emission, and also in otherrespects implemented in the same spirit, rather than according to the pure QED description.
Normally QED and QCD showers do not appear mixed. The most notable exception
is resolved photoproduction (in ep) and resolved γγ events (in e+ e− ), i.e. shower historiesof the type e → γ → q. Here the Q2 scales need not be ordered at the interface, i.e.
the last e → eγ branching may well have a larger Q2 than the first q → qg one, and thebranching γ → q does not even have a strict parton-shower interpretation for the vector
dominance model part of the photon parton distribution. This kind of configurations
is best described by the ’gamma/lepton’ machinery for having a flux of virtual photons
inside the lepton, see section 7.1.4. In this case, no initial-state radiation has currently
been implemented for the electron (or µ or τ ). The one inside the virtual-photon system
is considered with the normal algorithm, but with the lower cut-off scale modified by thephoton virtuality, see MSTP(66).
An older description still lives on, although no longer as the recommended one. There,
these issues are currently not addressed in full. Rather, based on the x selected for theparton (quark or gluon) at the hard scattering, the xγ is selected once and for all in therange x < xγ < 1, according to the distribution implied by eq. (54). The QCD parton
shower is then traced backwards from the hard scattering to the QCD shower initiator at
t0 . No attempt is made to perform the full QED shower, but rather the beam-remnant
treatment (see section 11.1) is used to find the q (or g) remnant that matches the q (org) QCD shower initiator, with the electron itself considered as a second beam remnant.
10.3.3

Transverse evolution

We have above seen that two parton lines may be defined, stretching back from thehard scattering to the initial incoming hadron wavefunctions at small Q2 . Specifically,
all parton flavours i, virtualities Q2 and energy fractions x may be found. The exact
kinematical interpretation of the x variable is not unique, however. For partons withsmall virtualities and transverse momenta, essentially all definitions agree, but differences
may appear for branchings close to the hard scattering.
In first-order QED [Ber85] and in some simple QCD toy models [Got86], one may showthat the ‘correct’ choice is the ‘ŝ approach’. Here one requires that ŝ = x1 x2 s, both at thehard-scattering scale and at any lower scale, i.e. ŝ(Q2 ) = x1 (Q2 ) x2 (Q2 ) s, where x1 andx2 are the x values of the two resolved partons (one from each incoming beam particle) at
the given Q2 scale. In practice this means that, at a branching with the splitting variablez, the total ŝ has to be increased by a factor 1/z in the backwards evolution. It also meansthat branchings on the two incoming legs have to be interleaved in a single monotonic
sequence of Q2 values of branchings. A problem with this x interpretation is that it is notquite equivalent with an ms definition of parton densities [Col00], or any other standard
definition. In practice, effects should not be large from this mismatch.
For a reconstruction of the complete kinematics in this approach, one should startwith the hard scattering, for which ŝ has been chosen according to the hard-scatteringmatrix element. By backwards evolution, the virtualities Q21 = −m21 and Q22 = −m22 of
the two interacting partons are reconstructed. Initially the two partons are considered intheir common c.m. frame, coming in along the ±z directions. Then the four-momentum
vectors have the non-vanishing components
E1,2 =ŝ ± (Q22 − Q21 )
√
,
2 ŝ

s

pz1 = −pz2 =(ŝ + Q21 + Q22 )2 − 4Q21 Q22
,
4ŝ

306

(192)

with (p1 + p2 )2 = ŝ.
If, say, Q21 > Q22 , then the branching 3 → 1 + 4, which produced parton 1, is the onethat took place closest to the hard scattering, and the one to be reconstructed first. With
the four-momentum p3 known, p4 = p3 − p1 is automatically known, so there are four
degrees of freedom. One corresponds to a trivial azimuthal angle around the z axis. The
z splitting variable for the 3 → 1 + 4 vertex is found as the same time as Q21 , and provides
the constraint (p3 + p2 )2 = ŝ/z. The virtuality Q23 is given by backwards evolution ofparton 3.
One degree of freedom remains to be specified, and this is related to the possibility
that parton 4 initiates a time-like parton shower, i.e. may have a non-zero mass. The
maximum allowed squared mass m2max,4 is found for a collinear branching 3 → 1 + 4. In
terms of the combinations
s1 = ŝ + Q22 + Q21 ,ŝs3 =+ Q22 + Q23 ,zqr1 =s21 − 4Q22 Q21 ,q

s23 − 4Q22 Q23 ,(193)

s1 s3 − r1 r3− Q21 − Q23 ,
2Q22

(194)

r3 =
one obtains
m2max,4 =which, for the special case of Q22 = 0, reduces to(m2max,4 =

Q21
− Q23
z

)(

ŝ
ŝ
−
2
ŝ + Q1 ŝ/z + Q23

)

.

(195)

These constraints on m4 are only the kinematical ones, in addition coherence phenomena
could constrain the mmax,4 values further. Some options of this kind are available; thedefault one is to require additionally that m24 ≤ Q21 , i.e. lesser than the space-like virtualityof the sister parton.
With the maximum virtuality given, the final-state showering machinery may be used
to give the development of the subsequent cascade, including the actual mass m24 , with0 ≤ m24 ≤ m2max,4 . The evolution is performed in the c.m. frame of the two ‘resolved’
partons, i.e. that of partons 1 and 2 for the branching3 → 1 + 4, and parton 4 is assumed√
to have a nominal energy Enom,4 = (1/z − 1) ŝ/2. (Slight modifications appear if parton
4 has a non-vanishing mass mq or m` .)Using the relation m24 = (p3 − p1 )2 , the momentum of parton 3 may now be found as
)

(

E3
pz3
p2⊥,3

1
ŝ
+ Q22 − Q21 − m24 ,
= √
z
2 ŝ
1
=
{s3 − 2E2 E3 } ,2pz1
no (s s + r r )/2 − Q2 (Q2 + Q2 + m2 )
1 3
1 3
4
3
1
2
= m2max,4 − m24
.
r12

(196)

The requirement that m24 ≥ 0 (or ≥ m2f for heavy flavours) imposes a constraint on
allowed z values. This constraint cannot be included in the choice of Q21 , where it logicallybelongs, since it also depends on Q22 and Q23 , which are unknown at this point. It is fairlyrare (in the order of 10% of all events) that a disallowed z value is generated, and when it307happens it is almost always for one of the two branchings closest to the hard interaction:for Q22 = 0 eq. (195) may be solved to yield z ≤ ŝ/(ŝ + Q21 − Q23 ), which is a more severe
cut for ŝ small and Q21 large. Therefore an essentially bias-free way of coping is to redo
completely any initial-state cascade for which this problem appears.
This completes the reconstruction of the 3 → 1 + 4 vertex. The subsystem made outof partons 3 and 2 may now be boosted to its rest frame and rotated to bring partons 3
and 2 along the ±z directions. The partons 1 and 4 now have opposite and compensatingtransverse momenta with respect to the event axis. When the next vertex is considered,either the one that produces parton 3 or the one that produces parton 2, the 3–2 subsystem
will fill the function the 1–2 system did above, e.g. the rôle of ŝ = ŝ12 in the formulae above
is now played by ŝ32 = ŝ12 /z. The internal structure of the 3–2 system, i.e. the branching3 → 1 + 4, appears nowhere in the continued description, but has become ‘unresolved’.
It is only reflected in the successive rotations and boosts performed to bring back the
new endpoints to their common rest frame. Thereby the hard-scattering subsystem 1–2
builds up a net transverse momentum and also an overall rotation of the hard-scattering
subsystem.
After a number of steps, the two outermost partons have virtualities Q2 < Q20 andthen the shower is terminated and the endpoints assigned Q2 = 0. Up to small corrections
from primordial k⊥ , discussed in section 11.1, a final boost will bring the partons fromtheir c.m. frame to the overall c.m. frame, where the x values of the outermost partonsagree also with the light-cone definition. The combination of several rotations and boosts
implies that the two colliding partons have a nontrivial orientation: when boosted backto their rest frame, they will not be oriented along the z axis. This new orientation isthen inherited by the final state of the collision, including resonance decay products.
10.3.4

Other initial-state shower aspects

In the formulae above, Q2 has been used as argument for αs , and not only as the space-likevirtuality of partons. This is one possibility, but in fact loop calculations tend to indicatethat the proper argument for αs is not Q2 but p2⊥ = (1 − z)Q2 [Bas83]. The variable p⊥does have the interpretation of transverse momentum, although it is only exactly so fora branching a → bc with a and c massless and Q2 = −m2b , and with z interpreted aslight-cone fraction of energy and momentum. The use of αs ((1 − z)Q2 ) is default in the
program.
Angular ordering is included in the shower evolution by default. However, as alreadymentioned, the physics is much more complicated than for time-like showers, and so thisoption should only be viewed as a first approximation. In the code the quantity orderedis an approximation of p⊥ /p ≈ sin θ. (An alternative would have been p⊥ /pL ≈ tan θ, but
this suffers from instability problems.)
In flavour excitation processes, a c (or b) quark enters the hard scattering and should
be reconstructed by the shower as coming from a g → cc (or g → bb) branching. Here anx value for the incoming c above Q2c /(Q2c + m2c ), where Q2c is the space-like virtuality of
the c, does not allow a kinematical reconstruction of the gluon branching with an xg < 1,
and is thus outside the allowed phase space. Such events (with some safety margin) arerejected. Currently they will appear in PYSTAT(1) listings in the ‘Fraction of events thatfail fragmentation cuts’, which is partly misleading, but has the correct consequence of
suppressing the physical cross section. Further, the Q2 value of the backwards evolutionof a c quark is by force kept above m2c , so as to ensure that the branching g → cc is not‘forgotten’ by evolving Q2 below Q20 . Thereby the possibility of having a c in the beam
remnant proper is eliminated [Nor98]. Warning: as a consequence, flavour excitation is
not at all possible too close to threshold. If the KFIN array in PYSUBS is set so as to requirea c (or b) on either side, and the phase space is closed for such a c to come from a g → cc
branching, the program will enter an infinite loop.
308

For proton beams, say, any c or b quark entering the hard scattering has to come froma preceding gluon splitting. This is not the case for a photon beam, since a photon has ac and b valence quark content. Therefore the above procedure need not be pursued there,
but c and b quarks may indeed appear as beam remnants.
As we see, the initial-state showering algorithm leads to a net boost and rotation of
the hard-scattering subsystems. The overall final state is made even more complex by theadditional final-state radiation. In principle, the complexity is very physical, but it may
still have undesirable side effects. One such, discussed further in section 9.2, is that it isvery difficult to generate events that fulfil specific kinematics conditions, since kinematics
is smeared and even, at times, ambiguous.
A special case is encountered in Deeply Inelastic Scattering in ep collisions. Here the
DIS x and Q2 values are defined in terms of the scattered electron direction and energy,
and therefore are unambiguous (except for issues of final-state photon radiation close to
the electron direction). Neither initial- nor final-state showers preserve the kinematics ofthe scattered electron, however, and hence the DIS x and Q2 are changed. In principle,
this is perfectly legitimate, with the caveat that one then also should use different sets ofparton distributions than ones derived from DIS, since these are based on the kinematicsof the scattered lepton and nothing else. Alternatively, one might consider showering
schemes that leave x and Q2 unchanged. In [Ben88] detailed modifications are presented
that make a preservation possible when radiation off the incoming and outgoing electron
is neglected, but these are not included in the current version of Pythia. Instead thecurrent ’gamma/lepton’ machinery explicitly separates off the e → eγ vertex from thecontinued fate of the photon.
The only reason for using the older machinery, such as process 10, is that this is still
the only place where weak charged and neutral current effects can be considered. Whatis available there, as an option, is a simple machinery which preserves x and Q2 from theeffects of QCD radiation, and also from those of primordial k⊥ and the beam-remnant
treatment, as follows. After the showers have been generated, the four-momentum of
the scattered lepton is changed to the expected one, based on the nominal x and Q2
values. The azimuthal angle of the lepton is maintained when the transverse momentum
is adjusted. Photon radiation off the lepton leg is not fully accounted for, i.e. it is assumed
that the energy of final-state photons is added to that of the scattered electron for thedefinition of x and Q2 (this is the normal procedure for parton-distribution definitions).
The change of three-momentum on the lepton side of the event is balanced by the
final-state partons on the hadron side, excluding the beam remnant but including all thepartons both from initial- and final-state showering. The fraction of three-momentum
shift taken by each parton is proportional to its original light-cone momentum in the
direction of the incoming lepton, i.e. to E ∓ pz for a hadron moving in the ± direction.
This procedure guarantees momentum but not energy conservation. For the latter, one
additional degree of freedom is needed, which is taken to be the longitudinal momentumof the initial-state shower initiator. As this momentum is modified, the change is sharedby the final-state partons on the hadron side, according to the same light-cone fractions
as before (based on the original momenta). Energy conservation requires that the totalchange in final-state parton energies plus the change in lepton side energy equals thechange in initiator energy. This condition can be turned into an iterative procedure tofind the initiator momentum shift.
Sometimes the procedure may break down. For instance, an initiator with x > 1 may
be reconstructed. If this should happen, the x and Q2 values of the event are preserved,
but new initial- and final-state showers are generated. After five such failures, the event
is completely discarded in favour of a new kinematical setup.
Kindly note that the four-momenta of intermediate partons in the shower history are
not being adjusted. In a listing of the complete event history, energy and momentum
need then not be conserved in shower branchings. This mismatch could be fixed up, if309

need be.
The scheme presented above should not be taken too literally, but is rather intended
as a contrast to the more sophisticated schemes already on the market, if one would liketo understand whether the kind of conservation scheme chosen does affect the observable
physics.
10.3.5Matrix-element matching

In Pythia 6.1, matrix-element matching was introduced for the initial-state shower description of initial-state radiation in the production of a single colour-singlet resonance,such as γ ∗ /Z0 /W± [Miu99] (for Higgs production, see below). The basic idea is to map
the kinematics between the PS and ME descriptions, and to find a correction factor thatcan be applied to hard emissions in the shower so as to bring agreement with the matrixelement expression. The Pythia shower kinematics definitions are based on Q2 as thespace-like virtuality of the parton produced in a branching and z as the factor by which
the ŝ of the scattering subsystem is reduced by the branching. Some simple algebra thenshows that the two qq0 → gW± emission rates disagree by a factor
R

qq0 →gW(dσ̂/dt̂)MEt̂2 + û2 + 2m2W ŝ(ŝ, t̂) ==,ŝ2 + m4W
(dσ̂/dt̂)PS(197)

which is always between 1/2 and 1. The shower can therefore be improved in two ways,
relative to the old description. Firstly, the maximum virtuality of emissions is raised from
Q2max ≈ m2W to Q2max = s, i.e. the shower is allowed to populate the full phase space(referred to as a ‘power shower’ in [Ple05, Ska05]). Secondly, the emission rate for the
final (which normally also is the hardest) q → qg emission on each side is corrected by
the factor R(ŝ, t̂) above, so as to bring agreement with the matrix-element rate in thehard-emission region. In the backwards evolution shower algorithm [Sjö85], this is thefirst branching considered.
The other possible O(αs ) graph is qg → q0 W± , where the corresponding correction
factor is(dσ̂/dt̂)MEŝ2 + û2 + 2m2W t̂
Rqg→q0 W (ŝ, t̂) =
,
(198)
=
(ŝ − m2W )2 + m4W(dσ̂/dt̂)PS
which lies between 1 and 3. A probable reason for the lower shower rate here is that theshower does not explicitly simulate the s-channel graph qg → q∗ → q0 W. The g → qq
branching therefore has to be preweighted by a factor of 3 in the shower, but otherwise
the method works the same as above. Obviously, the shower will mix the two alternative
branchings, and the correction factor for a final branching is based on the current type.
The reweighting procedure prompts some other changes in the shower. In particular,û < 0 translates into a constraint on the phase space of allowed branchings, not implemented before Pythia 6.1. Here û = Q2 − ŝold (1 − z)/z = Q2 − ŝnew (1 − z), where theassociation with the û variable is relevant if the branching is reinterpreted in terms of a
2 → 2 scattering. Usually such a requirement comes out of the kinematics, and therefore
is imposed eventually anyway. The corner of emissions that do not respect this requirement is that where the Q2 value of the space-like emitting parton is little changed andthe z value of the branching is close to unity. (That is, such branchings are kinematicallyallowed, but since the mapping to matrix-element variables would assume the first parton
to have Q2 = 0, this mapping gives an unphysical û, and hence no possibility to impose amatrix-element correction factor.) The correct behaviour in this region is beyond leadinglog predictivity. It is mainly important for the hardest emission, i.e. with largest Q2 . The
effect of this change is to reduce the total amount of emission by a non-negligible amountwhen no matrix-element correction is applied. (This can be confirmed by using the special
option MSTP(68) = -1.) For matrix-element corrections to be applied, this requirement
310

must be used for the hardest branching, and then whether it is used or not for the softer
ones is less relevant.
Our published comparisons with data on the p⊥W spectrum show quite a good agreement with this improved simulation [Miu99]. A worry was that an unexpectedly large
primordial k⊥ , around 4 GeV, was required to match the data in the low-p⊥Z region.
However, at that time we had not realized that the data were not fully unsmeared. The
required primordial k⊥ therefore drops by about a factor of two [Bál01]. This number isstill uncomfortably large, but not too dissimilar from what is required in various resummation descriptions.
The method can also be used for initial-state photon emission, e.g. in the processe+ e− → γ ∗ /Z0 . There the old default Q2max = m2Z allowed no emission at large p⊥ ,>p⊥ ∼mZ at LEP2. This is now corrected by the increased Q2max = s, and using the R ofeq. (197) with mW → mZ .
The above method does not address the issue of next-to-leading order corrections to
the total W cross section. Rather, the implicit assumption is that such corrections, coming
mainly from soft- and virtual-gluon effects, largely factorize from the hard-emission effects.
That is, that the p⊥ shape obtained in our approach will be rather unaffected by nextto-leading order corrections (when used both for the total and the high-p⊥ cross section).
A rescaling by a common K factor could then be applied by hand at the end of theday. However, the issue is not clear. Alternative approaches have been proposed, wheremore sophisticated matching procedures are used also to get the next-to-leading ordercorrections to the cross section integrated into the shower formalism [Mre99].
A matching can also be attempted for other processes than the ones above. Currently
a matrix-element correction factor is also used for g → gg and q → gq branchings in the
gg → h0 process, in order to match on to the gg → gh0 and qg → qh0 matrix elements[Ell88]. The loop integrals of Higgs production are quite complex, however, and therefore
only the expressions obtained in the limit of a heavy top quark is used as a starting point
to define the ratios of gg → gh0 and qg → qh0 to gg → h0 cross sections. (Whereas
the gg → h0 cross section by itself contains the complete expressions.) In this limit, thecompact correction factors
Rgg→gh0 (ŝ, t̂) =(dσ̂/dt̂)MEŝ4 + t̂4 + û4 + m8h=2(ŝ2 − m2h (ŝ − m2h ))2(dσ̂/dt̂)PS(199)(dσ̂/dt̂)MEŝ2 + û2= 2ŝ + (ŝ − m2h )2(dσ̂/dt̂)PS

(200)

and
Rqg→qh0 (ŝ, t̂) =can be derived. Even though they are clearly not as reliable as the above expressions forγ ∗ /Z0 /W± , they should hopefully represent an improved description relative to having nocorrection factor at all. For this reason they are applied not only for the Standard Model
Higgs, but for all the three Higgs states h0 , H0 and A0 . The Higgs correction factors arealways in the comfortable range between 1/2 and 1.
Note that a third process, qq → gh0 does not fit into the pattern of the other two. The
above process cannot be viewed as a showering correction to a lowest-order qq → h0 one:since the q is assumed (essentially) massless there is no pointlike coupling. The graphabove instead again involved a top loop, coupled to the initial state by a single s-channel
gluon. The final-state gluon is necessary to balance colours in the process, and therefore
the cross section is vanishing in the p⊥ → 0 limit.
10.3.6

A new p⊥ -ordered initial-state shower

In parallel with the introduction of a new p⊥ -ordered final-state shower, also a new p⊥ ordered initial-state shower is introduced. Thus the old PYSSPA routine is complemented311

with the new PYPTIS one. The advantage of a p⊥ -ordered evolution becomes especiallyapparent when multiple interactions are considered in the next section, and transverse
momentum can be used as a common ordering variable for multiple interactions andinitial-state radiation, thereby allowing ‘interleaved evolution’. At the level of one single
interaction, much of the formalism closely resembles the virtuality-ordered one presented
above. In this section we therefore only give an overview of the new main features of thealgorithm, with further details found in [Sjö04a].
Also the new initial-state showers are constructed by backwards evolution, starting at
the hard interaction and successively reconstructing preceding branchings. To simplify
the merging with first-order matrix elements, z is again defined by the ratio of ŝ before andafter an emission. For a massless parton branching into one space-like with virtuality Q2and one with mass m, this gives p2⊥ = Q2 −z(ŝ+Q2 )(Q2 +m2 )/ŝ, or p2⊥ = (1−z)Q2 −zQ4 /ŝ
for m = 0. Here ŝ is the squared invariant mass after the emission, i.e. excluding theemitted on-mass-shell parton.
The last term, zQ4 /ŝ, while normally expected to be small, gives a nontrivial relationship between p2⊥ and Q2 , e.g. with two possible Q2 solutions for a given p2⊥ . The
second solution corresponds to a parton being emitted, at very large angles, in the ‘backwards’ direction, where emissions from the incoming parton on the other side of the event
should dominate. Based on such physics considerations, and in order to avoid the resulting technical problems, the evolution variable is picked to be p2⊥evol = (1 − z)Q2 .
Also here p⊥evol sets the scale for the running αs . Once selected, the p2⊥evol is translated
into an actual Q2 by the inverse relation Q2 = p2⊥evol /(1 − z), with trivial Jacobian:dQ2 /Q2 dz = dp2⊥evol /p2⊥evol dz. From Q2 the correct p2⊥ , including the zQ4 /ŝ term, canbe constructed.
Emissions on the two incoming sides are interspersed to form a single falling p⊥ sequence, p⊥max > p⊥1 > p⊥2 > . . . > p⊥min . That is, the p⊥ of the latest branchingconsidered sets the starting scale of the downwards evolution on both sides, with the next
branching occurring at the side that gives the largest such evolved p⊥ .
In a branching a → bc, the newly reconstructed mother a is assumed to have vanishingmass — a heavy quark would have to be virtual to exist inside a proton, so it makes
no sense to put it on mass shell. The previous mother b, which used to be massless,
now acquires the space-like virtuality Q2 and the correct p⊥ previously mentioned, andkinematics has to be adjusted accordingly.
In the old algorithm, the b kinematics was not constructed until its space-like virtuality
had been set, and so four-momentum was explicitly conserved at each shower branching.
In the new algorithm, this is no longer the case. (A corresponding change occurs between
the old and new time-like showers, as noted above.) Instead it is the set of partons
produced by this mother b and the current mother d on the other side of the event thatcollectively acquire the p⊥ of the new a → bc branching. Explicitly, when the b is pushed
off-shell, the d four-momentum is modified accordingly, such that their invariant mass isretained. Thereafter a set of rotations and boosts of the whole b + d-produced systembring them to the frame where b has the desired p⊥ and d is restored to its correctfour-momentum.
Matrix-element corrections can be applied to the first, i.e. hardest in p⊥ , branching on
both sides of the event, to improve the accuracy of the high-p⊥ description. Also several
other aspects are directly inherited from the old algorithm.
The evolution of massive quarks (charm and bottom) is more messy than in the timelike case. A sensible procedure has been worked out, however, including modified splitting
kernels and kinematics [Sjö04a].
In addition to the normal sharp cutoff at a p⊥min scale, a new option has been included
to smoothly regularise the divergence at around a given regularisation scale p⊥0 , i.e. similar
to what is being done for taming the rise of the multiple interactions cross section. The
physical motivation is that, since at zero momentum transfer the beam hadron behaves
312like a colour singlet object, then at low scales the effective coupling of partons to coloured
probes should be reduced, to take into account that this ‘colour screening’ is not included
at all in the lowest-order branching and interaction matrix elements.

10.4

Routines and Common-Block Variables

In this section we collect information on how to use the initial- and final-state showering
routines. Of these PYSHOW for final-state radiation is the more generally interesting, since
it can be called to let a user-defined parton configuration shower. The same applies forthe new PYPTFS routine. PYSSPA, on the other hand, is so intertwined with the general
structure of a Pythia event that it is of little use as a stand-alone product, and so should
only be accessed via PYEVNT. Similarly PYPTIS should not be called directly. Instead
PEVNW should be called, which in its turn calls PYEVOL for the interleaved evolution of
initial-state radiation with PYPTIS and multiple interactions with PYPTMI.
CALL PYSHOW(IP1,IP2,QMAX)
Purpose: to generate time-like parton showers, conventional or coherent. The performance of the program is regulated by the switches MSTJ(38) - MSTJ(50) andparameters PARJ(80) - PARJ(90). In order to keep track of the colour flow
information, the positions K(I,4) and K(I,5) have to be organized properly forshowering partons. Inside the Pythia programs, this is done automatically, but
for external use proper care must be taken.
IP1 > 0, IP2 = 0 : generate a time-like parton shower for the parton in line IP1 incommon block PYJETS, with maximum allowed mass QMAX. With only one parton
at hand, one cannot simultaneously conserve both energy and momentum: wehere choose to conserve energy and jet direction, while longitudinal momentum(along the jet axis) is not conserved.
IP1 > 0, IP2 > 0 : generate time-like parton showers for the two partons in lines IP1and IP2 in the common block PYJETS, with maximum allowed mass for eachparton QMAX. For shower evolution, the two partons are boosted to their c.m.
frame. Energy and momentum is conserved for the pair of partons, although notfor each individually. One of the two partons may be replaced by a nonradiating
particle, such as a photon or a diquark; the energy and momentum of this particle
will then be modified to conserve the total energy and momentum.
IP1 > 0, -80 ≤ IP2 < 0 : generate time-like parton showers for the -IP2 (at most80) partons in lines IP1, IP1+1, . . . IPI-IP2-1 in the common block PYJETS,
with maximum allowed mass for each parton QMAX. The actions for IP2 = -1
and IP2 = -2 correspond to what is described above, but additionally larger
numbers may be used to generate the evolution starting from three or more given
partons. Then the partons are boosted to their c.m. frame, the direction of the
momentum vector is conserved for each parton individually and energy for thesystem as a whole. It should be understood that the uncertainty in this option
is larger than for two-parton systems, and that a number of the sophisticatedfeatures (such as coherence with the incoming colour flow) are not implemented.
IP1 > 0, IP2 = -100 : generate a four-parton system, where a history starting fromtwo partons has already been constructed as discussed in section 10.2.7. Includingintermediate partons this requires 8 lines. This option is used in PY4JET, whereas
you would normally not want to use it directly yourself.
QMAX : the maximum allowed mass of a radiating parton, i.e. the starting value for thesubsequent evolution. (In addition, the mass of a single parton may not exceed
its energy, the mass of a parton in a system may not exceed the invariant mass313

of the system.)
CALL PYPTFS(NPART,IPART,PTMAX,PTMIN,PTGEN)
Purpose: to generate a p⊥ -ordered time-like final-state parton shower. The performanceof the program is regulated by the switches MSTJ(38), MSTJ(41), MSTJ(45),MSTJ(46) and MSTJ(47), and parameters PARJ(80) PARJ(81), PARJ(82),
PARJ(83) and PARJ(90), i.e. only a subset of the ones available with PYSHOW.
In order to keep track of the colour flow information, the positions K(I,4) andK(I,5) have to be organized properly for showering partons. Inside the Pythia
programs, this is done automatically, but for external use proper care must be
taken.
NPART : the number of partons in the system to be showered. Must be at least 2, andcan be as large as required (modulo the technical limit below). Is updated atreturn to be the new number of partons, after the shower.
IPART : array, of dimension 500 (cf. the Les Houches Accord for user processes), in which
the positions of the relevant partons are stored. To allow the identification of
matrix elements, (the showered copies of) the original resonance decay products,
if any, should be stored in the first two slots, IPART(1) and IPART(2). Is updated
at return to be the new list of partons, by adding new particles at the end. Thus,
for q → qg the quark position is updated and the gluon added, while for g → gg
and g → qq the choice of original and new is arbitrary.
PTMAX : upper scale of shower evolution. An absolute limit is set by kinematical constraints inside each ‘dipole’.
PTMIN : lower scale of shower evolution. For QCD evolution, an absolute lower limit is(3)
set by PARJ(82)/2 or 1.1 × ΛQCD , whichever is larger. For QED evolution, an
absolute lower limit is set by PARJ(83)/2 or PARJ(90)/2. Normally one would
therefore set PTMIN = 0D0 to run the shower to its intended lower cutoff.
PTGEN : returns the hardest p⊥ generated; if none then PTGEN = 0.
Note 1: the evolution is factorized, so that a set of successive calls, where the PTMIN
scale and the NPART and IPART output of one call becomes the PTMAX scale andthe NPART and IPART input of the next, gives the same result (on the average) as
one single call for the full p⊥ range. In particular, the IPART(1) and IPART(2)entries continue to point to (the showered copies of) the original decay productsof a resonance if they did so to begin with.
Note 2: in order to read a shower listing, note that each branching now lists three ‘new’
partons. The first two are the daughters of the branching, and point back to thebranching mother. The third is the recoiling parton after it has taken the recoil,and points back to itself before the recoil. The total energy and momentum isconserved from the mother and original recoil to the three new partons, but not
separately between the mother and its two daughters.
Note 3: the shower is not (yet) set up to allow showers with a fixed αs nor handle
radiation in baryon-number-violating decays. Neither is there any provision formodels with a scalar gluon or Abelian vector gluons.
Note 4: the PYPTFS can also be used as an integrated element of a normal Pythia
run, in places where PYSHOW would otherwise be used. This is achieved by setting MSTJ(41) = 11 or = 12. Then PYSHOW will call PYPTFS, provided that theshowering system consists of at least two partons and that the forced four-partonshower option is not used (IP2 = -100). PTMAX is then chosen to be half of the
QMAX scale of the PYSHOW call, and PTMIN is chosen to zero (which means the default lower limits will be used). This works nicely e.g. in e+ e− annihilation andfor resonance decays. Currently it is not so convenient for hadronic events: there314

is not yet a matching to avoid double-counting between initial- and final-stateradiation, and sidebranch time-like evolution in space-like showers is currently
handled by evolving one parton with PYSHOW, which PYPTFS is not set up for.
Note 5: for simplicity, all partons are evolved from a common PTMAX scale. The formalism easily accommodates separate PTMAX scales for each parton, but we haveavoided this for now so as not to complicate the routine unnecessarily for generaluse.
FUNCTION PYMAEL(NI,X1,X2,R1,R2,ALPHA)
Purpose: returns the ratio of the first-order gluon emission rate normalized to the lowestorder event rate, eq. (181). An overall factor CF αs /2π is omitted, since therunning of αs probably is done better in shower language anyway.
NI :code of the matrix element to be used, see Table 26. In each group of four codesin that table, the first is for the 1 case, the second for the γ5 one, the third foran arbitrary mixture, see ALPHA below, and the last for 1 ± γ5 .
X1, X2 : standard energy fractions of the two daughters.
R1, R2 : mass of the two daughters normalized to the mother mass.
ALPHA: fraction of the no-γ5 (i.e. vector/scalar/. . . ) part of the cross section; a freeparameter for the third matrix element option of each group in Table 26 (13, 18,
23, 28, . . . ).
SUBROUTINE PYADSH(NFIN)
Purpose: to administrate a sequence of final-state showers for external processes, where
the order normally is that all resonances have decayed before showers are considered, and therefore already existing daughters have to be boosted when theirmothers radiate or take the recoil from radiation.
NFIN : line in the event record of the last final-state entry to consider.
SUBROUTINE PYSSPA(IPU1,IPU2)
Purpose: to generate the space-like showers of the initial-state radiation in the ‘old’,
virtuality-ordered model. The performance of the program is regulated by theswitches MSTP(61) - MSTP(69) and parameters PARP(61) - PARP(68).
IPU1, IPU2 : positions of the two partons entering the hard scattering, from which the
backwards evolution is initiated.
SUBROUTINE PYPTIS(MODE,PT2NOW,PT2CUT,PT2,IFAIL)
Purpose: to generate the space-like showers of the initial-state radiation in the ‘new’,transverse-momentum-ordered model. The performance of the program is regulated by the switches MSTP(61), MSTP(62), MSTP(68), MSTP(69), MSTP(70) andMSTP(72), and parameters PARP(61), PARP(62) and PARP(64) i.e. only a subsetof the ones available with PYSSPA, but also with a few new extensions.
MODE : whether initialization (−1), trial emission (0) or kinematics of accepted branching(+1).
PT2NOW : starting (max) p2⊥ scale for evolution.
PT2CUT : lower limit for evolution.
PT2 : result of evolution. Generated p2⊥ for trial emission.
IFAIL : status return code. IFAIL = 0 when all is well.
315

Note:

a few non-standard options have not been implemented, such as evolution with
fixed αs .

SUBROUTINE PYMEMX(MECOR,WTFF,WTGF,WTFG,WTGG)
Purpose: to set the maximum of the ratio of the correct matrix element to the one
implied by the space-like parton shower.
MECOR : kind of hard-scattering process, 1 for f+f → γ ∗ /Z0 /W± / . . . vector gauge bosons,
2 for g + g → h0 /H0 /A0 .
WTFF, WTGF, WTFG, WTGG : maximum weights for f → f (+g/γ), g/γ → f (+f), f →g/γ (+f) and g → g (+g), respectively.
SUBROUTINE PYMEWT(MECOR,IFLCB,Q2,Z,PHIBR,WTME)
Purpose: to calculate the ratio of the correct matrix element to the one implied by the
space-like parton shower.
MECOR : kind of hard-scattering process, 1 for f+f → γ ∗ /Z0 /W± / . . . vector gauge bosons,
2 for g + g → h0 /H0 /A0 .
IFLCB : kind of branching, 1 for f → f (+g/γ), 2 for g/γ → f (+f), 3 for f → g/γ (+f)
and 4 for g → g (+g).
Q2, Z : Q2 and z values of shower branching under consideration.
PHIBR : ϕ azimuthal angle of the shower branching; may be overwritten inside routine.
WTME : calculated matrix element correction weight, used in the acceptance/rejection of
the shower branching under consideration.
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
Purpose: to give access to a number of status codes and parameters which regulate theperformance of Pythia. Most parameters are described in section 14.3; here
only those related to PYSHOW and PYPTFS are described.
MSTJ(38) : (D = 0) matrix element code NI for PYMAEL; as in MSTJ(47). If nonzero, theMSTJ(38) value overrides MSTJ(47), but is then set = 0 in the PYSHOW or PYPTFS
call. The usefulness of this switch lies in processes where sequential decays occur
and thus there are several showers, each requiring its matrix element. Therefore
MSTJ(38) can be set in the calling routine when it is known, and when not set
one defaults back to the attempted matching procedure of MSTJ(47) = 3 (e.g.).
MSTJ(40) : (D = 0) possibility to suppress the branching probability for a branchingq → qg (or q → qγ) of a quark produced in the decay of an unstable particle
with width Γ, where this width has to be specified by you in PARJ(89). The
algorithm used is not exact, but still gives some impression of potential effects.
This switch, valid for PYSHOW, ought to have appeared at the end of the current
list of shower switches (after MSTJ(50)), but because of lack of space it appears
immediately before.
= 0 : no suppression, i.e. the standard parton-shower machinery.
= 1 : suppress radiation by a factor χ(ω) = Γ2 /(Γ2 + ω 2 ), where ω is the energy ofthe gluon (or photon) in the rest frame of the radiating dipole. Essentiallythis means that hard radiation with ω > Γ is removed.
= 2 : suppress radiation by a factor 1 − χ(ω) = ω 2 /(Γ2 + ω 2 ), where ω is theenergy of the gluon (or photon) in the rest frame of the radiating dipole.
Essentially this means that soft radiation with ω < Γ is removed.

316

MSTJ(41) : (D = 2) type of branchings allowed in shower.
= 0 : no branchings at all, i.e. shower is switched off.
= 1 : QCD type branchings of quarks and gluons.
= 2 : also emission of photons off quarks and leptons; the photons are assumed on
the mass shell.
= 3 : QCD type branchings of quarks and gluons, and also emission of photons
off quarks, but leptons do not radiate (unlike = 2). Is not implemented for
PYPTFS.
= 10 : as = 2, but enhance photon emission by a factor PARJ(84). This option
is unphysical, but for moderate values, PARJ(84)≤ 10, it may be used toenhance the prompt photon signal in qq events. The normalization of theprompt photon rate should then be scaled down by the same factor. The
dangers of an improper use are significant, so do not use this option if you
do not know what you are doing. Is not implemented for PYPTFS.
= 11 : QCD type branchings of quarks and gluons, like = 1, but if PYSHOW is calledwith a parton system that PYPTFS can handle, the latter routine is called
to do the shower. If PYPTFS is called directly by the user, this option isequivalent to = 1.
= 12 : also emission of photons off quarks and leptons, like = 2, but if PYSHOW iscalled with a parton system that PYPTFS can handle, the latter routine iscalled to do the shower. If PYPTFS is called directly by the user, this option
is equivalent to = 2.
MSTJ(42) : (D = 2) branching mode, especially coherence level, for time-like showers in
PYSHOW.
= 1 : conventional branching, i.e. without angular ordering.
= 2 : coherent branching, i.e. with angular ordering.
= 3 : in a branching a → bg, where mb is nonvanishing, the decay angle is reduced by a factor (1 + (m2b /m2a )(1 − z)/z)−1 , thereby taking into accountmass effects in the decay [Nor01]. Therefore more branchings are acceptablefrom an angular ordering point of view. In the definition of the angle in
aqg → qq branchings, the naı̈ve massless expression is reduced by a factor
1 − 4m2q /m2g , which can be motivated by a corresponding actual reductionin the p⊥ by mass effects. The requirement of angular ordering then kills
fewer potential g → qq branchings, i.e. the rate of such comes up. Theg → gg branchings are not changed from = 2. This option is fully within
the range of uncertainty that exists.
= 4 : as = 3 for a → bg and g → gg branchings, but no angular ordering requirement conditions at all are imposed on g → qq branchings. This is an
unrealistic extreme, and results obtained with it should not be overstressed.
However, for some studies it is of interest. For instance, it not only givesa much higher rate of charm and bottom production in showers, but also
affects the kinematical distributions of such pairs.
= 5 : new ‘intermediate’ coherence level [Nor01], where the consecutive gluon
emissions off the original pair of branching partons is not constrained byangular ordering at all. The subsequent showering of such a gluon is angular ordered, however, starting from its production angle. At LEP energies,
this gives almost no change in the total parton multiplicity, but this multiplicity now increases somewhat faster with energy than before, in better
agreement with analytical formulae. (The PYSHOW algorithm overconstrains
the shower by ordering emissions in mass and then vetoing increasing angles.
This is a first simple attempt to redress the issue.) Other branchings as in= 2.
= 6 : ‘intermediate’ coherence level as = 5 for primary partons, unchanged for317

g → gg and reduced angle for g → qq and secondary q → qg as in = 3.
‘intermediate’ coherence level as = 5 for primary partons, unchanged forg → gg, reduced angle for secondary q → qg as in = 3 and no angularordering for g → qq as in = 4.
MSTJ(43) : (D = 4) choice of z definition in branchings in PYSHOW.
= 1 : energy fraction in grandmother’s rest frame (‘local, constrained’).
= 2 : energy fraction in grandmother’s rest frame assuming massless daughters,with energy and momentum reshuffled for massive ones (‘local, unconstrained’).
= 3 : energy fraction in c.m. frame of the showering partons (‘global, constrained’).
= 4 : energy fraction in c.m. frame of the showering partons assuming masslessdaughters, with energy and momentum reshuffled for massive ones (‘global,
unconstrained’).
MSTJ(44) : (D = 2) choice of αs scale for shower in PYSHOW.
= 0 : fixed at PARU(111) value.
= 1 : running with Q2 = m2 /4, m mass of decaying parton, Λ as stored in
PARJ(81) (natural choice for conventional showers).
= 2 : running with Q2 = z(1 − z)m2 , i.e. roughly p2⊥ of branching, Λ as stored in
PARJ(81) (natural choice for coherent showers).
= 3 : while p2⊥ is used as αs argument in q → qg and g → gg branchings, as in= 2, instead m2 /4 is used as argument for g → qq ones. The argument isthat the soft-gluon resummation results suggesting the p2⊥ scale [Ama80] in
the former processes is not valid for the latter one, so that any multiple ofthe mass of the branching parton is a perfectly valid alternative. The m2 /4ones then gives continuity with p2⊥ for z = 1/2. Furthermore, with thischoice, it is no longer necessary to have the requirement of a minimum p⊥ inbranchings, else required in order to avoid having αs blow up. Therefore, inthis option, that cut has been removed for g → qq branchings. Specifically,
when combined with MSTJ(42) = 4, it is possible to reproduce the simple
1 + cos2 θ angular distribution of g → qq branchings, which is not possible
in any other approach. (However it may give too high a charm and bottom
production rate in showers [Nor01].)= 4 : p2⊥ as in = 2, but scaled down by a factor (1 − m2b /m2a )2 for a branchinga → bg with b massive, in an attempt better to take into account the masseffect on kinematics.
= 5 : as for = 4 for q → qg, unchanged for g → gg and as = 3 for g → qq.
MSTJ(45) : (D = 5) maximum flavour that can be produced in shower by g → qq; also
used to determine the maximum number of active flavours in the αs factor inparton showers (here with a minimum of 3).
MSTJ(46) : (D = 3) nonhomogeneous azimuthal distributions in a shower branching.
= 0 : azimuthal angle is chosen uniformly.
= 1 : nonhomogeneous azimuthal angle in gluon decays due to a kinematicsdependent effective gluon polarization. Not meaningful for scalar model,
i.e. then same as = 0.
= 2 : nonhomogeneous azimuthal angle in gluon decay due to interference withnearest neighbour (in colour). Not meaningful for Abelian model, i.e. thensame as = 0.
= 3 : nonhomogeneous azimuthal angle in gluon decay due to both polarization(= 1) and interference (= 2). Not meaningful for Abelian model, i.e. thensame as = 1. Not meaningful for scalar model, i.e. then same as = 2.
Note : PYPTFS only implements nonhomogeneities related to the gluon spin, and so
options 0 and 2 are equivalent, as are 1 and 3.
= 7 :

318

MSTJ(47) : (D = 3) matrix-element-motivated corrections to the gluon shower emission
rate in generic processes of the type a → bcg. Also, in the massless fermion
approximation, with an imagined vector source, to the lowest-order qqγ, `+ `− γor `ν` γ matrix elements, i.e. more primitive than for QCD radiation.
= 0 : no corrections.
= 1 - 5 : yes; try to match to the most relevant matrix element and default back to
an assumed source (e.g. a vector for a qq pair) if the correct mother particle
cannot be found.
= 6 - : yes, match to the specific matrix element code NI = MSTJ(47) of the PYMAEL
function; see Table 26.
Warning : since a process may contain sequential decays involving several different kinds of matrix elements, it may be dangerous to fix MSTJ(47) to a
specialized value > 5; see MSTJ(38) above.
MSTJ(48) : (D = 0) possibility to impose maximum angle for the first branching in a
PYSHOW shower.
= 0 : no explicit maximum angle.
= 1 : maximum angle given by PARJ(85) for single showering parton, by PARJ(85)and PARJ(86) for pair of showering partons.
MSTJ(49) : (D = 0) possibility to change the branching probabilities in PYSHOW according to some alternative toy models (note that the Q2 evolution of αs may well
be different in these models, but that only the MSTJ(44) options are at yourdisposal).
= 0 : standard QCD branchings.
= 1 : branchings according to a scalar gluon theory, i.e. the splitting kernels in theevolution equations are, with a common factor αs /(2π) omitted, Pq→qg =(2/3)(1 − z), Pg→gg = PARJ(87), Pg→qq = PARJ(88) (for each separate
flavour). The couplings of the gluon have been left as free parameters,
since they depend on the colour structure assumed. Note that, since a
spin 0 object decays isotropically, the gluon splitting kernels contain no z
dependence.
= 2 : branchings according to an Abelian vector gluon theory, i.e. the colour
factors are changed (compared with QCD) according to CF = 4/3 → 1,
NC = 3 → 0, TR = 1/2 → 3. Note that an Abelian model is not expected
to contain any coherence effects between gluons, so that one should normally use MSTJ(42) = 1 and MSTJ(46) = 0 or 1. Also, αs is expected toincrease with increasing Q2 scale, rather than decrease. No such αs option
is available; the one that comes closest is MSTJ(44) = 0, i.e. a fix value.
MSTJ(50) : (D = 3) possibility to introduce colour coherence effects in the first branching of a PYSHOW final-state shower. Only relevant when colour flows throughfrom the initial to the final state, i.e. mainly for QCD parton–parton scattering
processes.
= 0 : none.
= 1 : impose an azimuthal anisotropy. Does not apply when the intermediate
state is a resonance, e.g., in a t → bW+ decay the radiation off the b quark
is not restricted.
= 2 : restrict the polar angle of a branching to be smaller than the scattering angleof the relevant colour flow. Does not apply when the intermediate state is aresonance.
= 3 : both azimuthal anisotropy and restricted polar angles. Does not apply when
the intermediate state is a resonance.
= 4 - 6 : as = 1 - 3, except that now also decay products of coloured resonances
are restricted in angle.
Note: for subsequent branchings the (polar) angular ordering is automatic
319

(MSTP(42) = 2) and MSTJ(46) = 3).
PARJ(80) : (D = 0.5) ‘parity’ mixing parameter, α value for the PYMAEL routine, to be
used when MSTJ(38) is nonvanishing.
PARJ(81) : (D = 0.29 GeV) Λ value in running αs for parton showers (see MSTJ(44)).
This is used in all user calls to PYSHOW, in the PYEEVT/PYONIA e+ e− routines, andin a resonance decay. It is not intended for other time-like showers, however, forwhich PARP(72) is used. This parameter ought to be reduced by about a factorof two for use with the PYPTFS routine.
PARJ(82) : (D = 1.0 GeV) invariant mass cut-off mmin of PYSHOW parton showers, below which partons are not assumed to radiate. For Q2 = p2⊥ (MSTJ(44) = 2)
PARJ(82)/2 additionally gives the minimum p⊥ of a branching. To avoid infinite
αs values, one must have PARJ(82)> 2×PARJ(81) for MSTJ(44) ≥ 1 (this isautomatically checked in the program, with 2.2×PARJ(81) as the lowest value
attainable). When the PYPTFS routine is called, it is twice the p⊥min cut.
PARJ(83) : (D = 1.0 GeV) invariant mass cut-off mmin used for photon emission in
PYSHOW parton showers, below which quarks are not assumed to radiate. The
function of PARJ(83) closely parallels that of PARJ(82) for QCD branchings, butthere is a priori no requirement that the two be equal. The cut-off for photon
emission off leptons is given by PARJ(90). When the PYPTFS routine is called, it
is twice the p⊥min cut.
PARJ(84) : (D = 1.) used for option MSTJ(41) = 10 as a multiplicative factor in theprompt photon emission rate in final-state parton showers. Unphysical but usefultechnical trick, so beware!
PARJ(85), PARJ(86) : (D = 10., 10.) maximum opening angles allowed in the firstbranching of parton showers; see MSTJ(48).
PARJ(87) : (D = 0.) coupling of g → gg in scalar gluon shower, see MSTJ(49) = 1.
PARJ(88) : (D = 0.) coupling of g → qq in scalar gluon shower (per quark species), see
MSTJ(49) = 1.
PARJ(89) : (D = 0. GeV) the width of the unstable particle studied for the MSTJ(40)> 0 options; to be set by you (separately for each PYSHOW call, if need be).
PARJ(90) : (D = 0.0001 GeV) invariant mass cut-off mmin used for photon emissionin PYSHOW parton showers, below which leptons are not assumed to radiate, cf.
PARJ(83) for radiation off quarks. When the PYPTFS routine is called, it is twice
the p⊥min cut. By making this separation of cut-off values, photon emission
off leptons becomes more realistic, covering a larger part of the phase space.
The emission rate is still not well reproduced for lepton–photon invariant masses
smaller than roughly twice the lepton mass itself.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
Purpose: to give access to status code and parameters which regulate the performanceof Pythia. Most parameters are described in section 9.3; here only those related
to PYSSPA/PYPTIS and PYSHOW/PYPTFS are described.
MSTP(22) : (D = 0) special override of normal Q2 definition used for maximum of partonshower evolution. This option only affects processes 10 and 83 (Deeply Inelastic
Scattering) and only in lepton–hadron events.
= 0 : use the scale as given in MSTP(32).
= 1 : use the DIS Q2 scale, i.e. −t̂.
= 2 : use the DIS W 2 scale, i.e. (−t̂)(1 −qx)/x.
= 3 : use the DIS Q × W scale, i.e. (−t̂) (1 − x)/x.
= 4 : use the scale Q2 (1 − x) max(1, ln(1/x)), as motivated by first-order matrix

320elements [Ing80, Alt78].
in all of these alternatives, a multiplicative factor is introduced by PARP(67)and PARP(71), as usual.
MSTP(61) : (D = 2) master switch for initial-state QCD and QED radiation.
= 0 : off.
= 1 : on for QCD radiation in hadronic events and QED radiation in leptonic
ones. (Not implemented for PYPTIS, equivalent to 2.).
= 2 : on for QCD and QED radiation in hadronic events and QED radiation in
leptonic ones.
MSTP(62) : (D = 3) level of coherence imposed on the space-like parton-shower evolution.
= 1 : none, i.e. neither Q2 values nor angles need be ordered in PYSSPA, while p2⊥
values are ordered in PYPTIS.
= 2 : Q2 values in PYSSPA and p2⊥ values in PYPTIS are strictly ordered, increasing
towards the hard interaction.
= 3 : Q2 /p2⊥ values and opening angles of emitted (on-mass-shell or time-like)partons are both strictly ordered, increasing towards the hard interaction.
MSTP(63) : (D = 2) structure of associated time-like showers, i.e. showers initiated byemission off the incoming space-like partons in PYSSPA.
= 0 : no associated showers are allowed, i.e. emitted partons are put on the mass
shell.
= 1 : a shower may evolve, with maximum allowed time-like virtuality set by thephase space only.
= 2 : a shower may evolve, with maximum allowed time-like virtuality set by phasespace or by PARP(71) times the Q2 value of the space-like parton created in
the same vertex, whichever is the stronger constraint.
= 3 : a shower may evolve, with maximum allowed time-like virtuality set by phasespace, but further constrained to evolve within a cone with opening angle(approximately) set by the opening angle of the branching where the showering parton was produced.
MSTP(64) : (D = 2) choice of αs and Q2 scale in space-like parton showers in PYSSPA.
= 0 : αs is taken to be fix at the value PARU(111).
= 1 : first-order running αs with argument PARP(63)Q2 .
2= 2 : first-order running αs with argument PARP(64)k⊥=PARP(64)(1 − z)Q2 .
MSTP(65) : (D = 1) treatment of soft-gluon emission in space-like parton-shower evolution in PYSSPA.
= 0 : soft gluons are entirely neglected.
= 1 : soft-gluon emission is resummed and included together with the hard radiation as an effective z shift.
MSTP(66) : (D = 5) choice of lower cut-off for initial-state QCD radiation in VMD oranomalous photoproduction events, and matching to primordial k⊥ .
= 0 : the lower Q2 cutoff is the standard one in PARP(62)2 .
= 1 : for anomalous photons, the lower Q2 cut-off is the larger of PARP(62)2 and
VINT(283) or VINT(284), where the latter is the virtuality scale for theγ → qq vertex on the appropriate side of the event. The VINT values are
selected logarithmically even between PARP(15)2 and the Q2 scale of theparton distributions of the hard process.
= 2 : extended option of the above, intended for virtual photons. For VMD pho2
scale oftons, the lower Q2 cut-off is the larger of PARP(62)2 and the Pint
the SaS parton distributions. For anomalous photons, the lower cut-off ischosen as for = 1, but the VINT(283) and VINT(284) are here selected log2
and the Q2 scale of the parton distributions
arithmically even between Pintof the hard process.
Note:

321

2
the k⊥ of the anomalous/GVMD component is distributed like 1/k⊥
between2
k0 and p⊥min (W ). Apart from the change of the upper limit, this option
works just like = 1.
22/(k⊥
+ Q2 /4)2 with k0 < k⊥ <= 4 : a stronger damping at large k⊥ , like dk⊥
2p⊥min (W ). Apart from this, it works like = 1.
= 5 : a k⊥ generated as in = 4 is added vectorially with a standard Gaussian k⊥generated like for VMD states. Ensures that GVMD has typical k⊥ ’s above
those of VMD, in spite of the large primordial k⊥ ’s implied by hadronic
physics. (Probably attributable to a lack of soft QCD radiation in parton
showers.)
MSTP(67) : (D = 2) possibility to introduce colour coherence effects in the first branching of the backwards evolution of an initial-state shower in PYSSPA; mainly of
relevance for QCD parton–parton scattering processes.
= 0 : none.
= 2 : restrict the polar angle of a branching to be smaller than the scattering angleof the relevant colour flow.
Note 1: azimuthal anisotropies have not yet been included.
Note 2: for subsequent branchings, MSTP(62) = 3 is used to restrict the (polar)
angular range of branchings.
MSTP(68) : (D = 3) choice of maximum virtuality scale and matrix-element matching
scheme for initial-state radiation. To this end, the basic scattering processes areclassified as belonging to one or several of the following categories (hard-coded
for each process):
ISQCD = 1 : QCD processes, i.e. processes for which the hard scattering scale should
normally set the limit for subsequent multiple interactions. Consists of processes 11, 12, 13, 28, 53 and 68.
ISQCD = 0 : Other processes. Multiple interactions normally allowed to populate
full phase space.
ISJETS = 1 : Processes of the X+jet type, i.e. processes for which the matrix element already contains one radiated jet. For such processes, as well as for
QCD processes, the scale of the already existing jet(s) should set the limit
for further parton-shower evolution.
ISJETS = 0 : Processes which do not contain parton-shower jets at leading order.
ISMECR = 1 : Processes for which matrix element merging to the X+jet rate havebeen implemented. This list contains the processes 1, 2, 141, 142, 144, 102,
152 and 157, i.e. single s-channel colourless gauge boson and Higgs production: γ ∗ /Z0 , W± , Z0 0 , W0 ± , R, h0 , H0 and H± . Here the maximum scale ofshower evolution is s, the total squared energy. The nearest branching oneither side of the hard scattering is corrected by the ratio of the first-ordermatrix-element weight to the parton-shower one, so as to obtain an improved
description. For gauge boson production, this branching can be of the types
q → q + g, f → f + γ, g → q + q or γ → f + f, while for Higgs production
it is g → g + g. See section 10.3.5 for a detailed description. Note that the
improvements apply both for incoming hadron and lepton beams.
ISMECR = 0 : Processes for which no such corrections are implemented.
Given this information, the following options are available:= 0 : maximum shower virtuality is the same as the Q2 choice for the parton
distributions, see MSTP(32). (Except that the multiplicative extra factor
PARP(34) is absent and instead PARP(67) can be used for this purpose.) No
matrix-element correction.
= 1 : as = 0 for most processes, but for processes of the ISMECR = 1 type themaximum evolution scale is the full CM energy, and ME corrections areapplied where available.

= 3 :

322

= 2 :

as = 0 for most processes, but for processes of the ISQCD = 0 and ISJETS= 0 types the maximum evolution scale is the full CM energy. No ME
corrections are applied.
= 3 : as = 2, but ME corrections are applied where available.
= -1 : as = 0, except that there is no requirement on û being negative. (Onlyapplies to the old PYSSPA shower.)
MSTP(69) : (D = 0) possibility to change Q2 scale for parton distributions from theMSTP(32) choice, especially for e+ e− .
= 0 : use MSTP(32) scale.
= 1 : in lepton-lepton collisions, the QED lepton-inside-lepton parton distributions are evaluated with s, the full squared c.m. energy, as scale.
= 2 : s is used as parton distribution scale also in other processes.
MSTP(70) : (D = 1) regularization scheme for ISR radiation when p⊥ → 0 in the newp⊥ -ordered evolution in PYPTIS.
= 0 : sharp cut-off at p⊥min =PARP(62)/2.
= 1 : sharp cut-off at p⊥min =PARP(81), rescaled with energy, the same as thep⊥min scale used for multiple interactions when MSTP(82) = 1.
= 2 : a smooth turnoff at p⊥0 =PARP(82), rescaled with energy, the same as
the p⊥0 scale used for multiple interactions when MSTP(82) > 1. Thus
dp2⊥ /p2⊥ → dp2⊥ /(p2⊥ + p2⊥0 ) and αs (p2⊥ ) → αs (p2⊥ + p2⊥0 ). Note that, eventhough one could in principle allow branching down to vanishing p⊥ thisway (with a highly suppressed rate), the algorithm is nonetheless forced tostop once the evolution has reached a scale equal to 1.1 times the 3-flavour
ΛQCD .
MSTP(71) : (D = 1) master switch for final-state QCD and QED radiation.
= 0 : off.
= 1 : on.
MSTP(72) : (D = 1) maximum scale for radiation off FSR dipoles stretched between ISR
partons in the new p⊥ -ordered evolution in PYPTIS.
= 0 : the p⊥max scale of FSR is set as the minimum of the p⊥ production scale ofthe two endpoint partons. Dipoles stretched to remnants do not radiate.
= 1 : the p⊥max scale of FSR is set as the p⊥ production scale of the respective
radiating parton. Dipoles stretched to remnants do not radiate.
= 2 : the p⊥max scale of FSR is set as the p⊥ production scale of the respective
radiating parton. Dipoles stretched to remnants can radiate (by emissions
off the perturbative-parton side, not the remnant one).
PARP(61) : (D = 0.25 GeV) Λ value used in space-like parton shower (see MSTP(64)).
This value may be overwritten, see MSTP(3).
PARP(62) : (D = 1. GeV) effective cut-off Q or k⊥ value (see MSTP(64)), below whichspace-like parton showers are not evolved. Primarily intended for QCD showers
in incoming hadrons, but also applied to q → qγ branchings.
PARP(63) : (D = 0.25) in space-like shower evolution the virtuality Q2 of a parton ismultiplied by PARP(63) for use as a scale in αs and parton distributions when
MSTP(64) = 1.
PARP(64) : (D = 1.) in space-like parton-shower evolution the squared transverse mo2
is multiplied by PARP(64) for use as a scale in αs andmentum evolution scale k⊥parton distributions when MSTP(64) = 2.
PARP(65) : (D = 2. GeV) effective minimum energy (in c.m. frame) of time-like or onshell parton emitted in space-like shower; see also PARP(66). For a hard subprocess moving in the rest frame of the hard process, this number is reduced roughly
by a factor 1/γ for the boost to the hard-scattering rest frame.
PARP(66) : (D = 0.001) effective lower cut-off on 1 − z in space-like showers, in addition323

to the cut implied by PARP(65).
PARP(67) : (D = 4.) the Q2 scale of the hard scattering (see MSTP(32)) is multiplied by
PARP(67) to define the maximum parton virtuality allowed in Q2 -ordered spacelike showers. This does not apply to s-channel resonances, where the m aximum
virtuality is set by m2 . It does apply to all user-defined processes,however. The
current default is based on Tevatron studies (see e.g. [Fie02]), while argumentsfrom a matching of scales in heavy-flavour production [Nor98] might suggest
unity. The range 1–4 should be considered free for variations.
PARP(68) : (D = 0.001 GeV) lower Q cut-off for QED space-like showers. Comes in
addition to a hardcoded cut that the Q2 is at least 2m2e , 2m2µ or 2m2τ , as the casemay be.
PARP(71) : (D = 4.) the Q2 scale of the hard scattering (see MSTP(32)) is multiplied by
PARP(71) to define the maximum parton virtuality allowed in time-like showers.
This does not apply to s-channel resonances, where the maximum virtuality is
set by m2 . Like for PARP(67) this number is uncertain.
PARP(72) : (D = 0.25 GeV) Λ value used in running αs for time-like parton showers,
except for showers in the decay of a resonance. (Resonance decay, e.g. γ ∗ /Z 0
decay, is instead set by PARJ(81).)
COMMON/PYPART/NPART,NPARTD,IPART(MAXNUP),PTPART(MAXNUP)
Purpose: to keep track of partons that can radiate in final-state showers.
NPART : the number of partons that may radiate, determining how much of IPART and
PTPART is currently in use.
NPARTD : dummy, to avoid some compiler warnings.
IPART : the line number in /PYJETS/ in which a radiating parton is stored.
PTPART : the p⊥ scale of the parton, from which it is to be evolved downwards in searchof a first branching.

324

11

Beam Remnants and Underlying Events

Each incoming beam particle may leave behind a beam remnant, which does not takeactive part in the initial-state radiation or hard-scattering process. If nothing else, these
remnants need to be put together and colour connected to the rest of the event. In addition, in hadron–hadron collisions, the composite nature of the two incoming hadrons
implies the possibility that several pairs of partons can enter into separate but simultaneous scatterings, ‘multiple interactions’. In some fraction of events, these additional
scatterings can be hard or semi-hard, but due to the infrared peaking of the cross section
the bulk of them should normally be fairly soft compared to the primary interaction. This
extra component gives a non-negligible contribution to the ‘underlying event’ structure,and thus to the total multiplicity. Finally, in high-luminosity colliders, it is possible tohave several collisions between beam particles in one and the same beam crossing, i.e.
pile-up events, which further act to build up the general particle production activity thatis to be observed by detectors. These three aspects are described in turn, with emphasison the middle one, that of multiple interactions within a single hadron–hadron collision.
Work is ongoing to improve the theoretical framework for multiple interactions.
Pythia therefore contains two partly separate approaches to this physics, which also
leads to two partly separate descriptions of beam remnants. To make matters worse,from a pedagogical point of view, also the description of initial- and final-state radiation
has been improved, with the transition from virtuality-ordered to transverse-momentumordered evolution, and with the interleaving of multiple interactions and initial-state emissions in one common p⊥ -ordered sequence. In total we therefore need to distinguish threemain scenarios.
• The ‘old model’, based on [Sjö87a], contains the old beam-remnant and showeringmachineries. It remains the default while the new one is being developed, for backwards reference and comparisons. For instance, the ‘Tune A’ [Fie02] incarnationof this model is commonly used at the Tevatron, and offers a convenient reference
against which other models can be compared, without the need to know how to doa full detector simulation. Furthermore, the newer models below are mainly being
developed for pp and pp collisions so far, so have barely been tested with mesonand fixed-energy resolved-photon beams, and is not at all integrated into a complete consistent set of photon interactions for a spectum of incoming energies andvirtualities.
• The ‘intermediate model’, developed in [Sjö04], attempts a more sophisticated description of correlations in flavour, colour, longitudinal and (primordial) transverse
momenta between the beam remnants and the shower initiators than offered by
the old one, and also introduces some other improvements. The virtuality-orderedshowers are the same as in the old model, but now each interaction can be associated with its own showering activity, while before only the hardest interaction
would be showered, for technical reasons. In practice, we do not expect much usageof the intermediate model: either people stay with the old or move right to the new
one below. We have kept it mainly so that results from our publications can be
reproduced.
• The ‘new model’, with further aspects described in [Sjö04a], where transversemomentum-ordered showers are introduced and multiple interactions and initialstate radiation are interleaved. The beam-remnant description introduced in theintermediate model stays essentially unchanged.
Several of the key multiple interactions aspects are common between the models, suchas the generation of kinematics of the multiple (semi)hard interactions and the impactparameter picture. Other aspects are addressed in a more or less similar spirit, but indifferent ways, such as the desire to reduce the string length relative to naı̈ve expectations.
Therefore the current section starts with a description of the old model, and thereafter325

outlines where the intermediate and new models differ.

11.1

Beam Remnants — Old Model

The initial-state radiation algorithm reconstructs one shower initiator in each beam. (Ifinitial-state radiation is not included, the initiator is nothing but the incoming parton
to the hard interaction.) Together the two initiators delineate an interaction subsystem,
which contains all the partons that participate in the initial-state showers, in the hardinteraction, and in the final-state showers. Left behind are two beam remnants which,to first approximation, just sail through, unaffected by the hard process. (The issue ofadditional interactions is covered in the next section.)
A description of the beam-remnant structure contains a few components. First, given
the flavour content of a (colour-singlet) beam particle, and the flavour and colour of theinitiator parton, it is possible to reconstruct the flavour and colour of the beam remnant.
Sometimes the remnant may be represented by just a single parton or diquark, but often
the remnant has to be subdivided into two separate objects. In the latter case it isnecessary to share the remnant energy and momentum between the two. Due to Fermi
motion inside hadron beams, the initiator parton may have a ‘primordial k⊥ ’ transverse
momentum motion, which has to be compensated by the beam remnant. If the remnant
is subdivided, there may also be a relative transverse momentum. In the end, total energy
and momentum has to be conserved. To first approximation, this is ensured within eachremnant separately, but some final global adjustments are necessary to compensate forthe primordial k⊥ and any effective beam-remnant mass.
11.1.1

Hadron Beams

Consider first a proton (or, with trivial modifications, any other baryon or antibaryon).
• If the initiator parton is a u or d quark, it is assumed to be a valence quark, andtherefore leaves behind a diquark beam remnant, i.e. either a ud or a uu diquark,in a colour antitriplet state. Relative probabilities for different diquark spins arederived within the context of the non-relativistic SU(6) model, i.e. flavour SU(3)times spin SU(2). Thus a ud is 3/4 ud0 and 1/4 ud1 , while a uu is always uu1 .
• An initiator gluon leaves behind a colour octet uud state, which is subdivided into acolour triplet quark and a colour antitriplet diquark. SU(6) gives the appropriate
subdivision, 1/2 of the time into u + ud0 , 1/6 into u + ud1 and 1/3 into d + uu1 .
• A sea quark initiator, such as an s, leaves behind a uuds four-quark state. The PDG
flavour coding scheme and the fragmentation routines do not foresee such a state, so
therefore it is subdivided into a meson plus a diquark, i.e. 1/2 into us+ud0 , 1/6 into
us + ud1 and 1/3 into ds + uu1 . Once the flavours of the meson are determined, thechoice of meson multiplet is performed as in the standard fragmentation description.
• Finally, an antiquark initiator, such as an s, leaves behind a uuds four-quark state,
which is subdivided into a baryon plus a quark. Since, to first approximation, thess pair comes from the branching g → ss of a colour octet gluon, the subdivision
uud + s is not allowed, since it would correspond to a colour-singlet ss. Therefore
the subdivision is 1/2 into ud0 s + u, 1/6 into ud1 s + u and 1/3 into uu1 s + d. A
baryon is formed among the ones possible for the given flavour content and diquark
spin, according to the relative probabilities used in the fragmentation. One could
argue for an additional weighting to count the number of baryon states available fora given diquark plus quark combination, but this has not been included.
One may note that any u or d quark taken out of the proton is automatically assumed
to be a valence quark. Clearly this is unrealistic, but not quite as bad as it might seem.
In particular, one should remember that the beam-remnant scenario is applied to theinitial-state shower initiators at a scale of Q0 ≈ 1 GeV and at an x value usually much326

larger than the x at the hard scattering. The sea quark contribution therefore normallyis small.
For a meson beam remnant, the rules are in the same spirit, but somewhat easier, sinceno diquark or baryons need be taken into account. Thus a valence quark (antiquark)
initiator leaves behind a valence antiquark (quark), a gluon initiator leaves behind avalence quark plus a valence antiquark, and a sea quark (antiquark) leaves behind ameson (which contains the partner to the sea parton) plus a valence antiquark (quark).
11.1.2

Photon Beams

A resolved photon is similar in spirit to a meson. A VMD photon is associated with eitherρ0 , ω, φ or J/ψ, and so corresponds to a well-defined valence flavour content. Since the ρ0and ω are supposed to add coherently, the uu : dd mixing is in the ratio 4 : 1. Similarly
a GVMD state is characterized by its qq classification, in rates according to e2q times amass suppression for heavier quarks.
In the older photon physics options, where a quark content inside an electron is obtained by a numerical convolution, one does not have to make the distinction between
valence and sea flavour. Thus any quark (antiquark) initiator leaves behind the matching
antiquark (quark), and a gluon leaves behind a quark + antiquark pair. The relative
quark flavour composition in the latter case is assumed proportional to e2q among lightflavours, i.e. 2/3 into u + u, 1/6 into d + d, and 1/6 into s + s. If one wanted to, one could
also have chosen to represent the remnant by a single gluon.
11.1.3

Lepton Beams

If no initial-state radiation is assumed, an electron (or, in general, a lepton or a neutrino)
leaves behind no beam remnant. Also when radiation is included, one would expect torecover a single electron with the full beam energy when the shower initiator is reconstructed. This does not have to happen, e.g. if the initial-state shower is cut off at a
non-vanishing scale, such that some of the emission at low Q2 values is not simulated.
Further, for purely technical reasons, the distribution of an electron inside an electron,fee (x, Q2 ), is cut off at x = 1 − 10−10 . This means that always, when initial-state radiation
is included, a fraction of at least 10−10 of the beam energy has to be put into one single
photon along the beam direction, to represent this not simulated radiation. The physics
is here slightly different from the standard beam-remnant concept, but it is handled withthe same machinery. Beam remnants can also appear when the electron is resolved withthe use of parton distributions, but initial-state radiation is switched off. Conceptually,
this is a contradiction, since it is the initial-state radiation that builds up the parton
distributions, but sometimes the combination is still useful. Finally, since QED radiation
has not yet been included in events with resolved photons inside electrons, also in thiscase effective beam remnants have to be assigned by the program.
The beam-remnant assignments inside an electron, in either of the cases above, is asfollows.
• An e− initiator leaves behind a γ remnant.
• A γ initiator leaves behind an e− remnant.
• An e+ initiator leaves behind an e− + e− remnant.
• A q (q) initiator leaves behind a q + e− (q + e− ) remnant.
• A g initiator leaves behind a g + e− remnant. One could argue that, in agreementwith the treatment of photon beams above, the remnant should be q + q + e− . The
program currently does not allow for three beam-remnant objects, however.

327

11.1.4

Primordial k⊥

It is customary to assign a primordial transverse momentum to the shower initiator, totake into account the motion of quarks inside the original hadron, basically as required by
the uncertainty principle. A number of the order of hk⊥ i ≈ mp /3 ≈ 300 MeV could therefore be expected. However, in hadronic collisions much higher numbers than that are often
required to describe data, typically of the order of 1 GeV at fixed-target energies[EMC87]
and 2 GeV at collider energies [Miu99, Bál01], if a Gaussian parameterization is used.
Thus, an interpretation as a purely nonperturbative motion inside a hadron is difficult tomaintain.
Instead a likely culprit is the initial-state shower algorithm. This is set up to cover theregion of hard emissions, but may miss out on some of the softer activity, which inherently
borders on nonperturbative physics. By default, the shower does not evolve down to scales
below Q0 = 1 GeV. Any shortfall in shower activity around or below this cutoff then has
to be compensated by the primordial k⊥ source, which thereby largely loses its original
meaning. One specific reason for such a shortfall is that the current initial-state showeralgorithm does not include non-ordered emissions in Q2 , as is predicted to occur especiallyat small x and Q2 within the BFKL/CCFM framework [Lip76, Cia87].
11.1.5

Remnant Kinematics

By the hard scattering and the initial-state radiation machinery, the shower initiator has
been assigned some fraction x of the four-momentum of the beam particle, leaving behind
1 − x to the remnant. If the remnant consists of two objects, this energy and momentum
has to be shared, somehow. For an electron in the old photoproduction machinery, thesharing is given from first principles: if, e.g., the initiator is a q, then that q was producedin the sequence of branchings e → γ → q, where xγ is distributed according to the
convolution in eq. (54). Therefore the q remnant takes a fraction χ = (xγ − x)/(1 − x) of
the total remnant energy, and the e takes 1 − χ.
For the other beam remnants, the relative energy-sharing variable χ is not known
from first principles, but picked according to some suitable parameterization. Normallyseveral different options are available, that can be set separately for baryon and meson
beams, and for hadron + quark and quark + diquark (or antiquark) remnants. In one
extreme are shapes in agreement with naı̈ve counting rules, i.e. where energy is shared
evenly between ‘valence’ partons. For instance, P(χ) = 2 (1 − χ) for the energy fraction
taken by the q in a q + qq remnant. In the other extreme, an uneven distribution could be
used, like in parton distributions, where the quark only takes a small fraction and mostis retained by the diquark. The default for a q + qq remnant is of an intermediate type,(1 − χ)3,
P(χ) ∝ q4
χ2 + c2min(201)

with cmin = 2hmq i/Ecm = (0.6 GeV)/Ecm providing a lower cut-off. The default when ahadron is split off to leave a quark or diquark remnant is to use the standard Lund symmetric fragmentation function. In general, the more uneven the sharing of the energy, theless the total multiplicity in the beam-remnant fragmentation. If no multiple interactions
are allowed, a rather even sharing is needed to come close to the experimental multiplicity(and yet one does not quite make it). With an uneven sharing there is room to generate
more of the total multiplicity by multiple interactions [Sjö87a].
In a photon beam, with a remnant q + q, the χ variable is chosen the same way it
would have been in a corresponding meson remnant.
Before the χ variable is used to assign remnant momenta, it is also necessary toconsider the issue of primordial k⊥ . The initiator partons are thus assigned each a k⊥
328

value, vanishing for an electron or photon inside an electron, distributed either according
to a Gaussian or an exponential shape for a hadron, and according to either of theseshapes or a power-like shape for a quark or gluon inside a photon (which may in its turnbe inside an electron). The interaction subsystem is boosted and rotated to bring it fromthe frame assumed so far, with each initiator along the ±z axis, to one where the initiators
have the required primordial k⊥ values.
The p⊥ recoil is taken by the remnant. If the remnant is composite, the recoil isevenly split between the two. In addition, however, the two beam remnants may be
given a relative p⊥ , which is then always chosen as for qi qi pairs in the fragmentation
description.
The χ variable is interpreted as a sharing of light-cone energy and momentum, i.e.
E + pz for the beam moving in the +z direction and E − pz for the other one. When the
two transverse masses m⊥1 and m⊥2 of a composite remnant have been constructed, thetotal transverse mass can therefore be found as
m2⊥ =

m2⊥1
m2
+ ⊥2 ,
χ
1−χ

(202)

if remnant 1 is the one that takes the fraction χ. The choice of a light-cone interpretationto χ means the definition is invariant under longitudinal boosts, and therefore does notdepend on the beam energy itself. A χ value close to the naı̈ve borders 0 or 1 can leadto an unreasonably large remnant m⊥ . Therefore an additional check is introduced, thatthe remnant m⊥ be smaller than the naı̈ve c.m. frame remnant energy, (1 − x)Ecm /2. If
this is not the case, a new χ and a new relative transverse momentum is selected.
Whether there is one remnant parton or two, the transverse mass of the remnant is
not likely to agree with 1 − x times the mass of the beam particle, i.e. it is not going to bepossible to preserve the energy and momentum in each remnant separately. One therefore
allows a shuffling of energy and momentum between the beam remnants from each of thetwo incoming beams. This may be achieved by performing a (small) longitudinal boost ofeach remnant system. Since there are two boost degrees of freedom, one for each remnant,and two constraints, one for energy and one for longitudinal momentum, a solution may
be found.
Under some circumstances, one beam remnant may be absent or of very low energy,
while the other one is more complicated. One example is Deeply Inelastic Scattering in ep
collisions, where the electron leaves no remnant, or maybe only a low-energy photon. It isclearly then not possible to balance the two beam remnants against each other. Therefore,
if one beam remnant has an energy below 0.2 of the beam energy, i.e. if the initiator parton
has x > 0.8, then the two boosts needed to ensure energy and momentum conservation
are instead performed on the other remnant and on the interaction subsystem. If there isa low-energy remnant at all then, before that, energy and momentum are assigned to theremnant constituent(s) so that the appropriate light-cone combination E ±pz is conserved,
but not energy or momentum separately. If both beam remnants have low energy, butboth still exist, then the one with lower m⊥ /E is the one that will not be boosted.

11.2

Multiple Interactions — Old ModelIn this section we present the original model [Sjö87a] to describe the possibility that several parton pairs undergo hard interactions in a hadron–hadron collision, and thereby
contribute to the overall event activity, in particular at low p⊥ . The same model can also
be used to describe the VMD γp events, where the photon interacts like a hadron. Manybasic features of this model, for instance the introduction of a p⊥ cutoff corresponding toan inverse colour screening distance, and the options for a non-trivial transverse density
structure in the incoming hadrons, carry over to the new scenario. It is therefore recom329

mended first to read this section, even if the objective should be to learn about the new
scenario.
It should from the onset be made clear that this is not an easy topic. In fact, in thefull event generation process, probably no other area is as poorly understood as this one.
The whole concept of multiple interactions has been very controversial, with contradictory
experimental conclusions [AFS87], but a CDF study [CDF97] some years ago started tobring more general acceptance, further accelerated by the underlying-event studies of R.D.
Field [Fie02].
The multiple interactions scenario presented here [Sjö87a] was the first detailed modelfor this kind of physics, and is still one of the very few available. We will present two
related but separate scenarios, one ‘simple’ model and one somewhat more sophisticated.
In fact, neither of them are all that simple, which may make the models look unattractive.
However, the world of hadron physics is complicated, and if we err, it is most likely inbeing too unsophisticated. The experience gained with the model(s), in failures as well as
successes, could be used as a guideline in the evolution of yet more detailed and accurate
models.
Our basic philosophy will be as follows. The total rate of parton–parton interactions,as a function of the transverse momentum scale p⊥ , is assumed to be given by perturbative
QCD. This is certainly true for reasonably large p⊥ values, but here we shall also extend
the perturbative parton–parton scattering framework into the low-p⊥ region. A regularization of the divergence in the cross section for p⊥ → 0 has to be introduced, however,which will provide us with the main free parameter of the model. Since each incoming
hadron is a composite object, consisting of many partons, there should exist the possibility of several parton pairs interacting when two hadrons collide. It is not unreasonable
to assume that the different pairwise interactions take place essentially independently ofeach other, and that therefore the number of interactions in an event is given by a Poisson
distribution. This is the strategy of the ‘simple’ scenario.
Furthermore, hadrons are not only composite but also extended objects, meaning thatcollisions range from very central to rather peripheral ones. Reasonably, the average number of interactions should be larger in the former than in the latter case. Whereas theassumption of a Poisson distribution should hold for each impact parameter separately,
the distribution in number of interactions should be widened by the spread of impact parameters. The amount of widening depends on the assumed matter distribution inside thecolliding hadrons. In the ‘complex’ scenario, different matter distributions are therefore
introduced.
11.2.1The basic cross sections

The QCD cross section for hard 2 → 2 processes, as a function of the p2⊥ scale, is given
by
Ã
!
Z
Z
k
XZ
dσ
t̂û
2
2 dσ̂ij2
dx1 dx2 dt̂ fi (x1 , Q ) fj (x2 , Q )=,δ p⊥ −(203)
dp2⊥ i,j,kŝ
dt̂
cf. section 7.2. Implicitly, from now on we are assuming that the ‘hardness’ of processes
is given by the p⊥ scale of the scattering. For an application of the formula above to smallp⊥ values, a number of caveats could be made. At low p⊥ , the integrals receive major
contributions from the small-x region, where parton distributions are poorly understoodtheoretically (Regge-limit behaviour, dense-packing problems etc. [Lev90]) and not yetmeasured. Different sets of parton distributions can therefore give numerically rather
different results for the phenomenology of interest. One may also worry about higherorder corrections to the jet rates, K factors, beyond what is given by parton-shower
corrections — one simple option we allow here is to evaluate αs of the hard-scattering
process at an optimized scale, such as αs (0.075p2⊥ ) [Ell86].
330

The hard-scattering cross section above some given p⊥min is given by
σhard (p⊥min ) =Z s/4
p2⊥mindσ
dp2⊥ .
2
dp⊥

(204)

Since the differential cross section diverges roughly like dp2⊥ /p4⊥ , σhard is also divergent forp⊥min → 0. We may compare this with the total inelastic, non-diffractive cross section
σnd (s) — elastic and diffractive events are not the topic of this section. At current collider
energies σhard (p⊥min ) becomes comparable with σnd for p⊥min ≈ 2–3 GeV, and at larger
energies this occurs at even larger p⊥min . This need not lead to contradictions: σhard
does not give the hadron–hadron cross section but the parton–parton one. Each of theincoming hadrons may be viewed as a beam of partons, with the possibility of having
several parton–parton interactions when the hadrons pass through each other. In thislanguage, σhard (p⊥min )/σnd (s) is simply the average number of parton–parton scatterings
above p⊥min in an event, and this number may well be larger than unity.
While the introduction of several interactions per event is the natural consequence ofallowing small p⊥min values and hence large σhard ones, it is not the solution of σhard (p⊥min )being divergent for p⊥min → 0: the average ŝ of a scattering decreases slower with p⊥minthan the number of interactions increases, so naı̈vely the total amount of scattered partonic
energy becomes infinite. One cut-off is therefore obtained via the need to introduce proper
multi-parton correlated parton distributions inside a hadron. This is not a part of thestandard perturbative QCD formalism and is therefore not built into eq. (204). In practice,even correlated parton-distribution functions seems to provide too weak a cut, i.e. oneis lead to a picture with too little of the incoming energy remaining in the small-angle
beam-jet region [Sjö87a].
A more credible reason for an effective cut-off is that the incoming hadrons are colour
neutral objects. Therefore, when the p⊥ of an exchanged gluon is made small and thetransverse wavelength correspondingly large, the gluon can no longer resolve the individual colour charges, and the effective coupling is decreased. This mechanism is not incontradiction with perturbative QCD calculations, which are always performed assuming
scattering of free partons (rather than partons inside hadrons), but neither does present
knowledge of QCD provide an understanding of how such a decoupling mechanism would
work in detail. In the simple model one makes use of a sharp cut-off at some scale p⊥min ,while a more smooth dampening is assumed for the complex scenario.
One key question is the energy-dependence of p⊥min ; this may be relevant e.g. forcomparisons of jet rates at different Tevatron/RHIC energies, and even more for any
extrapolation to LHC energies. The problem actually is more pressing now than at thetime of the original study [Sjö87a], since nowadays parton distributions are known tobe rising more steeply at small x than the flat xf (x) behaviour normally assumed forsmall Q2 before HERA. This translates into a more dramatic energy dependence of themultiple-interactions rate for a fixed p⊥min .
The larger number of partons should also increase the amount of screening, however, asconfirmed by toy simulations [Dis01]. As a simple first approximation, p⊥min is assumed toincrease in the same way as the total cross section, i.e. with some power ² ≈ 0.08 [Don92]
that, via reggeon phenomenology, should relate to the behaviour of parton distributions
at small x and Q2 . Thus the default in Pythia isµ

sp⊥min (s) = (1.9 GeV)
1 TeV2

¶0.08

(205)

for the simple model, with the same ansatz for p⊥0 in the impact-parameter-dependent
approach, except that then 1.9 GeV → 2.0 GeV. At any energy scale, the simplest criteria to fix p⊥min is to require the average charged multiplicity hnch i or the height of the(pseudo)rapidity ‘plateau’ dnch /dη|η=0 to agree with the experimentally determined one.
331

In general, there is quite a strong dependence of the multiplicity on p⊥min , with a lowerp⊥min corresponding to more multiple interactions and therefore a higher multiplicity.
This is one of the possible inputs into the 1.9 GeV and 2.0 GeV numbers, making useof UA5 data in the energy range 200–900 GeV [UA584]. The energy dependence inside
this range is also consistent with the chosen ansatz [But05]. However, clearly, neither theexperimental nor the theoretical precision is high enough to make too strong statements.
It should also be remembered that the p⊥min values are determined within the context ofa given calculation of the QCD jet cross section, and given model parameters within themultiple-interactions scenario. If anything of this is changed, e.g. the parton distributions
used, then p⊥min ought to be retuned accordingly.
11.2.2The simple model

In an event with several interactions, it is convenient to impose an ordering. The logical choice is to arrange the scatterings in falling sequence of x⊥ = 2p⊥ /Ecm . The ‘first’scattering is thus the hardest one, with the ‘subsequent’ (‘second’, ‘third’, etc.) successively softer. It is important to remember that this terminology is in no way related toany picture in physical time; we do not know anything about the latter. (In a simplified
picture with the incoming hadrons Lorentz-contracted into flat pancakes, the interactions
would tend to have a specelike separation, i.e. without meaningful time ordering.) In
principle, all the scatterings that occur in an event must be correlated somehow, naı̈vely
by momentum and flavour conservation for the partons from each incoming hadron, lessnaı̈vely by various quantum mechanical effects. When averaging over all configurationsof soft partons, however, one should effectively obtain the standard QCD phenomenology
for a hard scattering, e.g. in terms of parton distributions. Correlation effects, known
or estimated, can be introduced in the choice of subsequent scatterings, given that the‘preceding’ (harder) ones are already known.
With a total cross section of hard interactions σhard (p⊥min ) to be distributed among
σnd (s) (non-diffractive, inelastic) events, the average number of interactions per event isjust the ratio n = σhard (p⊥min )/σnd (s). As a starting point we will assume that all hadroncollisions are equivalent (no impact-parameter dependence), and that the different parton–
parton interactions take place completely independently of each other. The number ofscatterings per event is then distributed according to a Poisson distribution with meann. A fit to SppS collider multiplicity data [UA584] gave p⊥min ≈ 1.6 GeV (for parton
distributions in use at the time), which corresponds to n ≈ 1. For Monte Carlo generationof these interactions it is useful to definedσ1f (x⊥ ) =,(206)
σnd (s) dx⊥
with dσ/dx⊥ obtained by analogy with eq. (203). Then f (x⊥ ) is simply the probability
to have a parton–parton interaction at x⊥ , given that the two hadrons undergo a nondiffractive, inelastic collision.
The probability that the hardest interaction, i.e. the one with highest x⊥ , is at x⊥1 , isnow given by½ Z¾
f (x⊥1 ) exp −1x⊥1

f (x0⊥ ) dx0⊥

,

(207)

i.e. the naı̈ve probability to have a scattering at x⊥1 multiplied by the probability that
there was no scattering with x⊥ larger than x⊥1 . This is the familiar exponential dampening in radioactive decays, encountered e.g. in parton showers in section 10.1.2. Using
the same technique as in the proof of the veto algorithm, section 4.2, the probability tohave an i:th scattering at an x⊥i < x⊥i−1 < · · · < x⊥1 < 1 is found to bef (x⊥i )1(i − 1)!

µZ 1x⊥i

f (x0⊥ ) dx0⊥

¶i−1

332

½

exp −

Z 1
x⊥i

¾

f (x0⊥ ) dx0⊥

.

(208)

The total probability to have a scattering at a given x⊥ , irrespectively of it being thefirst, the second or whatever, obviously adds up to give back f (x⊥ ). The multiple interaction formalism thus retains the correct perturbative QCD expression for the scattering
probability at any given x⊥ .
With the help of the integral
F (x⊥ ) =Z 1x⊥f (x0⊥ ) dx0⊥ =1 Z s/4 dσ
dp2
σnd (s) sx2⊥ /4 dp2⊥ ⊥(209)(where we assume F (x⊥ ) → ∞ for x⊥ → 0) and its inverse F −1 , the iterative procedure
to generate a chain of scatterings 1 > x⊥1 > x⊥2 > · · · > x⊥i is given byx⊥i = F −1 (F (x⊥i−1 ) − ln Ri ) .

(210)

Here the Ri are random numbers evenly distributed between 0 and 1. The iterative
chain is started with a fictitious x⊥0 = 1 and is terminated when x⊥i is smaller thanx⊥min = 2p⊥min /Ecm . Since F and F −1 are not known analytically, the standard vetoalgorithm is used to generate a much denser set of x⊥ values, whereof only some areretained in the end. In addition to the p2⊥ of an interaction, it is also necessary to generate
the other flavour and kinematics variables according to the relevant matrix elements.
Whereas the ordinary parton distributions should be used for the hardest scattering,in order to reproduce standard QCD phenomenology, the parton distributions to be used
for subsequent scatterings must depend on all preceding x values and flavours chosen. We
do not know enough about the hadron wave function to write down such joint probability
distributions. To take into account the energy ‘already’ used in harder scatterings, a conservative approach is to evaluate the parton distributions, not at xi for the i:th scatteredparton from hadron, but at the rescaled value
xi
x0i =
.
(211)
Pi−1
1 − j=1 xj
This is our standard procedure in the simple model; we have tried a few alternatives
without finding any significantly different behaviour in the final physics.
In a fraction exp(−F (x⊥min )) of the events studied, there will be no hard scattering
above x⊥min when the iterative procedure in eq. (210) is applied. It is therefore also
necessary to have a model for what happens in events with no (semi)hard interactions.
The simplest possible way to produce an event is to have an exchange of a very soft
gluon between the two colliding hadrons. Without (initially) affecting the momentum
distribution of partons, the ‘hadrons’ become colour octet objects rather than colour
singlet ones. If only valence quarks are considered, the colour octet state of a baryon canbe decomposed into a colour triplet quark and an antitriplet diquark. In a baryon-baryon
collision, one would then obtain a two-string picture, with each string stretched from thequark of one baryon to the diquark of the other. A baryon-antibaryon collision would give
one string between a quark and an antiquark and another one between a diquark and an
antidiquark.
In a hard interaction, the number of possible string drawings are many more, and theoverall situation can become quite complex when several hard scatterings are present inan event. Specifically, the string drawing now depends on the relative colour arrangement,in each hadron individually, of the partons that are about to scatter. This is a subject
about which nothing is known. To make matters worse, the standard string fragmentation
description would have to be extended, to handle events where two or more valence quarks
have been kicked out of an incoming hadron by separate interactions. In particular, theposition of the baryon number would be unclear. Such issues will be further discussed
below, when we go on to describe more recent models, but in the original studies they
were sidestepped. Specifically, we assumed that, following the hardest interaction, allsubsequent interactions belong to one of three classes.
333• Scatterings of the gg → gg type, with the two gluons in a colour-singlet state, suchthat a double string is stretched directly between the two outgoing gluons, decoupledfrom the rest of the system.
• Scatterings gg → gg, but colour correlations assumed to be such that each of thegluons is connected to one of the strings ‘already’ present. Among the different possibilities of connecting the colours of the gluons, the one which minimizes the total
increase in string length is chosen. This is in contrast to the previous alternative,
which roughly corresponds to a maximization (within reason) of the extra string
length.
• Scatterings gg → qq, with the final pair again in a colour-singlet state, such that asingle string is stretched between the outgoing q and q.
By default, the three possibilities were assumed equally probable. (More recent studies[Fie02] have suggested the minimal string length topology to dominate, an issue well
worth studying further.) Note that the total jet rate is maintained at its nominal value,i.e. scatterings such as qg → qg are included in the cross section, but are replaced bya mixture of gg and qq events for string drawing issues. Only the hardest interaction
is guaranteed to give strings coupled to the beam remnants. One should not take thisapproach to colour flow too seriously — clearly it is a simplification — but the overall
picture does not tend to be very dependent on the particular choice you make.
Since a gg → gg or qq scattering need not remain of this character if initial- andfinal-state showers were to be included (e.g. it could turn into a qg-initiated process),radiation is only included for the hardest interaction. In practice, this need not be a serious
problem: except for the hardest interaction, which can be hard because of experimental
trigger conditions, it is unlikely for a parton scattering to be so hard that radiation plays
a significant rôle.
In events with multiple interactions, the beam-remnant treatment is slightly modified.
First the hard scattering is generated, with its associated initial- and final-state radiation, and next any additional multiple interactions. Only thereafter are beam remnants
attached to the initiator partons of the hardest scattering, using the same machinery
as before, except that the energy and momentum already taken away from the beam
remnants also include that of the subsequent interactions.
11.2.3

A model with varying impact parameters

Up to this point, it has been assumed that the initial state is the same for all hadron collisions, whereas in fact each collision also is characterized by a varying impact parameterb. Within the classical framework of the model reviewed here, b is to be thought of as a
distance of closest approach, not as the Fourier transform of the momentum transfer. A
small b value corresponds to a large overlap between the two colliding hadrons, and hence
an enhanced probability for multiple interactions. A large b, on the other hand, corresponds to a grazing collision, with a large probability that no parton–parton interactions
at all take place.
In order to quantify the concept of hadronic matter overlap, one may assume a spherically symmetric distribution of matter inside the hadron, ρ(x) d3 x = ρ(r) d3 x. For simplicity, the same spatial distribution is taken to apply for all parton species and momenta.
Several different matter distributions have been tried, and are available. We will here
concentrate on the most extreme one, a double Gaussian
(

1−β
r2
ρ(r) ∝
exp
−
a31
a21

)

(

β
r2
+ 3 exp − 2
a2
a2

)

.

(212)

This corresponds to a distribution with a small core region, of radius a2 and containing afraction β of the total hadronic matter, embedded in a larger hadron of radius a1 . While334

it is mathematically convenient to have the origin of the two Gaussians coinciding, thephysics could well correspond to having three disjoint core regions, reflecting the presenceof three valence quarks, together carrying the fraction β of the proton momentum. One
could alternatively imagine a hard hadronic core surrounded by a pion cloud. Such details
would affect e.g. the predictions for the t distribution in elastic scattering, but are not of
any consequence for the current topics. To be specific, the values β = 0.5 and a2 /a1 = 0.2
were picked as default values. It should be noted that the overall distance scale a1 neverenters in the subsequent calculations, since the inelastic, non-diffractive cross section
σnd (s) is taken from literature rather than calculated from the ρ(r).
Compared to other shapes, like a simple Gaussian, the double Gaussian tends to givelarger fluctuations, e.g. in the multiplicity distribution of minimum-bias events: a collisionin which the two cores overlap tends to have a strongly increased activity, while ones where
they do not are rather less active. One also has a biasing effect: hard processes are more
likely when the cores overlap, thus hard scatterings are associated with an enhanced
multiple interaction rate. This provides one possible explanation for the experimental‘pedestal effect’ [UA187]. Recent studies of CDF data [Fie02, Mor02] have confirmedthat indeed something more peaked than a single Gaussian is required to understand thetransition from minimum-bias to underlying-event activity.
For a collision with impact parameter b, the time-integrated overlap O(b) between thematter distributions of the colliding hadrons is given byZ

O(b) ∝

Zdt

d3 x ρ(x, y, z) ρ(x + b, y, z + t)(b2
(1 − β)2
exp
−
∝
2a21
2a21

)

(

2β(1 − β)
b2
+ 2
exp
−
a1 + a22
a12 + a22

)

(

β2
b2
+ 2 exp − 2
2a2
2a2

)

.(213)The necessity to use boosted ρ(x) distributions has been circumvented by a suitable scaletransformation of the z and t coordinates.
The overlap O(b) is obviously strongly related to the eikonal Ω(b) of optical models. We
have kept a separate notation, since the physics context of the two is slightly different: Ω(b)is based on the quantum mechanical scattering of waves in a potential, and is normallyused to describe the elastic scattering of a hadron-as-a-whole, while O(b) comes from apurely classical picture of point-like partons distributed inside the two colliding hadrons.
Furthermore, the normalization and energy dependence is differently realized in the twoformalisms.
The larger the overlap O(b) is, the more likely it is to have interactions between partonsin the two colliding hadrons. In fact, there should be a linear relationshiphñ(b)i = kO(b) ,(214)

where ñ = 0, 1, 2, . . . counts the number of interactions when two hadrons pass eachother with an impact parameter b. The constant of proportionality, k, is related to theparton–parton cross section and hence increases with c.m. energy.
For each given impact parameter, the number of interactions is assumed to be distributed according to a Poisson. If the matter distribution has a tail to infinity (as the
double Gaussian does), events may be obtained with arbitrarily large b values. In orderto obtain finite total cross sections, it is necessary to assume that each event contains
at least one semi-hard interaction. The probability that two hadrons, passing each otherwith an impact parameter b, will actually undergo a collision is then given by
Pint (b) = 1 − exp(−hñ(b)i) = 1 − exp(−kO(b)) ,(215)

according to Poisson statistics. The average number of interactions per event at impact
parameter b is nowhñ(b)ikO(b)hn(b)i ==,(216)
Pint (b)1 − exp(−kO(b))
335where the denominator comes from the removal of hadron pairs which pass without colliding, i.e. with ñ = 0.
The relationship hni = σhard /σnd was earlier introduced for the average number of interactions per non-diffractive, inelastic event. When averaged over all impact parameters,
this relation must still hold true: the introduction of variable impact parameters may give
more interactions in some events and less in others, but it does not affect either σhard or
σnd . For the former this is because the perturbative QCD calculations only depend on
the total parton flux, for the latter by construction. Integrating eq. (216) over b, one thenobtainsRR
hn(b)i Pint (b) d2 b
kO(b) d2 b
σhard
R
R
hni =
.
(217)
=
=
2
2
σnd
Pint (b) d b(1 − exp(−kO(b))) d b
For O(b), σhard and σnd given, with σhard /σnd > 1, k can thus always be found (numerically) by solving the last equality.
The absolute normalization of O(b) is not interesting in itself, but only the relative
variation with impact parameter. It is therefore useful to introduce an ‘enhancement
factor’ e(b), which gauges how the interaction probability for a passage with impact
parameter b compares with the average, i.e.
hñ(b)i = kO(b) = e(b) hkO(b)i .

(218)

The definition of the average hkO(b)i is a bit delicate, since the average number of interactions per event is pushed up by the requirement that each event contain at least one
interaction. However, an exact meaning can be given [Sjö87a].
With the knowledge of e(b), the f (x⊥ ) function of the simple model generalizes tof (x⊥ , b) = e(b) f (x⊥ ) .

(219)

The naı̈ve generation procedure is thus to pick a b according to the phase space d2 b, findthe relevant e(b) and plug in the resulting f (x⊥ , b) in the formalism of the simple model.
If at least one hard interaction is generated, the event is retained, else a new b is to befound. This algorithm would work fine for hadronic matter distributions which vanish
outside some radius, so that the d2 b phase space which needs to be probed is finite. Since
this is not true for the distributions under study, it is necessary to do better.
By analogy with eq. (207), it is possible to ask what the probability is to find thehardest scattering of an event at x⊥1 . For each impact parameter separately, the probability to have an interaction at x⊥1 is given by f (x⊥1 , b), and this should be multiplied by
the probability that the event contains no interactions at a scale x0⊥ > x⊥1 , to yield thetotal probability distribution½

Z 1
dPhardest= f (x⊥1 , b) exp −f (x0⊥ , b) dx0⊥
d2 b dx⊥1x⊥1½= e(b) f (x⊥1 ) exp −e(b)

Z 1

x⊥1

¾

f (x0⊥ ) dx0⊥

¾

.

(220)

If the treatment of the exponential is deferred for a moment, the distribution in b and x⊥1
appears in factorized form, so that the two can be chosen independently of each other.
In particular, a high-p⊥ QCD scattering or any other hard scattering can be selectedwith whatever kinematics desired for that process, and thereafter assigned some suitable‘hardness’ x⊥1 . With the b chosen according to e(b) d2 b, the neglected exponential can now
be evaluated, and the event retained with a probability proportional to it. From the x⊥1
scale of the selected interaction, a sequence of softer x⊥i values may again be generated
as in the simple model, using the known f (x⊥ , b). This sequence may be empty, i.e. theevent need not contain any further interactions.
336

It is interesting to understand how the algorithm above works. By selecting b according
to e(b) d2 b, i.e. O(b) d2 b, the primary b distribution is maximally biased towards small
impact parameters. If the first interaction is hard, by choice or by chance, the integralof the cross section above x⊥1 is small, and the exponential close to unity. The rejection
procedure is therefore very efficient for all standard hard processes in the program — one
may even safely drop the weighting with the exponential completely. The large e(b) value
is also likely to lead to the generation of many further, softer interactions. If, on the other
hand, the first interaction is not hard, the exponential is no longer close to unity, and many
events are rejected. This pulls down the efficiency for ‘minimum bias’ event generation.
Since the exponent is proportional to e(b), a large e(b) leads to an enhanced probability
for rejection, whereas the chance of acceptance is larger with a small e(b). Among events
where the hardest interaction is soft, the b distribution is therefore biased towards larger
values (smaller e(b)), and there is a small probability for yet softer interactions.
To evaluate the exponential factor, the program pretabulates the integral of f (x⊥ ) at
the initialization stage, and further increases the Monte Carlo statistics of this tabulation
as the run proceeds. The x⊥ grid is concentrated towards small x⊥ , where the integral
is large. For a selected x⊥1 value, the f (x⊥ ) integral is obtained by interpolation. After
multiplication by the known e(b) factor, the exponential factor may be found.
In this section, nothing has yet been assumed about the form of the dσ/dp⊥ spectrum. Like in the impact-parameter-independent case, it is possible to use a sharp cut-off
at some given p⊥min value. However, now each event is required to have at least one interaction, whereas before events without interactions were retained and put at p⊥ = 0. It istherefore aesthetically more appealing to assume a gradual turn-off, so that a (semi)hard
interaction can be rather soft part of the time. The matrix elements roughly diverge likeαs (p2⊥ ) dp2⊥ /p4⊥ for p⊥ → 0. They could therefore be regularized as follows. Firstly, toremove the 1/p4⊥ behaviour, multiply by a factor p4⊥ /(p2⊥ + p2⊥0 )2 . Secondly, replace thep2⊥ argument in αs by p2⊥ + p2⊥0 . If one has included a K factor by a rescaling of the αsargument, as mentioned earlier, replace 0.075 p2⊥ by 0.075 (p2⊥ + p2⊥0 ).
With these substitutions, a continuous p⊥ spectrum is obtained, stretching from p⊥ = 0
to Ecm /2. For p⊥ À p⊥0 the standard perturbative QCD cross section is recovered,
while values p⊥ ¿ p⊥0 are strongly damped. The p⊥0 scale, which now is the main freeparameter of the model, in practice comes out to be of the same order of magnitude as
the sharp cut-off p⊥min did, i.e. 1.5–2 GeV at collider energies, but typically about 10%
higher.
Above we have argued that p⊥min and p⊥0 should only have a slow energy dependence,and even allowed for the possibility of fixed values. For the impact-parameter-independent
picture this works out fine, with all events being reduced to low-p⊥ two-string ones when
the c.m. energy is reduced. In the variable-impact-parameter picture, the whole formalism
only makes sense if σhard > σnd , see e.g. eq. (217). Since σnd does not vanish withdecreasing energy, but σhard would do that for a fixed p⊥0 , this means that p⊥0 has tobe reduced significantly at low energies, possibly even more than implied by our assumed
energy dependence. The more ‘sophisticated’ model of this section therefore makes senseat collider energies, whereas it may not be well suited for applications at fixed-target
energies. There one should presumably attach to a picture of multiple soft Pomeron
exchanges.

11.3

Beam Remnants (and Multiple Interactions) — Intermediate ModelA new description of multiple interactions and beam remnants is introduced withPythia 6.3 [Sjö04]. It is more sophisticated than the old one, but unfortunately as
a consequence less robust. Therefore extra caution should be exercised. Currently it
should only be used for the study of single minimum-bias events or events underlying
337

hard processes in pp or pp collisions. It is not fully developed and tested for meson or
photon beams, and cannot be used with pile-up events, or with baryon-number-violating
SUSY events.
The basic scheme for the generation of the multiple interactions is kept from the oldmodels. That is, based on the standard 2 → 2 QCD matrix elements convoluted withstandard parton densities, a sequence of p⊥ -ordered interactions is generated: p⊥1 > p⊥2 >p⊥3 > . . . . The sequence is stopped at some lower cut-off scale p⊥min , or alternatively
the matrix elements are damped smoothly around and below a scale p⊥0 . An impactparameter-dependent picture of hadron-hadron interactions can lead to further elementsof variability between events. See above for details on the basic framework.
The main limitation of the old approach is that there was no way to handle complicated beam remnants, e.g. where two valence quarks had been kicked out. Therefore thestructure of all interactions subsequent to the first one, i.e. the one with largest p⊥ , had tobe substantially simplified. The introduction of junction fragmentation in [Sjö03] allowed
this restriction to be lifted.
11.3.1

Flavour and x Correlations

In the new beam-remnants approach, the flavour content of the remnant is bookkept,
and is used to determine possible flavours in consecutive interactions. Thus, the standard
parton densities are only used to describe the hardest interaction. Already in the oldmodel, the x scale of parton densities is rescaled in subsequent interactions, such thatthe new x0 = 1 corresponds to the remaining momentum rather than the original beam
momentum, eq. (211). But now the distributions are not only squeezed in this manner,
their shapes are also changed, as follows:• Whenever a valence quark is kicked out, the number of remaining valence quarks ofthat species is reduced accordingly. Thus, for a proton, the valence d distribution iscompletely removed if the valence d quark has been kicked out, whereas the valenceu distribution is halved when one of the two is kicked out. In cases where thevalence and sea u and d quark distributions are not separately provided from the
PDF libraries, it is assumed that the sea is flavour-antiflavour symmetric, so that
one can write e.g.
u(x, Q2 ) = uval (x, Q2 ) + usea (x, Q2 ) = uval (x, Q2 ) + u(x, Q2 ).

(221)

The parametrized u and u distributions are then used to find the relative probability
for a kicked-out u quark to be either valence or sea.
• When a sea quark is kicked out, it must leave behind a corresponding antisea parton
in the beam remnant, by flavour conservation. We call this a companion quark, andbookkeep it separately from the normal sea. In the perturbative approximation thesea quark qs and its companion qc (not to be confused with flavour labels) comefrom a gluon branching g → qs + qc , where it is implicitly understood that if qs is
a quark, qc is its antiquark, and vice versa. This branching often would not be in
the perturbative regime, but we choose to make a perturbative ansatz, and also toneglect subsequent perturbative evolution of the qc distribution. If so, the shape ofthe companion qc distribution as a function of xc , given a sea parton at xs , becomesqc (xc ; xs ) ∝g(xc + xs )Pg→qs qc (z),xc + xs

(222)

where
z =

xc
,
xc + xs
338

(223)

´
1³ 2
z + (1 − z)2 ,2(1 − x)ng(x) ∝; (n = MSTP(87))
x

Pg→qs qc (z) =

(224)
(225)

is chosen. (Remember that this is supposed to occur at some low Q2 scale, so g(x)above should not be confused with the high-Q2 gluon behaviour.)• The normalization of valence and companion distributions is fixed by the respective
number of quarks, i.e. the sum rules
Z xremZ xrem 00

qv (x) dx = nqv ,qc,i (x; xs,i ) dx = 1(226)(for each i),(227)

where xrem is the longitudinal momentum fraction left after the previous interactions
and nqv is the number of q valence quarks remaining. Gluon and sea distributions
do not have corresponding requirements. Therefore their normalization is adjusted,
up or down, so as to obtain overall momentum conservation in the parton densities,
i.e. to fulfil the remaining sum rule:
Z xrem ÃX0!

q(x) + g(x) dx = xrem

(228)

q

Detailed formulae may be found in [Sjö04]. However, in that article, the sea+gluon
rescaling factor, eq. (4.26), was derived assuming the momentum fraction taken by acompanion quark scaled like 1/X, where X is the total remaining momentum. As it happens, the momentum fraction, xs , taken by the sea quark that originally gave rise to thecompanion quark, is already taken into account in the definition of the companion distribution. Hence a factor (X + xs )/X should be introduced for each companion distribution
in eq. (4.26). With this change, the companions take up more ‘room’ in momentum space.
In fact, in extreme cases it is possible to create so many companions in the beam remnantthat the sum of their momenta is larger than allowed by momentum conservation, i.e. therescaling factor becomes negative. Note that this occurs extremely rarely, and only whenvery many multiple interactions have occurred in an event. Since the companions are now
taking up more room than allowed, we address the problem by scaling them (all of them)down by precisely the amount needed to restore momentum conservation.
The above parton-density strategy is not only used to pick a succession of hard interactions. Each interaction may now, unlike before, have initial- and final-state shower
activity associated with it. The initial-state shower is constructed by backwards evolution, using the above parton densities. Even if the hard scattering does not involve a
valence quark, say, the possibility exists that the shower will reconstruct back to one.
11.3.2

Colour Topologies

The second part of the model is to hook up the scattered partons to the beam remnants,
in colours, in transverse momenta and in longitudinal momenta. The order in whichthese choices are made is partly intertwined, and one has to consider several special cases.
In this description we only give an outline, evading many of the details. Especially theassignment of colours is physically uncertain and technically challenging.
Each of the two incoming beam hadrons can be viewed — post facto — as having
consisted of a set of ‘initiator’ and ‘remnant’ partons. An initiator is the ‘original’ partonthat starts the initial-state cascade that leads up to one of the hard interactions. Together
the initiators and remnants make up the original proton, so together they carry the339

proton net flavour content, and energy and momentum, and are in a colour singlet state.
Therefore the remnant partons are constrained to carry ‘what is left’ when the initiators
are removed. The one exception is that we do not attempt to conserve longitudinal
momentum (and thereby energy) exactly within each incoming proton, but only for thesystem as a whole. The reason here is that we have put all initiators and remnants on
mass shell, whereas a more proper treatment ought to have included (moderate) spacelike virtualities for them. Such virtualities would have dissipated in the hard interactions,and so not survived to the final state. The current choice therefore shortcuts a numberof technical details that in the end should not matter.
In an event with n multiple interactions, a corresponding set of n initiator partonsis defined for each of the two incoming beams. This also defines the left-behind flavour
content of the remnants. At most there could be n + 3 remnant partons for a protonwith its 3 valence flavours, but there could also be none, if all valence quarks have been
kicked out and no unmatched companions have been added. In the latter case a gluon
is inserted to represent the beam remnant and carry leftover momentum, but else we do
not introduce gluons in the beam remnant.
In the string model, the simplest representation of a baryon is the Y-shape topology,
where each of the three valence quarks is connected via its string piece to the central
junction (see section 12.2.5). This junction does not carry energy or momentum of its
own, but topologically it is the carrier of the baryon number. Under normal circumstances,the legs in the Y are quite short. Interactions may deform the topology, however. The
simplest example would be Deeply Inelastic Scattering, where one of the valence quarks
is kicked violently, so that one of the three strings of the Y will stretch out and fragment
into hadrons. In this topology the other two will remain so close to each other and to thejunction that they effectively act as a single unit, a diquark.
In a hadronic interaction, a valence quark is kicked out by a coloured gluon rather thana colourless photon. This colour exchange implies that the string from the junction will
no longer attach to the scattered quark but rather to some other quark or gluon, but theother two quarks still effectively form a diquark. When two or three valence quarks are
kicked out, the junction motion will become more complicated, and must be consideredin full. Equivalently, when all of the valence quarks have large relative momenta, bydefinition no two of them have a small invariant mass and hence a diquark description
would be inappropriate.
Multiple valence quark interactions are rare, however. The bulk of interactions aregluonic. In this case we can imagine that the gluon originates from an unresolved emission
off one of the valence quarks, i.e. that its anticolour is attached to one of the quarks andits colour attached to the junction. After the gluon is kicked out, colour lines will thenconnect this quark and the junction to partons in the final state of the interaction. If a
second gluon is kicked out, it can, in colour space, have been located either between thequark and the first gluon, or between this gluon and the junction, or between the junction
and one of the other two quarks. (We do not include the possibility that this gluon together
with the first one could have been radiated from the system as an overall colour singlet
system, i.e. we do not (yet) address diffractive event topologies in this framework.) If
all of these possibilities had equal probability, the junction would often have two or all
three legs reconnected, and the baryon number could be moved quite dramatically inthe event. Empirically, this does not appear to happen (?), and furthermore it could
be argued that perturbative and impact-parameter arguments both allow much of theactivity to be correlated in ‘hot spot’ regions that leave much of the rest of the proton
unaffected. Therefore a free suppression parameter is introduced, such that further gluons
preferably connect to a string piece that has already been disturbed. In this way, gluons
preferentially will be found on one of the three colour lines to the junction.
The order in which they appear along that line is more difficult to make statements
about. So far in our description, no consideration has been given to the resulting momen340

tum picture, specified after transverse and longitudinal momenta have been picked (seebelow). This in general implies that strings will be stretched criss-cross in the event, if
the initiator gluons are just randomly ordered along the string piece on which they sit.
It is unlikely that such a scenario catches all the relevant physics. More likely is that,among all the possible colour topologies for the final state, those that correspond to thesmaller total string length are favoured, all other aspects being the same. Several options
have been introduced that approach this issue from different directions (see MSTP(89) andMSTP(95)). One is to attach the initiator gluons preferentially in those places that orderthe hard-scattering systems in rapidity (the default), another to prefer the attachmentsthat will give rise to the smaller string lengths in the final state. A third option is to rearrange the colour flow between the final-state partons themselves, again giving preference
to those rearrangements which minimize the overall string length. So far no preferred
scenario has been identified.
A complication is the following. Two g + g → g + g scatterings each on their own may
have a perfectly sensible colour flow. Still, when the two initial gluons on each side ofthe event are attached to each other and to the rest of the remnants, the resulting colour
flow may become unphysical. Specifically the colour flow may ‘loop back’ on itself, suchthat a single gluon comes to form a separate colour singlet system. Such configurations
are rejected and alternative colour arrangements are tried.
Another, rare, occurence is that the two junctions of the event can come to be connected to each other via two strings (graphical representation: -j=j-, where each dashcorresponds to a string piece). Since we have not (yet) programmed a fragmentation
scheme for such events, we simply reject them and generate a new event instead.
So far, interactions with sea quarks have not been mentioned, either a quark-antiquark
pair that both scatter or a single sea scatterer with a leftover companion quark in theremnant. However, we have already argued that each such pair can be viewed as comingfrom the branching of some initial nonperturbative gluon. This gluon can now be attachedto the beam Y topology just like the gluons considered above, and therefore does not
introduce any new degrees of freedom. When then the colours are traced, it could well
happen that a companion quark together with two remaining valence quarks form aseparate colour singlet system. It is then likely that this system will be of low mass andcollapse to a single baryon. Such possibilities are optionally allowed (see MSTP(88)), andcorrespondingly a companion antiquark could form a meson together with a single valencequark. As already mentioned, diquarks can be formed from two valence quarks.
11.3.3

Primordial k⊥

Partons are expected to have primordial k⊥ values of the order of a few hundred MeVfrom Fermi motion inside the incoming hadrons. In reality, one notes a need for a larger
input in the context of shower evolution, either in parton showers or in resummation
descriptions. This is not yet understood, but could e.g. represent a breakdown of the
DGLAP evolution equations at small Q2 . Until a better solution has been found, we
therefore have reason to consider an effective ‘primordial k⊥ ’, at the level of the initiators,
larger than the one above. For simplicity, a parametrized Q-dependent widthÃ

!

2.1 GeV × Q
σ(Q) = max, PARJ(21)7 GeV + Q

(229)

is introduced, where σ is the width of the two-dimensional Gaussian distribution of the2
i = σ 2 ), Q is the scale of the hard interaction andinitiator primordial k⊥ (so that hk⊥
PARJ(21) is the standard fragmentation p⊥ width. The remnant partons correspond to
Q = 0 and thus hit the lower limit. Apart from the selection of each individual k⊥ , there
is also the requirement that the total k⊥ of a beam adds up to zero. Different strategies341

can be used here, from sharing the recoil of one parton uniformly among all other initiatorand remnant partons, to only sharing it among those initiator/remnant partons that havebeen assigned as nearest neighbours in colour space.
11.3.4Beam-Remnant Kinematics

The longitudinal momenta of the initiator partons have been defined by the x values
picked. The remaining longitudinal momentum is shared between the remnants in accordance with their character. A valence quark receives an x picked√
at random according toa small-Q2 valence-like parton density, proportional to (1 − x)a / x, where a = 2 for a u
quark in a proton and a = 3.5 for a d quark. A sea quark must be the companion of one of
the initiator quarks, and can have an x picked according to the qc (xc ; xs ) distribution introduced above. A diquark would obtain an x given by the sum of its constituent quarks.
(But with the possibility to enhance this x, to reflect the extra gluon cloud that could
go with such a bigger composite object.) If a baryon or meson is in the remnant, its x
is equated with the z value obtainable from the Lund symmetric fragmentation function,
again with the possibility of enhancing this x as for a diquark. A gluon only appears in
an otherwise empty remnant, and can thus be given x = 1. Once x values have been
picked for each of the remnants, an overall rescaling is performed such that the remnants
together carry the desired longitudinal momentum.

11.4

Multiple Interactions (and Beam Remnants) – New Model

Each multiple interaction is associated with its set of initial- and final-state radiation. In
the old model such radiation was only considered for the first, i.e. hardest, interaction.
The technical reason had to do with the inability to handle junction string topologies,
and therefore the need to simplify the description. In practice, it could be argued thatthe subsequent interactions would tend to be soft, near the lower p⊥min or p⊥0 scales,
and therefore not be associated with additional hard radiation. Nevertheless it was alimitation.
The new junction and beam-remnant description allows radiation to be associatedwith each interactions. In the intermediate model, this is done in a disjoint manner:for each interaction, all initial- and final-state radiation activity associated with it isconsidered in full before the next interaction is selected and the showers are still the old,
virtuality-ordered ones.
In the new model, the new, transverse-momentum-ordered showers are introduced.
Thus p⊥ becomes the common evolution scale both for multiple interactions (MI), initialstate radiation (ISR) and final-state radiation (FSR), although the technical definition oftransverse momentum is slightly different in the three cases.
One can argue that, to a good approximation, the addition of FSR can be deferred
until after ISR and MI have been considered in full. Specifically, FSR does not modify
the total amount of energy carried by perturbatively defined partons, it only redistributes
that energy among more partons. By contrast, both the addition of a further ISR branching and the addition of a further interaction implies more perturbative energy, takenfrom the limited beam-remnants reservoir. These two mechanisms therefore are in directcompetition with each other.
We have already advocated in favour of ordering multiple interactions in a sequence offalling p⊥ values. This does not correspond to an ordering in a physical time, but rather
to the requirement that the hardest interaction should be the one for which standard
parton densities should apply. Any corrections, e.g. from the kind of flavour correlations
already discussed, would have to be introduced by approximate prescriptions, that would
become increasingly uncertain as further interactions are considered.

342

We now advocate that, by the same reasoning, also ISR emissions should be interleavedwith the MI chain, in one common sequence of decreasing p⊥ values. That is, a hard second
interaction should be considered before a soft ISR branching associated with the hardest
interaction. This is made possible by the adoption of p⊥ as common evolution variable.
Thus, the standard parton densities are only used to describe the hardest interaction andthe ISR branchings that occur above the p⊥ -scales of any secondary interactions.
In passing, note that the old showers requires two matching parameters, Q2max,shower =f p2⊥,MI . These f values, typically in the range 1 to 4, separate for space-like and time-likeshowers, are there to compensate on the average for the extra z-dependent factors in therelations p2⊥ ≈ (1 − z)Q2 and p2⊥ ≈ z(1 − z)Q2 , respectively, so that the showers canstart from a p⊥ scale comparable with that of the interaction. In our new model, with
Q2shower ≈ p2⊥ , this matching is automatic, i.e. f = 1.
The evolution downwards in p⊥ can now be viewed as a generalization of the backwards
evolution philosophy [Sjö85]: given a configuration at some p⊥ resolution scale, what
can that configuration have come from at a lower scale? Technically, starting from ahard interaction, a common sequence of subsequent evolution steps — interactions andbranchings mixed — can therefore be found. Assuming that the latest step occurred atsome p⊥i−1 scale, this sets the maximum p⊥max = p⊥i−1 for the continued evolution. Whatcan happen next is then either a new interaction or a new ISR branching on one of thetwo incoming sides in one of the existing interactions. The probability distribution forp⊥ = p⊥i is given by
dP
=
dp⊥

Ã

dPMI X dPISR
+
dp⊥
dp⊥

!

Ã

exp −

Z p⊥i−1 Ãp⊥!

dPMI X dPISR
+
dp0⊥
0
0
dp⊥
dp⊥

!

(230)

in simplified notation. Technically,
the p⊥i can be found by selecting a new trial interactionRaccording to dPMI exp(− RdPMI ), and a trial ISR branching in each of the possible places
according to dPISR exp(− dPISR ). The one of all of these possibilities that occurs at
the largest p⊥ preempts the others, and is allowed to be realized. The whole process isiterated, until a lower cutoff is reached, below which no further interactions or branchings
are allowed.
If there were no flavour and momentum constraints linking the different subsystems,
it is easy to see that such an interleaved evolution actually is equivalent to considering
the ISR of each interaction in full before moving on to the next interaction. Competition
is introduced via the correlated parton densities already discussed. Thus distributions are
squeezed to be nonvanishing in a range x ∈ [0, X], where X < 1 represents the fractionof the original beam-remnant momentum still available
for an interaction or branching.
Pn−1
When a trial n’th interaction is considered, X = 1 − i=1 xi , where the sum runs over
all the already existing interactions. The xi are the respective momentum fractions of
the ISR shower initiators at the current resolution scale, i.e., an xi is increased each timean ISR branching is backwards-constructed on an incoming parton leg. Similarly, theflavour content is modified to take into account the partons already extracted by then − 1 previous interactions, including the effects of ISR branchings. When instead a trial
shower branching is considered, the X sum excludes the interaction under consideration,
since this energy is at the disposal of the interaction, and similarly for the flavour content.
The choice of p⊥max scale for this combined evolution is process-dependent, as before.
For minimum-bias QCD events the full phase space is allowed, while the p⊥ scale of a QCD
hard process sets the maximum for the continued evolution, in order not to doublecount.
When the hard process represents a possibility not present in the MI/ISR machinery —production of Z0 , top, or supersymmetry, say — again the full (remaining) phase space
is available, though for processes that contain explicit jets in the matrix element, suchas W+jet, the ISR evolution is restricted to be below the jet cutoff. Note that, wheninterfacing events from an external matrix element generator, special care has to be taken
to ensure that these scales are set consistently.
343

There is also the matter of a lower p⊥min scale. Customarily such scales are chosen
separately for ISR and MI, and typically lower for the former than the latter. Bothcutoffs are related to the resolution of the incoming hadronic wave function, however, andin the current formalism ISR and MI are interleaved, so it makes sense to use the same
regularization procedure.Therefore also the branching probability is smoothly turned off
at a p⊥0 scale, like for MI, but the ISR suppression factor is the square root of the
MI one, since only one Feynman vertex is involved in a shower branching relative tothe two of a hard process. Thus the αs (p2⊥ ) dp2⊥ /p2⊥ divergence in a branching is tamed
to αs (p2⊥0 + p2⊥ ) dp2⊥ /(p2⊥0 + p2⊥ ). The scale of parton densities in ISR and MI alike ismaintained at p2⊥ , however, the argument being that the actual evolution of the partonic
content is given by standard DGLAP evolution, and that it is only when this content is
to be resolved that a dampening is to be imposed. This also has the boon that flavour
thresholds appear where they are expected.
The cutoff for FSR is still kept separate and lower, since that scale deals with thematching between perturbative physics and the nonperturbative hadronization at longtime scales, and so has a somewhat different function.
The description of beam remnants offers the same scenarios as outlined for the intermediate model above, and as further described in [Sjö04a]. A more recent option, notfound there, is the following ‘colour annealing’ scenario [San05], available only for the
new model, through the options MSTP(95) = 2 - 5. It has been constructed to produce
similar effects as ‘Tune A’ and similar tunes of the old model, but is also in some sense
intended to be representative of the ‘most extreme’ case. It starts from the assumption
that, at hadronization time, no information from the perturbative colour history of theevent is relevant. Instead, what determines how hadronizing strings form between thepartons is a minimization of the total potential energy stored in these strings. That is,
the partons, regardless of their formation history, will tend to be colour connected to thepartons closest to them in momentum space, hence minimizing the total ‘string length’,
as measured by the so-called λ measure [And83a, Sjö03]. Technically, the model implementation starts by erasing the colour tags of all final-state coloured partons. It thenbegins an iterative procedure (which unfortunately can be quite time-consuming):
1. Loop over all final-state coloured partons
2. For each such parton,(i ) compute the λ measure for each possible string connection from that parton
to other ‘colour-compatible’ final-state partons which do not already have stringpieces connected to them (for MSTP(95) = 2 and MSTP(95) = 3 with the extra
condition that closed gluon loops are suppressed, or, with options MSTP(95) = 6
and MSTP(95) = 7 available from version 6.402, with the condition that connections
must be initiated from free triplets), and(ii ) store the connection with the smallest λ measure for later comparison.
3. Compare all the possible ‘minimal string pieces’ found, one for each parton. Selectthe largest of these to be carried out physically. (That parton is in some sense theone that is currently furthest away from all other partons.)
4. If any ‘dangling colour tags’ are left, repeat from 1.
5. At the end of the iteration, it is possible that the last parton is a gluon, and that
all other partons already form a complete colour singlet system. In this case, thegluon is simply attached between the two partons where its presence will increase
the total λ measure the least.
Finally, let it be re-emphasized that the issue of colour correllations and reconnections,especially in hadron collisions, is an extremely challenging one, about which very little is
known for certain at present (see e.g. the discussions in [San05] and references therein).
The present models are therefore in this respect best regarded as vast simplifications of apresumably much more complex physical picture.

344

11.4.1

Joined Interactions

When the backwards evolution of initial-state radiation traces the prehistory to hardinteractions, two partons participating in two separate hard scatterings may turn out tohave a common ancestor, joined interactions (JI).
The joined interactions are well-known in the context of the evolution of multipartondensities [Kon79], but have not been applied to a multiple-interactions framework. A full
implementation of the complete kinematics, intertwining MI, ISR and JI all possible ways,
is a major undertaking, worth the effort only if the expected effects are non-negligible.
Given the many uncertainties in all the other processes at play, one would otherwise
expect that the general tuning of MI/ISR/FSR/. . . to data would hide the effects of JI.
The current program can simulate the joining term in the evolution equations, andthereby estimate how often and at what p⊥ values joinings should occur. However, theactual kinematics has not been worked out, so the suggested joinings are never performed.
Instead the evolution is continued as if nothing had happened. Therefore this facility ismore for general guidance than for detailed studies.
(2)
To see how it works, define the two-parton density fbc (xb , xc , Q2 ) as the probability
to have a parton b at energy fraction xb and a parton c at energy fraction xc when the
proton is probed at a scale Q2 . The evolution equation for this distribution is(2)
dfbc (xb , xc , Q2 )ndQ2 αs ZZ
(2)
=
dx
dz
fac
(xa , xc , Q2 ) Pa→bd (z) δ(xb − zxa )
a
Q2 2π
(2)

+ fba (xb , xa , Q2 ) Pa→cd (z) δ(xc − zxa )o+ fa (xa , Q2 ) Pa→bc (z) δ(xb − zxa ) δ(xc − (1 − z)xa )

.

(231)

As usual, we assume implicit summation over the allowed flavour combinations. The
first two terms in the above expression are the standard ones, where b and c evolve
independently, up to flavour and momentum conservation constraints, and are alreadytaken into account in the ISR framework. It is the last term that describes the new
possibility of two evolution chains having a common ancestry.
Rewriting this into a backwards-evolution probability, the last term gives
dPbc (xb , xc , Q2 ) ='¯¯¯ dQ2 ¯ α
xa fa (xa , Q2 )¯¯ sz(1 − z)Pa→bc (z)¯ 2 ¯¯ Q ¯ 2π x x f (2) (x , x , Q2 )b c bcbc¯¯¯ dQ2 ¯ α
xa fa (xa , Q2 )
¯
¯ s
¯ 2 ¯
z(1 − z)Pa→bc (z) ,¯ Q ¯ 2π xb fb (xb , Q2 ) xc fc (xc , Q2 )

(232)

(2)

introducing the approximation fbc (xb , xc , Q2 ) ' fb (xb , Q2 ) fc (xc , Q2 ) to put the equationin terms of more familiar quantities. Just like for MI and normal ISR, a form factor isgiven by integration over the relevant Q2 range and exponentiation. In practice, eq. (230)
is complemented by one more term given by the above probability.

11.5

Pile-up Events

In high-luminosity colliders, there is a non-negligible probability that one single bunch
crossing may produce several separate events, so-called pile-up events. This in particularapplies to future pp colliders like LHC, but one could also consider e.g. e+ e− colliders
with high rates of γγ collisions. The program therefore contains an option, currently only
applicable to hadron–hadron collisions, wherein several events may be generated and put
one after the other in the event record, to simulate the full amount of particle production
a detector might be facing.
345

The program needs to know the assumed luminosity per bunch–bunch crossing, expressed in mb−1 . Multiplied by the cross section for pile-up processes studied, σpile , thisgives the average number of collisions per beam crossing, n. These pile-up events aretaken to be of the minimum-bias type, with diffractive and elastic events included or not(and a further subdivision of diffractive events into single and double). This means that
σpile may be either σtot , σtot − σel or σtot − σel − σdiffr . Which option to choose depends
on the detector: most detectors would not be able to observe elastic pp scattering, andtherefore it would be superfluous to generate that kind of events. In addition, we allow for
the possibility that one interaction may be of a rare kind, selected freely by you. There is
no option to generate two ‘rare’ events in the same crossing; normally the likelihood forthat kind of occurrences should be small.
If only minimum-bias type events are generated, i.e. if only one cross section is involved in the problem, then the number of events in a crossing is distributed according
to a Poisson with the average number n as calculated above. The program actually will
simulate only those beam crossings where at least one event occurs, i.e. not consider
the fraction exp(−n) of zero-event crossings. Therefore the actually generated average
number of pile-up events is hni = n/(1 − exp(−n)).
Now instead consider the other extreme, where one event is supposed be rare, with across section σrare much smaller than σpile , i.e. f ≡ σrare /σpile ¿ 1. The probability thata bunch crossing will give i events, whereof one of the rare kind, now is
Pi = f i exp(−n)

nini−1= f n exp(−n).
i!
(i − 1)!

(233)

The naı̈ve Poisson is suppressed by a factor f , since one of the events is rare rather thanof the normal kind, but enhanced by a factor i, since any one of the i events may be therare one. As the equality shows, the probability distribution is now a Poisson in i − 1:
in a beam crossing which produces one rare event, the multiplicity of additional pile-up
events is distributed according to a Poisson with average number n. The total average
number of events thus is hni = n + 1.
Clearly, for processes with intermediate cross sections, n σrare /σpile ' 1, also the average number of events will be intermediate, and it is not allowed to assume only one event
to be of the ‘rare’ type. We do not consider that kind of situations.
Each pileup event can be assigned a separate collision vertex within the envelope
provided by the colliding beams, see MSTP(151). Only simple Gaussian shapes in space
and time are implemented internally, however. If this is too restrictive, you would haveto assign interaction points yourself, and then shift each event separately by the required
amount in space and time.
When the pile-up option is used, one main limitation is that event records may become
very large when several events are put one after the other, so that the space limit in the
PYJETS common block is reached. It is possible to expand the dimension of the common
block, see MSTU(4) and MSTU(5), but only up to about 20 000 entries, which may notalways be enough, especially not for LHC. Simplifications like switching off π 0 decay may
help keep down the size, but also has its limits.
For practical reasons, the program will only allow an n up to 120. The multiplicity
distribution is truncated above 200, or when the probability for a multiplicity has fallen
below 10−6 , whichever occurs sooner. Also low multiplicities with probabilities below 10−6
are truncated.

11.6

Common-Block Variables and Routines

Of the routines used to generate beam remnants, multiple interactions and pile-up events,
none are intended to be called directly by the user. The only way to regulate these aspects346

is therefore via the variables in the PYPARS common block. Therefore most emphasis isput on the latter, while routines and additional commonblocks are listed more briefly.
COMMON/PYPARS/MSTP(200),PARP(200),MSTI(200),PARI(200)
Purpose: to give access to a number of status codes and parameters which regulate
the performance of Pythia. Most parameters are described in section 9.3; here
only those related to beam remnants, multiple interactions and pile-up events aredescribed. If the default values, below denoted by (D = . . . ), are not satisfactory,
they must in general be changed before the PYINIT call. Exceptions, i.e. variables
which can be changed for each new event, are denoted by (C).
MSTP(81) : (D = 1) master switch for multiple interactions (MI), and also for the associated treatment of initial- and final-state showers and beam remnants. Its
meaning depends on whether PYEVNT (old and intermediate models) or PYEVNW(new model) are called.
= 0 : MI off; old model (new model if PYEVNW called directly).
= 1 : MI on; old model (new model if PYEVNW called directly).
= 10 : MI off; intermediate model (new model if PYEVNW called directly).
= 11 : MI on; intermediate model (new model if PYEVNW called directly).
= 20 : MI off; new model for PYEVNT and PYEVNW alike.
= 21 : MI on; new model for PYEVNT and PYEVNW alike.
Note: when PYEVNT is called for the options = 20 and = 21, the one and only action
this routine takes is to hand on execution to PYEVNW. Whenever, anywhere
else in this manual, a distinction is made between PYEVNT and PYEVNW, for =20 and = 21 it is the PYEVNW rules that apply. The only reason for hooking
up PYEVNT and PYEVNW this way is to allow a uniform interface to someexperimental setups, where the only way to steer program execution is by acards file.
Warning: many parameters have to be tuned differently for the old and new scenarios, such as PARP(81) - PARP(84), PARP(89) and PARP(90), and others are
specific to each scenario. In addition, the optimal parameter values dependon the choice of parton densities and so on. Therefore you must pick a
consistent set of values, rather than simply changing MSTP(81) byitself.
An example, for the old multiple interactions scenario, is R.D. Field’s Tune
A [Fie02] (which agrees with the current default, except for PARP(90)):
MSTP(81) = 1, MSTP(82) = 4, PARP(67) = 4.0, PARP(82) = 2.0,
PARP(83) = 0.5, PARP(84) = 0.4, PARP(85) = 0.9, PARP(86) = 0.95,
PARP(89) = 1800.0 and PARP(90) = 0.25, with CTEQ5L (default).
The same values cannot be used with the new scenario available with PYEVNW.
Specifically, the fact that each multiple interaction here can radiate on its
own, which only is the case for the first one in PYEVNT, means that each gives
more activity. Thus fewer interactions, i.e. a higher p⊥0 scale, is required.
Also other parameters are used in the new scenario. No tune of equivalentquality to Tune A is available so far. An example of a possible set is (wheresome are default values while others need to be set):
New model, smooth ISR, high FSR: MSTP(81) = 21, MSTP(70) = 2,
MSTP(72) = 2;p⊥0 and reconnect: PARP(82) = 2.5D0, MSTP(95) = 1, PARP(78) = 1.3D0;
ExpOfPow(1.8) overlap profile: MSTP(82) = 5, PARP(83) = 1.8D0;
Reference energy and rescaling pace: PARP(89) = 1800D0,
PARP(90) = 0.25D0;

347

ΛFSR scale: PARJ(81) = 0.14D0;
Beam remnants: MSTP(89) = 1, MSTP(88) = 0, PARP(79) = 2D0,
PARP(80) = 0.01D0.
MSTP(82) : (D = 4) structure of multiple interactions. For QCD processes, used down top⊥ values below p⊥min , it also affects the choice of structure for the one hard/semihard interaction.
= 0 : simple two-string model without any hard interactions. Toy model only!
= 1 : multiple interactions assuming the same probability in all events, with anabrupt p⊥min cut-off at PARP(81). (With a slow energy dependence given
by PARP(89) and PARP(90).) This option has not been implemented for the
new model in PYEVNW.
= 2 : multiple interactions assuming the same probability in all events, with acontinuous turn-off of the cross section at p⊥0 =PARP(82). (With a slow
energy dependence given by PARP(89) and PARP(90).)= 3 : multiple interactions assuming a varying impact parameter and a hadronicmatter overlap consistent with a Gaussian matter distribution, with a continuous turn-off of the cross section at p⊥0 =PARP(82). (With a slow energy
dependence given by PARP(89) and PARP(90).)= 4 : multiple interactions assuming a varying impact parameter and a hadronicmatter overlap consistent with a double Gaussian matter distribution given
by PARP(83) and PARP(84), with a continuous turn-off of the cross section
at p⊥0 =PARP(82). (With a slow energy dependence given by PARP(89) andPARP(90).)= 5 : multiple interactions assuming a varying impact parameter and a hadronicmatter overlap O(b) ∝ exp(−bd ). This shape does not have to correspondto a simple functional form for the matter distributions themselves. The
power d =PARP(83) (note changed meaning of this parameter relative to
the MSTP(82) = 4 option) can be varied continuously, with a lower cutoff
at 0.4 for technical reasons. The physically interersting range is between 1
and 2, i.e. between an exponential and a Gaussian. As above, there is acontinuous turn-off of the cross section at p⊥0 =PARP(82). (With a slow
energy dependence given by PARP(89) and PARP(90).)
Note 1: for MSTP(82) ≥ 2 and CKIN(3) > PARP(82) (modulo the slow energy dependence noted above), cross sections given with PYSTAT(1) may be somewhat
too large, since (for reasons of efficiency) the probability factor that the hardinteraction is indeed the hardest in the event is not included in the cross sections. It is included in the event selection, however, so the events generated
are correctly distributed. For CKIN(3) values a couple of times larger than
PARP(82) this ceases to be a problem.
Note 2: the PARP(81) and PARP(82) values are sensitive to the choice of parton distributions, ΛQCD , etc., in the sense that a change in the latter variables leads
to a net change in the multiple-interaction rate, which has to be compensatedby a retuning of PARP(81) or PARP(82) if one wants to keep the net multipleinteraction structure the same. The default PARP(81) and PARP(82) values
are consistent with the other default values give, i.e. parton distributions of
the proton etc.
Note 3: the multiple interactions combination of MSTP(81) = 0 (or = 10, = 20)
and MSTP(82) ≥ 2 is not so common, and certainly not intended to simulate
realistic events, but it is an allowed way to obtain events with only thehardest interaction of those the events would contain with the corresponding
MSTP(81) = 1 scenario. If also CKIN(3) is set larger than the respective p⊥0
scale (see PARP(82)), however, the program misbehaves. To avoid this,MSTP(82) is now set to 1 in such cases, in PYINIT. This may require some348

extra care if a run contains a loop over various MSTP(81) and MSTP(82)values.
Note 4: for technical reasons, options 2 and above have not been implemented forγp and γγ physics, i.e. there only the option 1 is available for multiple
interactions. Furthermore (and partly related), it is not possible, at least
not without some precautions, to run the MSTP(82) ≥ 2 options with varying
beam energies, see 9.8.
MSTP(83) : (D = 100) number of Monte Carlo generated phase-space points per bin(whereof there are 20) in the initialization (in PYMULT) of multiple interactionsfor MSTP(82) ≥ 2.
MSTP(84) : (D = 1) switch for initial-state radiation in interactions after the first one
in the intermediate and new models. (In the old model, no such radiation is
implemented.)= 0 : off.
= 1 : on, provided that MSTP(61) = 1.
Note : initial-state radiation in the first (hardest) interaction is not affected by
MSTP(84), but only by MSTP(61).
MSTP(85) : (D = 1) switch for final-state radiation in interactions after the first one
in the intermediate and new models. (In the old model, no such radiation is
implemented.)= 0 : off.
= 1 : on, provided that MSTP(71) = 1.
Note : final-state radiation in the first (hardest) interaction is not affected by
MSTP(85), but only by MSTP(71).
MSTP(86) : (D = 2) requirements on multiple interactions based on the hardness scaleof the main process.
= 1 : the main collision is harder than all the subsequent ones. This is the oldbehaviour, preserved for reasons of backwards compatibility, and most of
the time quite sensible, but with dangers as follows.
The traditional multiple interactions procedure is to let the main interaction
set the upper p⊥ scale for subsequent multiple interactions. For QCD, thisis a matter of avoiding double-counting. Other processes normally are hard,so the procedure is then also sensible. However, for a soft main interaction,
further softer interactions are hardly possible, i.e. multiple interactions are
more or less killed. Such a behaviour could be motivated by the rejected
events instead appearing as part of the interactions underneath a normal
QCD hard interaction, but in practice the latter mechanism is not implemented. (And would have been very inefficient to work with, had it been.)
For MSTP(82) ≥ 3 it is even worse, since also the events themselves are likely
to be rejected in the impact-parameter selection stage. Thus the spectrumof main events that survive is biased, with the low-p⊥ , soft tail suppressed.
Furthermore, even when events are rejected by the impact-parameter procedure, this is not reflected in the cross section for the process, as it shouldhave been. Results may thus be misleading.
= 2 : when the main process is of the QCD jets type (the same as those in multiple
interactions) subsequent interactions are requested to be softer, but for otherprocesses no such requirement exists.
= 3 : no requirements at all that multiple interactions have to be softer than themain interactions (of dubious use for QCD processes but intended for crosschecks).
Note: process cross sections are unreliable whenever the main process does restrict
subsequent interactions, and the main process can become soft. For QCD
jet studies in this region it is then better to put CKIN(3) = 0 and get the349

‘correct’ total cross section.
MSTP(87) : (D = 4) when a sea quark (antiquark) is picked from a hadron at some xs
value in the new model, there has to be a companion sea antiquark (quark) atsome other xc value. The xc distribution is assumed given by a convolution ofa mother gluon distribution g(x = xs + xc ) with the perturbative g → q + q
DGLAP splitting kernel. The simple ansatz g(x) = N (1 − x)n /x is used, where
N is a normalization constant and n = MSTP(87). MSTP(87) thus controls thelarge-x behaviour of the assumed gluon distribution. Only integers MSTP(87)= 0 - 4 are available; values below or above this range are set at the lower orupper limit, respectively.
MSTP(88) : (D = 1) strategy for the collapse of a quark–quark–junction configuration
to a diquark, or a quark–quark–junction–quark configuration to a baryon, in abeam remnant in the new model.
= 0 : only allowed when valence quarks only are involved.
= 1 : sea quarks can be used for diquark formation, but not for baryon formation.
= 2 : sea quarks can be used also for baryon formation.
MSTP(89) : (D = 1) Selection of method for colour connections in the initial stateof the new model. Note that all options respect the suppression provided by
PARP(80).
= 0 : random.
= 1 : the hard-scattering systems are ordered in rapidity. The initiators on eachside are connected so as to minimize the rapidity difference between neighbouring systems.
= 2 : each connection is chosen so as to minimize an estimate of the total string
length resulting from it. (This is the most technically complicated, andhence a computationally slow approach.)
MSTP(90) : (D = 0) strategy to compensate the ‘primordial k⊥ ’ assigned to a partonshower initiator or beam-remnant parton in the new model.
= 0 : all other such partons compensate uniformly.
= 1 : compensation spread out across colour chain as (1/2)n , where n is numberof steps the parton is removed in the chain.
= 2 : nearest colour neighbours only compensate.
MSTP(91) : (D = 1) (C) primordial k⊥ distribution in hadron. See MSTP(93) for photon.
= 0 : no primordial k⊥ .
= 1 : Gaussian, width given in PARP(91), upper cut-off in PARP(93).
= 2 : exponential, width given in PARP(92), upper cut-off in PARP(93). Not available in the new model.
26= 3 : distribution proportional to 1/(k⊥+3σ 2 /2)3 , i.e. with 1/k⊥tails. RMS width
given in PARP(91) and upper cutoff in PARP(93). Not available in the oldmodel.
= 4 : flat distribution (on limited interval), RMS width given in PARP(91) andupper cutoff in PARP(93). Not available in the old model.
= 11 : As 1, but width depends on the scale Q of the hard interaction, being the
maximum of the fragmentation width and 2.1 × Q/(7. + Q). Not available
in the old model.
= 13 : As 3, but the width scales with Q as for 11.
= 14 : As 4, but the width scales with Q as for 11.
Note: when multiple interactions are switched on, the distribution used for thesubsequent interactions vanishes in the old model, and has a RMS width
forced equal to the fragmentation one in the new model, but the shape still
follows the choice of MSTP(91).
MSTP(92) : (D = 3) (C) energy partitioning in hadron or resolved-photon remnant, when350

this remnant is split into two jets in the old model. (For a splitting into a hadronplus a jet, see MSTP(94).) The energy fraction χ taken by one of the two objects,
with conventions as described for PARP(94) and PARP(96), is chosen according to
the different distributions below. Here cmin = 0.6 GeV/Ecm ≈ 2hmq i/Ecm .
= 1 : 1 for meson or resolved photon, 2(1 − χ) for baryon, i.e. simple counting
rules.
= 2 : (k + 1)(1 − χ)k , with k given
by PARP(94) or PARP(96).
q= 3 : proportional to (1−χ)k / 4 χ2 + c2min , with k given by PARP(94) or PARP(96).
q

proportional to (1−χ)k / χ2 + c2min , with k given by PARP(94) or PARP(96).
proportional to (1 − χ)k /(χ2 + c2min )b/2 , with k given by PARP(94) or
PARP(96), and b by PARP(98).
MSTP(93) : (D = 1) (C) primordial k⊥ distribution in photon, either it is one of theincoming particles or inside an electron.
= 0 : no primordial k⊥ .
= 1 : Gaussian, width given in PARP(99), upper cut-off in PARP(100).
= 2 : exponential, width given in PARP(99), upper cut-off in PARP(100).
22 2
2) , with k⊥0 in PARP(99) and upper k⊥= 3 : power-like of the type dk⊥
/(k⊥0
+ k⊥
cut-off in PARP(100).
222= 4 : power-like of the type dk⊥
/(k⊥0
+ k⊥
), with k⊥0 in PARP(99) and upper k⊥
cut-off in PARP(100).
22
2= 5 : power-like of the type dk⊥
/(k⊥0
+ k⊥
), with k⊥0 in PARP(99) and upper k⊥
cut-off given by the p⊥ of the hard process or by PARP(100), whichever issmaller.
Note: for options 1 and 2 the PARP(100) value is of minor importance, once
PARP(100)ÀPARP(99). However, options 3 and 4 correspond to distribu2
tions with infinite hk⊥
i if the k⊥ spectrum is not cut off, and therefore the
PARP(100) value is as important for the overall distribution as is PARP(99).
MSTP(94) : (D = 3) (C) energy partitioning in hadron or resolved-photon remnant, when
this remnant is split into a hadron plus a remainder-jet in the old model. Theenergy fraction χ is taken by one of the two objects, with conventions as described
below or for PARP(95) and PARP(97).
= 1 : 1 for meson or resolved photon, 2(1 − χ) for baryon, i.e. simple counting
rules.
= 2 : (k + 1)(1 − χ)k , with k given by PARP(95) or PARP(97).
= 3 : the χ of the hadron is selected according to the normal fragmentation function used for the hadron in jet fragmentation, see MSTJ(11). The possibilityof a changed fragmentation function shape in diquark fragmentation (seePARJ(45)) is not included.
= 4 : as = 3, but the shape is changed as allowed in diquark fragmentation (seePARJ(45)); this change is here also allowed for meson production. (This
option is not so natural for mesons, but has been added to provide the sameamount of freedom as for baryons).
MSTP(95) : (D = 1) selection of method for colour reconnections in the final state in
the new model. For MSTP(95) = 1, the amount of reconnections is controlled
by PARP(78). For the remaining ‘colour annealing’ options, the strength was
not variable before version 6.402. In subsequent versions, the probability for agiven string piece to retain its colour history and hence not participate in theannealing is
Pkeep = (1 − PARP(78))nMI ,
(234)
= 4 :
= 5 :

where nMI is the number of interactions that occurred in the current event, making the reconnection probability for any given string piece larger in events withmany interactions. Note that the meaning of this switch differs between events351

generated with the new model (in PYEVNW) and ones generated with the intermediate model (in PYEVNT). For the former case, the reconnections are performed
after multiple interactions and the initial-state showers have been generated,but before the final-state showers, whereas in the latter case also the final-state
showers are generated before reconnections are tried. (In the old model, instead PARP(85) and PARP(86) regulate colour connections, but in a differentway.)= 0 : off.
= 1 : on. (NB: reconnections are still only allowed if there was actually more than
one interaction in the given event, (1) since the colour flow of 1 interaction
should be defined entirely by the shower, and (2) since one otherwise obtainsa (probably unwanted) component of diffractive topologies.)= 2 : on for hadron–hadron collisions simulated with PYEVNW, else off. Is based on
the new approach described in section 11.4, where an attempt is made tominimize the total string length λ. In this variant, closed loops of colourconnected gluons are only accepted if all else fails. In Pythia 6.402 andlater, strength of effect is controlled by PARP(78) as described above.
= 3 : on for all collisions simulated with PYEVNW, else off. Same model as = 2.
= 4 : same as = 2, but closed loops of colour-connected gluons are not suppressed.
= 5 : same as = 3, but closed loops of colour-connected gluons are not suppressed.
= 6 : same as = 2, but only free triplets (including also gluons with one stringpiece already attached) are allowed to initiate string pieces.
= 7 : same as = 3, but only free triplets (including also gluons with one stringpiece already attached) are allowed to initiate string pieces.
MSTP(96) : (D = 0) joined interactions (JI) on/off. JI is the possibility that partonsentering different multiple interactions turn out to stem from a common motherwhen the initial-state showers are traced backwards.
= 0 : no joinings allowed.
= 1 : p⊥ scales of joinings are found, but joinings are not actually performed.
= 2 : (joinings allowed — not yet implemented).
Note: at present, only the splitting kernels have been implemented, allowing to estimate the overall rates and p⊥ spectra of joinings, but the actual kinematics
has not yet been worked out, so that only ‘trial joinings’ can be generated.
Even if switched on, joinings will therefore not at present occur physicallyin the generated events.
MSTP(131) : (D = 0) master switch for pile-up events, i.e. several independent hadron–hadron interactions generated in the same bunch–bunch crossing, with the events
following one after the other in the event record.
= 0 : off, i.e. only one event is generated at a time.
= 1 : on, i.e. several events are allowed in the same event record. Information on
the processes generated may be found in MSTI(41) - MSTI(50).
MSTP(132) : (D = 4) the processes that are switched on for pile-up events. The first event
may be set up completely arbitrarily, using the switches in the PYSUBS common
block, while all the subsequent events have to be of one of the ‘inclusive’ processes
which dominate the cross section, according to the options below. It is thus notpossible to generate two rare events in the pile-up option.
= 1 : low-p⊥ processes (ISUB = 95) only. The low-p⊥ model actually used, bothin the hard event and in the pile-up events, is the one set by MSTP(81) etc.
This means that implicitly also high-p⊥ jets can be generated in the pile-up
events.
= 2 : low-p⊥ + double diffractive processes (ISUB = 95 and 94).
= 3 : low-p⊥ + double diffractive + single diffractive processes (ISUB = 95, 94, 93
and 92).
352

= 4 :

low-p⊥ + double diffractive + single diffractive + elastic processes, together
corresponding to the full hadron–hadron cross section (ISUB = 95, 94, 93,
92 and 91).
MSTP(133) : (D = 0) multiplicity distribution of pile-up events.
= 0 : selected by you, before each PYEVNT call, by giving the MSTP(134) value.
= 1 : a Poisson multiplicity distribution in the total number of pile-up events. Thisis the relevant distribution if the switches set for the first event in PYSUBS givethe same subprocesses as are implied by MSTP(132). In that case the mean
number of events per beam crossing is n = σpile ×PARP(131), where σpile isthe sum of the cross section for allowed processes. Since bunch crossings
which do not give any events at all (probability exp(−n)) are not simulated,
the actual average number per PYEVNT call is hni = n/(1 − exp(−n)).
= 2 : a biased distribution, as is relevant when one of the events to be generated
is assumed to belong to an event class with a cross section much smaller
than the total hadronic cross section. If σrare is the cross section for this rare
process (or the sum of the cross sections of several rare processes) and σpile
the cross section for the processes allowed by MSTP(132), then define n =σpile ×PARP(131) and f = σrare /σpile . The probability that a bunch crossing
will give i events is then Pi = f i exp(−n) ni /i!, i.e. the naı̈ve Poisson issuppressed by a factor f since one of the events will be rare rather thanfrequent, but enhanced by a factor i since any of the i events may be therare one. Only beam crossings which give at least one event of the requiredrare type are simulated, and the distribution above normalized accordingly.
Note: for practical reasons, it is required that n < 120, i.e. that an average beamcrossing does not contain more than 120 pile-up events. The multiplicity
distribution is truncated above 200, or when the probability for a multiplicity
has fallen below 10−6 , whichever occurs sooner. Also low multiplicities withprobabilities below 10−6 are truncated. See also PARI(91) - PARI(93).
MSTP(134) : (D = 1) a user-selected multiplicity, i.e. total number of pile-up events, tobe generated in the next PYEVNT call when MSTP(133) = 0. May be reset foreach new event, but must be in the range 1 ≤MSTP(134)≤ 200.
PARP(78) : (D = 0.025) parameter controlling the amount of colour reconnection in
the final state (the F and F 0 parameters in [Sjö04, Sjö04a]), when MSTP(95)=1.
Since Pythia version 6.402 PARP(78) also controls the amount of colour reconnection for the ‘colour annealing’ scenarios MSTP(95) > 1, but then has adifferent meaning than described here, see the MSTP(95) manual entry for details. For MSTP(95) = 1, try a fraction PARP(78) of the total number of possible
reconnections. Perform all reconnections which reduce the total string length
and which are consistent with the choice of strategy in MSTP(90). If at least one
reconnection is successfully made, loop back and try again, else keep the finalstate topology arrived to at this point. Note that a random colour reconnection
is tested each time, so that the same reconnection may be tried twice and some
not at all even when PARP(78) = 1D0. Thus, values somewhat larger than 1D0
are necessary if the ‘most extreme case’ scenario is desired. Also note that themeaning of this parameter will most likely be changed in future updates. At
present, it merely represents a crude way of turning up and down the amount ofcolour reconnections going on in the final state.
PARP(79) : (D = 2.0) enhancement factor applied when assigning x to composite systems(e.g. diquarks) in the beam remnant in the new model. Modulo a global rescaling(= normalization) of all the BR parton x values, the x of a composite system is
PARP(79) times the trivial sum over x values of its individual constituents.
PARP(80) : (D = 0.1) when colours of partons kicked out from a beam remnant are to353

be attached to the remnant, PARP(80) gives a suppression for the probability of
attaching those partons to the colour line between two partons which themselves
both lie in the remnant. A smaller value thus corresponds to a smaller probability
that several string pieces will go back and forth between the beam remnant andthe hard-scattering systems.
PARP(81) : (D = 1.9 GeV) effective minimum transverse momentum p⊥min for multiple
interactions with MSTP(82) = 1, at the reference energy scale PARP(89), withthe degree of energy rescaling given by PARP(90). The optimal value depends ona number of other assumptions, especially which parton distributions are being
used. The default is intended for CTEQ 5L.
PARP(82) : (D = 2.0 GeV) regularization scale p⊥0 of the transverse-momentum spectrum for multiple interactions with MSTP(82) ≥ 2, at the reference energy scale
PARP(89), with the degree of energy rescaling given by PARP(90). (Current
default based on the MSTP(82) = 4 option, without any change of MSTP(2) orMSTP(33).) The optimal value depends on a number of other assumptions, especially which parton distributions are being used. The default is intended for
CTEQ 5L.
PARP(83), PARP(84) : (D = 0.5, 0.4) parameters of the assumed matter overlap between the two colliding hadrons. For MSTP(82) = 4, a double Gaussian matter
distribution of the form given in eq. (212) is used, i.e. with a core of radius
PARP(84) of the main radius and containing a fraction PARP(83) of the total
hadronic matter. For MSTP(82) = 5 the hadronic matter overlap (i.e. the convolution of the two colliding matter distributions) is O(b) ∝ exp(−bd ), with powerd =PARP(83) while PARP(84 is unused. The interesting range of PARP(83) values
is different in the two cases, so this parameter should always be updated when
MSTP(82) is changed.
PARP(85) : (D = 0.9) probability that an additional interaction in the old multipleinteraction formalism gives two gluons, with colour connections to ‘nearest neighbours’ in momentum space.
PARP(86) : (D = 0.95) probability that an additional interaction in the old multipleinteraction formalism gives two gluons, either as described in PARP(85) or as aclosed gluon loop. Remaining fraction is supposed to consist of quark–antiquark
pairs.
PARP(87), PARP(88) : (D = 0.7, 0.5) in order to account for an assumed dominance ofvalence quarks at low transverse momentum scales, a probability is introduced
that a gg-scattering according to naı̈ve cross section is replaced by a qq one; thisis used only for MSTP(82) ≥ 2. The probability is parameterized as P = a(1 −(p2⊥ /(p2⊥ + b2 ))2 ), where a =PARP(87) and b =PARP(88)×PARP(82) (including
the slow energy rescaling of the p⊥0 parameter).
PARP(89) : (D = 1800. GeV) reference energy scale, at which PARP(81) and PARP(82)give the p⊥min and p⊥0 values directly. Has no physical meaning in itself, but is
PARP(90)
would have beenused for convenience only. (A form p⊥min = PARP(81)Ecm
equally possible but then with a less transparent meaning of PARP(81).) For
studies of the p⊥min dependence at some specific energy it may be convenient tochoose PARP(89) equal to this energy.
PARP(90) : (D = 0.16) power of the energy-rescaling term of the p⊥min and p⊥0 paramePARP(90). The default value is inspired byters, which are assumed proportional to Ecm
2×0.082², which= Ecm
the rise of the total cross section by the pomeron term, s² = Ecm
is not inconsistent with the small-x behaviour. It is also reasonably consistent
with the energy-dependence implied by a comparison with the UA5 multiplicity
distributions at 200 and 900 GeV [UA584]. PARP(90) = 0 is an allowed value,i.e. it is possible to have energy-independent parameters.
PARP(91) : (D = 2. GeV/c) (C) width of Gaussian primordial k⊥ distribution in354

2side hadron for MSTP(91) = 1, i.e. exp(−k⊥/σ 2 ) k⊥ dk⊥ with σ =PARP(91) and22hk⊥ i =PARP(91) .
Warning: the current default value is much larger than could be explained in purely
nonperturbative terms. It is thus likely that the large value compensates
for imperfections in the perturbative description and, as such, will have anenergy and process dependence. It may therefore be necessary to vary thisparameter for an optimal description.
PARP(92) : (D = 0.40 GeV/c) (C) width parameter of exponential primordial k⊥
distribution inside hadron for MSTP(91) = 2, i.e. exp(−k⊥ /σ) k⊥ dk⊥ with22
σ =PARP(92)√ and hk⊥ i = 6×PARP(92) . Thus one should put PARP(92) ≈PARP(91)/ 6 to have continuity with the option above.
PARP(93) : (D = 5. GeV/c) (C) upper cut-off for primordial k⊥ distribution inside
hadron.
PARP(94) : (D = 1.) (C) for MSTP(92) ≥ 2 this gives the value of the parameter k forthe case when a meson or resolved-photon remnant is split into two fragments(which is which is chosen at random).
PARP(95) : (D = 0.) (C) for MSTP(94) = 2 this gives the value of the parameter k forthe case when a meson or resolved-photon remnant is split into a meson and aspectator fragment jet, with χ giving the energy fraction taken by the meson.
PARP(96) : (D = 3.) (C) for MSTP(92) ≥ 2 this gives the value of the parameter k forthe case when a nucleon remnant is split into a diquark and a quark fragment,with χ giving the energy fraction taken by the quark jet.
PARP(97) : (D = 1.) (C) for MSTP(94) = 2 this gives the value of the parameter k forthe case when a nucleon remnant is split into a baryon and a quark jet or ameson and a diquark jet, with χ giving the energy fraction taken by the quark
jet or meson, respectively.
PARP(98) : (D = 0.75) (C) for MSTP(92) = 5 this gives the power of an assumed basic
1/χb behaviour in the splitting distribution, with b =PARP(98).
PARP(99) : (D = 1. GeV/c) (C) width parameter of primordial k⊥ distribution insidephoton; exact meaning depends on MSTP(93) value chosen (cf. PARP(91) andPARP(92) above).
PARP(100) : (D = 5. GeV/c) (C) upper cut-off for primordial k⊥ distribution insidephoton.
PARP(131) : (D = 0.01 mb−1 ) in the pile-up events scenario, PARP(131) gives the assumed luminosity per bunch–bunch crossing, i.e. if a subprocess has a crosssection σ, the average number of events of this type per bunch–bunch crossing
is n = σ×PARP(131). PARP(131) may be obtained by dividing the integrated
luminosity over a given time (1 s, say) by the number of bunch–bunch crossingsthat this corresponds to. Since the program will not generate more than 200
pile-up events, the initialization procedure will crash if n is above 120.
Further subroutines and commonblocks:
SUBROUTINE PYMULT(MMUL) : to generate semi-hard interactions according to the oldmultiple-interaction formalism.
SUBROUTINE PYREMN(IPU1,IPU2) : to add on target remnants and include primordialk⊥ according to the old beam-remnant treatment.
SUBROUTINE PYMIGN(MMUL) : initialises multiple interactions. Also generates multiple
interaction kinematics and flavours in the ‘intermediate model’ (equivalently to
PYMULT in the ‘old model’).
SUBROUTINE PYEVOL(MODE,PT2MAX,PT2MIN) : initializes and evolves an event consistingof a single 2 → 2 hard scattering down in p2⊥ from some PT2MAX scale down to
PT2MIN, with interleaved multiple interactions and initial-state radiation according to the new model.
MODE = -1 : initialize interleaved MI/ISR/JI evolution first time for each event.

355

Determine physical PT2MAX and PT2MIN scales.
MODE = 0 : (re-)initialize ISR/MI/JI evolution: resets relevant parameters (saved
during the first call). Is re-done every time an evolution must be restarted.
(This only applies to a restart of the entire evolution, not to the case when
PYEVOL is called multiple times for factorized evolution.)
MODE = 1 : evolve ISR/MI/JI from PT2MAX to PT2MIN.
MODE = 2 : finalize MI/ISR/JI evolution.
Note: multiple calls to PYEVOL using MODE = 1 can be made, with the PT2MIN
scale of one call being the PT2MAX scale of the next, producing a sequence offactorized evolution steps, if so desired. (Only relevant for some specialized
applications like L-CKKW style PS/ME matching [Cat01].)
SUBROUTINE PYPTMI(MODE,PT2NOW,PT2CUT,PT2,IFAIL) : initialises, generates, and accepts additional trial interactions in the new model. Also collapses the flavour
wave function of newly created quarks (from interactions or ISR branchings) intovalence, sea or companion quarks.
MODE = -1 : initialize MI from scratch.
MODE = 0 : generate trial interaction. Start at PT2NOW, solve Sudakov for PT2, abort
if below PT2CUT.
MODE = 1 : accept interaction at PT2NOW and store variables.
MODE = 2 : decide sea/valence/companion for kicked-out quark at PT2NOW.
PT2NOW : starting (max) p2⊥ scale for evolution.
PT2CUT : lower limit for evolution.
PT2 : result of evolution. Generated p2⊥ for trial interaction.
IFAIL : status return code.
= 0: all is well.
< 0: phase space exhausted, generation to be terminated.
> 0: additional interaction vetoed, but continue evolution.
SUBROUTINE PYMIHK : finds left-behind remnant flavour content and hooks up the colour
flow between the hard scattering and remnants in the new model (part of PYREMN
in the old scheme).
SUBROUTINE PYCTTR(I,KCS,IEND) : auxiliary to PYMIHK. Traces the colour flow in the
Pythia event record and translates it into a structure (/PYCTAG/) based on Les
Houches Accord style colour tags.
SUBROUTINE PYMIHG(JCP1,JCG1,JCP2,JCG2) : auxiliary routine to PYMIHK used to test
initial-state colour connections. Keeps track of which colour tags have beencollapsed with each other and tests whether a given new collapse will result insinglet gluons. Uses /PYCBLS/ to communicate with PYMIHK.
SUBROUTINE PYMIRM : picks primordial k⊥ and shares longitudinal momentum among
beam remnants in the new model (part of PYREMN in the old scheme).
FUNCTION PYFCMP(XC,XS,NPOW) : gives the approximate xf (x; xs ) distribution of a companion quark.
FUNCTION PYPCMP(XS,NPOW) : gives the approximate momentum integral of acompanion-quark xf (x) distribution.
COMMON/PYINTM/KFIVAL(2,3),NMI(2),IMI(2,800,2),NVC(2,-6:6),&XASSOC(2,-6:6,240),XPSVC(-6:6,-1:240),PVCTOT(2,-1:1),&XMI(2,240),Q2MI(240),IMISEP(0:240)
Purpose: to carry information relevant for multiple interactions in the new model.
KFIVAL(JS,J) : enumeration of the initial valence quark content on side JS. A baryon
requires all three slots J, while a meson only requires the first two, so that
KFIVAL(JS,3) = 0. The contents are initially zero for a γ, π 0 , K0S and KL , and356

are only set once a valence quark is kicked out. (For instance, a π 0 can be either
uu or dd, only a scattering will tell.)NMI(JS) : total number of scattered and remnant partons on side JS, including hypothesized gluons that each branch to a sea (anti)quark and its companion.
IMI(JS,I,1) : position in the /PYJETS/ event record of a scattered or remnant parton,or a hypothesized gluon that branches to a sea (anti)quark and its companion.
Here JS gives the side and 1 ≤ I ≤ NMI(JS) enumerates the partons.
IMI(JS,I,2) : nature of the partons described above.
= 0 : a valence quark or a gluon.
> 0 : parton is part of a sea quark-antiquark pair, and IMI(JS,I,2) gives the
position in the event record of the companion.
< 0 : flag that parton is a sea quark for which no companion has yet been defined;
useful at intermediate stages but should not be found in the final event.
NVC(JS,IFL) : total number of unmatched sea quarks on side JS of flavour IFL.
XASSOC(JS,IFL,I) : the x value of an unmatched sea quark, on side JS of flavour IFL
and with 1 ≤ I ≤ NVC(JS,IFL). When a companion quark is found then bothXASSOC and NVC are updated.
XPSVC(IFL,J) : subdivision of the parton density xf (x) for flavour IFL obtained by a
PYPDFU call (with MINT(30) = side JS).
J = -1 : sea contribution.
J = 0 : valence contribution.
1 ≤ J ≤ NVC(JS,IFL) : sea companion contribution from each of the above unmatched sea quarks.
PVCTOT(JS,J) : momentum fraction, i.e. integral of xf (x), inside the remaining hadronon side JS.
J = -1 : total carried originally by valence quarks.
J = 0 : total carried by valence quarks that have been removed.
J = 1 : total carried by unmatched sea companion quarks.
XMI(JS,I) : x value of incoming parton on side JS for interaction number I, 1 ≤ I ≤MINT(31). When there is an initial-state shower, x refers to the initiator of thisshower rather than to the incoming parton at the hard interaction. The x values
are normalized to the full energy of the incoming hadron, i.e. are not rescaled byprevious interactions.
Q2MI(I) : the Q2 scale of the hard interaction number I, 1 ≤ I ≤ MINT(31). This is
the scale stored in VINT(54), i.e. the Q2 used to evaluate parton densities at thehard scattering.
IMISEP(I) : last line in the event record filled by partons belonging to the interaction
number I, 1 ≤ I ≤ MINT(31). Thus interaction I is stored in lines IMISEP(I-1)+ 1 through IMISEP(I), where IMISEP(0) has been set so that this holds also
for I = 1.
COMMON/PYCBLS/MCO(4000,2),NCC,JCCO(4000,2),JCCN(4000,2),MACCPT
Purpose: to store temporary colour-tag information while hooking up initial state; used
to communicate between PYMIHK and PYMIHG.
MCO(I,J) : original colour tag J of parton I before initial-state connection procedure
was started.
NCC : number of colour tags that have so far been collapsed.
JCCO(I,J) : colour tag collapse array, I ≤ NCC. The I’th collapse is between colour tags
JCCO(I,1) and JCCO(I,2).
JCCN(I,J) : temporary storage for collapse being tested. If collapse is accepted, JCCO
is set equal to JCCN.

357

COMMON/PYCTAG/NCT, MCT(4000,2)
Purpose: to carry Les Houches Accord style colour tag information for partons in theevent record.
NCT : number of colour lines in the event, from 1 to NCT.
MCT(I,1) : colour tag of parton I in /PYJETS/.
MCT(I,2) : anticolour tag of parton I in /PYJETS/.
COMMON/PYISMX/MIMX,JSMX,KFLAMX,KFLCMX,KFBEAM(2),NISGEN(2,240),&PT2MX,PT2AMX,ZMX,RM2CMX,Q2BMX,PHIMX
Purpose: to carry information on hardest initial-state radiation trial branching so far,during the interleaved generation with multiple interactions in PYEVOL. All quantities except NISGEN refer to this hardest trial branching so far.
MIMX : interaction number.
JSMX : side. JSMX determines which evolution step has ‘won’ in the PYEVOL evolution.
Its possible values are:= -1 : no evolution step was found above the current PT2MIN.
= 0 : an additional interaction should be generated.
= 1 : an ISR branching on side 1 (interaction number MIMX).
= 2 : an ISR branching on side 2 (interaction number MIMX).
= 3 : a joining on side 1 (interactions MJN1MX and MJN2MX).
= 4 : a joining on side 2 (interactions MJN1MX and MJN2MX).
KFLAMX : mother flavour.
KFLCMX : time-like sister flavour.
KFBEAM(JS) : KF code of beam on side JS.
NISGEN(JS,MI) : number of already generated ISR branchings on interaction number
MI, side JS.
PT2MX : evolution p2⊥ scale.
PT2AMX : physical p2⊥ value.
ZMX : z energy sharing fraction.
RM2CMX : time-like sister mass-squared.
Q2BMX : space-like virtuality.
PHIMX : azimuthal (φ) angle.
COMMON/PYISJN/MJN1MX,MJN2MX,MJOIND(2,240)
Purpose: to carry information on possible joinings, i,e. when two separate showers arereconstructed to a common ancestor in the backwards evolution.

358

12

Fragmentation

The main fragmentation option in Pythia is the Lund string scheme, but some simple
independent fragmentation options are also available. These latter options should not
be taken too seriously, since we know that independent fragmentation does not provide
a consistent alternative, but occasionally one may like to compare string fragmentation
with something else.
The subsequent four sections give further details; the first one on flavour selection,
which is common to the two approaches, the second on string fragmentation, the third
on independent fragmentation, while the fourth and final contains information on a few
other issues.
The Lund fragmentation model is described in [And83], where all the basic ideas arepresented and earlier papers [And79, And80, And82, And82a] summarized. The details
given there on how a multiparton jet system is allowed to fragment are out of date,however, and for this one should turn to [Sjö84]. Also the ‘popcorn’ baryon production
mechanism is not covered, see [And85], and [Edé97] for a more sophisticated version. The
most recent comprehensive description of the Lund model is found in [And98]. Reviewsof fragmentation models in general may be found in [Sjö88, Sjö89].

12.1

Flavour Selection

In either string or independent fragmentation, an iterative approach is used to describe
the fragmentation process. Given an initial quark q = q0 , it is assumed that a new q1 q1
pair may be created, such that a meson q0 q1 is formed, and a q1 is left behind. This q1
may at a later stage pair off with a q2 , and so on. What need be given is thus the relative
probabilities to produce the various possible qi qi pairs, uu, dd, ss, etc., and the relative
probabilities that a given qi−1 qi quark pair combination forms a specific meson, e.g. forud either π + , ρ+ or some higher state.
In Pythia, to first approximation it is assumed that the two aspects can be factorized,
i.e. that it is possible first to select a qi qi pair, without any reference to allowed physicalmeson states, and that, once the qi−1 qi flavour combination is given, it can be assigned
to a given meson state with total probability unity. Corrections to this factorized ansatz
will come especially in the baryon sector.
12.1.1

Quark flavours and transverse momenta

In order to generate the quark–antiquark pairs qi qi which lead to string breakups, theLund model invokes the idea of quantum mechanical tunnelling, as follows. If the qi andqi have no (common) mass or transverse momentum, the pair can classically be created at
one point and then be pulled apart by the field. If the quarks have mass and/or transverse
momentum, however, the qi and qi must classically be produced at a certain distance sothat the field energy between them can be transformed into the sum of the two transverse
masses m⊥ . Quantum mechanically, the quarks may be created in one point (so as to keep
the concept of local flavour conservation) and then tunnel out to the classically allowed
region. In terms of a common transverse mass m⊥ of the qi and the qi , the tunnellingprobability is given by
Ã

πm2⊥
exp −
κ

!

Ã

!

Ã

πm2
πp2
= exp −
exp − ⊥
κ
κ

!

,

(235)

where the string tension κ ≈ 1 GeV/fm ≈ 0.2 GeV2 .
The factorization of the transverse momentum and the mass terms leads to a flavourindependent Gaussian spectrum for the px and py components of qi qi pairs. Since the359

string is assumed to have no transverse excitations, this p⊥ is locally compensated between
the quark and the antiquark of the pair. The p⊥ of a meson qi−1 qi is given by the vector
sum of the√ p⊥ ’s of the qi−1 and qi constituents, which implies Gaussians in px and py witha width 2 that of the quarks themselves. The assumption of a Gaussian shape may be
a good first approximation, but there remains the possibility of non-Gaussian tails, thatcan be important in some situations.
In a perturbative QCD framework, a hard scattering is associated with gluon radiation,and further contributions to what is naı̈vely called fragmentation p⊥ comes from unresolved radiation. This is used as an explanation why the experimental hp⊥ i is somewhat
higher than obtained with the formula above.
The formula also implies a suppression of heavy quark production u : d : s : c ≈1 : 1 : 0.3 : 10−11 . Charm and heavier quarks are hence not expected to be produced in thesoft fragmentation. Since the predicted flavour suppressions are in terms of quark masses,
which are notoriously difficult to assign (should it be current algebra, or constituent, ormaybe something in between?), the suppression of ss production is left as a free parameterin the program: uu : dd : ss = 1 : 1 : γs , where by default γs = 0.3. At least qualitatively,
the experimental value agrees with theoretical prejudice. Currently there is no production
at all of heavier flavours in the fragmentation process, but only in the hard process or as
part of the shower evolution.
12.1.2

Meson production

Once the flavours qi−1 and qi have been selected, a choice is made between the possible
multiplets. The relative composition of different multiplets is not given from first principles, but must depend on the details of the fragmentation process. To some approximation
one would expect a negligible fraction of states with radial excitations or non-vanishingorbital angular momentum. Spin counting arguments would then suggest a 3:1 mixture
between the lowest lying vector and pseudoscalar multiplets. Wave function overlap arguments lead to a relative enhancement of the lighter pseudoscalar states, which is more
pronounced the larger the mass splitting is [And82a].
In the program, six meson multiplets are included. If the nonrelativistic classification
scheme is used, i.e. mesons are assigned a valence quark spin S and an internal orbital
angular momentum L, with the physical spin s denoted J, J = L + S, then the multipletsare:• L = 0, S = 0, J = 0: the ordinary pseudoscalar meson multiplet;• L = 0, S = 1, J = 1: the ordinary vector meson multiplet;• L = 1, S = 0, J = 1: an axial vector meson multiplet;• L = 1, S = 1, J = 0: the scalar meson multiplet;• L = 1, S = 1, J = 1: another axial vector meson multiplet; and• L = 1, S = 1, J = 2: the tensor meson multiplet.
Each multiplet has the full five-flavour setup of 5 × 5 states included in the program.
Some simplifications have been made; for instance there is no mixing included between
the two axial vector multiplets.
In the program, the spin S is first chosen to be either 0 or 1. This is done according toparameterized relative probabilities, where the probability for spin 1 by default is taken
to be 0.5 for a meson consisting only of u and d quark, 0.6 for one which contains s as
well, and 0.75 for quarks with c or heavier quark, in accordance with the deliberations
above.
By default, it is assumed that L = 0, such that only pseudoscalar and vector mesons
are produced. For inclusion of L = 1 production, four parameters can be used, one togive the probability that a S = 0 state also has L = 1, the other three for the probability
that a S = 1 state has L = 1 and J either 0, 1, or 2. Experimentally a non-negligible

360

rate of L = 1 production is observed. This is visible in the appropriate invariant-mass
plots, but does not have a significant impact on event shapes in general, i.e. comparably
good descriptions can be obtained with and without L = 1 mesons. The reason is likelythat the mass spectrum of intermediate string pieces, as reconstructed from the primary
hadrons, is not too dissimilar from a smeared-out setup with higher mesonic states. Thatis, the string model already implicitly contains higher-excited mesons.
For the flavour-diagonal meson states uu, dd and ss, it is also necessary to include
mixing into the physical mesons. This is done according to a parameterization, based on
the mixing angles given in the Review of Particle Properties [PDG88]. In particular, thedefault choices correspond to1
1(uu + dd) − √ ss ;2
2
1
1
(uu + dd) + √ ss ;
η0 =
2
2
1
ω = √ (uu + dd)
2
φ = ss ,
η =

(236)

i.e. ideal mixing in the ω − φ system and θP ∼ −10 deg in the η − η 0 system. In theπ 0 − η − η 0 system, no account is thus taken of the difference in masses, an approximation
which seems to lead to an overestimate of η 0 rates [ALE92]. Therefore parameters havebeen introduced to allow an additional ‘brute force’ suppression of η and η 0 states.
12.1.3

Baryon production

The mechanism for meson production follows rather naturally from the simple picture ofa meson as a short piece of string between two q/q endpoints. There is no unique recipe togeneralize this picture to baryons. The program actually contains three different scenarios:
diquark, simple popcorn, and advanced popcorn. In the diquark model the baryon andantibaryon are always produced as nearest neighbours along the string, while mesons may(but need not) be produced in between in the popcorn scenarios. The simpler popcorn
alternative admits at most one intermediate meson, while the advanced one allows many.
Further differences may be found, but several aspects are also common between the three
scenarios. Below they are therefore described in order of increasing sophistication. Finallythe application of the models to baryon remnant fragmentation, where a diquark originallysits at one endpoint of the string, is discussed.
Diquark picture
Baryon production may, in its simplest form, be obtained by assuming that any flavour
qi given above could represent either a quark or an antidiquark in a colour triplet state.
Then the same basic machinery can be run through as above, supplemented with theprobability to produce various diquark pairs. In principle, there is one parameter for eachdiquark, but if tunnelling is still assumed to give an effective description, mass relations
can be used to reduce the effective number of parameters. There are three main ones
appearing in the program:• the relative probability to pick a qq diquark rather than a q;
• the extra suppression associated with a diquark containing a strange quark (overand above the ordinary s/u suppression factor γs ); and• the suppression of spin 1 diquarks relative to spin 0 ones (apart from the factor of
3 enhancement of the former based on counting the number of spin states).
The extra strange diquark suppression factor comes about since what appears in the
exponent of the tunnelling formula is m2 and not m, so that the diquark and the strange
quark suppressions do not factorize.
361

Only two baryon multiplets are included, i.e. there are no L = 1 excited states. The
two multiplets are:• S = J = 1/2: the ‘octet’ multiplet of SU(3);• S = J = 3/2: the ‘decuplet’ multiplet of SU(3).
In contrast to the meson case, different flavour combinations have different numbers ofstates available: for uuu only ∆++ , whereas uds may become either Λ, Σ0 or Σ∗0 .
An important constraint is that a baryon is a symmetric state of three quarks, neglecting the colour degree of freedom. When a diquark and a quark are joined to form abaryon, the combination is therefore weighted with the probability that they form a symmetric three-quark state. The program implementation of this principle is to first selecta diquark at random, with the strangeness and spin 1 suppression factors above included,
but then to accept the selected diquark with a weight proportional to the number ofstates available for the quark-diquark combination. This means that, were it not for thetunnelling suppression factors, all states in the SU(6) (flavour SU(3) times spin SU(2))
56-multiplet would become equally populated. Of course also heavier baryons may comefrom the fragmentation of e.g. c quark jets, but although the particle classification scheme
used in the program is SU(10), i.e. with five flavours, all possible quark-diquark combinations can be related to SU(6) by symmetry arguments. As in the case for mesons, one
could imagine an explicit further suppression of the heavier spin 3/2 baryons.
In case of rejection, one again chooses between a diquark or a quark. If choosing
diquark, a new baryon is selected and tested, etc. (In versions earlier than Pythia 6.106,
the algorithm was instead to always produce a new diquark if the previous one had been
rejected. However, the probability that a quark will produce a baryon and a antidiquarkis then flavour independent, which is not in agreement with the model.) Calling thetunnelling factor for diquark D TD , the number of spin states σD and the SU(6) factor
for D and a quark q SUD,q , the model prediction for the (q → B + D)/(q → M + q0 )ratio isP (qq) XSq =TD σD SUD,q .
(237)
P (q) D
(Neglecting this flavour dependence e.g. leads to an enhancement of the Ω− relative toprimary proton production with approximately a factor 1.2,Pusing Jetset 7.4 default
values.) Since the chosen algorithm implies the normalization D TD σD = 1 and SUD,q ≤1, the final diquark production rate is somewhat reduced from the P (qq)/P (q) input value.
When a diquark has been fitted into a symmetrical three-particle state, it should
not suffer any further SU(6) suppressions. Thus the accompanying antidiquark should‘survive’ with probability 1. When producing a quark to go with a previously produced
diquark, this is achieved by testing the configuration against the proper SU(6) factor,
but in case of rejection keep the diquark and pick a new quark, which then is tested, etc.
There is no obvious corresponding algorithm available when a quark from one side anda diquark from the other are joined to form the last hadron of the string. In this case thequark is a member of a pair, in which the antiquark already has formed a specific hadron.
Thus the quark flavour cannot be reselected. One could consider the SU(6) rejection
as a major joining failure, and restart the fragmentation of the original string, but thenthe already accepted diquark does suffer extra SU(6) suppression. In the program thejoining of a quark and a diquark is always accepted.
Simple popcorn
A more general framework for baryon production is the ‘popcorn’ one [And85], inwhich diquarks as such are never produced, but rather baryons appear from the successive
production of several qi qi pairs. The picture is the following. Assume that the original q
is red r and the q is r. Normally a new q1 q1 pair produced in the field would also be rr,
so that the q1 is pulled towards the q end and vice versa, and two separate colour-singlet
systems qq1 and q1 q are formed. Occasionally, the q1 q1 pair may be e.g. gg (g = green),362

in which case there is no net colour field acting on either q1 or q1 . Therefore, the paircannot gain energy from the field, and normally would exist only as a fluctuation. If q1
moves towards q and q1 towards q, the net field remaining between q1 and q1 is bb (b =blue; g + r = b if only colour triplets are assumed). In this central field, an additional q2 q2
pair can be created, where q2 now is pulled towards qq1 and q2 towards qq1 , with no net
colour field between q2 and q2 . If this is all that happens, the baryon B will be made up
out of q1 , q2 and some q4 produced between q and q1 , and B of q1 , q2 and some q5 , i.e.
the B and B will be nearest neighbours in rank and share two quark pairs. Specifically,
q1 will gain energy from q2 in order to end up on mass shell, and the tunnelling formulafor an effective q1 q2 diquark is recovered.
Part of the time, several bb colour pair productions may take place between the q1
and q1 , however. With two production vertices q2 q2 and q3 q3 , a central meson q2 q3 may
be formed, surrounded by a baryon q4 q1 q2 and an antibaryon q3 q1 q5 . We call this a
BM B configuration to distinguish it from the q4 q1 q2 + q2 q1 q5 BB configuration above.
For BM B the B and B only share one quark–antiquark pair, as opposed to two for
BB configurations. The relative probability for a BM B configuration is given by theuncertainty relation suppression for having the q1 and q1 sufficiently far apart that ameson may be formed in between. The suppression of the BM B system is estimated by|∆F |2 ≈ exp(−2µ⊥ M⊥ /κ)(238)

where µ⊥ and M⊥ is the transverse mass of q1 and the meson, respectively. Strictly
speaking, also configurations like BM M B, BM M M B, etc. should be possible, but since
the total invariant M⊥ grows rapidly with the number of mesons, the probability for thisis small in the simple model. Further, since larger masses corresponds to longer stringpieces, the production of pseudoscalar mesons is favoured over that of vector ones. If only
BB and BM B states are included, and if the probability for having a vector meson M
is not suppressed extra, two partly compensating errors are made (since a vector meson
typically decays into two or more pseudoscalar ones).
In total, the flavour iteration procedure therefore contains the following possible subprocesses (plus, of course, their charge conjugates):• q1 → q2 + (q1 q2 ) meson;• q1 → q2 q3 + (q1 q2 q3 ) baryon;• q1 q2 → q3 + (q1 q2 q3 ) baryon;• q1 q2 → q1 q3 + (q2 q3 ) meson;with the constraint that the last process cannot be iterated to obtain several mesons in
between the baryon and the antibaryon.
When selecting flavours for qq → M + qq0 , the quark coming from the accepted qq
is kept, and the other member of qq0 , as well as the spin of qq0 , is chosen with weights
taking SU(6) symmetry into account. Thus the flavour of qq is not influenced by SU(6)factors for qq0 , but the flavour of M is.
Unfortunately, the resulting baryon production model has a fair number of parameters, which would be given by the model only if quark and diquark masses were known
unambiguously. We have already mentioned the s/u ratio and the qq/q one; the latterhas to be increased from 0.09 to 0.10 for the popcorn model, since the total number ofpossible baryon production configurations is lower in this case (the particle produced between the B and B is constrained to be a meson). With the improved SU(6) treatment
introduced in Pythia 6.106, a rejected q → B + qq may lead to the splitting q → M + q0
instead. This calls for an increase of the qq/q input ratio by approximately 10%. For the
popcorn model, exactly the same parameters as already found in the diquark model areneeded to describe the BB configurations. For BM B configurations, the square root of a
suppression factor should be applied if the factor is relevant only for one of the B and B,
e.g. if the B is formed with a spin 1 ‘diquark’ q1 q2 but the B with a spin 0 diquark q1 q3 .
363

Additional parameters include the relative probability for BM B configurations, which isassumed to be roughly 0.5 (with the remaining 0.5 being BB), a suppression factor forhaving a strange meson M between the B and B (as opposed to having a lighter nonstrange one) and a suppression factor for having a ss pair (rather than a uu one) shared
between the B and B of a BM B configuration. The default parameter values are basedon a combination of experimental observation and internal model predictions.
In the diquark model, a diquark is expected to have exactly the same transverse momentum distribution as a quark. For BM B configurations the situation is somewhat more
unclear, but we have checked that various possibilities give very similar results. The option implemented in the program is to assume no transverse momentum at all for the q1 q1
pair shared by the B and B, with all other pairs having the standard Gaussian spectrumwith local momentum conservation. This means that the B and B p⊥ ’s are uncorrelated
in a BM B configuration and (partially) anticorrelated in the BB configurations, with thesame mean transverse momentum for primary baryons as for primary mesons.
Advanced popcorn
In [Edé97], a revised popcorn model is presented, where the separate production of
the quarks in an effective diquark is taken more seriously. The production of a qq pairwhich breaks the string is in this model determined by eq. (235), also when ending up in adiquark. Furthermore, the popcorn model is re-implemented in such a way that eq (238)
could be used explicitly in the Monte Carlo. The two parametersβq ≡ 2 hµ⊥q i /κ, or βu and ∆β ≡ βs − βu ,(239)then govern both the diquark and the intermediate meson production. In this algorithm,
configurations like BM M B etc. are considered in a natural way. The more independent
production of the diquark partons implies a moderate suppression of spin 1 diquarks.
Instead the direct suppression of spin 3/2 baryons, in correspondence to the suppressionof vector mesons relative to pseudo-scalar ones, is assumed to be important. Consequently,
a suppression of Σ-states relative to Λ0 is derived from the spin 3/2 suppression parameter.
Several new routines have been added, and the diquark code has been extended withinformation about the curtain quark flavour, i.e. the qq pair that is shared between thebaryon and antibaryon, but this is not visible externally. Some parameters are no longer
used, while others have to be given modified values. This is described in section 14.3.1.
Baryon remnant fragmentation
Occasionally, the endpoint of a string is not a single parton, but a diquark or antidiquark, e.g. when a quark has been kicked out of a proton beam particle. One could
consider fairly complex schemes for the resulting fragmentation. One such [And81] wasavailable in Jetset version 6 but is no longer found here. Instead the same basic scheme
is used as for diquark pair production above. Thus a qq diquark endpoint is fragmented
just as if produced in the field behind a matching qq flavour, i.e. either the two quarksof the diquark enter into the same leading baryon, or else a meson is first produced, containing one of the quarks, while the other is contained in the baryon produced in the next
step.
Similarly, the revised algorithm for baron production can be applied to endpoint diquarks, though only with some care [Edé97]. The suppression factor for popcorn mesons
is derived from the assumption of colour fluctuations violating energy conservation andthus being suppressed by the Heisenberg uncertainty principle. When splitting an original diquark into two more independent quarks, the same kind of energy shift does not
obviously emerge. One could still expect large separations of the diquark constituents
to be suppressed, but the shape of this suppression is beyond the scope of the model.
For simplicity, the same kind of exponential suppression as in the ”true popcorn” case is
implemented in the program. However, there is little reason for the strength of the suppression to be exactly the same in the different situations. Thus the leading rank meson
production in a diquark jet is governed by a new β parameter, which is independent of364

the popcorn parameters βu and ∆β in eq. (239). Furthermore, in the process (original
diquark → baryon+q) the spin 3/2 suppression should not apply at full strength. This
suppression factor stems from the normalization of the overlapping q and q wavefunctions
in a newly produced qq pair, but in the process considered here, two out of three valencequarks already exist as an initial condition of the string.
The diquark picture above is not adequate when two or more quarks are kicked outof a proton. Below, in section 12.2.5, we will introduce string junctions to describe such
topologies.

12.2

String Fragmentation

An iterative procedure can also be used for other aspects of the fragmentation. This ispossible because, in the string picture, the various points where the string breaks arecausally disconnected. Whereas the space–time picture in the c.m. frame is such thatslow particles (in the middle of the system) are formed first, this ordering is Lorentzframe-dependent and hence irrelevant. One may therefore make the convenient choice ofstarting an iteration process at the ends of the string and proceeding towards the middle.
The string fragmentation scheme is rather complicated for a generic multiparton state.
In order to simplify the discussion, we will therefore start with the simple qq process, andonly later survey the complications that appear when additional gluons are present. (Thisdistinction is made for pedagogical reasons, in the program there is only one generalpurpose algorithm).
12.2.1

Fragmentation functions

Assume a qq jet system, in its c.m. frame, with the quark moving out in the +z direction
and the antiquark in the −z one. We have discussed how it is possible to start the flavour
iteration from the q end, i.e. pick a q1 q1 pair, form a hadron qq1 , etc. It has also been
noted that the tunnelling mechanism is assumed to give a transverse momentum p⊥ foreach new qi qi pair created, with the p⊥ locally compensated between the qi and the qi
member of the pair, and with a Gaussian distribution in px and py separately. In the
program, this is regulated by one parameter, which gives the root-mean-square p⊥ of aquark. Hadron transverse momenta are obtained as the sum of p⊥ ’s of the constituent qi
and qi+1 , where a diquark is considered just as a single quark.
What remains to be determined is the energy and longitudinal momentum of thehadron. In fact, only one variable can be selected independently, since the momentum of
the hadron is constrained by the already determined hadron transverse mass m⊥ ,(E + pz )(E − pz ) = E 2 − p2z = m2⊥ = m2 + p2x + p2y .

(240)

In an iteration from the quark end, one is led (by the desire for longitudinal boost invariance and other considerations) to select the z variable as the fraction of E + pz taken by
the hadron, out of the available E + pz . As hadrons are split off, the E + pz (and E − pz )left for subsequent steps is reduced accordingly:(E + pz )new = (1 − z)(E + pz )old ,m2⊥(E − pz )new = (E − pz )old −.
z(E + pz )old(241)

The fragmentation function f (z), which expresses the probability that a given z ispicked, could in principle be arbitrary — indeed, several such choices can be used inside
the program, see below.
If one, in addition, requires that the fragmentation process as a whole should look
the same, irrespectively of whether the iterative procedure is performed from the q end
365

or the q one, ‘left–right symmetry’, the choice is essentially unique [And83a]: the ‘Lund
symmetric fragmentation function’,µ11−z
f (z) ∝ z aα
z
z

¶aβ

Ã

bm2
exp − ⊥
z

!

.

(242)

There is one separate parameter a for each flavour, with the index α corresponding to the‘old’ flavour in the iteration process, and β to the ‘new’ flavour. It is customary to put
all aα,β the same, and thus arrive at the simplified expression
f (z) ∝ z −1 (1 − z)a exp(−bm2⊥ /z) .

(243)

In the program, only two separate a values can be given, that for quark pair production
and that for diquark one. In addition, there is the b parameter, which is universal.
The explicit mass dependence in f (z) implies a harder fragmentation function forheavier hadrons. The asymptotic behaviour of the mean z value for heavy hadrons ishzi ≈ 1 −

1+a
.
bm2⊥

(244)

Unfortunately it seems this predicts a somewhat harder spectrum for B mesons thanobserved in data. However, Bowler [Bow81] has shown, within the framework of the
Artru–Mennessier model [Art74], that a massive endpoint quark with mass mQ leads toa modification of the symmetric fragmentation function, due to the fact that the string
area swept out is reduced for massive endpoint quarks, compared with massless ditto.
The Artru–Mennessier model in principle only applies for clusters with a continuous mass
spectrum, and does not allow an a term (i.e. a ≡ 0); however, it has been shown [Mor89]
that, for a discrete mass spectrum, one may still retain an effective a term. In the program
an approximate form with an a term has therefore been used:f (z) ∝µ

12

z 1+rQ bmQ

z aα

1−z
z

¶aβ

Ã

bm2
exp − ⊥
z

!

.

(245)

In principle the prediction is that rQ ≡ 1, but so as to be able to extrapolate smoothly
between this form and the original Lund symmetric one, it is possible to pick rQ separately
for c and b hadrons.
For future reference we note that the derivation of f (z) as a by-product also gives theprobability distribution in proper time τ of qi qi breakup vertices. In terms of Γ = (κτ )2 ,this distribution isP(Γ) dΓ ∝ Γa exp(−bΓ) dΓ ,(246)
with the same a and b as above. The exponential decay allows an interpretation in termsof an area law for the colour flux [And98].
Many different other fragmentation functions have been proposed, and a few are available as options in the program.
• The Field–Feynman parameterization [Fie78],f (z) = 1 − a + 3a(1 − z)2 ,(247)

with default value a = 0.77, is intended to be used only for ordinary hadrons madeout of u, d and s quarks.
• Since there are indications that the shape above is too strongly peaked at z = 0,
instead a shape likef (z) = (1 + c)(1 − z)c(248)
may be used.
366

• Charm and bottom data clearly indicate the need for a harder fragmentation function for heavy flavours. The best known of these is the Peterson/SLAC formula[Pet83]
1
f (z) ∝ ³
(249)
²Q ´2 ,1z 1− z − 1−zwhere ²Q is a free parameter, expected to scale between flavours like ²Q ∝ 1/m2Q .
• As a crude alternative, that is also peaked at z = 1, one may use
f (z) = (1 + c)z c .

(250)• In [Edé97], it is argued that the quarks responsible for the colour fluctuations in
stepwise diquark production cannot move along the light-cones. Instead there is anarea of possible starting points for the colour fluctuation, which is essentially givenby the proper time of the vertex squared. By summing over all possible starting
points, one obtains the total weight for the colour fluctuation. The result is arelative suppression of diquark vertices at early times, which is found to be of theform 1 − exp(−ρΓ), where Γ ≡ κ2 τ 2 and ρ ≈ 0.7GeV−2 . This result, and especiallythe value of ρ, is independent of the fragmentation function, f (z), used to reach a
specific Γ-value. However, if using a f (z) which implies a small average value hΓi,the program implementation is such that a large fraction of the q → B +qq attempts
will be rejected. This dilutes the interpretation of the input P (qq)/P (q) parameter,
which needs to be significantly enhanced to compensate for the rejections.
A property of the Lund Symmetric Fragmentation Function is that the first vertices
produced near the string ends have a lower hΓi than central vertices. Thus an effectof the low-Γ suppression is a relative reduction of the leading baryons. The effect issmaller if the baryon is very heavy, as the large mass implies that the first vertex
almost reaches the central region. Thus the leading-baryon suppression effect isreduced for c- and b jets.
12.2.2

Joining the jets

The f (z) formula above is only valid, for the breakup of a jet system into a hadron plus
a remainder-system, when the remainder mass is large. If the fragmentation algorithm
were to be used all the way from the q end to the q one, the mass of the last hadron to beformed at the q end would be completely constrained by global energy and momentum
conservation, and could not be on its mass shell. In theory it is known how to take such
effects into account [Edé00], but the resulting formulae are wholly unsuitable for Monte
Carlo implementation.
The practical solution to this problem is to carry out the fragmentation both from theq and the q end, such that for each new step in the fragmentation process, a random choice
is made as to from what side the step is to be taken. If the step is on the q side, then z
is interpreted as fraction of the remaining E + pz of the system, while z is interpreted as
E − pz fraction for a step from the q end. At some point, when the remaining mass of thesystem has dropped below a given value, it is decided that the next breakup will produce
two final hadrons, rather than a hadron and a remainder-system. Since the momenta of
two hadrons are to be selected, rather than that of one only, there are enough degrees of
freedom to have both total energy and total momentum completely conserved.
The mass at which the normal fragmentation process is stopped and the final two
hadrons formed is not actually a free parameter of the model: it is given by the requirement that the string everywhere looks the same, i.e. that the rapidity spacing of the final
two hadrons, internally and with respect to surrounding hadrons, is the same as elsewherein the fragmentation process. The stopping mass, for a given setup of fragmentation parameters, has therefore been determined in separate runs. If the fragmentation parameters367are changed, some retuning should be done but, in practice, reasonable changes can bemade without any special arrangements.
Consider a fragmentation process which has already split off a number of hadrons fromthe q and q sides, leaving behind a a qi qj remainder system. When this system breaks by
the production of a qn qn pair, it is decided to make this pair the final one, and produce
the last two hadrons qi qn and qn qj , if22((E + pz )(E − pz ))remaining = Wrem
< Wmin.

(251)

The Wmin is calculated according toWmin = (Wmin0 + mqi + mqj + k mqn ) (1 ± δ) .

(252)

Here Wmin0 is the main free parameter, typically around 1 GeV, determined to give a flat
rapidity plateau (separately for each particle species), while the default k = 2 corresponds
to the mass of the final pair being taken fully into account. Smaller values may also be
considered, depending on what criteria are used to define the ‘best’ joining of the q andthe q chain. The factor 1 ±δ, by default evenly distributed between 0.8 and 1.2, signifies asmearing of the Wmin value, to avoid an abrupt and unphysical cut-off in the invariant mass
distribution of the final two hadrons. Still, this distribution will be somewhat different
from that of any two adjacent hadrons elsewhere. Due to the cut there will be no tail up
to very high masses; there are also fewer events close to the lower limit, where the two
hadrons are formed at rest with respect to each other.
This procedure does not work all that well for heavy flavours, since it does not fully
take into account the harder fragmentation function encountered. Therefore, in addition
to the check above, one further test is performed for charm and heavier flavours, as follows.
If the check above allows more particle production, a heavy hadron qi qn is formed, leaving
a remainder qn qj . The range of allowed z values, i.e. the fraction of remaining E + pz that
may be taken by the qi qn hadron, is constrained away from 0 and 1 by the qi qn mass andminimal mass of the qn qj system. The limits of the physical z range is obtained whenthe qn qj system only consists of one single particle, which then has a well-determined(0)transverse mass m⊥ . From the z value obtained with the infinite-energy fragmentation
function formulae, a rescaled z 0 value between these limits is given by



1

v
u






(0)2
(0)2 2
(0)2

2
2

m⊥nj
m⊥nj
m2⊥in m⊥nj u
m
m
u
⊥in
⊥in
0
z = 1 + 2 − 2 + t1 − 2 − 2  − 4 2
(2z
−
1)
2

2
Wrem Wrem
Wrem Wrem
Wrem Wrem. (253)(0)

From the z 0 value, the actual transverse mass m⊥nj ≥ m⊥nj of the qn qj system may be
calculated. For more than one particle to be produced out of this system, the requirementÃm2⊥nj = (1 − z 0 )

m2
2
Wrem
− ⊥in
0
z

!

> (mqj + Wmin0 )2 + p2⊥(254)

has to be fulfilled. If not, the qn qj system is assumed to collapse to one single particle.
The consequence of the procedure above is that, the more the infinite energy fragmentation function f (z) is peaked close to z = 1, the more likely it is that only two particles
are produced. The procedure above has been constructed so that the two-particle fraction
can be calculated directly from the shape of f (z) and the (approximate) mass spectrum,
but it is not unique. For the symmetric Lund fragmentation function, a number of alternatives tried all give essentially the same result, whereas other fragmentation functions
may be more sensitive to details.
368

Assume now that two final hadrons have been picked. If the transverse mass of theremainder-system is smaller than the sum of transverse masses of the final two hadrons,
the whole fragmentation chain is rejected, and started over from the q and q endpoints.
This does not introduce any significant bias, since the decision to reject a fragmentation
chain only depends on what happens in the very last step, specifically that the next-to-laststep took away too much energy, and not on what happened in the steps before that.
If, on the other hand, the remainder-mass is large enough, there are two kinematicallyallowed solutions for the final two hadrons: the two mirror images in the rest frame of theremainder-system. Also the choice between these two solutions is given by the consistency
requirements, and can be derived from studies of infinite-energy jets. The probability forthe reverse ordering, i.e. where the rapidity and the flavour orderings disagree, is givenby the area law asPreverse =1
1 + eb∆where ∆ = Γ2 − Γ1 =

q

2
2
2
2 − m22(Wrem⊥in + m⊥nj ) − 4m⊥in m⊥nj .

(255)
For the Lund symmetric fragmentation function, b is the familiar parameter, whereas forother functions the b value becomes an effective number to be fitted to the behaviour
when not in the joining region.
When baryon production is included, some particular problems arise (also see section
12.1.3). First consider BB situations. In the naı̈ve iterative scheme, away from the middleof the event, one already has a quark and is to choose a matching diquark flavour or theother way around. In either case the choice of the new flavour can be done taking into
account the number of SU(6) states available for the quark-diquark combination. For acase where the final qn qn breakup is an antidiquark-diquark one, the weights for forming
qi qn and qn qi enter at the same time, however. We do not know how to handle thisproblem; what is done is to use weights as usual for the qi qn baryon to select qn , but thenconsider qn qi as given (or the other way around with equal probability). If qn qi turns
out to be an antidiquark-diquark combination, the whole fragmentation chain is rejected,
since we do not know how to form corresponding hadrons. A similar problem arises, andis solved in the same spirit, for a BM B configuration in which the B (or B) was chosen
as third-last particle. When only two particles remain to be generated, it is obviously toolate to consider having a BM B configuration. This is as it should, however, as can befound by looking at all possible ways a hadron of given rank can be a baryon.
While some practical compromises have to be accepted in the joining procedure, thefact that the joining takes place in different parts of the string in different events means
that, in the end, essentially no visible effects remain.
12.2.3String motion and infrared stability

We have now discussed the string fragmentation scheme for the fragmentation of a simple
qq jet system. In order to understand how these results generalize to arbitrary jet systems,
it is first necessary to understand the string motion for the case when no fragmentation
takes place. In the following we will assume that quarks as well as gluons are massless,but the generalisation to massive quarks is relatively straightforward.
For a qq event viewed in the c.m. frame, with total energy W , the partons startmoving out back-to-back, carrying half the energy each. As they move apart, energy
and momentum is lost to the string. When the partons are a distance W/κ apart, all
the energy is stored in the string. The partons now turn around and come together
again with the original momentum vectors reversed. This corresponds to half a periodof the full string motion; the second half the process is repeated, mirror-imaged. For
further generalizations to multiparton systems, a convenient description of the energy
and momentum flow is given in terms of ‘genes’ [Art83], infinitesimal packets of the fourmomentum given up by the partons to the string. Genes with pz = E, emitted from the q369

end in the initial stages of the string motion above, will move in the q direction with the
speed of light, whereas genes with pz = −E given up by the q will move in the q direction.
Thus, in this simple case, the direction of motion for a gene is just opposite to that ofa free particle with the same four-momentum. This is due to the string tension. If thesystem is not viewed in the c.m. frame, the rules are that any parton gives up genes with
four-momentum proportional to its own four-momentum, but the direction of motion of
any gene is given by the momentum direction of the genes it meets, i.e. that were emitted
by the parton at the other end of that particular string piece. When the q has lost allits energy, the q genes, which before could not catch up with q, start impinging on it,
and the q is pulled back, accreting q genes in the process. When the q and q meet in theorigin again, they have completely traded genes with respect to the initial situation.
A 3-jet qqg event initially corresponds to having a string piece stretched between q and
g and another between g and q. Gluon four-momentum genes are thus flowing towards
the q and q. Correspondingly, q and q genes are flowing towards the g. When the gluon
has lost all its energy, the g genes continue moving apart, and instead a third string region
is formed in the ‘middle’ of the total string, consisting of overlapping q and q genes. The
two ‘corners’ on the string, separating the three string regions, are not of the gluon-kink
type: they do not carry any momentum.
If this third region would only appear at a time later than the typical time scale forfragmentation, it could not affect the sharing of energy between different particles. Thisis true in the limit of high energy, well separated partons. For a small gluon energy,
on the other hand, the third string region appears early, and the overall drawing of thestring becomes fairly 2-jet-like, since the third string region consists of q and q genes
and therefore behaves exactly as a string pulled out directly between the q and q. In
the limit of vanishing gluon energy, the two initial string regions collapse to naught, andthe ordinary 2-jet event is recovered [Sjö84]. Also for a collinear gluon, i.e. θqg (or θqg )small, the stretching becomes 2-jet-like. In particular, the q string endpoint first moves
out a distance pq /κ losing genes to the string, and then a further distance pg /κ, a firsthalf accreting genes from the g and the second half re-emitting them. (This latter half
actually includes yet another string piece; a corresponding piece appears at the q end,such that half a period of the system involves five different string regions.) The end result
is, approximately, that a string is drawn out as if there had only been a single parton withenergy |pq + pg |, such that the simple 2-jet event again is recovered in the limit θqg → 0.
These properties of the string motion are the reason why the string fragmentation scheme
is ‘infrared safe’ with respect to soft or collinear gluon emission.
The discussions for the 3-jet case can be generalized to the motion of a string with q
and q endpoints and an arbitrary number of intermediate gluons. For n partons, whereofn − 2 gluons, the original string contains n − 1 pieces. Anytime one of the original gluons
has lost its energy, a new string region is formed, delineated by a pair of ‘corners’. As
the extra ‘corners’ meet each other, old string regions vanish and new are created, so that
half a period of the string contains 2n2 − 6n + 5 different string regions. Each of theseregions can be understood simply as built up from the overlap of (opposite-moving) genes
from two of the original partons, according to well-specified rules.
12.2.4

Fragmentation of multiparton systems

The full machinery needed for a multiparton system is very complicated, and is describedin detail in [Sjö84]. The following outline is far from complete, and is complicated nonetheless. The main message to be conveyed is that a Lorentz covariant algorithm exists forhandling an arbitrary parton configuration, but that the necessary machinery is more
complex than in either cluster or independent fragmentation.
Assume n partons, with ordering along the string, and related four-momenta, givenby q(p1 )g(p2 )g(p3 ) · · · g(pn−1 )q(pn ). The initial string then contains n − 1 separate pieces.
370

The string piece between the quark and its neighbouring gluon is, in four-momentum(1)(1)
space, spanned by one side with four-momentum p+ = p1 and another with p− = p2 /2.
The factor of 1/2 in the second expression comes from the fact that the gluon shares its
energy between two string pieces. The indices ‘+’ and ‘−’ denotes direction towards theq and q end, respectively. The next string piece, counted from the quark end, is spanned
(2)
(2)
(n−1)
by p+ = p2 /2 and p− = p3 /2, and so on, with the last one being p+= pn−1 /2 and(n−1)
p−= pn .
(i)(i)2
For the algorithm to work, it is important that all p± be light-cone-like, i.e. p± = 0.
Since gluons are massless, it is only the two endpoint quarks which can cause problems.
The procedure here is to create new p± vectors for each of the two endpoint regions,defined to be linear combinations of the old p± ones for the same region, with coefficientsdetermined so that the new vectors are light-cone-like. De facto, this corresponds toreplacing a massive quark at the end of a string piece with a massless quark at the end ofa somewhat longer string piece. With the exception of the added fictitious piece, whichanyway ends up entirely within the heavy hadron produced from the heavy quark, thestring motion remains unchanged by this.
In the continued string motion, when new string regions appear as time goes by, the(j)first such string regions that appear can be represented as being spanned by one p+ and(k)another p− four-vector, with j and k not necessarily adjacent. For instance, in the qgq(3)(1)
case, the ‘third’ string region is spanned by p+ and p− . Later on in the string evolution
history, it is also possible to have regions made up of two p+ or two p− momenta. These
appear when an endpoint quark has lost all its original momentum, has accreted themomentum of a gluon, and is now re-emitting this momentum. In practice, these regions(j) (k)may be neglected. Therefore only pieces made up by a (p+ , p− ) pair of momenta areconsidered in the program.
The allowed string regions may be ordered in an abstract parameter plane, where the(j, k) indices of the four-momentum pairs define the position of each region along thetwo (parameter plane) coordinate axes. In this plane the fragmentation procedure canbe described as a sequence of steps, starting at the quark end, where (j, k) = (1, 1), andending at the antiquark one, (j, k) = (n−1, n−1). Each step is taken from an ‘old’ qi−1 qi−1
pair production vertex, to the production vertex of a ‘new’ qi qi pair, and the string piece
between these two string breaks represent a hadron. Some steps may be taken within
one and the same region, while others may have one vertex in one region and the othervertex in another region. Consistency requirements, like energy-momentum conservation,dictates that vertex j and k region values be ordered in a monotonic sequence, and thatthe vertex positions are monotonically ordered inside each region. The four-momentum ofeach hadron can be read off, for p+ (p− ) momenta, by projecting the separation between(i)the old and the new vertex on to the j (k) axis. If the four-momentum fraction of p±(i)taken by a hadron is denoted x± , then the total hadron four-momentum is given by
p=

j2
X
j=j1

(j) (j)x+ p + +k2X(k) (k)2 k2 )
2 k2 )
1 k1 )
1 k1 ),+ py2 ê(j+ px2 ê(j+ py1 ê(jx− p− + px1 ê(j
y
x
y
x

(256)

k=k1
(j)

for a step from region (j1 , k1 ) to region (j2 , k2 ). By necessity, x+ is unity for a j1 < j < j2 ,(k)and correspondingly for x− .
The (px , py ) pairs are the transverse momenta produced at the two string breaks,
and the (êx , êy ) pairs four-vectors transverse to the string directions in the regions of therespective string breaks:êx(jk)2 = ê(jk)2= −1 ,
y
(j)

(k)

(jk)
êx(jk) êy(jk) = ê(jk)x,y p+ = êx,y p− = 0 .

371

(257)

The fact that the hadron should be on mass shell, p2 = m2 , puts one constraint on
where a new breakup may be, given that the old one is already known, just as eq. (240)
did in the simple 2-jet case. The remaining degree of freedom is, as before, to be given by
the fragmentation function f (z). The interpretation of the z is only well-defined for a step
entirely constrained to one of the initial string regions, however, which is not enough. In
the 2-jet case, the z values can be related to the proper times of string breaks, as follows.
The variable Γ = (κτ )2 , with κ the string tension and τ the proper time between the
production vertex of the partons and the breakup point, obeys an iterative relation of thekindΓ0 = 0 ,ΓiÃm2= (1 − zi ) Γi−1 + ⊥i
zi

!

.

(258)

Here Γ0 represents the value at the q and q endpoints, and Γi−1 and Γi the values atthe old and new breakup vertices needed to produce a hadron with transverse mass m⊥i ,and with the zi of the step chosen according to f (zi ). The proper time can be definedin an unambiguous way, also over boundaries between the different string regions, so for
multijet events the z variable may be interpreted just as an auxiliary variable needed todetermine the next Γ value. (In the Lund symmetric fragmentation function derivation,the Γ variable actually does appear naturally, so the choice is not as arbitrary as it may
seem here.) The mass and Γ constraints together are sufficient to determine where thenext string breakup is to be chosen, given the preceding one in the iteration scheme. In
reality, several ambiguities remain, however none of these are of importance to the overall
picture.
The algorithm for finding the next breakup then works something like follows. Picka hadron, p⊥ , and z, and calculate the next Γ. If the old breakup is in the region (j, k),
and if the new breakup is also assumed to be in the same region, then the m2 and Γ(k)(j)
constraints can be reformulated in terms of the fractions x+ and x− the hadron must(k)(j)
take of the total four-vectors p+ and p− :
(j)

(k)

(j) (k)(j)(k)(j) (k)m2 = c1 + c2 x+ + c3 x− + c4 x+ x− ,
Γ = d1 + d2 x+ + d3 x− + d4 x+ x− .

(259)

Here the coefficients cn are fairly simple expressions, obtainable by squaring eq. (256),
while dn are slightly more complicated in that they depend on the position of the oldstring break, but both the cn and the dn are explicitly calculable. What remains is an(j)(k)
equation system with two unknowns, x+ and x− . The absence of any quadratic terms is(i)2
due to the fact that all p± = 0, i.e. to the choice of a formulation based on light-cone-likelongitudinal vectors. Of the two possible solutions to the equation system (elimination of
one variable gives a second degree equation in the other), one is unphysical and can bediscarded outright. The other solution is checked for whether the x± values are actuallyinside the physically allowed region, i.e. whether the x± values of the current step, plus
whatever has already been used up in previous steps, are less than unity. If yes, a solution
has been found. If no, it is because the breakup could not take place inside the region
studied, i.e. because the equation system was solved for the wrong region. One therefore
has to change either index j or index k above by one step, i.e. go to the next nearest
string region. In this new region, a new equation system of the type in eq. (259) may be
written down, with new coefficients. A new solution is found and tested, and so on untila physically acceptable solution is found. The hadron four-momentum is now given by
an expression of the type (256). The breakup found forms the starting point for the newstep in the fragmentation chain, and so on. The final joining in the middle is done as in
the 2-jet case, with minor extensions.
372

12.2.5

Junction topologies

When several valence quarks are kicked out of an incoming proton, or when baryon number
is violated, another kind of string topology can be produced. In its simplest form, it canbe illustrated by the decay χ̃01 → udd which can take place in baryon-number-violatingsupersymmetric theories. If we assume that the ‘colour triplet’ string kind encountered
above is the only basic building block, we are led to a Y-shaped string topology, with aquark at each end and a ‘junction’ where the strings meet. This picture is similar to that
adopted, for instance, in models of baryon wavefunctions [Art75]. The following gives a(very) brief overview of a detailed model of junction fragmentation developed in [Sjö03].
As the quarks move out, also the string junction would move, so as to minimize thetotal string energy. It is at rest in a frame where the opening angle between any pair ofquarks is 120◦ , so that the forces acting on the junction cancel. In such a frame, eachof the three strings would fragment pretty much as ordinary strings, e.g. in a back-toback qq pair of jets, at least as far as reasonably high-momentum particles are concerned.
Thus an iterative procedure can be used, whereby the leading q is combined with a newly
produced q1 , to form a meson and leave behind a remainder-jet q1 . (As above, this has
nothing to do with the ordering in physical time, where the fragmentation process again
starts in the middle and spreads outwards.) Eventually, when little energy is left, the threeremainders qi qj qk form a single baryon, which thus has a reasonably small momentum inthe rest frame of the junction. We see that the junction thereby implicitly comes to be
the carrier of the net baryon number of the system. Further baryon production can welloccur at higher momenta in each of the three jets, but then always in pairs of a baryon
and an antibaryon. While the fragmentation principles as such are clear, the technicaldetails of the joining of the jets become more complicated than in the qq case. Someapproximations can be made that allow a reasonably compact and efficient algorithm,
which gives sensible results [Sjö03]. Specifically, two of the strings, preferably the ones
with lowest energy, can be fragmented until their remaining energy is below some cut-off
value. In the actual implementation, one of the two is required to have rather little energy
left, while the other could have somewhat more. At this point, the two remainder flavours
are combined into one effective diquark, which is assigned all the remaining energy andmomentum. The final string piece, between this diquark and the third quark, can now be
considered as described for simple qq strings above.
Among the additional complications are that the diquark formed from the leftovers
may have a larger momentum than energy and thereby nominally may be space-like. If
only by a little, it normally would not matter, but in extreme cases the whole final string
may come to have a negative squared mass. Such configurations are rejected on grounds ofbeing unphysical and the whole fragmentation procedure is restarted from the beginning.
As above, the fragmentation procedure can be formulated in a Lorentz-frameindependent manner, given the four-vector that describes the motion of the junction.
Therefore, while the fragmentation picture is simpler to visualize in the rest frame of thejunction, one may prefer to work in the rest frame of the system or in the lab frame, as
the case may be.
Each of the strings considered above normally would not go straight from the junction
to an endpoint quark, rather it would wind its way via a number of intermediate gluons,in the neutralino case generated by bremsstrahlung in the decay. It is straightforward touse the same formalism as for other multiparton systems to extend the description above
to such cases. The one complication is that the motion of the junction may become more
complicated, especially when the emission of reasonably soft gluons is considered. Thiscan be approximated by a typical mean motion during the hadronization era [Sjö03].
The most general string topology foreseen is one with two junctions, i.e. a >−< topology. Here one junction would be associated with a baryon number and the other with anantibaryon one. There would be two quark ends, two antiquark ones, and five string pieces373

(including the one between the two junctions) that each could contain an arbitrary number of intermediate gluons. Such topologies can arise, for instance, in pp collisions wherea gluon exchange ties together the two beam baryon junctions, or in prompt BNV decaysof t̃ squarks in e+ e− collisions. A further complication here is that it should in principle
be possible for the junction and antijunction to meet and annihilate, producing a system
instead with two separate qq strings and no junctions. This aspect is also modeled in theimplementation, by selecting the topology that minimises the total potential energy, as
defined in [Sjö03]. Omitting details here, the resulting behaviour is that junctions withlittle effective relative motion tend to annihilate, whereas junctions at large velocities
tend to remain intact, with corresponding consequences for the presence or absence of‘junction baryons’.

12.3

Independent Fragmentation

The independent fragmentation (IF) approach dates back to the early seventies [Krz72],
and gained widespread popularity with the Field–Feynman paper [Fie78]. Subsequently,
IF was the basis for two programs widely used in the early PETRA/PEP days, the Hoyer
et al. [Hoy79] and the Ali et al. [Ali80] programs. Pythia has as (non-default) options awide selection of independent fragmentation algorithms. The historical application of IF
has been to matrix-element descriptions of jet production in e+ e− annihilation, i.e. for 2-,
3- and 4-parton configurations. In principle, the options made available in Pythia could
be used for any event topology, but in practice it is strongly recommended not to use IF
except for the above-mentioned case, and then only as a strawman model.
12.3.1

Fragmentation of a single jet

In the IF approach, it is assumed that the fragmentation of any system of partons can be
described as an incoherent sum of independent fragmentation procedures for each parton
separately. The process is to be carried out in the overall c.m. frame of the jet system,with each jet fragmentation axis given by the direction of motion of the corresponding
parton in that frame.
Exactly as in string fragmentation, an iterative ansatz can be used to describe thesuccessive production of one hadron after the next. Assume that a quark is kicked out bysome hard interaction, carrying a well-defined amount of energy and momentum. This
quark jet q is split into a hadron qq1 and a remainder-jet q1 , essentially collinear witheach other. New quark and hadron flavours are picked as already described. The sharingof energy and momentum is given by some probability distribution f (z), where z is thefraction taken by the hadron, leaving 1 − z for the remainder-jet. The remainder-jet isassumed to be just a scaled-down version of the original jet, in an average sense. The
process of splitting off a hadron can therefore be iterated, to yield a sequence of hadrons.
In particular, the function f (z) is assumed to be the same at each step, i.e. independentof remaining energy. If z is interpreted as the fraction of the jet E + pL , i.e. energy plus
longitudinal momentum with respect to the jet axis, this leads to a flat central rapidity
plateau dn/dy for a large initial energy.
Fragmentation functions can be chosen among those listed above for string fragmentation, but also here the default is the Lund symmetric fragmentation function.
The normal z interpretation means that a choice of a z value close to 0 corresponds to aparticle moving backwards, i.e. with pL < 0. It makes sense to allow only the production ofparticles with pL > 0, but to explicitly constrain z accordingly would destroy longitudinalinvariance. The most straightforward way out is to allow all z values but discard hadrons
with pL < 0. Flavour, transverse momentum and E + pL carried by these hadrons are‘lost’ for the forward jet. The average energy of the final jet comes out roughly right
this way, with a spread of 1–2 GeV around the mean. The jet longitudinal momentum374

is decreased, however, since the jet acquires an effective mass during the fragmentation
procedure. For a 2-jet event this is as it should be, at least on average, because also themomentum of the compensating opposite-side parton is decreased.
Flavour is conserved locally in each qi qi splitting, but not in the jet as a whole. Firstof all, there is going to be a last meson qn−1 qn generated in the jet, and that will leavebehind an unpaired quark flavour qn . Independent fragmentation does not specify the fateof this quark. Secondly, also a meson in the middle of the flavour chain may be selectedwith such a small z value that it obtains pL < 0 and is rejected. Thus a u quark jet of
charge 2/3 need not only gives jets of charge 0 or 1, but also of −1 or +2, or even higher.
Like with the jet longitudinal momentum above, one could imagine this compensated byother jets in the event.
It is also assumed that transverse momentum is locally conserved, i.e. the net p⊥ of theqi qi pair as a whole is assumed to be vanishing. The p⊥ of the qi is taken to be a Gaussianin the two transverse degrees of freedom separately, with the transverse momentum of a
hadron obtained by the sum of constituent quark transverse momenta. The total p⊥ of ajet can fluctuate for the same two reasons as discussed above for flavour. Furthermore, insome scenarios one may wish to have the same p⊥ distribution for the first-rank hadron qq1
as for subsequent ones, in which case also the original q should be assigned an unpairedp⊥ according to a Gaussian.
Within the IF framework, there is no unique recipe for how gluon jet fragmentation
should be handled. One possibility is to treat it exactly like a quark jet, with the initialquark flavour chosen at random among u, u, d, d, s and s, including the ordinary s quark
suppression factor. Since the gluon is supposed to fragment more softly than a quark
jet, the fragmentation function may be chosen independently. Another common option
is to split the g jet into a pair of parallel q and q ones, sharing the energy, e.g. as in a
perturbative branching g → qq, i.e. f (z) ∝ z 2 + (1 − z)2 . The fragmentation function
could still be chosen independently, if so desired. Further, in either case the fragmentationp⊥ could be chosen to have a different mean.
12.3.2

Fragmentation of a jet system

In a system of many jets, each jet is fragmented independently. Since each jet by itself
does not conserve the flavour, energy and momentum, as we have seen, then neither doesa system of jets. At the end of the generation, special algorithms are therefore used topatch this up. The choice of approach has major consequences, e.g. for event shapes andαs determinations [Sjö84a].
Little attention is usually given to flavour conservation, and we only offer one scheme.
When the fragmentation of all jets has been performed, independently of each other, thenet initial flavour composition, i.e. number of u quarks minus number of u quarks etc., iscompared with the net final flavour composition. In case of an imbalance, the flavours of
the hadron with lowest three-momentum are removed, and the imbalance is re-evaluated.
If the remaining imbalance could be compensated by a suitable choice of new flavours
for this hadron, flavours are so chosen, a new mass is found and the new energy can beevaluated, keeping the three-momentum of the original hadron. If the removal of flavours
from the hadron with lowest momentum is not enough, flavours are removed from theone with next-lowest momentum, and so on until enough freedom is obtained, whereafter
the necessary flavours are recombined at random to form the new hadrons. Occasionally
one extra qi qi pair must be created, which is then done according to the customary
probabilities.
Several different schemes for energy and momentum conservation have been devised.
One [Hoy79] is to conserve transverse momentum locally within each jet, so that the final
momentum vector of a jet is always parallel with that of the corresponding parton. Thenlongitudinal momenta may be rescaled separately for particles within each jet, such that375

the ratio of rescaled jet momentum to initial parton momentum is the same in all jets.
Since the initial partons had net vanishing three-momentum, so do now the hadrons. The
rescaling factors may be chosen such that also energy comes out right. Another common
approach [Ali80] is to boost the event to the frame where the total hadronic momentum
is vanishing. After that, energy conservation can be obtained by rescaling all particle
three-momenta by a common factor.
The number of possible schemes is infinite. Two further options are available in the
program. One is to shift all particle three-momenta by a common amount to give net
vanishing momentum, and then rescale as before. Another is to shift all particle threemomenta, for each particle by an amount proportional to the longitudinal mass withrespect to the imbalance direction, and with overall magnitude selected to give momentum
conservation, and then rescale as before. In addition, there is a choice of whether to treat
separate colour singlets (like qq0 and q0 q in a qqq0 q0 event) separately or as one single big
system.
A serious conceptual weakness of the IF framework is the issue of Lorentz invariance.
The outcome of the fragmentation procedure depends on the coordinate frame chosen,a problem circumvented by requiring fragmentation always to be carried out in the c.m.
frame. This is a consistent procedure for 2-jet events, but only a technical trick for
multijets.
It should be noted, however, that a Lorentz covariant generalization of the independent
fragmentation model exists, in which separate ‘gluon-type’ and ‘quark-type’ strings areused, the Montvay scheme [Mon79]. The ‘quark string’ is characterized by the ordinary
string constant κ, whereas a ‘gluon string’ is taken to have a string constant κg . If κg > 2κ
it is always energetically favourable to split a gluon string into two quark ones, and theordinary Lund string model is recovered. Otherwise, for a 3-jet qqg event the threedifferent string pieces are joined at a junction. The motion of this junction is given by thevector sum of string tensions acting on it. In particular, it is always possible to boost an
event to a frame where this junction is at rest. In this frame, much of the standard naı̈ve
IF picture holds for the fragmentation of the three jets; additionally, a correct treatment
would automatically give flavour, momentum and energy conservation. Unfortunately,
the simplicity is lost when studying events with several gluon jets. In general, each event
will contain a number of different junctions, resulting in a polypod shape with a numberof quark and gluons strings sticking out from a skeleton of gluon strings. With the shiftof emphasis from three-parton to multi-parton configurations, the simple option existingin Jetset 6.3 therefore is no longer included.
A second conceptual weakness of IF is the issue of collinear divergences. In a partonshower picture, where a quark or gluon is expected to branch into several reasonably
collimated partons, the independent fragmentation of one single parton or of a bunch ofcollinear ones gives quite different outcomes, e.g. with a much larger hadron multiplicity
in the latter case. It is conceivable that a different set of fragmentation functions could beconstructed in the shower case in order to circumvent this problem (local parton–hadron
duality [Dok89] would correspond to having f (z) = δ(z − 1)).

12.4

Other Fragmentation Aspects

Here some further aspects are considered.
12.4.1

Small-mass systems

A hadronic event is conventionally subdivided into sets of partons that form separate
colour singlets. These sets are represented by strings, that e.g. stretch from a quark end
via a number of intermediate gluons to an antiquark end. Three string-mass regions may
be distinguished for the hadronization.
376

1. Normal string fragmentation. In the ideal situation, each string has a large invariant
mass. Then the standard iterative fragmentation scheme above works well. In
practice, this approach can be used for all strings above some cut-off mass of a few
GeV.
2. Cluster decay. If a string is produced with a small invariant mass, maybe only
two-body final states are kinematically accessible. The traditional iterative Lund
scheme is then not applicable. We call such a low-mass string a cluster, and consider
it separately from above. The modelling is still intended to give a smooth match on
to the standard string scheme in the high-cluster-mass limit [Nor98].
3. Cluster collapse. This is the extreme case of the above situation, where the string
mass is so small that the cluster cannot decay into two hadrons. It is then assumed
to collapse directly into a single hadron, which inherits the flavour content of thestring endpoints. The original continuum of string/cluster masses is replaced by adiscrete set of hadron masses. Energy and momentum then cannot be conserved
inside the cluster, but must be exchanged with the rest of the event [Nor98].
String systems below a threshold mass are handled by the cluster machinery. In it, anattempt is first made to produce two hadrons, by having the string break in the middle
by the production of a new qq pair, with flavours and hadron spins selected according to
the normal string rules. If the sum of the hadron masses is larger than the cluster mass,repeated attempts can be made to find allowed hadrons; the default is two tries. If an
allowed set is found, the angular distribution of the decay products in the cluster rest
frame is picked isotropically near the threshold, but then gradually more elongated along
the string direction, to provide a smooth match to the string description at larger masses.
This also includes a forward–backward asymmetry, so that each hadron is preferentiallyin the same hemisphere as the respective original quark it inherits.
If the attempts to find two hadrons fail, one single hadron is formed from the given
flavour content. The basic strategy thereafter is to exchange some minimal amount ofenergy and momentum between the collapsing cluster and other string pieces in the neighbourhood. The momentum transfer can be in either direction, depending on whether the
hadron is lighter or heavier than the cluster it comes from. When lighter, the excess momentum is split off and put as an extra ‘gluon’ on the nearest string piece, where ‘nearest’is defined by a space–time history-based distance measure. When the hadron is heavier,momentum is instead borrowed from the endpoints of the nearest string piece.
The free parameters of the model can be tuned to data, especially to the significant
asymmetries observed between the production of D and D mesons in π − p collisions, withhadrons that share some of the π − flavour content very much favoured at large xF in theπ − fragmentation region [Ada93]. These spectra and asymmetries are closely related tothe cluster collapse mechanism, and also to other effects of the colour topology of theevent (‘beam drag’) [Nor98]. The most direct parameters are the choice of compensation scheme (MSTJ(16)), the number of attempts to find a kinematically valid two-bodydecay (MSTJ(16)) and the border between cluster and string descriptions (PARJ(32)).
Also many other parameters enter the description, however, such as the effective charm
mass (PMAS(4,1)), the quark constituent masses (PARF(101) - PARF(105)), the beamremnant structure (MSTP(91) - MSTP(94) and PARP(91) - PARP(100)) and the standard string fragmentation parameters.
The cluster collapse is supposed to be a part of multiparticle production. It is notintended for exclusive production channels, and may there give quite misleading results.
For instance, a cc quark pair produced in a γγ collision could well be collapsed to asingle J/ψ if the invariant mass is small enough, even though the process γγ → J/ψ intheory is forbidden by spin–parity–charge considerations. Furthermore, properties suchas strong isospin are not considered in the string fragmentation picture (only its third
component, i.e. flavour conservation), neither when one nor when many particles areproduced. For multiparticle states this should matter little, since the isospin then will be377

duly randomized, but properly it would forbid the production of several one- or two-bodystates that currently are generated.
12.4.2

Interconnection Effects

The widths of the W, Z and t are all of the order of 2 GeV. A Standard Model Higgs witha mass above 200 GeV, as well as many supersymmetric and other beyond-the-Standard
Model particles would also have widths in the multi-GeV range. Not far from threshold,the typical decay times τ = 1/Γ ≈ 0.1 fm ¿ τhad ≈ 1 fm. Thus hadronic decay systems
overlap, between a resonance and the underlying event, or between pairs of resonances,
so that the final state may not contain independent resonance decays.
So far, studies have mainly been performed in the context of W pair production at
LEP2. Pragmatically, one may here distinguish three main eras for such interconnection:
1. Perturbative: this is suppressed for gluon energies ω > Γ by propagator/timescaleeffects; thus only soft gluons may contribute appreciably.
2. Nonperturbative in the hadroformation process: normally modelled by a colour
rearrangement between the partons produced in the two resonance decays and in
the subsequent parton showers.
3. Nonperturbative in the purely hadronic phase: best exemplified by Bose–Einsteineffects; see next section.
The above topics are deeply related to the unsolved problems of strong interactions:confinement dynamics, 1/NC2 effects, quantum mechanical interferences, etc. Thus they
offer an opportunity to study the dynamics of unstable particles, and new ways to probe
confinement dynamics in space and time [Gus88a, Sjö94], but they also threaten to limit
or even spoil precision measurements.
The reconnection scenarios outlined in [Sjö94a] are now available, plus also an option
along the lines suggested in [Gus94]. (A toy model presented in [San05] and intended
mostly for hadron collisions is also available, as described in section 11.4.) Currently
they can only be invoked in process 25, e+ e− → W+ W− → q1 q2 q3 q4 , which is the mostinteresting one for the foreseeable future. (Process 22, e+ e− → γ ∗ /Z0 γ ∗ /Z0 → q1 q1 q2 q2
can also be used, but the travel distance is calculated based only on the Z0 propagator
part. Thus, the description in scenarios I, II and II0 below would not be sensible e.g. fora light-mass γ ∗ γ ∗ pair.) If normally the event is considered as consisting of two separate
colour singlets, q1 q2 from the W+ and q3 q4 from the W− , a colour rearrangement cangive two new colour singlets q1 q4 and q3 q2 . It therefore leads to a different hadronic final
state, although differences usually turn out to be subtle and difficult to isolate [Nor97].
When also gluon emission is considered, the number of potential reconnection topologies
increases.
Since hadronization is not understood from first principles, it is important to rememberthat we deal with model building rather than exact calculations. We will use the standard
Lund string fragmentation model [And83] as a starting point, but have to extend it
considerably. The string is here to be viewed as a Lorentz covariant representation of alinear confinement field.
The string description is entirely probabilistic, i.e. any negative-sign interference effects are absent. This means that the original colour singlets q1 q2 and q3 q4 may transmute
to new singlets q1 q4 and q3 q2 , but that any effects e.g. of q1 q3 or q2 q4 dipoles are absent.
In this respect, the nonperturbative discussion is more limited in outlook than a corresponding perturbative one. However, note that dipoles such as q1 q3 do not correspond tocolour singlets, and can therefore not survive in the long-distance limit of the theory, i.e.
they have to disappear in the hadronization phase.
The imagined time sequence is the following. The W+ and W− fly apart from theircommon production vertex and decay at some distance. Around each of these decay vertices, a perturbative parton shower evolves from an original qq pair. The typical distance378

that a virtual parton (of mass m ∼ 10 GeV, say, so that it can produce separate jets in
the hadronic final state) travels before branching is comparable with the average W+ W−
separation, but shorter than the fragmentation time. Each W can therefore effectively
be viewed as instantaneously decaying into a string spanned between the partons, from aquark end via a number of intermediate gluons to the antiquark end. The strings expand,
both transversely and longitudinally, at a speed limited by that of light. They eventually
fragment into hadrons and disappear. Before that time, however, the string from theW+ and the one from the W− may overlap. If so, there is some probability for a colour
reconnection to occur in the overlap region. The fragmentation process is then modified.
The Lund string model does not constrain the nature of the string fully. At one
extreme, the string may be viewed as an elongated bag, i.e. as a flux tube without any
pronounced internal structure. At the other extreme, the string contains a very thin core,a vortex line, which carries all the topological information, while the energy is distributed
over a larger surrounding region. The latter alternative is the chromoelectric analogue
to the magnetic flux lines in a type II superconductor, whereas the former one is moreakin to the structure of a type I superconductor. We use them as starting points for twocontrasting approaches, with nomenclature inspired by the superconductor analogy.
In scenario I, the reconnection probability is proportional to the space–time volume
over which the W+ and W− strings overlap, with saturation at unit probability. This
probability is calculated as follows. In the rest frame of a string piece expanding along
the ±z direction, the colour field strength is assumed to be given byn2Ω(x, t) = exp −(x2 + y 2 )/2rhadon2θ(t − |x|) exp −(t2 − z 2 )/τfrag

o

.

(260)

The first factor gives a Gaussian fall-off in the transverse directions, with a string width
rhad ≈ 0.5 fm of typical hadronic dimensions. The time retardation factor θ(t − |x|)
ensures that information on the decay of the W spreads outwards with the speed of light.
The last factor gives the probability that the string has not yet fragmented at a given
proper time along the string axis, with τfrag ≈ 1.5 fm. For a string piece e.g. from the W+
decay, this field strength has to be appropriately rotated, boosted and displaced to theW+ decay vertex. In addition, since the W+ string can be made up of many pieces, the+string field strength Ω+max (x, t) is defined as the maximum of all the contributing Ω ’s in
the given point. The probability for a reconnection to occur is now given byµPrecon = 1 − exp −kI¶Z−
d3 x dt Ω+max (x, t) Ωmax (x, t)

,

(261)

where kI is a free parameter. If a reconnection occurs, the space–time point for this−reconnection is selected according to the differential probability Ω+max (x, t) Ωmax (x, t). Thisdefines the string pieces involved and the new colour singlets.
In scenario II it is assumed that reconnections can only take place when the coreregions of two string pieces cross each other. This means that the transverse extent ofstrings can be neglected, which leads to considerable simplifications compared with theprevious scenario. The position of a string piece at time t is described by a one-parameterset x(t, α), where 0 ≤ α ≤ 1 is used to denote the position along the string. To findwhether two string pieces i and j from the W+ and W− decays cross, it is sufficient to−+−solve the equation system x+i (t, α ) = xj (t, α ) and to check that this (unique) solution isin the physically allowed domain. Further, it is required that neither string piece has had
2},time to fragment, which gives two extra suppression factors of the form exp{−τ 2 /τfragwith τ the proper lifetime of each string piece at the point of crossing, i.e. as in scenario I.
If there are several string crossings, only the one that occurs first is retained. The II0
scenario is a variant of scenario II, with the requirement that a reconnection is allowed
only if it leads to a reduction of the string length.

379

Other models include a simplified implementation of the ‘GH’ model [Gus94], where
the reconnection is selected solely based on the criterion of a reduced string length. The‘instantaneous’ and ‘intermediate’ scenarios are two toy models. In the former (which isequivalent to that in [Gus88a]) the two reconnected systems q1 q4 and q3 q2 are immediately
formed and then subsequently shower and fragment independently of each other. In thelatter, a reconnection occurs between the shower and fragmentation stages. One has tobear in mind that the last two ‘optimistic’ (from the connectometry point of view) toy
approaches are oversimplified extremes and are not supposed to correspond to the true
nature. These scenarios may be useful for reference purposes, but are already excluded
by data.
While interconnection effects are primarily viewed as hadronization physics, their implementation is tightly coupled to the event generation of a few specific processes, and not
to the generic hadronization machinery. Therefore the relevant main switch MSTP(115)and parameters PARP(115) - PARP(120) are described in section 9.3.
12.4.3

Bose–Einstein effects

A crude option for the simulation of Bose–Einstein effects is included since long, but isturned off by default. In view of its shortcomings, alternative descriptions have been
introduced that try to overcome at least some of them [Lön95].
The detailed BE physics is not that well understood, see e.g. [Lör89]. What is offeredis an algorithm, more than just a parameterization (since very specific assumptions andchoices have been made), and yet less than a true model (since the underlying physics
picture is rather fuzzy). In this scheme, the fragmentation is allowed to proceed as usual,and so is the decay of short-lived particles like ρ. Then pairs of identical particles, π +say, are considered one by one. The Qij value of a pair i and j is evaluated,
q

Qij =

(pi + pj )2 − 4m2 ,(262)

where m is the common particle mass. A shifted (smaller) Q0ij is then to be found suchthat the (infinite statistics) ratio f2 (Q) of shifted to unshifted Q distributions is given by
the requested parameterization. The shape may be chosen either exponential or Gaussian,f2 (Q) = 1 + λ exp (−(Q/d)r ) ,r = 1 or 2 .

(263)(In fact, the distribution has to dip slightly below unity at Q values outside the Boseenhancement region, from conservation of total multiplicity.) If the inclusive distributionof Qij values is assumed given√ just by phase space, at least at small relative momentum
then, with d3 p/E ∝ Q2 dQ/ Q2 + 4m2 , Q0ij is found as the solution to the equation
Z Qij
0

√

Z Q0
ij
Q2 dQ
Q2 dQ
√
f2 (Q)=.
Q2 + 4m2
Q2 + 4m2
0(264)

The change of Qij can be translated into an effective shift of the three-momenta of thetwo particles, if one uses as extra constraint that the total three-momentum of each pair
be conserved in the c.m. frame of the event. Only after all pairwise momentum shifts
have been evaluated, with respect to the original momenta, are these momenta actuallyshifted, for each particle by the (three-momentum) sum of evaluated shifts. The totalenergy of the event is slightly reduced in the process, which is compensated by an overall
rescaling of all c.m.-frame momentum vectors. It can be discussed which are the particles
to involve in this rescaling. Currently the only exceptions to using everything are leptons
and neutrinos coming from resonance decays (such as W’s) and photons radiated by
leptons (also in initial-state radiation). Finally, the decay chain is resumed with more
long-lived particles like π 0 .
380Two comments can be made. The Bose–Einstein effect is here interpreted almost as aclassical force acting on the ‘final state’, rather than as a quantum mechanical phenomenonon the production amplitude. This is not a credo, but just an ansatz to make things
manageable. Also, since only pairwise interactions are considered, the effects associatedwith three or more nearby particles tend to get overestimated. (More exact, but also
more time-consuming methods may be found in [Zaj87].) Thus the input λ may haveto be chosen smaller than what one wants to get out. (On the other hand, many of thepairs of an event contains at least one particle produced in some secondary vertex, like a
D decay. This reduces the fraction of pairs which may contribute to the Bose–Einsteineffects, and thus reduces the potential signal.) This option should therefore be used withcaution, and only as a first approximation to what Bose–Einstein effects can mean.
Probably the largest weakness of the above approach is the issue how to conserve thetotal four-momentum. It preserves three-momentum locally, but at the expense of notconserving energy. The subsequent rescaling of all momenta by a common factor (in therest frame of the event) to restore energy conservation is purely ad hoc. For studies of asingle Z0 decay, it can plausibly be argued that such a rescaling does minimal harm. The
same need not hold for a pair of resonances. Indeed, studies [Lön95] show that this global
rescaling scheme, which we will denote BE0 , introduces an artificial negative shift in thereconstructed W mass, making it difficult (although doable) to study the true BE effects
in this case. This is one reason to consider alternatives.
The global rescaling is also running counter to our philosophy that BE effects should
be local. To be more specific, we assume that the energy density of the string is a fixed
quantity. To the extent that a pair of particles have their four-momenta slightly shifted,
the string should act as a ‘commuting vessel’, providing the difference to other particles
produced in the same local region of the string. What this means in reality is still notcompletely specified, so further assumptions are necessary. In the following we discuss
four possible algorithms, whereof the last two are based strictly on the local conservation
aspect above, while the first two are attempting a slightly different twist to the locality
concept. All are based on calculating an additional shift δrlk for some pairs of particles,
where particles k and l need not be identical bosons. In the end each particle momentumPP
will then be shifted to p0i = pi + j6=i δpji + α k6=i δrki , with the parameter α adjusted
separately for each event so that the total energy is conserved.
In the first approach we emulate the criticism of the global event weight methods withweights always above unity, as being intrinsically unstable. It appears more plausible thatweights fluctuate above and below unity. For instance, the simple pair symmetrization
weight is 1+cos(∆x·∆p), with the 1+λ exp(−Q2 R2 ) form only obtained after integration
over a Gaussian source. Non-Gaussian sources give oscillatory behaviours.
If weights above unity correspond to a shift of pairs towards smaller relative Q values,
the below-unity weights instead give a shift towards larger Q. One therefore is lead to a
picture where very nearby identical particles are shifted closer, those somewhat further
are shifted apart, those even further yet again shifted closer, and so on. Probably theoscillations dampen out rather quickly, as indicated both by data and by the global modelstudies. We therefore simplify by simulating only the first peak and dip. Furthermore, toinclude the desired damping and to make contact with our normal generation algorithm(for simplicity), we retain the Gaussian form, but the standard f2 (Q) = 1+λ exp(−Q2 R2 )is multiplied by a further factor 1 + αλ exp(−Q2 R2 /9). The factor 1/9 in the exponential,
i.e. a factor 3 difference in the Q variable, is consistent with data and also with what one
might expect from a dampened cos form, but should be viewed more as a simple ansatz
than having any deep meaning.
In the algorithm, which we denote BE3 , δrji is then non-zero only for pairs of identicalbosons, and is calculated in the same way as δpji , with the additional factor 1/9 in the
exponential. As explained above, the δrji shifts are then scaled by a common factor αthat ensures total energy conservation. It turns out that the average α needed is ≈ −0.2.
381

The negative sign is exactly what we want to ensure that δrji corresponds to shifting apair apart, while the order of α is consistent with the expected increase in the number
of affected pairs when a smaller effective radius R/3 is used. One shortcoming of themethod, as implemented here, is that the input f2 (0) is not quite 2 for λ = 1 but rather(1 + λ)(1 + αλ) ≈ 1.6. This could be solved by starting off with an input λ somewhat
above unity.
The second algorithm, denoted BE32 , is a modification of the BE3 form intended togive f2 (0) = 1 + λ. The ansatz isnon

f2 (Q) = 1 + λ exp(−Q2 R2 )³

´o

1 + αλ exp(−Q2 R2 /9) 1 − exp(−Q2 R2 /4)

,

(265)

applied to identical pairs. The combination exp(−Q2 R2 /9) (1 − exp(−Q2 R2 /4)) can beviewed as a Gaussian, smeared-out representation of the first dip of the cos function. As
a technical trick, the δrji are found as in the BE3 algorithm and thereafter scaled down by
the 1 − exp(−Q2 R2 /4) factor. (This procedure does not quite reproduce the formalismof eq. (264), but comes sufficiently close for our purpose, given that the ansatz form in
itself is somewhat arbitrary.) One should note that, even with the above improvement
relative to the BE3 scheme, the observable two-particle correlation is lower at small Q
than in the BE0 algorithm, so some further tuning of λ could be required. In this scheme,
hαi ≈ −0.25.
In the other two schemes, the original form of f2 (Q) is retained, and the energy is
instead conserved by picking another pair of particles that are shifted apart appropriately.
That is, for each pair of identical particles i and j, a pair of non-identical particles, k andl, neither identical to i or j, is found in the neighbourhood of i and j. For each shift δpji , acorresponding δrlk is found so that the total energy and momentum in the i, j, k, l system
is conserved. However, the actual momentum shift of a particle is formed as the vector
sum of many contributions, so the above pair compensation mechanism is not perfect.
The mismatch is reflected in a non-unit value α used to rescale the δrlk terms.
The k, l pair should be the particles ‘closest’ to the pair affected by the BE shift, in thespirit of local energy conservation. One option would here have been to ‘look behind thescenes’ and use information on the order of production along the string. However, once
decays of short-lived particles are included, such an approach would still need arbitrary
further rules. We therefore stay with the simplifying principle of only using the produced
particles.
Looking at W+ W− (or Z0 Z0 ) events and a pair i, j with both particles from the sameW, it is not obvious whether the pair k, l should also be selected only from this W or if all
possible pairs should be considered. Below we have chosen the latter as default behaviour,
but the former alternative is also studied below.
One obvious measure of closeness is small invariant mass. A first choice would thenbe to pick the combination that minimizes the invariant mass mijkl of all four particles.
However, such a procedure does not reproduce the input f2 (Q) shape very well: both thepeak height and peak width are significantly reduced, compared with what happens in
the BE0 algorithm. The main reason is that either of k or l may have particles identicalto itself in its local neighbourhood. The momentum compensation shift of k is at random,
more or less, and therefore tends to smear the BE signal that could be introduced relative
to k’s identical partner. Note that, if k and its partner are very close in Q to start
with, the relative change δQ required to produce a significant BE effect is very small,approximately δQ ∝ Q. The momentum compensation shift on k can therefore easily
become larger than the BE shift proper.
It is therefore necessary to disfavour momentum compensation shifts that break up
close identical pairs. One alternative would have been to share the momentum conservation shifts suitably inside such pairs. We have taken a simpler course, by introducing
a suppression factor 1 − exp(−Q2k R2 ) for particle k, where Qk is the Q value between k382and its nearest identical partner. The form is fixed such that a Qk = 0 is forbidden andthen the rise matches the shape of the BE distribution itself. Specifically, in the third
algorithm, BEm , the pair k, l is chosen so that the measure
Wijkl =

(1 − exp(−Q2k R2 ))(1 − exp(−Q2l R2 ))m2ijkl(266)

is maximized. The average α value required to rescale for the effect of multiple shifts is
0.73, i.e. somewhat below unity.
The BEλ algorithm is inspired by the so-called λ measure [And89] (not the be confusedwith the λ parameter of f2 (Q)). It corresponds to a string length in the Lund string
fragmentation framework. It can be shown that partons in a string are colour-connected
in a way that tends to minimize this measure. The same is true for the ordering of the
produced hadrons, although with large fluctuations. As above, having identical particles
nearby to k, l gives undesirable side effects. Therefore the selection is made so that
Wijkl =

(1 − exp(−Q2k R2 ))(1 − exp(−Q2l R2 ))min(12 permutations) (mij mjk mkl , mij mjl mlk , . . .)(267)

is maximized. The denominator is intended to correspond to exp(λ). For cases whereparticles i and j comes from the same string, this would favour compensating the energy
using particles that are close by and in the same string. We find hαi ≈ 0.73, as above.
The main switches and parameters affecting the Bose–Einstein algorithm are MSTJ(51)- MSTJ(57) and PARJ(91) - PARJ(96).

383

13

Particles and Their Decays

Particles are the building blocks from which events are constructed. We here use theword ‘particle’ in its broadest sense, i.e. including partons, resonances, hadrons, and soon, subgroups we will describe in the following. Each particle is characterized by somequantities, such as charge and mass. In addition, many of the particles are unstable andsubsequently decay. This section contains a survey of the particle content of the programs,
and the particle properties assumed. In particular, the decay treatment is discussed. Someparticle and decay properties form part already of the hard subprocess description, andare therefore described in sections 6, 7 and 8.

13.1

The Particle Content

In order to describe both current and potential future physics, a number of different
particles are needed. A list of some particles, along with their codes, is given in section
5.1. Here we therefore emphasize the generality rather than the details.
Four full generations of quarks and leptons are included in the program, although
indications from LEP strongly suggest that only three exist in Nature. The PDG naming
convention for the fourth generation is to repeat the third one but with a prime: b0 , t0 τ 0and ντ0 . Quarks may appear either singly or in pairs; the latter are called diquarks andare characterized by their flavour content and their spin. A diquark is always assumed to
be in a colour antitriplet state.
The colour neutral hadrons may be build up from the five lighter coloured quarks (anddiquarks). Six full meson multiplets are included and two baryon ones, see section 12.1.
In addition, K0S and K0L are considered as separate particles coming from the ‘decay’ of0
K0 and K (or, occasionally, produced directly).
Other particles from the Standard Model include the gluon g, the photon γ, theintermediate gauge bosons Z0 and W± , and the standard Higgs h0 . Non-standard particles
include additional gauge bosons, Z00 and W0± , additional Higgs bosons H0 , A0 and H± ,a leptoquark LQ a horizontal gauge boson R0 , technicolor and supersymmetric particles,
and more.
From the point of view of usage inside the programs, particles may be subdivided into
three classes, partly overlapping.
1. A parton is generically any object which may be found in the wave function of theincoming beams, and may participate in initial- or final-state showers. This includes
what is normally meant by partons, i.e. quarks and gluons, but here also leptons andphotons. In a few cases other particles may be classified as partons in this sense.
2. A resonance is an unstable particle produced as part of the hard process, and where
the decay treatment normally is also part of the hard process. Resonance partial
widths are perturbatively calculable, and therefore it is possible to dynamicallyrecalculate branching ratios as a function of the mass assigned to a resonance. Resonances includes particles like the Z0 and other massive gauge bosons and Higgs
particles, in fact everything with a mass above the b quark and additionally also alighter γ ∗ .
3. Hadrons and their decay products, i.e. mesons and baryons produced either in the
fragmentation process, in secondary decays or as part of the beam-remnant treatment, but not directly as part of the hard process (except in a few special cases).
Hadrons may be stable or unstable. Branching ratios are not assumed perturbatively
calculable, and can therefore be set freely. Also leptons and photons produced indecays belong to this class. In practice, this includes everything up to and including
b quarks in mass (except a light γ ∗ , see above).
Usually the subdivision above is easy to understand and gives you the control you
would expect. Thus the restriction on the allowed decay modes of a resonance will directly384affect the cross section of a process, while this is not the case for an ordinary hadron,since in the latter case there is no precise theory knowledge on the set of decay modes
and branching ratios.

13.2

Masses, Widths and Lifetimes

13.2.1MassesQuark masses are not particularly well defined. In the program it is necessary to makeuse of three kinds of masses, kinematical, running current algebra ones and constituent
ones. The first ones are relevant for the kinematics in hard processes, e.g. in gg → cc,
and are partly fixed by such considerations [Nor98]. The second define couplings to Higgs
particles, and also other mass-related couplings in models for physics beyond the Standard
Model. Both these kinds directly affect cross sections in processes. Constituent masses,
finally, are used to derive the masses of hadrons, for some not yet found ones, and e.g. togauge the remainder-mass below which the final two hadrons are to be produced in string
fragmentation.
The first set of values are the ones stored in the standard mass array PMAS. The starting values of the running masses are stored in PARF(91) - PARF(96), with the runningcalculated in the PYMRUN function. Constituent masses are also stored in the PARF array,
above position 101. We maintain this distinction for the five first flavours, and partly for
top, using the following values by default:
quark kinematical current algebra mass constituent massd0.33 GeV
0.0099 GeV
0.325 GeV
u0.33 GeV
0.0056 GeV
0.325 GeVs
0.5 GeV
0.199 GeV
0.5 GeVc
1.5 GeV
1.23 GeV
1.6 GeVb
4.8 GeV
4.17 GeV
5.0 GeVt
175 GeV
165 GeV—For top no constituent mass is defined, since it does not form hadrons. For hypothetical
fourth generation quarks only one set of mass values is used, namely the one in PMAS.
Constituent masses for diquarks are defined as the sum of the respective quark masses.
The gluon is always assumed massless.
Particle masses, when known, are taken from ref. [PDG96]. Hypothesized particles,
such as fourth generation fermions and Higgs bosons, are assigned some not unreasonable
set of default values, in the sense of where you want to search for them in the not-toodistant future. Here it is understood that you will go in and change the default values
according to your own opinions at the beginning of a run.
The total number of hadrons in the program is very large, whereof some are not yet
discovered (like charm and bottom baryons). There the masses are built up, when needed,from the constituent masses. For this purpose one uses formulae of the type [DeR75]m = m0 +X

mi + k m2di

X hσ i · σ j i
i<j

mi mj

,

(268)

i.e. one constant term, a sum over constituent masses and a spin-spin interaction term foreach quark pair in the hadron. The constants m0 and k are fitted from known masses,
treating mesons and baryons separately. For mesons with orbital angular momentum
L = 1 the spin-spin coupling is assumed vanishing, and only m0 is fitted. One may also
define ‘constituent diquarks masses’ using the formula above, with a k value 2/3 that ofbaryons. The default values are:385

multiplet
m0k
pseudoscalars and vectors
0.
0.16 GeV
axial vectors (S = 0)0.50 GeV
0.
scalars0.45 GeV
0.
axial vectors (S = 1)
0.55 GeV
0.
tensors
0.60 GeV0.
baryons
0.11 GeV 0.048 GeV
diquarks
0.077 GeV 0.048 GeV.
Unlike earlier versions of the program, the actual hadron values are hardcoded, i.e. areunaffected by any change of the charm or bottom quark masses.
13.2.2

Widths

A width is calculated perturbatively for those resonances which appear in the Pythia
hard-process generation machinery. The width is used to select masses in hard processes
according to a relativistic Breit–Wigner shape. In many processes the width is allowed
to be ŝ-dependent, see section 7.3.
Other particle masses, as discussed so far, have been fixed at their nominal value. We
now have to consider the mass broadening for short-lived particles such as ρ, K∗ or ∆.
Compared to the Z0 , it is much more difficult to describe the ρ resonance shape, sincenonperturbative and threshold effects act to distort the naı̈ve shape. Thus the ρ mass islimited from below by its decay ρ → ππ, but also from above, e.g. in the decay φ → ρπ.
Normally thus the allowed mass range is set by the most constraining decay chains. Somerare decay modes, specifically ρ0 → ηγ and a2 → η 0 π, are not allowed to have full impact,
however. Instead one accepts an imperfect rendering of the branching ratio, as some lowmass ρ0 /a2 decays of the above kind are rejected in favour of other decay channels. In
some decay chains, several mass choices are coupled, like in a2 → ρπ, where also the a2
has a non-negligible width. Finally, there are some extreme cases, like the f0 , which has anominal mass below the KK threshold, but a tail extending beyond that threshold, andtherefore a non-negligible branching ratio to the KK channel.
In view of examples like these, no attempt is made to provide a full description. Insteada simplified description is used, which should be enough to give the general smearing of
events due to mass broadening, but maybe not sufficient for detailed studies of a specificresonance. By default, hadrons are therefore given a mass distribution according to a
non-relativistic Breit–Wigner
P(m) dm ∝1
dm .
(m − m0 )2 + Γ2 /4(269)

Leptons and resonances not taken care of by the hard process machinery are distributed
according to a relativistic Breit–Wigner
P(m2 ) dm2 ∝1(m2 − m20 )2 + m20 Γ2

dm2 .

(270)

Here m0 and Γ are the nominal mass and width of the particle. The Breit–Wigner shapeis truncated symmetrically, |m − m0 | < δ, with δ arbitrarily chosen for each particle sothat no problems are encountered in the decay chains. It is possible to switch off the mass
broadening, or to use either a non-relativistic or a relativistic Breit–Wigners everywhere.
The f0 problem has been ‘solved’ by shifting the f0 mass to be slightly above the KK
threshold and have vanishing width. Then kinematics in decays f0 → KK is reasonably
well modelled. The f0 mass is too large in the f0 → ππ channel, but this does not reallymatter, since one anyway is far above threshold here.

386

13.2.3

Lifetimes

Clearly the lifetime and the width of a particle are inversely related. For practical applications, however, any particle with a non-negligible width decays too close to its production
vertex for the lifetime to be of any interest. In the program, the two aspects are therefore
considered separately. Particles with a non-vanishing nominal proper lifetime τ0 = hτ iare assigned an actual lifetime according toP(τ ) dτ ∝ exp(−τ /τ0 ) dτ ,(271)

i.e. a simple exponential decay is assumed. Since the program uses dimensions where thespeed of light c ≡ 1, and space dimensions are in mm, then actually the unit of cτ0 is mm
and of τ0 itself mm/c ≈ 3.33 × 10−12 s.
If a particle is produced at a vertex v = (x, t) with a momentum p = (p, E) and a
lifetime τ , the decay vertex position is assumed to bev0 = v + τ

p
,
m

(272)

where m is the mass of the particle. With the primary interaction (normally) in theorigin, it is therefore possible to construct all secondary vertices in parallel with theordinary decay treatment.
The formula above does not take into account any detector effects, such as a magnetic
field. It is therefore possible to stop the decay chains at some suitable point, and leave
any subsequent decay treatment to the detector simulation program. One may selectthat particles are only allowed to decay if they have a nominal lifetime τ0 shorter thansome given value or, alternatively, if their decay vertices x0 are inside some spherical or
cylindrical volume around the origin.

13.3

DecaysSeveral different kinds of decay treatment are used in the program, depending on thenature of the decay. Not discussed here are the decays of resonances which are handled
as part of the hard process.
13.3.1

Strong and electromagnetic decays

The decays of hadrons containing the ‘ordinary’ u, d and s quarks into two or threeparticles are known, and branching ratios may be found in [PDG96]. (At least for thelowest-lying states; the four L = 1 meson multiplets are considerably less well known.)
We normally assume that the momentum distributions are given by phase space. There
are a few exceptions, where the phase space is weighted by a matrix-element expression,as follows.
In ω and φ decays to π + π − π 0 , a matrix element of the form
|M|2 ∝ |pπ+ × pπ− |2(273)

is used, with the pπ the pion momenta in the rest frame of the decay. (Actually, what iscoded is the somewhat more lengthy Lorentz invariant form of the expression above.)
Consider the decay chain P0 → P1 +V → P1 +P2 +P3 , with P representing pseudoscalarmesons and V a vector one. Here the decay angular distribution of the V in its rest frameis|M|2 ∝ cos2 θ02 ,(274)
where θ02 is the angle between P0 and P2 . The classical example is D → K∗ π → Kππ. If
the P1 is replaced by a γ, the angular distribution in the V decay is instead ∝ sin2 θ02 .
387

In Dalitz decays, π 0 or η → e+ e− γ, the mass m∗ of the e+ e− pair is selected according
to
∗2

∗2

P(m ) dm

dm∗2
∝
m∗2

Ã

2m2
1 + ∗2em! s

4m2
1 − ∗2e
m

Ã

m∗2
1− 2
mπ,η

!3

1
(m2ρ − m∗2 )2 + m2ρ Γ2ρ

.

(275)
The last factor, the VMD-inspired ρ propagator, is negligible for π decay. Once the m∗
has been selected, the angular distribution of the e+ e− pair is given by
0

n

0

o

n

o

|M|2 ∝ (m∗2 −2m2e ) (pγ pe+ )2 + (pγ pe− )2 +4m2e (pγ pe+ )(pγ pe− ) + (pγ pe+ )2 + (pγ pe− )2 .
(276)
Also a number of simple decays involving resonances of heavier hadrons, e.g. Σ0c →−∗−Λ+→ B− γ are treated in the same way as the other two-particle decays.
c π or B
13.3.2

Weak decays of the τ lepton

For the τ lepton, an explicit list of decay channels has been put together, which includes
channels with up to five final-state particles, some of which may be unstable and subsequently decay to produce even larger total multiplicities.
The leptonic decays τ − → ντ `− ν ` , where ` is e or µ, are distributed according to thestandard V − A matrix element|M|2 = (pτ pν ` )(p` pντ ) .

(277)(The corresponding matrix element is also used in µ decays, but normally the µ is assumed
stable.)In τ decays to hadrons, the hadrons and the ντ are distributed according to phasespace times the factor xν (3 − xν ), where xν = 2Eν /mτ in the rest frame of the τ . The
latter factor is the ντ spectrum predicted by the parton level V − A matrix element, andtherefore represents an attempt to take into account that the ντ should take a larger
momentum fraction than given by phase space alone.
The probably largest shortcoming of the τ decay treatment is that no polarization
effects are included, i.e. the τ is always assumed to decay isotropically. Usually this is
not correct, since a τ is produced polarized in Z0 and W± decays. The PYTAUD routine
provides a generic interface to an external τ decay library, such as Tauola [Jad91], wheresuch effects could be handled (see also MSTJ(28)).
13.3.3

Weak decays of charm hadrons

The charm hadrons have a mass in an intermediate range, where the effects of the naı̈ve
V − A weak decay matrix element is partly but not fully reflected in the kinematics offinal-state particles. Therefore different decay strategies are combined. We start withhadronic decays, and subsequently consider semileptonic ones.
+
For the four ‘main’ charm hadrons, D+ , D0 , D+s and Λc , a number of branching ratios
are already known. The known branching ratios have been combined with reasonableguesses, to construct more or less complete tables of all channels. For hadronic decays ofD0 and D+ , where rather much is known, all channels have an explicitly listed particle
content. However, only for the two-body decays and some three-body decays is resonanceproduction properly taken into account. It means that the experimentally measured
branching ratio for a Kππ decay channel, say, is represented by contributions from a
direct Kππ channel as well as from indirect ones, such as K∗ π and Kρ. For a channel likeKππππ, on the other hand, not all possible combinations of resonances (many of which
would have to be off mass shell to have kinematics work out) are included. This is more or388

less in agreement with the philosophy adopted in the PDG tables [PDG92]. For D+s andΛ+knowledge
isratherincomplete,and
only
two-body
decay
channelsarelisted.
Finalcstates with three or more hadron are only listed in terms of a flavour content.
The way the program works, it is important to include all the allowed decay channels
up to a given multiplicity. Channels with multiplicity higher than this may then be
generated according to a simple flavour combination scheme. For instance, in a D+s decay,
the normal quark content is ssud, where one s is the spectator quark and the others comefrom the weak decay of the c quark. The spectator quark may also be annihilated, like inD+s → ud. The flavour content to make up one or two hadrons is therefore present fromthe onset. If one decides to generate more hadrons, this means new flavour-antiflavour
pairs have to be generated and combined with the existing flavours. This is done using
the same flavour approach as in fragmentation, section 12.1.
In more detail, the following scheme is used.
+
1. The multiplicity is first selected. The D+s and Λc multiplicity is selected according
to a distribution described further below. The program can also be asked to generate
decays of a predetermined multiplicity.
2. One of the non-spectator flavours is selected at random. This flavour is allowed
to ‘fragment’ into a hadron plus a new remaining flavour, using exactly the same
flavour generation algorithm as in the standard jet fragmentation, section 12.1.
3. Step 2 is iterated until only one or two hadrons remain to be generated, depending onwhether the original number of flavours is two or four. In each step one ‘unpaired’flavour is replaced by another one as a hadron is ‘peeled off’, so the number of
unpaired flavours is preserved.
4. If there are two flavours, these are combined to form the last hadron. If there are
four, then one of the two possible pairings into two final hadrons is selected at
random. To find the hadron species, the same flavour rules are used as when final
flavours are combined in the joining of two jets.
5. If the sum of decay product masses is larger than the mass of the decaying particle,
the flavour selection is rejected and the process is started over at step 1. Normallya new multiplicity is picked, but for D0 and D+ the old multiplicity is retained.
6. Once an acceptable set of hadrons has been found, these are distributed according
to phase space.
The picture then is one of a number of partons moving apart, fragmenting almost like
jets, but with momenta so low that phase-space considerations are enough to give theaverage behaviour of the momentum distribution. Like in jet fragmentation, endpoint
flavours are not likely to recombine with each other. Instead new flavour pairs are createdin between them. One should also note that, while vector and pseudoscalar mesons areproduced at their ordinary relative rates, events with many vectors are likely to fail instep 5. Effectively, there is therefore a shift towards lighter particles, especially at large
multiplicities.
When a multiplicity is to be picked,√ this is done according to a Gaussian distribution,centered at c+nq /4 and with a width c, with the final number rounded off to the nearest
integer. The value for the number of quarks nq is 2 or 4, as described above, andPµ

m − mqc = c1 ln
c2

¶

,

(278)

where m is the hadron mass and c1 and c2 have been tuned to give a reasonable descriptionof multiplicities. There is always some lower limit for the allowed multiplicity; if a number
smaller than this is picked the choice is repeated. Since two-body decays are explicitly+
enumerated for D+s and Λc , there the minimum multiplicity is three.
Semileptonic branching ratios are explicitly given in the program for all the four particles discussed here, i.e. it is never necessary to generate the flavour content using the389

fragmentation description. This does not mean that all branching ratios are known; a fairamount of guesswork is involved for the channels with higher multiplicities, based on aknowledge of the inclusive semileptonic branching ratio and the exclusive branching ratios
for low multiplicities.
In semileptonic decays it is not appropriate to distribute the lepton and neutrino
momenta according to phase space. Instead the simple V − A matrix element is used, inthe limit that decay product masses may be neglected and that quark momenta can be
replaced by hadron momenta. Specifically, in the decay H → `+ ν` h, where H is a charm
hadron and h and ordinary hadron, the matrix element|M|2 = (pH p` )(pν ph )(279)

is used to distribute the products. It is not clear how to generalize this formula whenseveral hadrons are present in the final state. In the program, the same matrix element
is used as above, with ph replaced by the total four-momentum of all the hadrons. This
tends to favour a low invariant mass for the hadronic system compared with naı̈ve phasespace.
There are a few charm hadrons, such as Ξc and Ωc , which decay weakly but are so rarethat little is known about them. For these a simplified generic charm decay treatment isused. For hadronic decays only the quark content is given, and then a multiplicity and aflavour composition is picked at random, as already described. Semileptonic decays are
assumed to produce only one hadron, so that V −A matrix element can be simply applied.
13.3.4

Weak decays of bottom hadrons

Some exclusive branching ratios now are known for B decays. In this version, the B0 ,B+ , B0s and Λ0b therefore appear in a similar vein to the one outlined above for D+s and+Λc above. That is, all leptonic channels and all hadronic two-body decay channels are
explicitly listed, while hadronic channels with three or more particles are only given interms of a quark content. The Bc is exceptional, in that either the bottom or the charm
quark may decay first, and in that annihilation graphs may be non-negligible. Leptonic
and semileptonic channels are here given in full, while hadronic channels are only listed
in terms of a quark content, with a relative composition as given in [Lus91]. No separate
branching ratios are set for any of the other weakly decaying bottom hadrons, but insteada pure ‘spectator quark’ model is assumed, where the decay of the b quark is the same in all
hadrons and the only difference in final flavour content comes from the spectator quark.
Compared to the charm decays, the weak decay matrix elements are given somewhat
larger importance in the hadronic decay channels.
In semileptonic decays b → c`− ν ` the c quark is combined with the spectator antiquark
or diquark to form one single hadron. This hadron may be either a pseudoscalar, a vector
or a higher resonance (tensor etc.). The relative fraction of the higher resonances has been
picked to be about 30%, in order to give a leptonic spectrum in reasonable experimentwith data. (This only applies to the main particles B0 , B+ , B0s and Λ0b ; for the rest the
choice is according to the standard composition in the fragmentation.) The overall process
is therefore H → h`− ν ` , where H is a bottom antimeson or a bottom baryon (rememberthat B is the one that contains a b quark), and the matrix element used to distributemomenta is|M|2 = (pH pν )(p` ph ) .
(280)
Again decay product masses have been neglected in the matrix element, but in the branching ratios the τ − ν τ channel has been reduced in rate, compared with e− ν e and µ− ν µ ones,
according to the expected mass effects. No CKM-suppressed decays b → u`− ν ` are currently included.
In most multi-body hadronic decays, e.g. b → cdu, the c quark is again combinedwith the spectator flavour to form one single hadron, and thereafter the hadron and the390

two quark momenta are distributed according to the same matrix element as above, with`− ↔ d and ν ` ↔ u. The invariant mass of the two quarks is calculated next. If thismass is so low that two hadrons cannot be formed from the system, the two quarks are
combined into one single hadron. Else the same kind of approach as in hadronic charm
decays is adopted, wherein a multiplicity is selected, a number of hadrons are formed andthereafter momenta are distributed according to phase space. The difference is that here
the charm decay product is distributed according to the V − A matrix element, and only
the rest of the system is assumed isotropic in its rest frame, while in charm decays all
hadrons are distributed isotropically.
Note that the c quark and the spectator are assumed to form one colour singlet andthe du another, separate one. It is thus assumed that the original colour assignments ofthe basic hard process are better retained than in charm decays. However, sometimes
this will not be true, and with about 20% probability the colour assignment is flippedaround so that cu forms one singlet. (In the program, this is achieved by changing theorder in which decay products are given.) In particular, the decay b → csc is allowed togive a cc colour-singlet state part of the time, and this state may collapse to a single J/ψ.
Two-body decays of this type are explicitly listed for B0 , B+ , B0s and Λ0b ; while other J/ψ
production channels appear from the flavour content specification.
00
The B0 –B and B0s –Bs systems mix before decay. This is optionally included. With aprobability
Ã!
xτ
2
Pflip = sin(281)
2 hτ ia B is therefore allowed to decay like a B, and vice versa. The mixing parameters are by00default xd = 0.7 in the B0 –B system and xs = 10 in the B0s –Bs one.
In the past, the generic B meson and baryon decay properties were stored for ‘particle’85, now obsolete but not yet removed. This particle contains a description of the free bquark decay, with an instruction to find the spectator flavour according to the particle
code of the actual decaying hadron.
13.3.5

Other decays

For onia spin 1 resonances, decay channels into a pair of leptons are explicitly given.
Hadronic decays of the J/ψ are simulated using the flavour generation model introduced
for charm. For Υ a fraction of the hadronic decays is into qq pairs, while the rest is into
ggg or ggγ, using the matrix elements of eq. (43). The ηc and ηb are both allowed to decay
into a gg pair, which then subsequently fragments. In Υ and ηb decays the partons areallowed to shower before fragmentation, but energies are too low for showering to have
any impact.
Default branching ratios are given for resonances like the Z0 , the W± , the t or the h0 .
When Pythia is initialized, these numbers are replaced by branching ratios evaluatedfrom the given masses. For Z0 and W± the branching ratios depend only marginally on themasses assumed, while effects are large e.g. for the h0 . In fact, branching ratios may varyover the Breit–Wigner resonance shape, something which is also taken into account in
the PYRESD description. Therefore the simpler resonance treatment of PYDECY is normallynot so useful, and should be avoided. When it is used, a channel is selected according to
the given fixed branching ratios. If the decay is into a qq pair, the quarks are allowed toshower and subsequently the parton system is fragmented.

391

14

The Fragmentation and Decay Program Elements

In this section we collect information on most of the routines and common-block variables
found in the fragmentation and decay descriptions of Pythia, plus a number of related
low-level tasks.

14.1

Definition of Initial Configuration or Variables

With the use of the conventions described for the event record, it is possible to specify any
initial jet/particle configuration. This task is simplified for a number of often occurring
situations by the existence of the filling routines below. It should be noted that manyusers do not come in direct contact with these routines, since that is taken care of byhigher-level routines for specific processes, particularly PYEVNT and PYEEVT.
Several calls to the routines can be combined in the specification. In case one call isenough, the complete fragmentation/decay chain may be simulated at the same time. At
each call, the value of N is updated to the last line used for information in the call, so if
several calls are used, they should be made with increasing IP number, or else N should
be redefined by hand afterwards.
The routine PYJOIN is very useful to define the colour flow in more complicated partonconfigurations; thereby one can bypass the not-so-trivial rules for how to set the K(I,4)
and K(I,5) colour-flow information.
The routine PYGIVE contains a facility to set various common-block variables in acontrolled and documented fashion.
CALL PY1ENT(IP,KF,PE,THE,PHI)
Purpose: to add one entry to the event record, i.e. either a parton or a particle.
IP :normally line number for the parton/particle. There are two exceptions.
If IP = 0, line number 1 is used and PYEXEC is called.
If IP < 0, line -IP is used, with status code K(-IP,2) = 2 rather than 1; thus
a parton system may be built up by filling all but the last parton of the systemwith IP < 0.
KF :parton/particle flavour code.
PE :parton/particle energy. If PE is smaller than the mass, the parton/particle istaken to be at rest.
THE, PHI : polar and azimuthal angle for the momentum vector of the parton/particle.
CALL PY2ENT(IP,KF1,KF2,PECM)
Purpose: to add two entries to the event record, i.e. either a 2-parton system or two
separate particles.
IP :normally line number for the first parton/particle, with the second in line IP +1. There are two exceptions.
If IP = 0, lines 1 and 2 are used and PYEXEC is called.
If IP < 0, lines -IP and -IP + 1 are used, with status code K(I,1) = 3, i.e. withspecial colour connection information, so that a parton shower can be generated
by a PYSHOW call, followed by a PYEXEC call, if so desired (only relevant forpartons).
KF1, KF2 : flavour codes for the two partons/particles.
PECM : (= Ecm ) the total energy of the system.
Remark: the system is given in the c.m. frame, with the first parton/particle going out
in the +z direction.
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CALL PY3ENT(IP,KF1,KF2,KF3,PECM,X1,X3)
Purpose: to add three entries to the event record, i.e. either a 3-parton system or three
separate particles.
IP :normally line number for the first parton/particle, with the other two in IP + 1
and IP + 2. There are two exceptions.
If IP = 0, lines 1, 2 and 3 are used and PYEXEC is called.
If IP < 0, lines -IP through -IP + 2 are used, with status code K(I,1) = 3,
i.e. with special colour connection information, so that a parton shower can begenerated by a PYSHOW call, followed by a PYEXEC call, if so desired (only relevant
for partons).
KF1, KF2, KF3: flavour codes for the three partons/particles.
PECM : (Ecm ) the total energy of the system.
X1, X3 : xi = 2Ei /Ecm , i.e. twice the energy fraction taken by the i’th parton. Thusx2 = 2 − x1 − x3 , and need not be given. Note that not all combinations of xi
are inside the physically allowed region.
Remarks : the system is given in the c.m. frame, in the xz-plane, with the first parton
going out in the +z direction and the third one having px > 0. A system mustbe given in the order of colour flow, so qgq and qgq are allowed but qqg not.
Thus x1 and x3 come to correspond to what is normally called x1 and x2 , i.e. thescaled q and q energies.
CALL PY4ENT(IP,KF1,KF2,KF3,KF4,PECM,X1,X2,X4,X12,X14)
Purpose: to add four entries to the event record, i.e. either a 4-parton system or four
separate particles (or, for qqq0 q0 events, two 2-parton systems).
IP :normally line number for the first parton/particle, with the other three in lines
IP + 1, IP + 2 and IP + 3. There are two exceptions.
If IP = 0, lines 1, 2, 3 and 4 are used and PYEXEC is called.
If IP < 0, lines -IP through -IP + 3 are used, with status code K(I,1) = 3,
i.e. with special colour connection information, so that a parton shower can begenerated by a PYSHOW call, followed by a PYEXEC call, if so desired (only relevant
for partons).
KF1,KF2,KF3,KF4 : flavour codes for the four partons/particles.
PECM : (= Ecm ) the total energy of the system.
X1,X2,X4 : xi = 2Ei /Ecm , i.e. twice the energy fraction taken by the i’th parton. Thus
x3 = 2 − x1 − x2 − x4 , and need not be given.
2
X12,X14 : xij = 2pi pj /Ecm
, i.e. twice the four-vector product of the momenta for partons i and j, properly normalized. With the masses known, other xij may beconstructed from the xi and xij given. Note that not all combinations of xi andxij are inside the physically allowed region.
Remarks: the system is given in the c.m. frame, with the first parton going out in the +z
direction and the fourth parton lying in the xz-plane with px > 0. The second
parton will have py > 0 and py < 0 with equal probability, with the third partonbalancing this py (this corresponds to a random choice between the two possible
stereoisomers). A system must be given in the order of colour flow, e.g. qggq
and qq0 q0 q.
CALL PYJOIN(NJOIN,IJOIN)
Purpose: to connect a number of previously defined partons into a string configuration.
393

Initially the partons must be given with status codes K(I,1) = 1, 2 or 3. Afterwards the partons all have status code 3, i.e. are given with full colour-flow
information. Compared to the normal way of defining a parton system, the partons need therefore not appear in the same sequence in the event record as they
are assumed to do along the string. It is also possible to call PYSHOW for all orsome of the entries making up the string formed by PYJOIN.
NJOIN: the number of entries that are to be joined by one string.
IJOIN: an one-dimensional array, of size at least NJOIN. The NJOIN first numbers are thepositions of the partons that are to be joined, given in the order the partons are
assumed to appear along the string. If the system consists entirely of gluons, thestring is closed by connecting back the last to the first entry.
Remarks: only one string (i.e. one colour singlet) may be defined per call, but one is atliberty to use any number of PYJOIN calls for a given event. The program will
check that the parton configuration specified makes sense, and not take any action unless it does. Note, however, that an initially sensible parton configuration
may become nonsensical, if only some of the partons are reconnected, while theothers are left unchanged.
CALL PYGIVE(CHIN)
Purpose: to set the value of any variable residing in the commmonblocks PYJETS,PYDAT1, PYDAT2, PYDAT3, PYDAT4, PYDATR, PYSUBS, PYPARS, PYINT1, PYINT2,
PYINT3, PYINT4, PYINT5, PYINT6, PYINT7, PYINT8, PYMSSM, PYMSRV, or PYTCSM.
This is done in a more controlled fashion than by directly including the common blocks in your program, in that array bounds are checked and the old andnew values for the variable changed are written to the output for reference. An
example how PYGIVE can be used to parse input from a file is given in section
3.6.
CHIN : character expression of length at most 100 characters, with requests for variables
to be changed, stored in the form
variable1=value1;variable2=value2;variable3=value3. . . .
Note that an arbitrary number of instructions can be stored in one call if separated by semicolons, and that blanks may be included anyplace. An exclamation
mark is recognized as the beginning of a comment, which is not to be processed.
Normal parsing is resumed at the next semicolon (if any remain). An example
would beCALL PYGIVE(’MSEL=16!Higgs production;PMAS(25,1)=115.!h0 mass’)
The variablei may be any single variable in the Pythia common blocks, and thevaluei must be of the correct integer, real or character (without extra quotes)
type. Array indices and values must be given explicitly, i.e. cannot be variablesin their own right. The exception is that the first index can be preceded by a C,
signifying that the index should be translated from normal KF to compressed KC
code with a PYCOMP call; this is allowed for the KCHG, PMAS, MDCY, CHAF and MWID
arrays.
If a valuei is omitted, i.e. with the construction variable=, the current value iswritten to the output, but the variable itself is not changed.
The writing of info can be switched off by MSTU(13) = 0.
Note 1: the checks on array bounds are hardwired into this routine. Therefore, if youchange array dimensions and MSTU(3), MSTU(6) and/or MSTU(7), as allowed byother considerations, these changes will not be known to PYGIVE. Normally thisshould not be a problem, however.
Note 2: in the first PYGIVE call, the Pythia header will be printed, if it was not already,394

except if the command is to set MSTU(12) = 12345.

14.2

The Physics Routines

Once the initial parton/particle configuration has been specified (by the routines in theprevious section or, more commonly, by higher-level routines such as PYEVNT) and default
parameter values changed, if so desired, only a PYEXEC call is necessary to simulate thewhole fragmentation and decay chain. Therefore a normal user will not directly see any of
the other routines in this section. Some of them could be called directly, but the dangerof faulty usage is then non-negligible.
The PYTAUD routine provides an optional interface to an external τ decay library, wherepolarization effects could be included. It is up to you to write the appropriate calls, as
explained at the end of this section.
CALL PYEXEC
Purpose: to administrate the fragmentation and decay chain. PYEXEC may be called
several times, but only entries which have not yet been treated (more precisely,
which have 1 ≤K(I,1)≤ 10) can be affected by further calls. This may apply if more partons/particles have been added by you, or if particles previously
considered stable are now allowed to decay. The actions that will be taken during a PYEXEC call can be tailored extensively via the PYDAT1 - PYDAT3 common
blocks, in particular by setting the MSTJ values suitably.
SUBROUTINE PYPREP(IP) : to rearrange parton-shower end products (marked with
K(I,1) = 3) sequentially along strings; also to (optionally) allow small parton
systems to collapse into two particles or one only, in the latter case with energy
and momentum to be shuffled elsewhere in the event; also to perform checks
that e.g. flavours of colour-singlet systems make sense. In the new multiple interactions scenario this procedure may fail frequently, e.g. in the processing ofjunctions, and new tries have to be made, up to five times.
SUBROUTINE PYSTRF(IP) : to generate the fragmentation of an arbitrary colour-singlet
parton system according to the Lund string fragmentation model. One of theabsolutely central routines of Pythia.
SUBROUTINE PYINDF(IP) : to handle the fragmentation of a parton system according
to independent fragmentation models, and implement energy, momentum andflavour conservation, if so desired. Also the fragmentation of a single parton,
not belonging to a parton system, is considered here. (This is of course physicalnonsense, but may sometimes be convenient for specific tasks.)
SUBROUTINE PYDECY(IP) : to perform a particle decay, according to known branchingratios or different kinds of models, depending on our level of knowledge. Various
matrix elements are included for specific processes.
SUBROUTINE PYKFDI(KFL1,KFL2,KFL3,KF) : to generate a new quark or diquark flavour
and to combine it with an existing flavour to give a hadron.
KFL1: incoming flavour.
KFL2: extra incoming flavour, e.g. for formation of final particle, where the flavours
are completely specified. Is normally 0.
KFL3: newly created flavour; is 0 if KFL2 is non-zero.
KF:
produced hadron. Is 0 if something went wrong (e.g. inconsistent combination of incoming flavours).
SUBROUTINE PYPTDI(KFL,PX,PY) : to give transverse momentum, e.g. for a qq pair created in the colour field, according to independent Gaussian distributions in px
and py .
395

SUBROUTINE PYZDIS(KFL1,KFL3,PR,Z) : to generate the longitudinal scaling variable
z in jet fragmentation, either according to the Lund symmetric fragmentation
function, or according to a choice of other shapes.
SUBROUTINE PYBOEI : to include Bose–Einstein effects according to a simple parameterization. By default, this routine is not called. If called from PYEXEC, this is done
after the decay of short-lived resonances, but before the decay of long-lived ones.
This means the routine should never be called directly by you, nor would effects
be correctly simulated if decays are switched off. See MSTJ(51) - MSTJ(57) for
switches of the routine.
FUNCTION PYMASS(KF) : to give the mass for a parton/particle.
SUBROUTINE PYNAME(KF,CHAU) : to give the parton/particle name (as a string of type
CHARACTER CHAU*16). The name is read out from the CHAF array.
FUNCTION PYCHGE(KF) : to give three times the charge for a parton/particle. The value
is read out from the KCHG(KC,1) array.
FUNCTION PYCOMP(KF) : to give the compressed parton/particle code KC for a given KF
code, as required to find entry into mass and decay data tables. Also checks
whether the given KF code is actually an allowed one (i.e. known by the program),and returns 0 if not. Note that KF may be positive or negative, while the resulting
KC code is never negative.
Internally PYCOMP uses a binary search in a table, with KF codes arranged inincreasing order, based on the KCHG(KC,4) array. This table is constructed thefirst time PYCOMP is called, at which time MSTU(20) is set to 1. In case of auser change of the KCHG(KC,4) array one should reset MSTU(20) = 0 to force
a re-initialization at the next PYCOMP call (this is automatically done in PYUPDA
calls). To speed up execution, the latest (KF,KC) pair is kept in memory andchecked before the standard binary search.
SUBROUTINE PYERRM(MERR,MESSAG) : to keep track of the number of errors and warnings
encountered, write out information on them, and abort the program in case of
too many errors.
FUNCTION PYANGL(X,Y) : to calculate the angle from the x and y coordinates.
SUBROUTINE PYLOGO : to write a title page for the Pythia programs. Called by
PYLIST(0).
SUBROUTINE PYTIME(IDATI) : to give the date and time, for use in PYLOGO and elsewhere. Since Fortran 77 does not contain a standard way of obtaining thisinformation, the routine is dummy, to be replaced by you. Some commented-out
examples are given, e.g. for Fortran 90 or the GNU Linux libU77. The output
is given in an integer array ITIME(6), with components year, month, day, hour,
minute and second. If there should be no such information available on a system,
it is acceptable to put all the numbers above to 0.
CALL PYTAUD(ITAU,IORIG,KFORIG,NDECAY)
Purpose: to act as an interface between the standard decay routine PYDECY and a usersupplied τ lepton decay library such as Tauola [Jad91]. The latter librarywould normally know how to handle polarized τ ’s, given the τ helicity as input,
so one task of the interface routine is to construct the τ polarization/helicity
from the information available. Input to the routine (from PYDECY) is provided
in the first three arguments, while the last argument and some event record
information have to be set before return. To use this facility you have to set theswitch MSTJ(28), include your own interface routine PYTAUD and see to it thatthe dummy routine PYTAUD is not linked. The dummy routine is there only to
avoid unresolved external references when no user-supplied interface is linked.
396

ITAU : line number in the event record where the τ is stored. The four-momentum of
this τ has first been boosted back to the rest frame of the decaying mother andthereafter rotated to move out along the +z axis. It would have been possible
to also perform a final boost to the rest frame of the τ itself, but this has been
avoided so as not to suppress the kinematics aspect of close-to-threshold production (e.g. in B decays) vs. high-energy production (e.g. in real W decays).
The choice of frame should help the calculation of the helicity configuration.
After the PYTAUD call, the τ and its decay products will automatically be rotated and boosted back. However, seemingly, the event record does not conserve
momentum at this intermediate stage.
IORIG : line number where the mother particle to the τ is stored. Is 0 if the mother is
not stored. This does not have to mean the mother is unknown. For instance,
in semileptonic B decays the mother is a W± with known four-momentum pW =pτ + pντ , but there is no W line in the event record. When several copies ofthe mother is stored (e.g. one in the documentation section of the event recordand one in the main section), IORIG points to the last. If a branchings like
τ → τ γ occurs, the ‘grandmother’ is given, i.e. the mother of the direct τ before
branching.
KFORIG : flavour code for the mother particle. Is 0 if the mother is unknown. The
mother would typically be a resonance such as γ ∗ /Z0 (23), W± (±24), h0 (25),or H± (±37). Often the helicity choice would be clear just by the knowledge ofthis mother species, e.g., W± vs. H± . However, sometimes further complications
may exist. For instance, the KF code 23 represents a mixture of γ ∗ and Z0 ; aknowledge of the mother mass (in P(IORIG,5)) would here be required to makethe choice of helicities. Further, a W± or Z0 may either be (predominantly)
transverse or longitudinal, depending on the production process under study.
NDECAY : the number of decay products of the τ ; to be given by the user routine. You
must also store the KF flavour codes of those decay products in the positions
K(I,2), N + 1 ≤ I ≤ N + NDECAY, of the event record. The corresponding fivemomentum (momentum, energy and mass) should be stored in the associated
P(I,J) positions, 1 ≤ J ≤ 5. The four-momenta are expected to add up to thefour-momentum of the τ in position ITAU. You should not change the N value or
any of the other K or V values (neither for the τ nor for its decay products) since
this is automatically done in PYDECY.

14.3

The General Switches and Parameters

The common block PYDAT1 contains the main switches and parameters for the fragmentation and decay treatment, but also for some other aspects. Here one may control indetail what the program is to do, if the default mode of operation is not satisfactory.
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
Purpose: to give access to a number of status codes and parameters which regulate
the performance of the program as a whole. Here MSTU and PARU are related
to utility functions, as well as a few parameters of the Standard Model, while
MSTJ and PARJ affect the underlying physics assumptions. Some of the variables
in PYDAT1 are described elsewhere, and are therefore here only reproduced as
references to the relevant sections. This in particular applies to many coupling
constants, which are found in section 9.4, and switches of the older dedicatede+ e− machinery, section 6.3.
MSTU(1) - MSTU(3) : variables used by the event study routines, section 15.1.
397

MSTU(4) : (D = 4000) number of lines available in the common block PYJETS. Should
always be changed if the dimensions of the K and P arrays are changed by you,but should otherwise never be touched. Maximum allowed value is 10000, unless
MSTU(5) is also changed.
MSTU(5) : (D = 10000) is used in building up the special colour-flow information storedin K(I,4) and K(I,5) for K(I,3) = 3, 13 or 14. The generic form for j = 4 or
5 is
K(I,j)= 2×MSTU(5)2 ×MCFR+MSTU(5)2 ×MCTO+MSTU(5)×ICFR+ICTO,
with notation as in section 5.2. One should always have MSTU(5) ≥ MSTU(4).
On a 32 bit machine, values MSTU(5) > 20000 may lead to overflow problems,and should be avoided.
MSTU(6) : (D = 500) number of KC codes available in the KCHG, PMAS, MDCY, and CHAF
arrays; should be changed if these dimensions are changed.
MSTU(7) : (D = 8000) number of decay channels available in the MDME, BRAT and KFDP
arrays; should be changed if these dimensions are changed.
MSTU(10) : (D = 2) use of parton/particle masses in filling routines (PY1ENT, PY2ENT,PY3ENT, PY4ENT).
= 0 : assume the mass to be zero.
= 1 : keep the mass value stored in P(I,5), whatever it is. (This may be used
e.g. to describe kinematics with off-mass-shell partons).
= 2 : find masses according to mass tables as usual.
MSTU(11) - MSTU(12) : variables used by the event study routines, section 15.1.
MSTU(13) : (D = 1) writing of information on variable values changed by a PYGIVE
call.
= 0 : no information is provided.
= 1 : information is written to standard output.
MSTU(14) : variable used by the event study routines, section 15.1.
MSTU(15) : (D = 0) decides how PYLIST shows empty lines, which are interspersed
among ordinary particles in the event record.
= 0 : do not print lines with K(I,1) ≤ 0.
= 1 : do not print lines with K(I,1) < 0.
= 2 : print all lines.
MSTU(16) : (D = 1) choice of mother pointers for the particles produced by a fragmenting
parton system.
= 1 : all primary particles of a system point to a line with KF = 92 or 93, forstring or independent fragmentation, respectively, or to a line with KF = 91
if a parton system has so small a mass that it is forced to decay into one
or two particles. The two (or more) shower initiators of a showering parton
system point to a line with KF = 94. The entries with KF = 91–94 in theirturn point back to the predecessor partons, so that the KF = 91–94 entries
form a part of the event history proper.
= 2 : although the lines with KF = 91–94 are present, and contain the correct
mother and daughter pointers, they are not part of the event history proper,
in that particles produced in string fragmentation point directly to either of
the two endpoint partons of the string (depending on the side they were generated from), particles produced in independent fragmentation point to therespective parton they were generated from, particles in small mass systems
point to either endpoint parton, and shower initiators point to the original
on-mass-shell counterparts. Also the daughter pointers bypass the KF = 91–
94 entries. In independent fragmentation, a parton need not produce any
particles at all, and then have daughter pointers 0.
When junctions are present, the related primary baryon points back to thejunction around which it is produced. More generally, consider a system398

stored as q1 q2 Jq3 . In the listing of primary hadrons coming from this system,
the first ones will have K(I,3) pointers back to q1 , either they come from thehadronization of the q1 or q2 string pieces. (In principle the hadrons could
be classified further, but this has not been done.) Then comes the junction
baryon, which may be followed by hadrons again pointing back to q1 . Thisis because the final string piece, between the junction and q3 , is fragmented
from both ends with a joining to conserve overall energy and momentum.
(Typically the two shortest strings are put ahead of the junction.) Moreproperly these hadrons also could have pointed back to the junction, butthen the special status of the junction baryon would have been lost. The
final hadrons point to q3 , and have fragmented off this end of the stringpiece in to the junction. The above example generalizes easily to topologies
with two junctions, e.g. q1 q2 JJq3 q4 , where now particles in the central JJ
string would point either to q1 or q4 depending on production side.
Note : MSTU(16) should not be changed between the generation of an event andthe translation of this event record with a PYHEPC call, since this may givean erroneous translation of the event history.
MSTU(17) : (D = 0) storage option for MSTU(90) and associated information on z values
for heavy-flavour production.
= 0 : MSTU(90) is reset to zero at each PYEXEC call. This is the appropriate course
if PYEXEC is only called once per event, as is normally the case when you do
not yourself call PYEXEC.
= 1 : you have to reset MSTU(90) to zero yourself before each new event. This is
the appropriate course if several PYEXEC calls may appear for one event, i.e.
if you call PYEXEC directly.
MSTU(19) : (D = 0) advisory warning for unphysical flavour setups in PY2ENT, PY3ENT
or PY4ENT calls.
= 0 : yes.
= 1 : no; MSTU(19) is reset to 0 in such a call.
MSTU(20) : (D = 0) flag for the initialization status of the PYCOMP routine. A value 0
indicates that tables should be (re)initialized, after which it is set 1. In case
you change the KCHG(KC,4) array you should reset MSTU(20) = 0 to force a
re-initialization at the next PYCOMP call.
MSTU(21) : (D = 2) check on possible errors during program execution. Obviously no
guarantee is given that all errors will be caught, but some of the most trivial
user-caused errors may be found.
= 0 : errors do not cause any immediate action, rather the program will try tocope, which may mean e.g. that it runs into an infinite loop.
= 1 : parton/particle configurations are checked for possible errors. In case ofproblem, an exit is made from the misbehaving subprogram, but the generation of the event is continued from there on. For the first MSTU(22) errors
a message is printed; after that no messages appear.
= 2 : parton/particle configurations are checked for possible errors. In case ofproblem, an exit is made from the misbehaving subprogram, and subsequently from PYEXEC. You may then choose to correct the error, and continue the execution by another PYEXEC call. For the first MSTU(22) errors
a message is printed, after that the last event is printed and execution is
stopped.
MSTU(22) : (D = 10) maximum number of errors that are printed.
MSTU(23) : (I) count of number of errors experienced to date. Is not updated for errors
in a string system containing junctions. (Since errors occasionally do happen
there, and are difficult to eliminate altogether.)
MSTU(24) : (R) type of latest error experienced; reason that event was not generated in399

full. Is reset at each PYEXEC call.
= 0 : no error experienced.
= 1 : program has reached end of or is writing outside PYJETS memory.
= 2 : unknown flavour code or unphysical combination of codes; may also be
caused by erroneous string connection information.
= 3 : energy or mass too small or unphysical kinematical variable setup.
= 4 : program is caught in an infinite loop.
= 5 : momentum, energy or charge was not conserved (even allowing for machine
precision errors, see PARU(11)); is evaluated only after event has been generated in full, and does not apply when independent fragmentation without
momentum conservation was used.
= 6 : error call from outside the fragmentation/decay package (e.g. the e+ e− routines).
= 7 : inconsistent particle data input in PYUPDA (MUPDA = 2,3) or other PYUPDArelated problem.
= 8 : problems in more peripheral service routines.
= 9 : various other problems.
MSTU(25) : (D = 1) printing of warning messages.
= 0 : no warnings are written.
= 1 : first MSTU(26) warnings are printed, thereafter no warnings appear.
MSTU(26) : (D = 10) maximum number of warnings that are printed.
MSTU(27) : (I) count of number of warnings experienced to date.
MSTU(28) : (R) type of latest warning given, with codes parallelling those for MSTU(24),but of a less serious nature.
MSTU(29) : (I) denotes the presence (1) or not (0) of a junction in the latest system
studied. Used to decide whether to update the MSTU(23) counter in case oferrors.
MSTU(30) : (I) count of number of errors experienced to date, equivalent to MSTU(23)except that it is also updated for errors in a string system containing junctions.
MSTU(31) : (I) number of PYEXEC calls in present run.
MSTU(32) - MSTU(33) : variables used by the event-study routines, section 15.1.
MSTU(41) - MSTU(63) : switches for event-analysis routines, see section 15.5.
MSTU(70) - MSTU(80) : variables used by the event study routines, section 15.1.
MSTU(90) : number of heavy-flavour hadrons (i.e. hadrons containing charm or bottom)
produced in the fragmentation stage of the current event, for which the positionsin the event record are stored in MSTU(91) - MSTU(98) and the z values in
the fragmentation in PARU(91) - PARU(98). At most eight values will be stored(normally this is no problem). No z values can be stored for those heavy hadrons
produced when a string has so small mass that it collapses to one or two particles,
nor for those produced as one of the final two particles in the fragmentation ofa string. If MSTU(17) = 1, MSTU(90) should be reset to zero by you before each
new event, else this is done automatically.
MSTU(91) - MSTU(98) : the first MSTU(90) positions will be filled with the line numbersof the heavy-flavour hadrons produced in the current event. See MSTU(90) for
additional comments. Note that the information is corrupted by calls to PYEDIT
with options 0–5 and 21–23; calls with options 11–15 work, however.
MSTU(101) - MSTU(118) : switches related to couplings, see section 9.4.
MSTU(121) - MSTU(125) : internally used in the advanced popcorn code, see section
14.3.1.
MSTU(131) - MSTU(140) : internally used in the advanced popcorn code, see section
14.3.1.
MSTU(161), MSTU(162) : information used by event-analysis routines, see section 15.5.

400

PARU(1) : (R) π ≈ 3.141592653589793.
PARU(2) : (R) 2π ≈ 6.283185307179586.
PARU(3) : (D = 0.197327) conversion factor for GeV−1 → fm or fm−1 → GeV.
PARU(4) : (D = 5.06773) conversion factor for fm → GeV−1 or GeV → fm−1 .
PARU(5) : (D = 0.389380) conversion factor for GeV−2 → mb or mb−1 → GeV2 .
PARU(6) : (D = 2.56819) conversion factor for mb → GeV−2 or GeV2 → mb−1 .
PARU(11) : (D = 0.001) relative error, i.e. non-conservation of momentum and energy
divided by total energy, that may be attributable to machine precision problems
before a physics error is suspected (see MSTU(24) = 5).
PARU(12) : (D = 0.09 GeV2 ) effective cut-off in squared mass, below which partons
may be recombined to simplify (machine precision limited) kinematics of string
fragmentation. (Default chosen to be of the order of a light quark mass, or halfa typical light meson mass.)
PARU(13) : (D = 0.01) effective angular cut-off in radians for recombination of partons,
used in conjunction with PARU(12).
PARU(21) : (I) contains the total energy W of all first-generation partons/particles after
a PYEXEC call; to be used by the PYP function for I > 0, J = 20–25.
PARU(41) - PARU(63) : parameters for event-analysis routines, see section 15.5.
PARU(91) - PARU(98) : the first MSTU(90) positions will be filled with the fragmentation z values used internally in the generation of heavy-flavour hadrons —how these are translated into the actual energies and momenta of the observed
hadrons is a complicated function of the string configuration. The particle withz value stored in PARU(i) is to be found in line MSTU(i) of the event record. See
MSTU(90) and MSTU(91) - MSTU(98) for additional comments.
PARU(101) - PARU(195) : various coupling constants and parameters related to couplings, see section 9.4.
MSTJ(1) : (D = 1) choice of fragmentation scheme.
= 0 : no jet fragmentation at all.
= 1 : string fragmentation according to the Lund model.
= 2 : independent fragmentation, according to specification in MSTJ(2) andMSTJ(3).
MSTJ(2) : (D = 3) gluon jet fragmentation scheme in independent fragmentation.
= 1 : a gluon is assumed to fragment like a random d, u or s quark or antiquark.
= 2 : as = 1, but longitudinal (see PARJ(43), PARJ(44) and PARJ(59)) and transverse (see PARJ(22)) momentum properties of quark or antiquark substituting for gluon may be separately specified.
= 3 : a gluon is assumed to fragment like a pair of a d, u or s quark and its
antiquark, sharing the gluon energy according to the Altarelli-Parisi splitting
function.
= 4 : as = 3, but longitudinal (see PARJ(43), PARJ(44) and PARJ(59)) and transverse (see PARJ(22)) momentum properties of quark and antiquark substituting for gluon may be separately specified.
MSTJ(3) : (D = 0) energy, momentum and flavour conservation options in independent
fragmentation. Whenever momentum conservation is described below, energy
and flavour conservation is also implicitly assumed.
= 0 : no explicit conservation of any kind.
= 1 : particles share momentum imbalance compensation according to their energy (roughly equivalent to boosting event to c.m. frame). This is similar to
the approach in the Ali et al. program [Ali80].
= 2 : particles share momentum imbalance compensation according to their longitudinal mass with respect to the imbalance direction.
= 3 : particles share momentum imbalance compensation equally.

401

= 4 :

transverse momenta are compensated separately within each jet, longitudinal momenta are rescaled so that ratio of final jet to initial parton momentumis the same for all the jets of the event. This is similar to the approach in
the Hoyer et al. program [Hoy79].
= 5 : only flavour is explicitly conserved.
= 6 - 10 : as = 1 - 5, except that above several colour singlet systems that followed immediately after each other in the event listing (e.g. qqqq) weretreated as one single system, whereas here they are treated as separate systems.
= -1 : independent fragmentation, where also particles moving backwards with respect to the jet direction are kept, and thus the amount of energy andmomentum mismatch may be large.
MSTJ(11) : (D = 4) choice of longitudinal fragmentation function, i.e. how large a fraction of the energy available a newly-created hadron takes.
= 1 : the Lund symmetric fragmentation function, see PARJ(41) - PARJ(45).
= 2 : choice of some different forms for each flavour separately, see PARJ(51) PARJ(59).
= 3 : hybrid scheme, where light flavours are treated with symmetric Lund (=1), but charm and heavier can be separately chosen, e.g. according to thePeterson/SLAC function (= 2).
= 4 : the Lund symmetric fragmentation function (= 1), for heavy endpoint
quarks modified according to the Bowler (Artru–Mennessier, Morris) space–time picture of string evolution, see PARJ(46).
= 5 : as = 4, but with possibility to interpolate between Bowler and Lund separately for c and b; see PARJ(46) and PARJ(47).
MSTJ(12) : (D = 2) choice of baryon production model.
= 0 : no baryon-antibaryon pair production at all; initial diquark treated as a unit.
= 1 : diquark-antidiquark pair production allowed; diquark treated as a unit.
= 2 : diquark-antidiquark pair production allowed, with possibility for diquark to
be split according to the ‘popcorn’ scheme.
= 3 : as = 2, but additionally the production of first rank baryons may be suppressed by a factor PARJ(19).
= 4 : as = 2, but diquark vertices suffer an extra suppression of the form 1 −exp(ρΓ), where ρ ≈ 0.7GeV−2 is stored in PARF(192).
= 5 : Advanced version of the popcorn model. Independent of PARJ(3 - 7). Instead depending on PARJ(8 - 10). When using this option PARJ(1) needs
to enhanced by approx. a factor 2 (i.e. it losses a bit of its normal meaning),
and PARJ(18) is suggested to be set to 0.19. See section 14.3.1 for further
details.
MSTJ(13) : (D = 0) generation of transverse momentum for endpoint quark(s) of single
quark jet or qq jet system (in multijet events no endpoint transverse momentumis ever allowed for).
= 0 : no transverse momentum for endpoint quarks.
= 1 : endpoint quarks obtain transverse momenta like ordinary qq pairs producedin the field (see PARJ(21)); for 2-jet systems the endpoints obtain balancing
transverse momenta.
MSTJ(14) : (D = 1) treatment of a colour-singlet parton system with a low invariantmass.
= 0 : no precautions are taken, meaning that problems may occur in PYSTRF (orPYINDF) later on. Warning messages are issued when low masses are encountered, however, or when the flavour or colour configuration appears to
be unphysical.
= 1 : small parton systems are allowed to collapse into two particles or, failing402

that, one single particle. Normally all small systems are treated this way,
starting with the smallest one, but some systems would require more workand are left untreated; they include diquark-antidiquark pairs below thetwo-particle threshold. See further MSTJ(16) and MSTJ(17).
= -1 : special option for PYPREP calls, where no precautions are taken (as for = 0),but, in addition, no checks are made on the presence of small-mass systems
or unphysical flavour or colour configurations; i.e. PYPREP only rearranges
colour strings.
MSTJ(15) : (D = 0) production probability for new flavours.
= 0 : according to standard Lund parameterization, as given by PARJ(1) PARJ(20).
= 1 : according to probabilities stored in PARF(201) - PARF(1960); note that nodefault values exist here, i.e. PARF must be set by you. The MSTJ(12) switch
can still be used to set baryon production mode, with the modification that
MSTJ(12) = 2 here allows an arbitrary number of mesons to be produced
between a baryon and an antibaryon (since the probability for diquark →meson + new diquark is assumed independent of prehistory).
MSTJ(16) : (D = 2) mode of cluster treatment (where a cluster is a low-mass string thatcan fragment to two particles at the most).
= 0 : old scheme. Cluster decays (to two hadrons) are isotropic. In cluster collapses (to one hadron), energy-momentum compensation is to/from the parton or hadron furthest away in mass.
= 1 : intermediate scheme. Cluster decays are anisotropic in a way that is intended
to mimic the Gaussian p⊥ suppression and string ‘area law’ of suppressed
rapidity orderings of ordinary string fragmentation. In cluster collapses,
energy-momentum compensation is to/from the string piece most closely
moving in the same direction as the cluster. Excess energy is put as an
extra gluon on this string piece, while a deficit is taken from both endpointsof this string piece as a common fraction of their original momentum.
= 2 : new default scheme. Essentially as = 1 above, except that an energy deficit
is preferentially taken from the endpoint of the string piece that is moving
closest in direction to the cluster.
MSTJ(17) : (D = 2) number of attempts made to find two hadrons that have a combined
mass below the cluster mass, and thus allow a cluster to decay to two hadrons
rather than collapse to one. Thus the larger MSTJ(17), the smaller the fractionof collapses. At least one attempt is always made, and this was the old default
behaviour.
MSTJ(18) : (D = 10) maximum number of times the junction rest frame is evaluatedwith improved knowledge of the proper energies. When the boost in an iteration
corresponds to a γ < 1.01 the iteration would be stopped sooner. An iterative
solution is required since the rest frame of the junction is defined by a vector sumof energies (see PARP(48)) that are assumed already known in this rest frame.
(In practice, this iteration is normally a minor effect, of more conceptual thanpractical impact.)
MSTJ(19) : (D = 0) in a string system containing two junctions (or, more properly, ajunction and an antijunction), there is a possibility for these two to disappear, byan ‘annihilation’ that gives two separate strings [Sjö03]. That is, a configuration
like q1 q2 JJq3 q4 can collapse to q1 q3 plus q2 q4 .
= 0 : the selection between the two alternatives is made dynamically, so as to pickthe string configuration with the smallest length.
= 1 : the two-junction topology always remains.
= 2 : the two-junction topology always collapses to two separate strings.
Note: the above also applies, suitably generalized, when parton-shower activity is403

included in the event. If the shower in between the two junctions comes tocontain a g → qq branching, however, the system inevitable is split into two
separate junction systems q1 q2 Jq plus qJq3 q4
MSTJ(21) : (D = 2) form of particle decays.
= 0 : all particle decays are inhibited.
= 1 : a particle declared unstable in the MDCY vector, and with decay channels
defined, may decay within the region given by MSTJ(22). A particle may
decay into partons, which then fragment further according to the MSTJ(1)value.
= 2 : as = 1, except that a qq parton system produced in a decay (e.g. of a B
meson) is always allowed to fragment according to string fragmentation,rather than according to the MSTJ(1) value (this means that momentum,
energy and charge are conserved in the decay).
MSTJ(22) : (D = 1) cut-off on decay length for a particle that is allowed to decay according to MSTJ(21) and the MDCY value.
= 1 : a particle declared unstable is also forced to decay.
= 2 : a particle is decayed only if its average proper lifetime is smaller than
PARJ(71).
= 3 : a particle is decayed only if the decay vertex is within a distance PARJ(72)of the origin.
= 4 : a particle is decayed only if the decay vertex is within a cylindrical volume
with radius PARJ(73) in the xy-plane and extent to ±PARJ(74) in the z
direction.
MSTJ(23) : (D = 1) possibility of having a shower evolving from a qq pair created as
decay products. This switch only applies to decays handled by PYDECY rather
than PYRESD, and so is of less relevance today.
= 0 : never.
= 1 : whenever the decay channel matrix-element code is MDME(IDC,2) = 4, 32,
33, 44 or 46, the two first decay products (if they are partons) are allowed
to shower, like a colour-singlet subsystem, with maximum virtuality givenby the invariant mass of the pair.
MSTJ(24) : (D = 2) particle masses.
= 0 : discrete mass values are used.
= 1 : particles registered as having a mass width in the PMAS vector are given a
mass according to a truncated Breit–Wigner shape, linear in m, eq. (269).
= 2 : as = 1, but gauge bosons (actually all particles with |KF| ≤ 100) are distributed according to a Breit–Wigner quadratic in m, as obtained frompropagators.
= 3 : as = 1, but Breit–Wigner shape is always quadratic in m, eq. (270).
MSTJ(26) : (D = 2) inclusion of B–B mixing in decays.
= 0 : no.
= 1 : yes, with mixing parameters given by PARJ(76) and PARJ(77). Mixing
decays are not specially marked.
= 2 : yes, as = 1, but a B (B) that decays as a B (B) is marked as K(I,1) = 12
rather than the normal K(I,1) = 11.
MSTJ(28) : (D = 0) call to an external τ decay library like Tauola. For this option to
be meaningful, it is up to you to write the appropriate interface and include thatin the routine PYTAUD, as explained in section 14.2.
= 0 : not done, i.e. the internal PYDECY treatment is used.
= 1 : done whenever the τ mother particle species can be identified, else the internal PYDECY treatment is used. Normally the mother particle should always
be identified, but it is possible for you to remove event history informationor to add extra τ ’s directly to the event record, and then the mother is not404

known.
= 2 : always done.
MSTJ(38) - MSTJ(50) : switches for time-like parton showers, see section 10.4.
MSTJ(51) : (D = 0) inclusion of Bose–Einstein effects.
= 0 : no effects included.
= 1 : effects included according to an exponential parameterization f2 (Q) =1+PARJ(92)× exp(−Q/PARJ(93)), where f2 (Q) represents the ratio of particle production at Q with Bose–Einstein effects to that without, and the relative momentum Q is defined by Q2 (p1 , p2 ) = −(p1 −p2 )2 = (p1 +p2 )2 −4m2 .
Particles with width broader than PARJ(91) are assumed to have time todecay before Bose–Einstein effects are to be considered.
= 2 : effects included according to a Gaussian parameterization f2 (Q) =1+PARJ(92)× exp(−(Q/PARJ(93))2 ), with notation and comments asabove.
MSTJ(52) : (D = 3) number of particle species for which Bose–Einstein correlations areto be included, ranged along the chain π + , π − , π 0 , K+ , K− , K0L , K0S , η and η 0 .
Default corresponds to including all pions (π + , π − , π 0 ), 7 to including all Kaonsas well, and 9 is maximum.
MSTJ(53) : (D = 0) In e+ e− → W+ W− , e+ e− → Z0 Z0 , or if PARJ(94) > 0 and there areseveral strings in the event, apply BE algorithm= 0 : on all pion pairs.
= 1 : only on pairs were both pions come from the same W/Z/string.
= 2 : only on pairs were the pions come from different W/Z/strings.
= -2 : when calculating balancing shifts for pions from same W/Z/string, only
consider pairs from this W/Z/string.
Note: if colour reconnections has occurred in an event, the distinction between
pions coming from different W/Z’s is lost.
MSTJ(54) : (D = 2) Alternative local energy compensation in the BE algorithm. (Notation in brackets refer to the one used in [Lön95].)= 0 : global energy compensation (BE0 ).
= 1 : compensate with identical pairs by negative BE enhancement with a thirdof the radius (BE3 ).
= 2 : ditto, but with the compensation constrained to vanish at Q = 0, by anadditional 1 − exp(−Q2 R2 /4) factor (BE32 ).
= -1 : compensate with pair giving the smallest invariant mass (BEm ).
= -2 : compensate with pair giving the smallest string length (BEλ ).
MSTJ(55) : (D = 0) Calculation of difference vector in the BE algorithm.
= 0 : in the lab frame.
= 1 : in the c.m. of the given pair.
MSTJ(56) : (D = 0) In e+ e− → W+ W− or e+ e− → Z0 Z0 include distance between W/Z’s
for BE calculation.
= 0 : radius is the same for all pairs.
= 1 : radius for pairs from different W/Z’s is R + δRWW (R + δRZZ ), where δR is
the generated distance between the decay vertices. (When considering W or
Z pairs with an energy well above threshold, this should give more realistic
results.)
MSTJ(57) : (D = 1) Penalty for shifting particles with close-by identical neighbours inlocal energy compensation, MSTJ(54) < 0.
= 0 : no penalty.
= 1 : penalty.
MSTJ(91) : (I) flag when generating gluon jet with options MSTJ(2) = 2 or 4 (then = 1,
else = 0).
MSTJ(92) : (I) flag that a qq or gg pair or a ggg triplet created in PYDECY should be405

allowed to shower, is 0 if no pair or triplet, is the entry number of the first parton
if a pair indeed exists, is the entry number of the first parton, with a − sign, if
a triplet indeed exists.
MSTJ(93) : (I) switch for PYMASS action. Is reset to 0 in PYMASS call.
= 0 : ordinary action.
= 1 : light (d, u, s, c, b) quark masses are taken from PARF(101) - PARF(105)rather than PMAS(1,1) - PMAS(5,1). Diquark masses are given as sum ofquark masses, without spin splitting term.
= 2 : as = 1. Additionally the constant terms PARF(121) and PARF(122) are
subtracted from quark and diquark masses, respectively.
MSTJ(101) - MSTJ(121) : switches for e+ e− event generation, see section 6.3.
PARJ(1) : (D = 0.10) is P(qq)/P(q), the suppression of diquark-antidiquark pair production in the colour field, compared with quark–antiquark production.
PARJ(2) : (D = 0.30) is P(s)/P(u), the suppression of s quark pair production in thefield compared with u or d pair production.
PARJ(3) : (D = 0.4) is (P(us)/P(ud))/(P(s)/P(d)), the extra suppression of strange
diquark production compared with the normal suppression of strange quarks.
PARJ(4) : (D = 0.05) is (1/3)P(ud1 )/P(ud0 ), the suppression of spin 1 diquarks compared with spin 0 ones (excluding the factor 3 coming from spin counting).
PARJ(5) : (D = 0.5) parameter determining relative occurrence of baryon production by
BM B and by BB configurations in the simple popcorn baryon production model,roughly P(BM B)/(P(BB) + P(BM B)) = PARJ(5)/(0.5+PARJ(5)). This andsubsequent baryon parameters are modified in the advanced popcorn scenario,see section 14.3.1.
PARJ(6) : (D = 0.5) extra suppression for having a ss pair shared by the B and B of a
BM B situation.
PARJ(7) : (D = 0.5) extra suppression for having a strange meson M in a BM B configuration.
PARJ(8) - PARJ(10) : used in the advanced popcorn scenario, see section 14.3.1.
PARJ(11) - PARJ(17) : parameters that determine the spin of mesons when formed infragmentation or decays.
PARJ(11) : (D = 0.5) is the probability that a light meson (containing u and dquarks only) has spin 1 (with 1. - PARJ(11) the probability for spin 0).
PARJ(12) : (D = 0.6) is the probability that a strange meson has spin 1.
PARJ(13) : (D = 0.75) is the probability that a charm or heavier meson has spin 1.
PARJ(14) : (D = 0.) is the probability that a spin = 0 meson is produced with anorbital angular momentum 1, for a total spin = 1.
PARJ(15) : (D = 0.) is the probability that a spin = 1 meson is produced with anorbital angular momentum 1, for a total spin = 0.
PARJ(16) : (D = 0.) is the probability that a spin = 1 meson is produced with anorbital angular momentum 1, for a total spin = 1.
PARJ(17) : (D = 0.) is the probability that a spin = 1 meson is produced with anorbital angular momentum 1, for a total spin = 2.
Note : the end result of the numbers above is that, with i = 11, 12 or 13, dependingon flavour content,
P(S = 0, L = 0, J = 0) = (1 − PARJ(i)) × (1 − PARJ(14)),
P(S = 0, L = 1, J = 1) = (1 − PARJ(i)) × PARJ(14),P(S = 1, L = 0, J = 1) = PARJ(i) × (1 − PARJ(15) − PARJ(16) − PARJ(17)),
P(S = 1, L = 1, J = 0) = PARJ(i) × PARJ(15),P(S = 1, L = 1, J = 1) = PARJ(i) × PARJ(16),P(S = 1, L = 1, J = 2) = PARJ(i) × PARJ(17),where S is the quark ‘true’ spin and J is the total spin, usually called the406spin s of the meson.
PARJ(18) : (D = 1.) is an extra suppression factor multiplying the ordinary SU(6)weight for spin 3/2 baryons, and hence a means to break SU(6) in addition
to the dynamic breaking implied by PARJ(2), PARJ(3), PARJ(4), PARJ(6) andPARJ(7).
PARJ(19) : (D = 1.) extra baryon suppression factor, which multiplies the ordinary
diquark-antidiquark production probability for the breakup closest to the endpoint of a string, but leaves other breaks unaffected. Is only used for MSTJ(12)= 3.
PARJ(21) : (D = 0.36 GeV) corresponds to the width σ in the Gaussian px and py
transverse momentum distributions for primary hadrons. See also PARJ(22) PARJ(24).
PARJ(22) : (D = 1.) relative increase in transverse momentum in a gluon jet generated
with MSTJ(2) = 2 or 4.
PARJ(23), PARJ(24) : (D = 0.01, 2.) a fraction PARJ(23) of the Gaussian transverse
momentum distribution is taken to be a factor PARJ(24) larger than input in
PARJ(21). This gives a simple parameterization of non-Gaussian tails to theGaussian shape assumed above.
PARJ(25) : (D = 1.) extra suppression factor for η production in fragmentation; if an ηis rejected a new flavour pair is generated and a new hadron formed.
PARJ(26) : (D = 0.4) extra suppression factor for η 0 production in fragmentation; if an
η 0 is rejected a new flavour pair is generated and a new hadron formed.
PARJ(31) : (D = 0.1 GeV) gives the remaining W+ below which the generation of asingle jet is stopped. (It is chosen smaller than a pion mass, so that no hadrons
moving in the forward direction are missed.)
PARJ(32) : (D = 1. GeV) is, with quark masses added, used to define the minimum
allowable energy of a colour-singlet parton system.
PARJ(33) - PARJ(34) : (D = 0.8 GeV, 1.5 GeV) are, together with quark masses, used
to define the remaining energy below which the fragmentation of a parton systemis stopped and two final hadrons formed. PARJ(33) is normally used, except for
MSTJ(11) = 2, when PARJ(34) is used.
PARJ(36) : (D = 2.) represents the dependence on the mass of the final quark pair for
defining the stopping point of the fragmentation. Is strongly correlated to the
choice of PARJ(33) - PARJ(35).
PARJ(37) : (D = 0.2) relative width of the smearing of the stopping point energy.
PARJ(39) : (D = 0.08 GeV−2 ) refers to the probability for reverse rapidity ordering of the
final two hadrons, according to eq. (255), for MSTJ(11) = 2 (for other MSTJ(11)values PARJ(42) is used).
PARJ(40): (D = 1.) possibility to modify the probability for reverse rapidity ordering of
the final two hadrons in the fragmentation of a string with PYSTRF, or from theonly two hadrons of a low-mass string considered in PYPREP. Modifies eq. (255)
to Preverse = 1/(1 + exp(PARJ(40)b∆)).
PARJ(41), PARJ(42) : (D = 0.3, 0.58 GeV−2 ) give the a and b parameters of the symmetric Lund fragmentation function for MSTJ(11) = 1, 4 and 5 (and MSTJ(11)= 3 for ordinary hadrons).
PARJ(43), PARJ(44) : (D = 0.5, 0.9 GeV−2 ) give the a and b parameters as above forthe special case of a gluon jet generated with IF and MSTJ(2) = 2 or 4.
PARJ(45) : (D = 0.5) the amount by which the effective a parameter in the Lund flavourdependent symmetric fragmentation function is assumed to be larger than thenormal a when diquarks are produced. More specifically, referring to eq. (242),
aα =PARJ(41) when considering the fragmentation of a quark and = PARJ(41)+ PARJ(45) for the fragmentation of a diquark, with corresponding expression
for aβ depending on whether the newly created object is a quark or diquark (for407

an independent gluon jet generated with MSTJ(2) = 2 or 4, replace PARJ(41) by
PARJ(43)). In the popcorn model, a meson created in between the baryon andantibaryon has aα = aβ =PARJ(41) + PARJ(45).
PARJ(46), PARJ(47) : (D = 2*1.) modification of the Lund symmetric fragmentation
for heavy endpoint quarks according to the recipe by Bowler, available when
MSTJ(11) = 4 or 5 is selected. The shape is given by eq. (245). If MSTJ(11) = 4
then rQ =PARJ(46) for both c and b, while if MSTJ(11) = 5 then rc =PARJ(46)and rb =PARJ(47). PARJ(46) and PARJ(47) thus provide a possibility to interpolate between the ‘pure’ Bowler shape, r = 1, and the normal Lund one, r = 0.
The additional modifications made in PARJ(43) - PARJ(45) are automaticallytaken into account, if necessary.
PARJ(48) : (D = 1.5 GeV) in defining the junction rest frame, the effective pull directionof a chain of partonsis defined by the vector sum of their momenta, multipliedPby a factor exp(− E/PARJ(48)), where the energy sum runs over all partonson the string between (but excluding) the given one and the junction itself. Theenergies should be defined in the junction rest frame, which requires an iterative
approximation, see MSTJ(18).
PARJ(49) : (D = 1. GeV) retry (up to 10 times) when both strings, to be joined in ajunction to form a new string endpoint, have a remaining energy above PARJ(49)(evaluated in the junction rest frame) after having been fragmented.
PARJ(50) : (D = 10. GeV) retry as above when either of the strings have a remaining energy above a random energy evenly distributed between PARJ(49) andPARJ(49) + PARJ(50) (drawn anew for each test).
PARJ(51) - PARJ(55) : (D = 3*0.77, −0.05, −0.005) give a choice of four possible ways
to parameterize the fragmentation function for MSTJ(11) = 2 (and MSTJ(11) =3 for charm and heavier). The fragmentation of each flavour KF may be chosen separately; for a diquark the flavour of the heaviest quark is used. With
c =PARJ(50+KF), the parameterizations are:0 ≤ c ≤ 1 : Field–Feynman, f (z) = 1 − c + 3c(1 − z)2 ;−1 ≤ c < 0 : Peterson/SLAC, f (z) = 1/(z(1 − 1/z − (−c)/(1 − z))2 );c > 1 : power peaked at z = 0, f (z) = (1 − z)c−1 ;c < −1 : power peaked at z = 1, f (z) = z −c−1 .
PARJ(59) : (D = 1.) replaces PARJ(51) - PARJ(53) for gluon jet generated withMSTJ(2) = 2 or 4.
PARJ(61) - PARJ(63) : (D = 4.5, 0.7, 0.) parameterizes the energy dependence ofthe primary multiplicity distribution in phase-space decays. The former twocorrespond to c1 and c2 of eq. (278), while the latter allows a further additive
term in the multiplicity specifically for onium decays.
PARJ(64) : (0.003 GeV) minimum kinetic energy in decays (safety margin for numerical precision errors). When violated, typically new masses would be selected
if particles have a Breit–Wigner width, or a new decay channel where that isrelevant.
PARJ(65) : (D = 0.5 GeV) mass which, in addition to the spectator quark or diquark
mass, is not assumed to partake in the weak decay of a heavy quark in a hadron.
This parameter was mainly intended for top decay and is currently not in use.
PARJ(66) : (D = 0.5) relative probability that colour is rearranged when two singlets
are to be formed from decay products. Only applies for MDME(IDC,2) = 11–30,
i.e. low-mass phase-space decays.
PARJ(71) : (D = 10 mm) maximum average proper lifetime cτ for particles allowed todecay in the MSTJ(22) = 2 option. With the default value, K0S , Λ, Σ− , Σ+ , Ξ− ,
Ξ0 and Ω− are stable (in addition to those normally taken to be stable), but
charm and bottom do still decay.
PARJ(72) : (D = 1000 mm) maximum distance from the origin at which a decay is408allowed to take place in the MSTJ(22) = 3 option.
√
PARJ(73) : (D = 100 mm) maximum cylindrical distance ρ = x2 + y 2 from the origin
at which a decay is allowed to take place in the MSTJ(22) = 4 option.
PARJ(74) : (D = 1000 mm) maximum z distance from the origin at which a decay isallowed to take place in the MSTJ(22) = 4 option.
0
PARJ(76) : (D = 0.7) mixing parameter xd = ∆M/Γ in B0 –B system.
0
PARJ(77) : (D = 10.) mixing parameter xs = ∆M/Γ in B0s –Bs system.
PARJ(80) - PARJ(90) : parameters for time-like parton showers, see section 10.4.
PARJ(91) : (D = 0.020 GeV) minimum particle width in PMAS(KC,2), above which particle decays are assumed to take place before the stage where Bose–Einstein effects
are introduced.
PARJ(92) : (D = 1.) nominal strength of Bose–Einstein effects for Q = 0, see MSTJ(51).
This parameter, often denoted λ, expresses the amount of incoherence in particle
production. Due to the simplified picture used for the Bose–Einstein effects, inparticular for effects from three nearby identical particles, the actual λ of thesimulated events may be larger than the input value.
PARJ(93) : (D = 0.20 GeV) size of the Bose–Einstein effect region in terms of the
Q variable, see MSTJ(51). The more conventional measure, in terms of theradius R of the production volume, is given by R = h̄/PARJ(93)≈ 0.2
fm×GeV/PARJ(93)=PARU(3)/PARJ(93).
PARJ(94) : (D = 0.0 GeV) Increase radius for pairs from different W/Z/strings.
< 0 : if MSTJ(56) = 1, the radius for pairs from different W/Z’s is increased toR + δRWW + PARU(3)/abs(PARJ(94)).
> 0 : the radius for pairs from different strings is increased to
R + PARU(3)/PARJ(94).
PARJ(95) : (R) Set to the energy imbalance after the BE algorithm, before rescaling of
momenta.
PARJ(96) : (R) Set to the α needed to retain energy-momentum conservation in each
event for relevant models.
PARJ(121) - PARJ(171) : parameters for e+ e− event generation, see section 6.3.
PARJ(180) - PARJ(195) : various coupling constants and parameters related to couplings, see section 9.4.
14.3.1

The advanced popcorn code for baryon production

In section 12.1.3 a new advanced popcorn code for baryon production model was presented,
based on [Edé97]. It partly overwrites and redefines the meaning of some of the parameters
above. Therefore the full description of these new options are given separately in thissection, together with a listing of the new routines involved.
In order to use the new options, a few possibilities are open.
• Use of the old diquark and popcorn models, MSTJ(12) = 1 and = 2, is essentiallyunchanged. Note, however, that PARJ(19) is available for an ad-hoc suppression of
first-rank baryon production.
• Use of the old popcorn model with new SU(6) weighting:• Set MSTJ(12) = 3.
• Increase PARJ(1) by approximately a factor 1.2 to retain about the same effective baryon production rate as in MSTJ(12) = 2.
• Note: the new SU(6) weighting e.g. implies that the total production rate ofcharm and bottom baryons is reduced.
• Use of the old flavour model with new SU(6) treatment and modified fragmentation
function for diquark vertices (which softens baryon spectra):• Set MSTJ(12) = 4.
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• Increase PARJ(1) by about a factor 1.7 and PARJ(5) by about a factor 1.2 torestore the baryon and popcorn rates of the MSTJ(12) = 2 default.
• Use of the new flavour model (automatically with modified diquark fragmentation
function.)• Set MSTJ(12) = 5.
• Increase PARJ(1) by approximately a factor 2.
• Change PARJ(18) from 1 to approx. 0.19.
• Instead of PARJ(3) - PARJ(7), tune PARJ(8), PARJ(9), PARJ(10) andPARJ(18). (Here PARJ(10) is used only in collisions having remnants of baryon
beam particles.)• Note: the proposed parameter values are based on a global fit to all baryon
production rates. This e.g. means that the proton rate is lower than in the
MSTJ(12) = 2 option, with current data somewhere in between. The PARJ(1)value would have to be about 3 times higher in MSTJ(12) = 5 than in = 2
to have the same total baryon production rate (=proton+neutron), but thenother baryon rates would not match at all.
• The new options MSTJ(12) = 4 and = 5 (and, to some extent, = 3) soften baryon
spectra in such a way that PARJ(45) (the change of a for diquarks in the Lundsymmetric fragmentation function) is available for a retune. It affects i.e. baryonantibaryon rapidity correlations and the baryon excess over antibaryons in quarkjets.
The changes in and additions to the common blocks are as follows.
MSTU(121) - MSTU(125) : Internal flags and counters; only MSTU(123) may be touchedby you.
MSTU(121) : Popcorn meson counter.
MSTU(122) : Points at the proper diquark production weights, to distinguish between ordinary popcorn and rank 0 diquark systems. Only needed if
MSTJ(12) = 5.
MSTU(123) : Initialization flag. If MSTU(123) is 0 in a PYKFDI call, PYKFIN is called
and MSTU(123) set to 1. Would need to be reset by you if flavour parameters
are changed in the middle of a run.
MSTU(124) : First parton flavour in decay call, stored to easily find random flavour
partner in a popcorn system.
MSTU(125) : Maximum number of popcorn mesons allowed in decay flavour generation. If a larger popcorn system passes the fake string suppressions, the error
KF = 0 is returned and the flavour generation for the decay is restarted.
MSTU(131) - MSTU(140) : Store of popcorn meson flavour codes in decay algorithm.
Purely internal.
MSTJ(12) : (D = 2) Main switch for choice of baryon production model. Suppression ofrank 1 baryons by a parameter PARJ(19) is no longer governed by the MSTJ(12)switch, but instead turned on by setting PARJ(19) < 1. Three new options areavailable:= 3 : as = 2, but additionally the production of first rank baryons may be suppressed by a factor PARJ(19).
= 4 : as = 2, but diquark vertices suffers an extra suppression of the form 1 −exp(ρΓ), where ρ ≈ 0.7 GeV−2 is stored in PARF(192).
= 5 : Advanced version of the popcorn model. Independent of PARJ(3 - 7). Instead depending on PARJ(8 - 10). When using this option PARJ(1) needs
to enhanced by approx. a factor 2 (i.e. it losses a bit of its normal meaning),
and PARJ(18) is suggested to be set to 0.19.
PARJ(8), PARJ(9) : (D = 0.6, 1.2 GeV−1 ) The new popcorn parameters βu and410δβ = βs − βu . Used to suppress popcorn mesons of total invariant mass M⊥
by exp(−βq ∗ M⊥ ). Larger PARJ(9) leads to a stronger suppression of popcorn systems surrounded by an ss pair, and also a little stronger suppression of
strangeness in diquarks.
PARJ(10) : (D = 0.6 GeV−1 ) Corresponding parameter for suppression of leading rankmesons of transverse mass M⊥ in the fragmentation of diquark jets, used if
MSTJ(12) = 5. The treatment of original diquarks is flavour independent, i.e.
PARJ(10) is used even if the diquark contains s or heavier quarks.
PARF(131) - PARF(190) : Different diquark and popcorn weights, calculated in PYKFIN,
which is automatically called from PYKFDI.
PARF(131) : Popcorn ratio BM B/BB in the old model.
PARF(132-134) : Leading rank meson ratio M B/B in the old model, for original
diquark with 0, 1 and 2 s-quarks, respectively.
PARF(135-137) : Colour fluctuation quark ratio, i.e. the relative probability thatthe heavier quark in a diquark fits into the baryon at the opposite side of
the popcorn meson. For sq, original sq and original cq diquarks, respectively.
PARF(138) : The extra suppression of strange colour fluctuation quarks, due to therequirement of surrounding a popcorn meson. (In the old model, it is simply
PARJ(6).)
PARF(139) : Preliminary suppression of a popcorn meson in the new model.
A system of N popcorn mesons is started with weight proportional to
PARF(139)N . It is then tested against the correct weight, derived fromthe mass of the system. For strange colour fluctuation quarks, the weight is
PARF(138)*PARF(139).
PARF(140) : Preliminary suppression of leading rank mesons in diquark strings, irrespective of flavour. Corresponds to PARF(139).
PARF(141-145) : Maximal SU(6) factors for different types of diquarks.
PARF(146) : Σ/Λ suppression if MSTJ(12) = 5, derived from PARJ(18).
PARF(151-190) : Production ratios for different diquarks. Stored in four groups,
handling q → BB, q → BM...B, qq → M B and finally qq → M B in thecase of original diquarks. In each group is stored:
1 : s/u colour fluctuation ratio.
2,3 : s/u ratio for the vertex quark if the colour fluctuation quark islight or strange, respectively.
4 : q/q0 vertex quark ratio if the colour fluctuation quark is light and= q.
5-7 : (spin 1)/(spin 0) ratio for su, us and ud, where the first flavour isthe colour fluctuation quark.
8-10 : Unused.
PARF(191) : (D = 0.2) Non-constituent mass in GeV of a ud0 diquark. Used in combination with diquark constituent mass differences to derive relative production
rates for different diquark flavours in the MSTJ(12) = 5 option.
PARF(192) : (D = 0.5) Parameter for the low-Γ suppression of diquark vertices in the
MSTJ(12)≥ 4 options. PARF(192) represents e−ρ , i.e. the suppression is of theform 1. - PARF(192)Γ , Γ in GeV2 .
PARF(193,194) : (I) Store of some popcorn weights used by the present popcorn system.
PARF(201-1400) : (I) Weights for every possible popcorn meson construction in the
MSTJ(12) = 5 option. Calculated from input parameters and meson masses in
PYKFIN. When q1 q2 → M +q1 q3 , the weights for M and the new diquark depends
not only on q1 and q2 : it is also important if this is a ‘true’ popcorn system, ora system which started with a diquark at the string end, and if M is the final
meson of the popcorn system, i.e. if the q1 q3 diquark will go into a baryon or not.

411

With five possible flavours for q1 and q2 this gives 80 different situations whenselecting M and q3 . However, quarks heavier than s only exist in the string
endpoints, and if more popcorn mesons are to be produced, the q1 q3 diquark
does not influence the weights and the q1 dependence reduces to what β factor(PARJ(8 - 10)) that is used. Then 40 distinct situations remains, i.e.:‘true popcorn’ final meson q1
q2
YES
YES
d,u,s
d,u,s
NO
<s,s
d,u,s
NO
YES
d,u,s,>s d,u,s,c,bNO
1 cased,u,s,c,bThis table also shows the order in which the situations are stored. E.g. situation
no. 1 is ‘YES,YES,d,d’, situation no.11 is ‘YES,NO,< s,u’.
In every situation q3 can be d, u or s. if q3 = q2 there are in the program threepossible flavour mixing states available for the meson. This gives five possible
meson flavours, and for each one of them there are six possible L, S spin states.
Thus 30 PARF positions are reserved for each situation, and these are used asfollows:
For each spin multiplet (in the same order as in PARF(1 - 60)) five positions
are reserved. First are stored the weights for the the q3 6= q2 mesons, with q3
in increasing order. If q2 > s, this occupies three spots, and the final two areunused. If q2 ≤ s, the final three spots are used for the diagonal states whenq3 = q 2 .
In summary, all common-block variables are completely internal, except MSTU(123),
MSTJ(12), PARJ(8) - PARJ(10) and PARF(191), PARF(192). Among these, PARF(191)
and PARF(192) should not need to be changed. MSTU(123) should be 0 when starting,and reset to 0 whenever changing a switch or parameter which influences flavour weightWith MSTJ(12) = 4, PARJ(5) may need to increase. With MSTJ(12) = 5, a preliminary
tune suggests PARJ(8) = 0.6, PARJ(9) = 1.2, PARJ(10) = 0.6, PARJ(1) = 0.20 and
PARJ(18) = 0.19.
Three new subroutines are added, but are only needed for internal use.
SUBROUTINE PYKFIN : to calculate a large set of diquark and popcorn weights from input
parameters. Is called from PYKFDI if MSTU(123) = 0. Sets MSTU(123) to 1.
SUBROUTINE PYNMES(KFDIQ) : to calculate number of popcorn mesons to be generated
in a popcorn system, or the number of leading rank mesons when fragmenting adiquark string. Stores the number in MSTU(121). Always returns 0 if MSTJ(12)< 2. Returns 0 or 1 if MSTJ(12) < 5.
KFDIQ : Flavour of the diquark in a diquark string. If starting a popcorn system
inside a string, KFDIQ is 0.
SUBROUTINE PYDCYK(KFL1,KFL2,KFL3,KF) : to generate flavours in the phase spacemodel of hadron decays, and in cluster decays. Is essentially the same as a
PYKFDI call, but also takes into account the effects of string dynamics in flavour
production in the MSTJ(12)≥ 4 options. This is done in order to get a reasonable interpretation of the input parameters also for hadron decays with these
options.
KFL1,KFL2,KFL3,KF : See SUBROUTINE PYKFDI.
Internally the diquark codes have been extended to store the necessary further popcorn
information. As before, an initially existing diquark has a code of the type 1000qa +100qb +2s + 1, where qa > qb . Diquarks created in the fragmentation process now have the longer
code 10000qc + 1000qa + 100qb + 2s + 1, i.e. one further digit is set. Here qc is the curtain
quark, i.e. the flavour of the quark-antiquark pair that is shared between the baryon
and the antibaryon, either qa or qb . The non-curtain quark, the other of qa and qb , may

412

have its antiquark partner in a popcorn meson. In case there are no popcorn mesons
this information is not needed, but is still set at random to be either of qa and qb . The
extended code is used internally in PYSTRF and PYDECY and in some routines called bythem, but is not visible in any event listings.

14.4

Further Parameters and Particle Data

The PYUPDA routine is the main tool for updating particle data tables. However, for decay
tables in the SUSY Les Houches Accord (SLHA) format, the PYSLHA routine should be
used instead. The following common blocks are maybe of a more peripheral interest, withthe exception of the MDCY array, which allows a selective inhibiting of particle decays, andsetting masses of not yet discovered particles, such as PMAS(25,1), the (Standard Model)
Higgs mass.
CALL PYUPDA(MUPDA,LFN)
Purpose: to give you the ability to update particle data, or to keep several versions ofmodified particle data for special purposes (e.g. bottom studies).
MUPDA : gives the type of action to be taken.
= 1 : write a table of particle data, that you then can edit at leisure. For ordinary
listing of decay data, PYLIST(12) should be used, but that listing could not
be read back in by the program.
For each compressed flavour code KC = 1–500, one line is written containing
the corresponding uncompressed KF flavour code (1X,I9) in KCHG(KC,4),
the particle and antiparticle names (2X,A16,2X,A16) in CHAF, the electric(I3), colour charge (I3) and particle/antiparticle distinction (I3) codes in
KCHG, the mass (F12.5), the mass width (F12.5), maximum broadening(F12.5) and average proper lifetime (1P,E13.5) in PMAS, the resonance width
treatment (I3) in MWID and the on/off decay switch (I3) in MDCY(KC,1).
After a KC line follows one line for each possible decay channel, containing
the MDME codes (10X,2I5), the branching ratio (F12.6) in BRAT, and the
KFDP codes for the decay products (5I10), with trailing 0’s if the number ofdecay products is smaller than 5.
= 2 : read in particle data, as written with = 1 and thereafter edited by you, anduse this data subsequently in the current run. This also means e.g. themapping between the full KF and compressed KC flavour codes. Reading isdone with fixed format, which means that you have to preserve the format
codes described for = 1 during the editing. A number of checks will be madeto see if input looks reasonable, with warnings if not. If some decay channel
is said not to conserve charge, it should be taken seriously. Warnings thatdecay is kinematically disallowed need not be as serious, since that particulardecay mode may not be switched on unless the particle mass is increased.
= 3 : read in particle data, like option 2, but use it as a complement to rather thana replacement of existing data. The input file should therefore only containnew particles and particles with changed data. New particles are added tothe bottom of the KC and decay channel tables. Changed particles retain
their KC codes and hence the position of particle data, but their old decay
channels are removed, this space is recuperated, and new decay channels
are added at the end. Thus also the decay channel numbers of unchanged
particles are affected.
= 4 : write current particle data as data lines, which can be edited into BLOCKDATA PYDATA for a permanent replacement of the particle data. This option

413

LFN :

is intended for the program author only, not for you.
the file number which the data should be written to or read from. You must see
to it that this file is properly opened for read or write (since the definition of filenames is platform dependent).

CALL PYSLHA(MUPDA,KF,IFAIL)
Purpose: to read and write SUSY spectra and/or decay tables conforming to the SLHA
format [Ska03]. This normally happens automatically during initialization, when
the appropriate switches have been set, see the examples in section 8.7, but
PYSLHA may also be used stand-alone, to update the decay table for any particle.
To do this, the user should call PYSLHA manually before the call to PYINIT, withMUPDA = 2 (see below) and KF the flavour code of the particle whose decay table
is to be updated. In case several decay tables are to be updated, separate calls
must be made for each particle.
MUPDA : gives the type of action to be taken.
= 1 : read in spectrum from Logical Unit Number IMSS(21). This call happens
automatically during initialization when IMSS(1) = 11. It is not intended
for manual use.
= 2 : read in decay table for particle code KF from Logical Unit Number IMSS(22).
Is performed automatically when IMSS(1) = 11 and IMSS(22)6=0, but only
for sparticles and higgs bosons. Any decay lines associated with a zero widthmother are ignored, giving a fast way of switching off decays without having
to comment out all the decay lines. This call may also be used stand-alone
to update the decay table of a particle.
= 3 : write out spectrum on Logical Unit Number IMSS(23). This call happens
automatically during initialization when IMSS(1)6=0 and IMSS(23)6=0. It is
not intended for manual use.
= 4 : write out decay table for particle code KF on Logical Unit Number IMSS(24).
This option is not supported yet.
KF :the flavour code of the particle for which decay table read-in or write-out isdesired, only relevant for MUPDA = 2, 4.
IFAIL : return code specifying whether the desired operation succeeded.
= 0 : Operation succeeded.
= 1 : Operation failed, e.g. if no decay table in the SLHA format matching thedesired KF code was found on the file.
COMMON/PYDAT2/KCHG(500,4),PMAS(500,4),PARF(2000),VCKM(4,4)
Purpose: to give access to a number of flavour-treatment constants or parameters andparticle/parton data. Particle data is stored by compressed code KC rather thanby the full KF code. You are reminded that the way to know the KC value is to
use the PYCOMP function, i.e. KC = PYCOMP(KF).
KCHG(KC,1) : three times particle/parton charge for compressed code KC.
KCHG(KC,2) : colour information for compressed code KC.
= 0 : colour-singlet particle.
= 1 : quark or antidiquark.
= -1 : antiquark or diquark.
= 2 : gluon.
KCHG(KC,3) : particle/antiparticle distinction for compressed code KC.
= 0 : the particle is its own antiparticle.

414

= 1 : a nonidentical antiparticle exists.
KCHG(KC,4) : equals the uncompressed particle code KF (always with a positive sign).
This gives the inverse mapping of what is provided by the PYCOMP routine.
PMAS(KC,1) : particle/parton mass m (in GeV) for compressed code KC.
PMAS(KC,2) : the total width Γ (in GeV) of an assumed symmetric Breit–Wigner mass
shape for compressed particle code KC.
PMAS(KC,3) : the maximum deviation (in GeV) from the PMAS(KC,1) value at which
the Breit–Wigner shape above is truncated. Is used in ordinary particle decays,
but not in the resonance treatment; cf. the CKIN variables.
PMAS(KC,4) : the average lifetime τ for compressed particle code KC, with cτ in mm, i.e.
τ in units of about 3.33 × 10−12 s.
PARF(1) - PARF(60) : give a parameterization of the dd–uu–ss flavour mixing in production of flavour-diagonal mesons. Numbers are stored in groups of 10, for thesix multiplets pseudoscalar, vector, axial vector (S = 0), scalar, axial vector(S = 1) and tensor, in this order; see section 12.1.2. Within each group, thefirst two numbers determine the fate of a dd flavour state, the second two thatof a uu one, the next two that of an ss one, while the last four are unused. Callthe numbers of a pair p1 and p2 . Then the probability to produce the state withsmallest KF code is 1 − p1 , the probability for the middle one is p1 − p2 and theprobability for the one with largest code is p2 , i.e. p1 is the probability to produce
either of the two ‘heavier’ ones.
PARF(61) - PARF(80) : give flavour SU(6) weights for the production of a spin 1/2
or spin 3/2 baryon from a given diquark–quark combination. Should not bechanged.
PARF(91) - PARF(96) : (D = 0.0099, 0.0056, 0.199, 1.23, 4.17, 165 GeV) default nominal quark masses, used to give the starting value for running masses calculated
in PYMRUN.
PARF(101) - PARF(105) : contain d, u, s, c and b constituent masses, in the past usedin mass formulae for undiscovered hadrons, and should not be changed.
PARF(111), PARF(112) : (D = 0.0, 0.11 GeV) constant terms in the mass formulae forheavy mesons and baryons, respectively (with diquark getting 2/3 of baryon).
PARF(113), PARF(114) : (D = 0.16, 0.048 GeV) factors which, together with ClebschGordan coefficients and quark constituent masses, determine the mass splitting
due to spin-spin interactions for heavy mesons and baryons, respectively. The
latter factor is also used for the splitting between spin 0 and spin 1 diquarks.
PARF(115) - PARF(118) : (D = 0.50, 0.45, 0.55, 0.60 GeV), constant mass terms, added
to the constituent masses, to get the mass of heavy mesons with orbital angular momentum L = 1. The four numbers are for pseudovector mesons withquark spin 0, and for scalar, pseudovector and tensor mesons with quark spin 1,
respectively.
PARF(121), PARF(122) : (D = 0.1, 0.2 GeV) constant terms, which are subtracted forquark and diquark masses, respectively, in defining the allowed phase space inparticle decays into partons (e.g. B0 → cdud).
PARF(201) - PARF(1960) : (D = 1760*0) relative probabilities for flavour productionin the MSTJ(15) = 1 option; to be defined by you before any Pythia calls. (Thestandard meaning is changed in the advanced baryon popcorn code, described in
section 14.3.1, where many of the PARF numbers are used for other purposes.)
The index in PARF is of the compressed form
120 + 80×KTAB1+25×KTABS+KTAB3.
Here KTAB1 is the old flavour, fixed by preceding fragmentation history, while
KTAB3 is the new flavour, to be selected according to the relevant relative
probabilities (except for the very last particle, produced when joining two jets,415

where both KTAB1 and KTAB3 are known). Only the most frequently appearing
quarks/diquarks are defined, according to the code 1 = d, 2 = u, 3 = s, 4 = c,
5 = b, 6 = t (obsolete!), 7 = dd1 , 8 = ud0 , 9 = ud1 , 10 = uu1 , 11 = sd0 ,12 = sd1 , 13 = su0 , 14 = su1 , 15 = ss1 , 16 = cd0 , 17 = cd1 , 18 = cu0 , 19 = cu1 ,20 = cs0 , 21 = cs1 , 22 = cc1 . These are thus the only possibilities for the newflavour to be produced; for an occasional old flavour not on this list, the ordinary
relative flavour production probabilities will be used.
Given the initial and final flavour, the intermediate hadron that is produced isalmost fixed. (Initial and final diquark here corresponds to ‘popcorn’ productionof mesons intermediate between a baryon and an antibaryon). The additional
index KTABS gives the spin type of this hadron, with
0 = pseudoscalar meson or Λ-like spin 1/2 baryon,
1 = vector meson or Σ-like spin 1/2 baryon,
2 = tensor meson or spin 3/2 baryon.
(Some meson multiplets, not frequently produced, are not accessible by thisparameterization.)
Note that some combinations of KTAB1, KTAB3 and KTABS do not correspondto a physical particle (a Λ-like baryon must contain three different quark flavours,a Σ-like one at least two), and that you must see to it that the corresponding
PARF entries are vanishing. One additional complication exists when KTAB3
and KTAB1 denote the same flavour content (normally KTAB3=KTAB1, but
for diquarks the spin freedom may give KTAB3=KTAB1±1): then a flavour
neutral meson is to be produced, and here dd, uu and ss states mix (heavier
flavour states do not, and these are therefore no problem). For these cases theordinary KTAB3 value gives the total probability to produce either of the mesons
possible, while KTAB3=23 gives the relative probability to produce the lightest
meson state (π 0 , ρ0 , a02 ), KTAB3=24 relative probability for the middle meson(η, ω, f20 ), and KTAB3 = 25 relative probability for the heaviest one (η 0 , φ,f200 ). Note that, for simplicity, these relative probabilities are assumed the same
whether initial and final diquark have the same spin or not; the total probability
may well be assumed different, however.
As a general comment, the sum of PARF values for a given KTAB1 need not
be normalized to unity, but rather the program will find the sum of relevant
weights and normalize to that. The same goes for the KTAB3=23–25 weights.
This makes it straightforward to use one common setup of PARF values and stillswitch between different MSTJ(12) baryon production modes, with the exceptionof the advanced popcorn scenarios.
VCKM(I,J) : squared matrix elements of the Cabibbo-Kobayashi-Maskawa flavour mixing matrix.
I :up type generation index, i.e. 1 = u, 2 = c, 3 = t and 4 = t0 .
J :down type generation index, i.e. 1 = d, 2 = s, 3 = b and 4 = b0 .
COMMON/PYDAT3/MDCY(500,3),MDME(8000,2),BRAT(8000),KFDP(8000,5)
Purpose: to give access to particle decay data and parameters. In particular, the
MDCY(KC,1) variables may be used to switch on or off the decay of a given
particle species, and the MDME(IDC,1) ones to switch on or off an individual decay channel of a particle. For quarks, leptons and gauge bosons, a number ofdecay channels are included that are not allowed for on-mass-shell particles, see
MDME(IDC,2) = 102. These channels are not directly used to perform decays,
but rather to denote allowed couplings in a more general sense, and to switch
on or off such couplings, as described elsewhere. Particle data is stored by com416

pressed code KC rather than by the full KF code. You are reminded that the way
to know the KC value is to use the PYCOMP function, i.e. KC = PYCOMP(KF).
MDCY(KC,1) : switch to tell whether a particle with compressed code KC may be allowed
to decay or not.
= 0 : the particle is not allowed to decay.
= 1 : the particle is allowed to decay (if decay information is defined below for theparticle).
Warning: these values may be overwritten for resonances in a PYINIT call, basedon the MSTP(41) option you have selected. If you want to allow resonancedecays in general but switch off the decay of one particular resonance, thisis therefore better done after the PYINIT call.
MDCY(KC,2) : gives the entry point into the decay channel table for compressed particle
code KC. Is 0 if no decay channels have been defined.
MDCY(KC,3) : gives the total number of decay channels defined for compressed particle code KC, independently of whether they have been assigned a non-vanishingbranching ratio or not. Thus the decay channels are found in positions
MDCY(KC,2) to MDCY(KC,2) + MDCY(KC,3) - 1.
MDME(IDC,1) : on/off switch for individual decay channel IDC. In addition, a channel
may be left selectively open; this has some special applications in the event
generation machinery. Effective branching ratios are automatically recalculated
for the decay channels left open. Also process cross sections are affected; seesection 7.6.2. If a particle is allowed to decay by the MDCY(KC,1) value, at least
one channel must be left open by you. A list of decay channels with current IDC
numbers may be obtained with PYLIST(12).
= -1 : this is a non-Standard Model decay mode, which by default is assumed not
to exist. Normally, this option is used for decays involving fourth generationor H± particles.
= 0 : channel is switched off.
= 1 : channel is switched on.
= 2 : channel is switched on for a particle but off for an antiparticle. It is also on
for a particle which is its own antiparticle, i.e. here it means the same as =1.
= 3 : channel is switched on for an antiparticle but off for a particle. It is off fora particle which is its own antiparticle.
= 4 : in the production of a pair of equal or charge conjugate resonances in
Pythia, say h0 → W+ W− , either one of the resonances is allowed to decay
according to this group of channels, but not both. If the two particles of thepair are different, the channel is on. For ordinary particles, not resonances,
this option only means that the channel is switched off.
= 5 : as = 4, but an independent group of channels, such that in a pair of equal or
charge conjugate resonances the decay of either resonance may be specified
independently. If the two particles in the pair are different, the channel isoff. For ordinary particles, not resonances, this option only means that thechannel is switched off.
Warning: the two values −1 and 0 may look similar, but in fact are quite different.
In neither case the channel so set is generated, but in the latter case thechannel still contributes to the total width of a resonance, and thus affects
both simulated line shape and the generated cross section when Pythia
is run. The value 0 is appropriate to a channel we assume exists, even if
we are not currently simulating it, while −1 should be used for channels
we believe do not exist. In particular, you are warned unwittingly to set
fourth generation channels 0 (rather than −1), since by now the support for417

a fourth generation is small.
Remark: all the options above may be freely mixed. The difference, for thosecases where both make sense, between using values 2 and 3 and using 4
and 5 is that the latter automatically include charge conjugate states, e.g.
h0 → W+ W− → e+ νe du or due− ν e , but the former only one of them. In
calculations of the joint branching ratio, this makes a factor 2 difference.
Example: to illustrate the above options, consider the case of a W+ W− pair. One
might then set the following combination of switches for the W:
channel value commentud
1
allowed for W+ and W− in any combination,us
0
never produced but contributes to W width,
cd2
allowed for W+ only,
3
allowed for W− only, i.e. properly W− → cs,
cstb
0never produced but contributes to W width
if the channel is kinematically allowed,+
νe e
4
allowed for one of W+ or W− , but not both,νµ µ+
4allowed for one of W+ or W− , but not both,and not in combination with νe e+ ,+ντ τ
5allowed for the other W, but not both,−νχ χ−1 not produced and does not contribute to W width.
A W+ W− final state ud + cs is allowed, but not its charge conjugate ud + cs,
since the latter decay mode is not allowed for a W+ . The combination
νe e+ + ν̄τ τ − is allowed, since the two channels belong to different groups,
but not νe e+ + ν̄µ µ− , where both belong to the same. Both ud + ν̄τ τ − andud + ντ τ + are allowed, since there is no clash. The full rulebook, for thiscase, is given by eq. (111). A term ri2 means channel i is allowed for W+
and W− simultaneously, a term ri rj that channels i and j may be combined,
and a term 2ri rj that channels i and j may be combined two ways, i.e. that
also a charge conjugate combination is allowed.
MDME(IDC,2) : information on special matrix-element treatment for decay channel IDC.
Is mainly intended for the normal-particle machinery in PYDECY, so many of thecodes are superfluous in the more sophisticated resonance decay treatment by
PYRESD, see section 2.1.2. In addition to the outline below, special rules apply forthe order in which decay products should be given, so that matrix elements andcolour flow is properly treated. One such example is the weak matrix elements,
which only will be correct if decay products are given in the right order. The
program does not police this, so if you introduce channels of your own and use
these codes, you should be guided by the existing particle data.
= 0 : no special matrix-element treatment; partons and particles are copied directly to the event record, with momentum distributed according to phasespace.
= 1 : ω and φ decays into three pions, eq. (273).
= 2 : π 0 or η Dalitz decay to γe+ e− , eq. (275).
= 3 : used for vector meson decays into two pseudoscalars, to signal non-isotropic
decay angle according to eq. (274), where relevant.
= 4 : decay of a spin 1 onium resonance to three gluons or to a photon and two
gluons, eq. (43). The gluons may subsequently develop a shower if MSTJ(23)= 1.
= 11 : phase-space production of hadrons from the quarks available.
= 12 : as = 11, but for onia resonances, with the option of modifying the multiplicity distribution separately.
= 13 : as = 11, but at least three hadrons to be produced (useful when the twobody decays are given explicitly).
418

= 14 : as = 11, but at least four hadrons to be produced.
= 15 : as = 11, but at least five hadrons to be produced.
= 22 - 30 : phase-space production of hadrons from the quarks available, with themultiplicity fixed to be MDME(IDC,2) - 20, i.e. 2–10.
= 31 : two or more quarks and particles are distributed according to phase space.
If three or more products, the last product is a spectator quark, i.e. sittingat rest with respect to the decaying hadron.
= 32 : a qq or gg pair, distributed according to phase space (in angle), and allowed
to develop a shower if MSTJ(23) = 1.
= 33 : a triplet qXq, where X is either a gluon or a colour-singlet particle; the final
particle (q) is assumed to sit at rest with respect to the decaying hadron,and the two first particles (q and X) are allowed to develop a shower if
MSTJ(23) = 1. Nowadays superfluous.
= 41 : weak decay, where particles are distributed according to phase space, multiplied by a factor from the expected shape of the momentum spectrum of
the direct product of the weak decay (the ντ in τ decay).
= 42 : weak decay matrix element for quarks and leptons. Products may be given
either in terms of quarks or hadrons, or leptons for some channels. If thespectator system is given in terms of quarks, it is assumed to collapse into
one particle from the onset. If the virtual W decays into quarks, these quarks
are converted to particles, according to phase space in the W rest frame, as
in = 11. Is intended for τ , charm and bottom.
= 43 : as = 42, but if the W decays into quarks, these will either appear as jets
or, for small masses, collapse into a one- or two-body system. Nowadays
superfluous.
= 44 : weak decay matrix element for quarks and leptons, where the spectator system may collapse into one particle for a small invariant mass. If the first two
decay products are a qq0 pair, they may develop a parton shower if MSTJ(23)= 1. Was intended for top and beyond, but is nowadays superfluous.
= 48 : as = 42, but require at least three decay products.
= 50 : (default behaviour, also obtained for any other code value apart from theones listed below) do not include any special threshold factors. That is,a decay channel is left open even if the sum of daughter nominal masses
is above the mother actual mass, which is possible if at least one of thedaughters can be pushed off the mass shell. Is intended for decay treatment
in PYRESD with PYWIDT calls, and has no special meaning for ordinary PYDECY
calls.
= 51 : a step threshold, i.e. a channel is switched off when the sum of daughter
nominal masses is above the mother actual mass. Is intended for decay
treatment in PYRESD with PYWIDT calls, and has no special meaning forordinary PYDECY calls. q= 52 : a β-factor threshold, i.e. (1 − m21 /m2 − m22 /m2 )2 − 4m21 m22 /m4 , assumingthat the values stored in PMAS(KC,2) and BRAT(IDC) did not include any
threshold effects at all. Is intended for decay treatment in PYRESD withPYWIDT calls, and has no special meaning for ordinary PYDECY calls.
= 53 : as = 52, but assuming that PMAS(KC,2) and BRAT(IDC) did include thethreshold effects, so that the weight should be the ratio of the β value at theactual mass to that at the nominal one. Is intended for decay treatment in
PYRESD with PYWIDT calls, and has no special meaning for ordinary PYDECY
calls.
= 101 : this is not a proper decay channel, but only to be considered as a continuation line for the decay product listing of the immediately preceding channel.
Since the KFDP array can contain five decay products per channel, with this419

code it is possible to define channels with up to ten decay products. It is
not allowed to have several continuation lines after each other.
= 102 : this is not a proper decay channel for a decaying particle on the mass shell(or nearly so), and is therefore assigned branching ratio 0. For a particle
off the mass shell, this decay mode is allowed, however. By including thischannel among the others, the switches MDME(IDC,1) may be used to allow or
forbid these channels in hard processes, with cross sections to be calculated
separately. As an example, γ → uu is not possible for a massless photon,
but is an allowed channel in e+ e− annihilation.
BRAT(IDC) : give branching ratios for the different decay channels. In principle, thesum of branching ratios for a given particle should be unity. Since the program
anyway has to calculate the sum of branching ratios left open by the MDME(IDC,1)
values and normalize to that, you need not explicitly ensure this normalization,
however. (Warnings are printed in PYUPDA(2) or PYUPDA(3) calls if the sum is
not unity, but this is entirely intended as a help for finding user mistypings.) For
decay channels with MDME(IDC,2) > 80 the BRAT values are dummy.
KFDP(IDC,J) : contain the decay products in the different channels, with five positions
J = 1 - 5 reserved for each channel IDC. The decay products are given following the standard KF code for partons and particles, with 0 for trailing empty
positions. Note that the MDME(IDC+1,2) = 101 option allows you to double themaximum number of decay product in a given channel from 5 to 10, with thefive latter products stored in KFDP(IDC+1,J).
COMMON/PYDAT4/CHAF(500,2)
CHARACTER CHAF*16
Purpose: to give access to character type variables.
CHAF(KC,1) : particle name according to KC code.
CHAF(KC,2) : antiparticle name according to KC code when an antiparticle exists, elseblank.

14.5

Miscellaneous Comments

The previous sections have dealt with the subroutine options and variables one at a time.
This is certainly important, but for a full use of the capabilities of the program, it is also
necessary to understand how to make different pieces work together. This is somethingthat cannot be explained fully in a manual, but must also be learnt by trial and error. Thissection contains some examples of relationships between subroutines, common blocks andparameters. It also contains comments on issues that did not fit in naturally anywhere
else, but still might be useful to have on record.
14.5.1

Interfacing to detector simulation

Very often, the output of the program is to be fed into a subsequent detector simulation
program. It therefore becomes necessary to set up an interface between the PYJETS
common block and the detector model. Preferably this should be done via the HEPEVT
standard common block, see section 5.4, but sometimes this may not be convenient. If a
PYEDIT(2) call is made, the remaining entries exactly correspond to those an ideal detector
could see: all non-decayed particles, with the exception of neutrinos. The translation of
momenta should be trivial (if need be, a PYROBO call can be made to rotate the ‘preferred’z direction to whatever is the longitudinal direction of the detector), and so should the420translation of particle codes. In particular, if the detector simulation program also uses
the standard Particle Data Group codes, no conversion at all is needed. The problem
then is to select which particles are allowed to decay, and how decay vertex information
should be used.
Several switches regulate which particles are allowed to decay. First, the master switch
MSTJ(21) can be used to switch on/off all decays (and it also contains a choice of how
fragmentation should be interfaced). Second, a particle must have decay modes definedfor it, i.e. the corresponding MDCY(KC,2) and MDCY(KC,3) entries must be non-zero forcompressed code KC = PYCOMP(KF). This is true for all colour neutral particles except
the neutrinos, the photon, the proton and the neutron. (This statement is actually not
fully correct, since irrelevant ‘decay modes’ with MDME(IDC,2) = 102 exist in some cases.)Third, the individual switch in MDCY(KC,1) must be on. Of all the particles with decay
modes defined, only µ± , π ± , K± and K0L are by default considered stable.
Finally, if MSTJ(22) does not have its default value 1, checks are also made on the
lifetime of a particle before it is allowed to decay. In the simplest alternative, MSTJ(22) =2, the comparison is based on the average lifetime, or rather cτ , measured in mm. Thus
if the limit PARJ(71) is (the default) 10 mm, then decays of K0S , Λ, Σ− , Σ+ , Ξ− , Ξ0 andΩ− are all switched off, but charm and bottom still decay. No cτ values below 1 µm aredefined. With the two options MSTJ(22) = 3 or 4, a spherical or cylindrical volume isdefined around the origin, and all decays taking place inside this volume are ignored.
Whenever a particle is in principle allowed to decay, i.e. MSTJ(21) and MDCY on, an
proper lifetime is selected once and for all and stored in V(I,5). The K(I,1) is then alsochanged to 4. For MSTJ(22) = 1, such a particle will also decay, but else it could remainin the event record. It is then possible, at a later stage, to expand the volume insidewhich decays are allowed, and do a new PYEXEC call to have particles fulfilling the newconditions (but not the old) decay. As a further option, the K(I,1) code may be put to5, signalling that the particle will definitely decay in the next PYEXEC call, at the vertex
position given (by you) in the V vector.
This then allows the Pythia decay routines to be used inside a detector simulation
program, as follows. For a particle which did not decay before entering the detector, its
point of decay is still well defined (in the absence of deflections by electric or magnetic
fields), eq. (272). If it interacts before that point, the detector simulation program is left
to handle things. If not, the V vector is updated according to the formula above, K(I,1)
is set to 5, and PYEXEC is called, to give a set of decay products, that can again be tracked.
A further possibility is to force particles to decay into specific decay channels; thismay be particularly interesting for charm or bottom physics. The choice of channels left
open is determined by the values of the switches MDME(IDC,1) for decay channel IDC(use PYLIST(12) to obtain the full listing). One or several channels may be left open; inthe latter case effective branching ratios are automatically recalculated without the needfor your intervention. It is also possible to differentiate between which channels are left
open for particles and which for antiparticles. Lifetimes are not affected by the exclusionof some decay channels. Note that, whereas forced decays can enhance the efficiency forseveral kinds of studies, it can also introduce unexpected biases, in particular when events
may contain several particles with forced decays, cf. section 7.6.2.
14.5.2

Parameter values

A non-trivial question is to know which parameter values to use. The default values
stored in the program are based on comparisons with LEP e+ e− → Z0 data at around91 GeV [LEP90], using a parton-shower picture followed by string fragmentation. Some
examples of more recent parameter sets are found in [Kno96]. If fragmentation is indeed
a universal phenomenon, as we would like to think, then the same parameters should
also apply at other energies and in other processes. The former aspect, at least, seems421

to be borne out by comparisons with lower-energy PETRA/PEP data and higher-energy
LEP2 data. Note, however, that the choice of parameters is intertwined with the choiceof perturbative QCD description. If instead matrix elements are used, a best fit to 30
GeV data would require the values PARJ(21) = 0.40, PARJ(41) = 1.0 and PARJ(42) =0.7. With matrix elements one does not expect an energy independence of the parameters, since the effective minimum invariant mass cut-off is then energy dependent, i.e.
so is the amount of soft gluon emission effects lumped together with the fragmentation
parameters. This is indeed confirmed by the LEP data. A mismatch in the perturbative
QCD treatment could also lead to small differences between different processes.
It is often said that the string fragmentation model contains a wealth of parameters.
This is certainly true, but it must be remembered that most of these deal with flavour
properties, and to a large extent factorize from the treatment of the general event shape.
In a fit to the latter it is therefore usually enough to consider the parameters of the perturbative QCD treatment, like Λ in αs and a shower cut-off Q0 (or αs itself and ymin , ifmatrix elements are used), the a and b parameter of the Lund symmetric fragmentation
function (PARJ(41) and PARJ(42)) and the width of the transverse momentum distribution (σ =PARJ(21)). In addition, the a and b parameters are very strongly correlated by
the requirement of having the correct average multiplicity, such that in a typical χ2 plot,
the allowed region corresponds to a very narrow but very long valley, stretched diagonallyfrom small (a,b) pairs to large ones. As to the flavour parameters, these are certainly
many more, but most of them are understood qualitatively within one single framework,that of tunnelling pair production of flavours.
Since the use of independent fragmentation has fallen in disrespect, it should bepointed out that the default parameters here are not particularly well tuned to the data.
This especially applies if one, in addition to asking for independent fragmentation, alsoasks for another setup of fragmentation functions, i.e. other than the standard Lundsymmetric one. In particular, note that most fits to the popular Peterson/SLAC heavyflavour fragmentation function are based on the actual observed spectrum. In a Monte
Carlo simulation, one must then start out with something harder, to compensate for theenergy lost by initial-state photon radiation and gluon bremsstrahlung. Since independent fragmentation is not collinear safe (i.e, the emission of a collinear gluon changes theproperties of the final event), the tuning is strongly dependent on the perturbative QCD
treatment chosen. All the parameters needed for a tuning of independent fragmentation
are available, however.

14.6

ExamplesA 10 GeV u quark jet going out along the +z axis is generated withCALL PY1ENT(0,2,10D0,0D0,0D0)
Note that such a single jet is not required to conserve energy, momentum or flavour. In
the generation scheme, particles with negative pz are produced as well, but these areautomatically rejected unless MSTJ(3) = -1. While frequently used in former days, theone-jet generation option is not of much current interest.
In e.g. a leptoproduction event a typical situation could be a u quark going out inthe +z direction and a ud0 target remnant essentially at rest. (Such a process can be
simulated by Pythia, but here we illustrate how to do part of it yourself.) The simplest
procedure is probably to treat the process in the c.m. frame and boost it to the lab frame
afterwards. Hence, if the c.m. energy is 20 GeV and the boost βz = 0.996 (corresponding
to xB = 0.045), then
CALL PY2ENT(0,2,2101,20D0)
CALL PYROBO(0,0,0D0,0D0,0D0,0D0,0.996D0)
422

The jets could of course also be defined and allowed to fragment in the lab frame withCALL PY1ENT(-1,2,223.15D0,0D0,0D0)
CALL PY1ENT(2,12,0.6837D0,3.1416D0,0D0)
CALL PYEXEC
Note here that the target diquark is required to move in the backwards direction with
E −pz = mp (1−xB ) to obtain the correct invariant mass for the system. This is, however,
only an artifact of using a fixed diquark mass to represent a varying target remnant mass,
and is of no importance for the fragmentation. If one wants a nicer-looking event record,
it is possible to use the following
CALL PY1ENT(-1,2,223.15D0,0D0,0D0)
MSTU(10)=1
P(2,5)=0.938D0*(1D0-0.045D0)
CALL PY1ENT(2,2101,0D0,0D0,0D0)
MSTU(10)=2
CALL PYEXECA 30 GeV uug event with Eu = 8 GeV and Eu = 14 GeV is simulated withCALL PY3ENT(0,2,21,-2,30D0,2D0*8D0/30D0,2D0*14D0/30D0)
The event will be given in a standard orientation with the u quark along the +z axis
and the u in the ±z, +x half plane. Note that the flavours of the three partons have to
be given in the order they are found along a string, if string fragmentation options are
to work. Also note that, for 3-jet events, and particularly 4-jet ones, not all setups ofkinematical variables x lie within the kinematically allowed regions of phase space.
All common-block variables can obviously be changed by including the corresponding
common block in the user-written main program. Alternatively, the routine PYGIVE canbe used to feed in values, with some additional checks on array bounds then performed.
A callCALL PYGIVE(’MSTJ(21)=3;PMAS(C663,1)=210.;CHAF(401,1)=funnyino;’//&’PMAS(21,4)=’)will thus change the value of MSTJ(21) to 3, the value of PMAS(PYCOMP(663),1) =PMAS(136,1) to 210., the value of CHAF(401,1) to ’funnyino’, and print the current
value of PMAS(21,4). Since old and new values of parameters changed are written to output, this may offer a convenient way of documenting non-default values used in a given
run. On the other hand, if a variable is changed back and forth frequently, the resulting
voluminous output may be undesirable, and a direct usage of the common blocks is thento be recommended (the output can also be switched off, see MSTU(13)).
A general rule of thumb is that none of the physics routines (PYSTRF, PYINDF, PYDECY,etc.) should ever be called directly, but only via PYEXEC. This routine may be called
repeatedly for one single event. At each call only those entries that are allowed to fragment
or decay, and have not yet done so, are treated. Thus
CALL PY2ENT(1,1,-1,20D0)
MSTJ(1)=0
MSTJ(21)=0
MDCY(PYCOMP(111),1)=0
CALL PYEXECMSTJ(1)=1
CALL PYEXECMSTJ(21)=2! fill 2 partons without fragmenting! inhibit jet fragmentation! inhibit particle decay! inhibit pi0 decay! will not do anything!
! partons will fragment, no decays!
423

CALL PYEXECCALL PYEXECMDCY(PYCOMP(111),1)=1
CALL PYEXEC! particles decay, except pi0! nothing new can happen!
! pi0’s decay

A partial exception to the rule above is PYSHOW. Its main application is for internal
use by PYEEVT, PYDECY, and PYEVNT, but it can also be directly called by you. Note thata special format for storing colour-flow information in K(I,4) and K(I,5) must then be
used. For simple cases, the PY2ENT can be made to take care of that automatically, by
calling with the first argument negative.
CALL PY2ENT(-1,1,-2,80D0)
CALL PYSHOW(1,2,80D0)
CALL PYEXEC! store d ubar with colour flow
! shower partons! subsequent fragmentation/decay

For more complicated configurations, PYJOIN should be used.
It is always good practice to list one or a few events during a run to check that the
program is working as intended. With
CALL PYLIST(1)all particles will be listed and in addition total charge, momentum and energy of stable entries will be given. For string fragmentation these quantities should be conserved
exactly (up to machine precision errors), and the same goes when running independent
fragmentation with one of the momentum conservation options. PYLIST(1) gives a format
that comfortably fits in an 80 column window, at the price of not giving the complete
story. With PYLIST(2) a more extensive listing is obtained, and PYLIST(3) also gives
vertex information. Further options are available, like PYLIST(12), which gives a list ofparticle data.
An event, as stored in the PYJETS common block, will contain the original partons andthe whole decay chain, i.e. also particles which subsequently decayed. If parton showers
are used, the amount of parton information is also considerable: first the on-shell partons
before showers have been considered, then a K(I,1) = 22 line with total energy of theshowering subsystem, after that the complete shower history tree-like structure, starting
off with the same initial partons (now off-shell), and finally the end products of the shower
rearranged along the string directions. This detailed record is useful in many connections,but if one only wants to retain the final particles, superfluous information may be removed
with PYEDIT. Thus e.g.
CALL PYEDIT(2)will leave you with the final charged and neutral particles, except for neutrinos (and someother weakly interacting, neutral particles).
The information in PYJETS may be used directly to study an event. Some usefuladditional quantities derived from these, such as charge and rapidity, may easily be found
via the PYK and PYP functions. Thus electric charge = PYP(I,6) (as integer, three times
charge = PYK(I,6)) and true rapidity y with respect to the z axis = PYP(I,17).
A number of utility (MSTU, PARU) and physics (MSTJ, PARJ) switches and parameters
are available in common block PYDAT1. All of these have sensible default values. Particle
data is stored in common blocks PYDAT2, PYDAT3 and PYDAT4. Note that the data in thearrays KCHG, PMAS, MDCY CHAF and MWID is not stored by KF code, but by the compressed
code KC. This code is not to be learnt by heart, but instead accessed via the conversion
function PYCOMP, KC = PYCOMP(KF).
In the particle tables, the following particles are considered stable: the photon, e± ,±µ , π ± , K± , K0L , p, p, n, n and all the neutrinos. It is, however, always possible to inhibit424

the decay of any given particle by putting the corresponding MDCY value zero or negative,e.g. MDCY(PYCOMP(310),1) = 0 makes K0S and MDCY(PYCOMP(3122),1) = 0 Λ stable. It
is also possible to select stability based on the average lifetime (see MSTJ(22)), or basedon whether the decay takes place within a given spherical or cylindrical volume aroundthe origin.
The Field–Feynman jet model [Fie78] is available in the program by changing thefollowing values: MSTJ(1) = 2 (independent fragmentation), MSTJ(3) = -1 (retain particles with pz < 0; is not mandatory), MSTJ(11) = 2 (choice of longitudinal fragmentation function, with the a parameter stored in PARJ(51) - PARJ(53)), MSTJ(12) = 0(no baryon production), MSTJ(13) = 1 (give endpoint quarks p⊥ as quarks created in
the field), MSTJ(24) = 0 (no mass broadening of resonances), PARJ(2) = 0.5 (s/u ratio for the production of new qq pairs), PARJ(11) = PARJ(12) = 0.5 (probability formesons to have spin 1) and PARJ(21) = 0.35 (width of Gaussian transverse momentum
distribution). In addition only d, u and s single quark jets may be generated following
the FF recipe. Today the FF ‘standard jet’ concept is probably dead and buried, so thenumbers above should more be taken as an example of the flexibility of the program, thanas something to apply in practice.
A wide range of independent fragmentation options are implemented, to be accessedwith the master switch MSTJ(1) = 2. In particular, with MSTJ(2) = 1 a gluon jet is assumed to fragment like a random d, d, u, u, s or s jet, while with MSTJ(2) = 3 the gluonis split into a dd, uu or ss pair of jets sharing the energy according to the Altarelli-Parisi
splitting function. Whereas energy, momentum and flavour is not explicitly conserved in
independent fragmentation, a number of options are available in MSTJ(3) to ensure this‘post facto’, e.g. MSTJ(3) = 1 will boost the event to ensure momentum conservation andthen (in the c.m. frame) rescale momenta by a common factor to obtain energy conservation, whereas MSTJ(3) = 4 rather uses a method of stretching the jets in longitudinal
momentum along the respective jet axis to keep angles between jets fixed.

425

15

Event Study and Analysis Routines

After an event has been generated, one may wish to list it, or process it further in various
ways. The first section below describes some simple study routines of this kind, while thesubsequent ones describe more sophisticated analysis routines.
To describe the complicated geometries encountered in multihadronic events, a numberof event measures have been introduced. These measures are intended to provide a global
view of the properties of a given event, wherein the full information content of the event
is condensed into one or a few numbers. A steady stream of such measures are proposedfor different purposes. Many are rather specialized or never catch on, but a few become
standards, and are useful to have easy access to. Pythia therefore contains a number ofroutines that can be called for any event, and that will directly access the event record
to extract the required information.
In the presentation below, measures have been grouped in three kinds. The first contains simple event shape quantities, such as sphericity and thrust. The second is jet finding
algorithms. The third is a mixed bag of particle multiplicities and compositions, factorial
moments and energy–energy correlations, put together in a small statistics package.
None of the measures presented here are Lorentz invariant. The analysis will be
performed in whatever frame the event happens to be given in. It is therefore up to you
to decide whether the frame in which events were generated is the right one, or whether
events beforehand should be boosted, e.g. to the c.m. frame. You can also decide which
particles you want to have affected by the analysis.

15.1

Event Study Routines

After a PYEVNT call, or another similar physics routine call, the event generated is stored in
the PYJETS common block, and whatever physical variable is desired may be constructedfrom this record. An event may be rotated, boosted or listed, and particle data may belisted or modified. Via the functions PYK and PYP the values of some frequently appearing
variables may be obtained more easily. As described in subsequent sections, also moredetailed event shape analyses may be performed simply.
CALL PYROBO(IMI,IMA,THE,PHI,BEX,BEY,BEZ)
Purpose: to perform rotations and Lorentz boosts (in that order, if both in the samecall) of jet/particle momenta and vertex position variables.
IMI, IMA : range of entries affected by transformation, IMI ≤ I ≤ IMA. If 0 or below, IMI
defaults to 1 and IMA to N. Lower and upper bounds given by positive MSTU(1)and MSTU(2) override the IMI and IMA values, and also the 1 to N constraint.
THE, PHI : standard polar coordinates θ, ϕ, giving the rotated direction of a momentum
vector initially along the +z axis.
BEX, BEY, BEZ : gives the direction and size β of a Lorentz boost, such that a particle
initially at rest will have p/E =β afterwards.
Remark: all entries in the range IMI - IMA (or modified as described above) are affected,
unless the status code of an entry is K(I,1)≤ 0.
CALL PYEDIT(MEDIT)
Purpose: to exclude unstable or undetectable jets/particles from the event record. One
may also use PYEDIT to store spare copies of events (specifically initial partonconfiguration) that can be recalled to allow e.g. different fragmentation schemes
to be run through with one and the same parton configuration. Finally, an event426which has been analysed with PYSPHE, PYTHRU or PYCLUS (see section 15.5) may
be rotated to align the event axis with the z direction.
MEDIT : tells which action is to be taken.
= 0 : empty (K(I,1) = 0) and documentation (K(I,1) > 20) lines are removed.
The jets/particles remaining are compressed in the beginning of the PYJETS
common block and the N value is updated accordingly. The event history is
lost, so that information stored in K(I,3), K(I,4) and K(I,5) is no longer
relevant.
= 1 : as = 0, but in addition all jets/particles that have fragmented/decayed(K(I,1) > 10) are removed.
= 2 : as = 1, but also all neutrinos and unknown particles (i.e. compressed code
KC= 0) are removed. Also the lowest-lying neutralino χ01 (code 1000022), thegraviton (39) and the gravitino (1000039) are treated on an equal footingwith neutrinos. Other similar but not foreseen particles would not be removed automatically, but would have to be put to K(I,1) > 10 by hand
before PYEDIT is called.
= 3 : as = 2, but also all uncharged, colour neutral particles are removed, leaving
only charged, stable particles (and unfragmented partons, if fragmentation
has not been performed).
= 5 : as = 0, but also all partons which have branched or been rearranged in aparton shower and all particles which have decayed are removed, leaving
only the fragmenting parton configuration and the final-state particles.
= 11 : remove lines with K(I,1) < 0. Update event history information (in K(I,3)- K(I,5)) to refer to remaining entries.
= 12 : remove lines with K(I,1) = 0. Update event history information (in K(I,3)- K(I,5)) to refer to remaining entries.
= 13 : remove lines with K(I,1) = 11, 12 or 15, except for any line with K(I,2)= 94. Update event history information (in K(I,3) - K(I,5)) to refer toremaining entries. In particular, try to trace origin of daughters, for which
the mother is decayed, back to entries not deleted.
= 14 : remove lines with K(I,1) = 13 or 14, and also any line with K(I,2) =94. Update event history information (in K(I,3) - K(I,5)) to refer toremaining entries. In particular, try to trace origin of rearranged jets backthrough the parton-shower history to the shower initiator.
= 15 : remove lines with K(I,1) > 20. Update event history information (inK(I,3) - K(I,5)) to refer to remaining entries.
= 16 : try to reconstruct missing daughter pointers of decayed particles from themother pointers of decay products. These missing pointers typically comefrom the need to use K(I,4) and K(I,5) also for colour flow information.
= 21 : all partons/particles in current event record are stored (as a spare copy) inbottom of common block PYJETS (can e.g. be done to save original partons
before calling PYEXEC).
= 22 : partons/particles stored in bottom of event record with = 21 are placed in
beginning of record again, overwriting previous information there (so thate.g. a different fragmentation scheme can be used on the same partons).
Since the copy at bottom is unaffected, repeated calls with = 22 can bemade.
= 23 : primary partons/particles in the beginning of event record are marked as not
fragmented or decayed, and number of entries N is updated accordingly. Is asimple substitute for = 21 plus = 22 when no fragmentation/decay products
precede any of the original partons/particles.
= 31 : rotate largest axis, determined by PYSPHE, PYTHRU or PYCLUS, to sit along
the z direction, and the second largest axis into the xz plane. For PYCLUS
427

it can be further specified to +z axis and xz plane with x > 0, respectively.
Requires that one of these routines has been called before.
= 32 : mainly intended for PYSPHE and PYTHRU, this gives a further alignment of
the event, in addition to the one implied by = 31. The ‘slim’ jet, definedas the side (z > 0 or z < 0) with the smallest summed p⊥ over square rootof number of particles, is rotated into the +z hemisphere. In the opposite
hemisphere (now z < 0), the side of x > 0 and x < 0 which has the largest
summed |pz | is rotated into the z < 0, x > 0 quadrant. Requires that PYSPHEor PYTHRU has been called before.
Remark: all entries 1 through N are affected by the editing. For options 0–5 lower andupper bounds can be explicitly given by MSTU(1) and MSTU(2).
CALL PYLIST(MLIST)
Purpose: to list an event, jet or particle data, or current parameter values.
MLIST : determines what is to be listed.
= 0 : writes a title page with program version number and last date of change; ismostly for internal use.
= 1 : gives a simple list of current event record, in an 80 column format suitablefor viewing directly in a standard terminal window. For each entry, thefollowing information is given: the entry number I, the parton/particle name(see below), the status code (K(I,1)), the flavour code KF (K(I,2)), theline number of the mother (K(I,3)), and the three-momentum, energy andmass (P(I,1) - P(I,5)). If MSTU(3) is non-zero, lines immediately after
the event record proper are also listed. A final line contains information on
total charge, momentum, energy and invariant mass.
The particle name is given by a call to the routine PYNAME. For an entry
which has decayed/fragmented (K(I,1) = 11 - 20), this particle name isgiven within parentheses. Similarly, a documentation line (K(I,1) = 21-30) has the name enclosed in expression signs (!. . . !) and an event/jet
axis information line the name within inequality signs (<. . . >). If the last
character of the name is a ‘?’, it is a signal that the complete name has
been truncated to fit in, and can therefore not be trusted; this is very rare.
For partons which have been arranged along strings (K(I,1) = 1, 2, 11 or
12), the end of the parton name column contains information about thecolour string arrangement: an A for the first entry of a string, an I for all
intermediate ones, and a V for the final one (a poor man’s rendering of avertical doublesided arrow, l).
It is possible to insert lines just consisting of sequences of ====== to separate
different sections of the event record, see MSTU(70) - MSTU(80).
= 2 : gives a more extensive list of the current event record, in a 132 column format, suitable for wide terminal windows. For each entry, the following information is given: the entry number I, the parton/particle name (with padding
as described for = 1), the status code (K(I,1)), the flavour code KF (K(I,2)),the line number of the mother (K(I,3)), the decay product/colour-flow
pointers (K(I,4), K(I,5)), and the three-momentum, energy and mass
(P(I,1) - P(I,5)). If MSTU(3) is non-zero, lines immediately after the
event record proper are also listed. A final line contains information on total charge, momentum, energy and invariant mass. Lines with only ======
may be inserted as for = 1.
= 3 : gives the same basic listing as = 2, but with an additional line for each entry
containing information on production vertex position and time (V(I,1) -428

V(I,4)) and, for unstable particles, proper lifetime (V(I,5)).
is a simpler version of the listing obtained with = 2,but excluding the momentum information, and instead providing the alternative Les Houches
style colour tags used for final-state colour reconnection in the new multiple
interactions scenario. It thus allows colour flow to be studied and debugged
in a ”leaner” format.
= 5 : gives a simple listing of the event record stored in the HEPEVT common block.
This is mainly intended as a tool to check how conversion with the PYHEPC
routine works. The listing does not contain vertex information, and theflavour code is not displayed as a name.
= 7 : gives a simple listing of the parton-level event record for an external process,as stored in the HEPEUP common block. This is mainly intended as a tool tocheck how reading from HEPEUP input works. The listing does not contain
lifetime or spin information, and the flavour code is not displayed as a name.
It also does not show other HEPEUP numbers, such as the event weight.
= 11 : provides a simple list of all parton/particle codes defined in the program,with KF code and corresponding particle name. The list is grouped by particle kind, and only within each group in ascending order.
= 12 : provides a list of all parton/particle and decay data used in the program.
Each parton/particle code is represented by one line containing KF flavour
code, KC compressed code, particle name, antiparticle name (where appropriate), electrical and colour charge and presence or not of an antiparticle(stored in KCHG), mass, resonance width and maximum broadening, average
proper lifetime (in PMAS) and whether the particle is considered stable ornot (in MDCY). Immediately after a particle, each decay channel gets one
line, containing decay channel number (IDC read from MDCY), on/off switch
for the channel, matrix element type (MDME), branching ratio (BRAT), anddecay products (KFDP). The MSTU(1) and MSTU(2) flags can be used to set
the range of KF codes for which particles are listed.
= 13 : gives a list of current parameter values for MSTU, PARU, MSTJ and PARJ, andthe first 200 entries of PARF. This is useful to keep check of which default
values were changed in a given run.
Remark: for options 1–3 and 12 lower and upper bounds of the listing can be explicitly
given by MSTU(1) and MSTU(2).
= 4 :KK = PYK(I,J)
Purpose: to provide various integer-valued event data. Note that many of the options
available (in particular I > 0, J ≥ 14) which refer to event history will not work
after a PYEDIT call. Further, the options 14–18 depend on the way the event
history has been set up, so with the explosion of different allowed formats these
options are no longer as safe as they may have been. For instance, option 16 canonly work if MSTU(16) = 2.
I = 0, J = : properties referring to the complete event.
= 1 : N, total number of lines in event record.
= 2 : total number of partons/particles remaining after fragmentation and decay.
= 6 : three times the total charge of remaining (stable) partons and particles.
I > 0, J = : properties referring to the entry in line no. I of the event record.
= 1 - 5 : K(I,1) - K(I,5), i.e. parton/particle status KS, flavour code KF andorigin/decay product/colour-flow information.
= 6 : three times parton/particle charge.
= 7 : 1 for a remaining entry, 0 for a decayed, fragmented or documentation entry.

429

= 8 := 9 := 10 := 11 := 12 := 13 := 14 := 15 := 16 := 17 := 18 := 19 := 20 := 21 := 22 :

KF code (K(I,2)) for a remaining entry, 0 for a decayed, fragmented or
documentation entry.
KF code (K(I,2)) for a parton (i.e. not colour neutral entry), 0 for a particle.
KF code (K(I,2)) for a particle (i.e. colour neutral entry), 0 for a parton.
compressed flavour code KC.
colour information code, i.e. 0 for colour neutral, 1 for colour triplet, −1 for
antitriplet and 2 for octet.
flavour of ‘heaviest’ quark or antiquark (i.e. with largest code) in hadron or
diquark (including sign for antiquark), 0 else.
generation number. Beam particles or virtual exchange particles are generation 0, original jets/particles generation 1 and then 1 is added for each stepin the fragmentation/decay chain.
line number of ancestor, i.e. predecessor in first generation (generation 0
entries are disregarded).
rank of a hadron in the jet it belongs to. Rank denotes the ordering inflavour space, with hadrons containing the original flavour of the jet having
rank 1, increasing by 1 for each step away in flavour ordering. All decay
products inherit the rank of their parent. Whereas the meaning of a firstrank hadron in a quark jet is always well-defined, the definition of higher
ranks is only meaningful for independently fragmenting quark jets. In othercases, rank refers to the ordering in the actual simulation, which may be oflittle interest.
generation number after a collapse of a parton system into one particle, with
0 for an entry not coming from a collapse, and −1 for entry with unknown
history. A particle formed in a collapse is generation 1, and then one is
added in each decay step.
number of decay/fragmentation products (only defined in a collective sense
for fragmentation).
origin of colour for showering parton, 0 else.
origin of anticolour for showering parton, 0 else.
position of colour daughter for showering parton, 0 else.
position of anticolour daughter for showering parton, 0 else.

PP = PYP(I,J)
Purpose: to provide various real-valued event data. Note that some of the options available (I > 0, J = 20 - 25), which are primarily intended for studies of systems
in their respective c.m. frame, requires that a PYEXEC call (directly or indirectly,
e.g. by PYEVNT) has been made for the current initial parton/particle configuration, but that the latest PYEXEC call has not been followed by a PYROBO one.
I = 0, J = : properties referring to the complete event.
= 1 - 4 : sum of px , py , pz and E, respectively, for the stable remaining entries.
= 5 : invariant mass of the stable remaining entries.
= 6 : sum of electric charge of the stable remaining entries.
I > 0, J = : properties referring to the entry in line no. I of the event record.
= 1 - 5 : P(I,1) - P(I,5), i.e. normally px , py , pz , E and m for jet/particle.
= 6 : electric charge e.
= 7 : squared momentum |p|2 = p2x + p2y + p2z .
= 8 : absolute momentum |p|.
= 9 : squared transverse momentum p2⊥ = p2x + p2y .
= 10 : transverse momentum p⊥ .
= 11 : squared transverse mass m2⊥ = m2 + p2x + p2y .
430

= 12 : transverse mass m⊥ .
= 13 - 14 : polar angle θ in radians (between 0 and π) or degrees, respectively.
= 15 - 16 : azimuthal angle ϕ in radians (between −π and π) or degrees, respectively.
= 17 : true rapidity y = (1/2) ln((E + pz )/(E − pz )).
= 18 : rapidity yπ obtained by assuming that the particle is a pion when calculating
the energy E, to be used in the formula above, from the (assumed known)
momentum p.
= 19 : pseudorapidity η = (1/2) ln((p + pz )/(p − pz )).
= 20 : momentum fraction xp = 2|p|/W , where W is the total energy of the event,i.e. of the initial jet/particle configuration.
= 21 : xF = 2pz /W (Feynman-x if system is studied in the c.m. frame).
= 22 : x⊥ = 2p⊥ /W .
= 23 : xE = 2E/W .
= 24 : z+ = (E + pz )/W .
= 25 : z− = (E − pz )/W .
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
Purpose: to give access to a number of status codes that regulate the behaviour of theevent study routines. The main reference for PYDAT1 is in section 14.3.
MSTU(1),MSTU(2) : (D = 0, 0) can be used to replace the ordinary lower and upper
limits (normally 1 and N) for the action of PYROBO, and most PYEDIT and PYLIST
calls. Are reset to 0 in a PYEXEC call.
MSTU(3) : (D = 0) number of lines with extra information added after line N. Is reset to
0 in a PYEXEC call, or in an PYEDIT call when particles are removed.
MSTU(11) : (D = 6) file number to which all program output is directed. It is yourresponsibility to see to it that the corresponding file is also opened for output.
MSTU(12) : (D = 0) writing of title page (version number and last date of change forPythia) on output file.
= 0 : (or anything else except 12345) title page is written at first occasion, at
which time one sets MSTU(12) = 12345.
= 12345 : no title page is written.
Note: since the PYLOGO routine that writes the title page also checks that BLOCKDATA PYDATA is loaded, the value 12345 has been chosen sufficiently exotic
that it will not have been set by mistake.
MSTU(32) : (I) number of entries stored with PYEDIT(21) call.
MSTU(33) : (I) if set 1 before a PYROBO call, the V vectors (in the particle range to berotated/boosted) are set 0 before the rotation/boost. MSTU(33) is set back to 0
in the PYROBO call.
MSTU(70) : (D = 0) the number of lines consisting only of equal signs (======) that are
inserted in the event listing obtained with PYLIST(1), PYLIST(2) or PYLIST(3),so as to distinguish different sections of the event record on output. At most 10
such lines can be inserted; see MSTU(71) - MSTU(80). Is reset at PYEDIT callswith arguments 0–5.
MSTU(71) - MSTU(80) : line numbers below which lines consisting only of equal signs(======) are inserted in event listings. Only the first MSTU(70) of the 10 allowed
positions are enabled.

431

15.2

Event Shapes

In this section we study general event shape variables: sphericity, thrust, Fox-Wolfram
moments, and jet masses. These measures are implemented in the routines PYSPHE,PYTHRU, PYFOWO and PYJMAS, respectively. While possible to calculate for any event, theirlargest use is in e+ e− → γ ∗ /Z0 → qq annihilation events, where the (unknown) qq axis
largely regulates the particle flow.
Each event is assumed characterized by the particle four-momentum vectors pi =
(pi , Ei ), with i = 1, 2, · · · , n an index running over the particles of the event.
15.2.1

Sphericity

The sphericity tensor is defined as [Bjo70]Xpαi pβii
S αβ = X|pi |2

,

(282)

i

where α, β = 1, 2, 3 corresponds to the x, y and z components. By standard diagonalization of S αβ one may find three eigenvalues λ1 ≥ λ2 ≥ λ3 , with λ1 + λ2 + λ3 = 1. The
sphericity of the event is then defined asS=

3(λ2 + λ3 ) ,
2

(283)

so that 0 ≤ S ≤ 1. Sphericity is essentially a measure of the summed p2⊥ with respect tothe event axis; a 2-jet event corresponds to S ≈ 0 and an isotropic event to S ≈ 1.
The aplanarity A, with definition A = 32 λ3 , is constrained to the range 0 ≤ A ≤ 12 .
It measures the transverse momentum component out of the event plane: a planar event
has A ≈ 0 and an isotropic one A ≈ 12 .
Eigenvectors vj can be found that correspond to the three eigenvalues λj of the sphericity tensor. The v1 one is called the sphericity axis (or event axis, if it is clear from thecontext that sphericity has been used), while the sphericity event plane is spanned by v1
and v2 .
The sphericity tensor is quadratic in particle momenta. This means that the sphericity
value is changed if one particle is split up into two collinear ones which share the original
momentum. Thus sphericity is not an infrared safe quantity in QCD perturbation theory.
A useful generalization of the sphericity tensor isX

S (r)αβ =i|pi |r−2 pαi pβiX,|pi |r(284)

iwhere r is the power of the momentum dependence. While r = 2 thus corresponds tosphericity, r = 1 corresponds to linear measures calculable in perturbation theory [Par78]:
β
X pα
i pi
i
S (1)αβ = X

|pi |
|pi |

.

(285)

i

Eigenvalues and eigenvectors may be defined exactly as before, and therefore also
equivalents of S and A. These have no standard names; we may call them linearized432sphericity Slin and linearized aplanarity Alin . Quantities derived from the linear matrix
that are standard in the literature are instead the combinations [Ell81]
C = 3(λ1 λ2 + λ1 λ3 + λ2 λ3 ) ,D = 27λ1 λ2 λ3 .

(286)
(287)

Each of these is constrained to be in the range between 0 and 1. Typically, C is used tomeasure the 3-jet structure and D the 4-jet one, since C is vanishing for a perfect 2-jet
event and D is vanishing for a planar event. The C measure is related to the second
Fox-Wolfram moment (see below), C = 1 − H2 .
Noninteger r values may also be used, and corresponding generalized sphericity andaplanarity measures calculated. While perturbative arguments favour r = 1, we know thatthe fragmentation ‘noise’, e.g. from transverse momentum fluctuations, is proportionately
larger for low-momentum particles, and so r > 1 should be better for experimental event
axis determinations. The use of too large an r value, on the other hand, puts all theemphasis on a few high-momentum particles, and therefore involves a loss of information.
It should then come as no surprise that intermediate r values, of around 1.5, gives the best
performance for event axis determinations in 2-jet events, where the theoretical meaning of
the event axis is well-defined. The gain in accuracy compared with the more conventional
choices r = 2 or r = 1 is rather modest, however.
15.2.2ThrustThe quantity thrust T is defined by [Bra64]
X|n · pi |i

T = max

X

|n|=1

|pi |

,

(288)

i

and the thrust axis v1 is given by the n vector for which maximum is attained. The
allowed range is 1/2 ≤ T ≤ 1, with a 2-jet event corresponding to T ≈ 1 and an isotropic
event to T ≈ 1/2.
In passing, we note that this is not the only definition found in the literature. The
definitions agree for events studied in the c.m. frame and where all particles are detected.
However, a definition like
¯
¯
¯
¯
¯X
¯X
¯
¯
¯
¯
¯
¯
¯
θ(n
·
p
)
p
¯
θ
p
¯
i
i
i i¯
¯
¯
¯
¯
i
T = 2 max i X= 2 max X

|pi |

|n|=1

|pi |

θi =0,1

i

(289)

i

(where θ(x) is the step function, θ(x) = 1 if x > 0, else θ(x) = 0) gives different results
than the one above if e.g. only charged particles are detected. It would even be possible
to have T > 1; to avoid such problems, often an extra fictitious particle is introduced tobalance the total momentum [Bra79].
Eq. (288) may be rewritten as
¯
¯
¯X
¯
¯
¯
²i pi ¯¯¯¯i
T = max X.

|pi |²i =±1(290)

i(This may also be viewed as applying eq. (289) to an event with 2n particles, n carrying
the momenta pi and n the momenta −pi , thus automatically balancing the momentum.)433

To find the thrust value and axis this way, 2n−1 different possibilities would have to betested. The reduction by a factor of 2 comes from T being unchanged when all ²i → −²i .
Therefore this approach rapidly becomes prohibitive. Other exact methods exist, which‘only’ require about 4n2 combinations to be tried.
In the implementation in Pythia, a faster alternative method is used, in which thethrust axis is iterated from a starting direction n(0) according toX²(n(j) · pi ) pin(j+1) = ¯¯ i

(291)

¯
¯
¯X
¯
²(n(j) · pi ) pi ¯¯¯¯i(where ²(x) = 1 for x > 0 and ²(x) = −1 for x < 0). It is easy to show that the related
thrust value will never decrease, T (j+1) ≥ T (j) . In fact, the method normally converges
in 2–4 iterations. Unfortunately, this convergence need not be towards the correct thrust
axis but is occasionally only towards a local maximum of the thrust function [Bra79]. We
know of no foolproof way around this complication, but the danger of an error may belowered if several different starting axes n(0) are tried and found to agree. These n(0) aresuitably constructed from the n0 (by default 4) particles with the largest momenta in theP0event, and the 2n −1 starting directions i ²i pi constructed from these are tried in fallingorder of the corresponding absolute momentum values. When a predetermined numberof the starting axes have given convergence towards the same (best) thrust axis this one
is accepted.
In the plane perpendicular to the thrust axis, a major [MAR79] axis and value may
be defined in just the same fashion as thrust, i.e.
X

Ma =

max

|n|=1, n·v1 =0|n · pi |

i

X

|pi |

.

(292)

i

In a plane more efficient methods can be used to find an axis than in three dimensions[Wu79], but for simplicity we use the same method as above. Finally, a third axis, theminor axis, is defined perpendicular to the thrust and major ones, and a minor value Mi
is calculated just as thrust and major. The difference between major and minor is calledoblateness, O = Ma − Mi . The upper limit on oblateness depends on the thrust value ina not-so-simple way. In general O ≈ 0 corresponds to an event symmetrical around thethrust axis and high O to a planar event.
As in the case of sphericity, a generalization to arbitrary momentum dependence mayeasily be obtained, here by replacing the pi in the formulae above by |pi |r−1 pi . Thispossibility is included, although so far it has not found any experimental use.
15.2.3Fox-Wolfram moments

The Fox-Wolfram moments Hl , l = 0, 1, 2, . . ., are defined by [Fox79]
Hl =X |pi | |pj |
i,j

2
Evis

Pl (cos θij ) ,(293)

where θij is the opening angle between hadrons i and j and Evis the total visible energyof the event. Note that also autocorrelations, i = j, are included. The Pl (x) are theLegendre polynomials,
P0 (x) = 1 ,434

P1 (x) = x ,1P2 (x) =(3x2 − 1) ,2
1
P3 (x) =(5x3 − 3x) ,2
1
P4 (x) =(35x4 − 30x2 + 3) .
8

(294)

To the extent that particle masses may be neglected, H0 ≡ 1. It is customary to normalize
the results to H0 , i.e. to give Hl0 = Hl /H0 . If momentum is balanced then H1 ≡ 0. 2-jet
events tend to give Hl ≈ 1 for l even and ≈ 0 for l odd.
15.2.4

Jet masses

The particles of an event may be divided into two classes. For each class a squared invariant mass may be calculated, M12 and M22 . If the assignment of particles is adjusted suchthat the sum M12 + M22 is minimized, the two masses thus obtained are called heavy andlight jet mass, MH and ML . It has been shown that these quantities are well behaved inperturbation theory [Cla79]. In e+ e− annihilation, the heavy jet mass obtains a contribution from qqg 3-jet events, whereas the light mass is non-vanishing only when 4-jet events
2also are included. In the c.m. frame of an event one has the limits 0 ≤ MH2 ≤ Ecm/3.
In general, the subdivision of particles tends to be into two hemispheres, separated bya plane perpendicular to an event axis. As with thrust, it is time-consuming to find theexact solution. Different approximate strategies may therefore be used. In the program,the sphericity axis is used to perform a fast subdivision into two hemispheres, and thus
into two preliminary jets. Thereafter one particle at a time is tested to determine whether
the sum M12 + M22 would be decreased if that particle were to be assigned to the otherjet. The procedure is stopped when no further significant change is obtained. Often theoriginal assignment is retained as it is, i.e. the sphericity axis gives a good separation.
This is not a full guarantee, since the program might get stuck in a local minimum which
is not the global one.

15.3

Cluster Finding

Global event measures, like sphericity or thrust, can only be used to determine the jet axes
for back-to-back 2-jet events. To determine the individual jet axes in events with threeor more jets, or with two (main) jets which are not back-to-back, cluster algorithms arecustomarily used. In these, nearby particles are grouped together into a variable numberof clusters. Each cluster has a well-defined direction, given by a suitably weighted averageof the constituent particle directions.
The cluster algorithms traditionally used in e+ e− and in pp physics differ in several
respects. The former tend to be spherically symmetric, i.e. have no preferred axis in space,and normally all particles have to be assigned to some jet. The latter pick the beam axis
as preferred direction, and make use of variables related to this choice, such as rapidity
and transverse momentum; additionally only a fraction of all particles are assigned tojets.
This reflects a difference in the underlying physics: in pp collisions, the beam remnants
found at low transverse momenta are not related to any hard processes, and therefore only
provide an unwanted noise to many studies. (Of course, also hard processes may produce
particles at low transverse momenta, but at a rate much less than that from soft or semihard processes.) Further, the kinematics of hard processes is, to a good approximation,
factorized into the hard subprocess itself, which is boost invariant in rapidity, and partondistribution effects, which determine the overall position of a hard scattering in rapidity.
435

Hence rapidity, azimuthal angle and transverse momentum is a suitable coordinate frame
to describe hard processes in.
In standard e+ e− annihilation events, on the other hand, the hard process c.m. frame
tends to be almost at rest, and the event axis is just about randomly distributed in space,i.e. with no preferred rôle for the axis defined by the incoming e± . All particle production
is initiated by and related to the hard subprocess. Some of the particles may be less easy
to associate with a specific jet, but there is no compelling reason to remove any of themfrom consideration.
This does not mean that the separation above is always required. 2γ events in e+ e−
may have a structure with ‘beam jets’ and ‘hard scattering’ jets, for which the pp typealgorithms might be well suited. Conversely, a heavy particle produced in pp collisions
could profitably be studied, in its own rest frame, with e+ e− techniques.
In the following, particles are only characterized by their three-momenta or, alternatively, their energy and direction of motion. No knowledge is therefore assumed ofparticle types, or even of mass and charge. Clearly, the more is known, the more sophisticated clustering algorithms can be used. The procedure then also becomes more
detector-dependent, and therefore less suitable for general usage.
Pythia contains two cluster finding routines. PYCLUS is of the e+ e− type and PYCELL
of the pp one. Each of them allows some variations of the basic scheme.
15.3.1Cluster finding in an e+ e− type of environment

The usage of cluster algorithms for e+ e− applications started in the late 1970’s. A numberof different approaches were proposed [Bab80], see review in [Mor98]. Of these, we will
here only discuss those based on binary joining. In this kind of approach, initially each
final-state particle is considered to be a cluster. Using some distance measure, the twonearest clusters are found. If their distance is smaller than some cut-off value, the twoclusters are joined into one. In this new configuration, the two clusters that are now
nearest are found and joined, and so on until all clusters are separated by a distancelarger than the cut-off. The clusters remaining at the end are often also called jets. Note
that, in this approach, each single particle belongs to exactly one cluster. Also note thatthe resulting jet picture explicitly depends on the cut-off value used. Normally the numberof clusters is allowed to vary from event to event, but occasionally it is more useful tohave the cluster algorithm find a predetermined number of jets (like 3).
The obvious choice for a distance measure is to use squared invariant mass, i.e. fortwo clusters i and j to define the distance to be
m2ij = (Ei + Ej )2 − (pi + pj )2 .

(295)

(Equivalently, one could have used the invariant mass as measure rather than its square;
this is just a matter of convenience.) In fact, a number of people (including one of
the authors) tried this measure long ago and gave up on it, since it turns out to havesevere instability problems. The reason is well understood: in general, particles tend tocluster closer in invariant mass in the region of small momenta. The clustering process
therefore tends to start in the center of the event, and only subsequently spread outwards
to encompass also the fast particles. Rather than clustering slow particles around thefast ones (where the latter naı̈vely should best represent the jet directions), the invariant
mass measure will tend to cluster fast particles around the slow ones.
Another instability may be seen by considering the clustering in a simple 2-jet event.
By the time that clustering has reached the level of three clusters, the ‘best’ the clustering
algorithm can possibly have achieved, in terms of finding three low-mass clusters, is tohave one fast cluster around each jet, plus a third slow cluster in the middle. In thelast step this third cluster would be joined with one of the fast ones, to produce two
final asymmetric clusters: one cluster would contain all the slow particles, also those that436

visually look like belonging to the opposite jet. A simple binary joining process, with nopossibility to reassign particles between clusters, is therefore not likely to be optimal.
The solution adopted [Sjö83] is to reject invariant mass as distance measure. Instead a
jet is defined as a collection of particles which have a limited transverse momentum withrespect to a common jet axis, and hence also with respect to each other. This picture isclearly inspired by the standard fragmentation picture, e.g. in string fragmentation. A
distance measure dij between two particles (or clusters) with momenta pi and pj should
thus not depend critically on the longitudinal momenta but only on the relative transverse
momentum. A number of such measures were tried, and the one eventually selected was
d2ij =

1
4 |pi | |pj |4 |pi |2 |pj |2 sin2 (θij /2)(|pi | |pj | − pi · pj )=.
2(|pi | + |pj |)2(|pi | + |pj |)2(296)

For small relative angle θij , where 2 sin(θij /2) ≈ sin θij and cos θij ≈ 1, this measure
reduces to|pi × pj |
dij ≈
,
(297)
|pi + pj |where ‘×’ represents the cross product. We therefore see that dij in this limit has thesimple physical interpretation as the transverse momentum of either particle with respect
to the direction given by the sum of the two particle momenta. Unlike the approximate
expression, however, dij does not vanish for two back-to-back particles, but is here more
related to the invariant mass between them.
The basic scheme is of the binary joining type, i.e. initially each particle is assumed to
be a cluster by itself. Then the two clusters with smallest relative distance dij are foundand, if dij < djoin , with djoin some predetermined distance, the two clusters are joined toone, i.e. their four-momenta are added vectorially to give the energy and momentum of
the new cluster. This is repeated until the distance between any two clusters is > djoin .
The number and momenta of these final clusters then represent our reconstruction of theinitial jet configuration, and each particle is assigned to one of the clusters.
To make this scheme workable, two further ingredients are introduced, however.
Firstly, after two clusters have been joined, some particles belonging to the new cluster may actually be closer to another cluster. Hence, after each joining, all particles inthe event are reassigned to the closest of the clusters. For particle i, this means that the
distance dij to all clusters j in the event has to be evaluated and compared. After allparticles have been considered, and only then, are cluster momenta recalculated to take
into account any reassignments. To save time, the assignment procedure is not iterated
until a stable configuration is reached, but, since all particles are reassigned at each step,
such an iteration is effectively taking place in parallel with the cluster joining. Only at
the very end, when all dij > djoin , is the reassignment procedure iterated to convergence— still with the possibility to continue the cluster joining if some dij should drop below
djoin due to the reassignment.
Occasionally, it may occur that the reassignment step leads to an empty cluster, i.e.
one to which no particles are assigned. Since such a cluster has a distance dij = 0 to
any other cluster, it is automatically removed in the next cluster joining. However, it
is possible to run the program in a mode where a minimum number of jets is to bereconstructed. If this minimum is reached with one cluster empty, the particle is foundwhich has largest distance to the cluster it belongs to. That cluster is then split into
two, namely the large-distance particle and a remainder. Thereafter the reassignment
procedure is continued as before.
Secondly, the large multiplicities normally encountered means that, if each particle
initially is to be treated as a separate cluster, the program will become very slow. Therefore a smaller number of clusters, for a normal e+ e− event typically 8–12, is constructed437

as a starting point for the iteration above, as follows. The particle with the highest momentum is found, and thereafter all particles within a distance dij < dinit from it, where
dinit ¿ djoin . Together these are allowed to form a single cluster. For the remaining particles, not assigned to this cluster, the procedure is iterated, until all particles have beenused up. Particles in the central momentum region, |p| < 2dinit are treated separately; if
their vectorial momentum sum is above 2dinit they are allowed to form one cluster, otherwise they are left unassigned in the initial configuration. The value of dinit , as long as
reasonably small, has no physical importance, in that the same final cluster configuration
will be found as if each particle initially is assumed to be a cluster by itself: the particles clustered at this step are so nearby anyway that they almost inevitably must enter
the same jet; additionally the reassignment procedure allows any possible ‘mistake’ to becorrected in later steps of the iteration. However, if computer time is not a problem, one
may well let dinit → 0 and circumvent this preclustering.
Thus the jet reconstruction depends on one single parameter, djoin , with a clearcut
physical meaning of a transverse momentum ‘jet-resolution power’. Neglecting smearing
from fragmentation, dij between two clusters of equal energy corresponds to half theinvariant mass of the two original partons. If one only wishes to reconstruct well separated
jets, a large djoin should be chosen, while a small djoin would allow the separation of close
jets, at the cost of sometimes artificially dividing a single jet into two. In particular, b
quark jets may here be a nuisance. The value of djoin to use for a fixed jet-resolution powerin principle should be independent of the c.m. energy of events, although fragmentation
effects may give a contamination of spurious extra jets that increases slowly with Ecm for
fixed djoin . Therefore values as low as a djoin = 2.5 GeV was acceptable at PETRA/PEP,
while 3–4 GeV may be better for applications at LEP and beyond.
This completes the description of the main option of the PYCLUS routine. Variations
are possible. One such is to skip the reassignment step, i.e. to make use only of the simple
binary joining procedure, without any possibility to reassign particles between jets. (This
option is included mainly as a reference, to check how important reassignment really is.)The other main alternative is to replace the distance measure used above with the ones
used in the JADE or Durham algorithms.
The JADE cluster algorithm [JAD86] is an attempt to save the invariant mass measure.
The distance measure is defined to be
yij =2Ei Ej (1 − cos θij )
.
2
Evis

(298)

2
Here Evis is the total visible energy of the event. The usage of Evisin the denominator2rather than Ecm tends to make the measure less sensitive to detector acceptance corrections; in addition the dimensionless nature of yij makes it well suited for a comparison of
results at different c.m. energies. For the subsequent discussions, this normalization willbe irrelevant, however.
The yij measure is very closely related to the squared mass distance measure: the twocoincide (up to the difference in normalization) if mi = mj = 0. However, consider a pairof particles or clusters with non-vanishing individual masses and a fixed pair mass. Then,the larger the net momentum of the pair, the smaller the yij measure. This somewhat
tends to favour clustering of fast particles, and makes the algorithm less unstable thanthe one based on true invariant mass.
The successes of the JADE algorithm are well known: one obtains a good agreement
between the number of partons generated on the matrix-element (or parton-shower) level
and the number of clusters reconstructed from the hadrons, such that QCD aspects like therunning of αs can be studied with a small dependence on fragmentation effects. Of course,
the insensitivity to fragmentation effects depends on the choice of fragmentation model.
Fragmentation effects are small in the string model, but not necessarily in independent438

fragmentation scenarios. Although independent fragmentation in itself is not credible,
this may be seen as a signal for caution.
One should note that the JADE measure still suffers from some of the diseases of thesimple mass measure (without reassignments), namely that particles which go in opposite
directions may well be joined into the same cluster. Therefore, while the JADE algorithm
is a good way to find the number of jets, it is inferior to the standard dij measure for adetermination of jet directions and energies [Bet92]. The dij measure also gives narrower
jets, which agree better with the visual impression of jet structure.
Later, the Durham algorithm was introduced [Cat91], which works as the JADE one
but with a distance measureỹij =

2 min(Ei2 , Ej2 )(1 − cos θij )
.
2
Ecm

(299)

Like the dij measure, this is a transverse momentum, but ỹij has the geometrical interpretation as the transverse momentum of the softer particle with respect to the directionof the harder one, while dij is the transverse momentum of either particle with respect
to the common direction given by the momentum vector sum. The two definitions agree
when one cluster is much softer than the other, so the soft gluon exponentiation provenfor the Durham measure also holds for the dij one.
The main difference therefore is that the standard PYCLUS option allows reassignments,
while the Durham algorithm does not. The latter is therefore more easily calculable
on the perturbative parton level. This point is sometimes overstressed, and one could
give counterexamples why reassignments in fact may bring better agreement with theunderlying perturbative level. In particular, without reassignments, one will make therecombination that seems the ‘best’ in the current step, even when that forces you tomake ‘worse’ choices in subsequent steps. With reassignments, it is possible to correct for
mistakes due to the too local sensitivity of a simple binary joining scheme.
15.3.2Cluster finding in a pp type of environment

The PYCELL cluster finding routines is of the kind pioneered by UA1 [UA183], and commonly used in pp physics. It is based on a choice of pseudorapidity η, azimuthal angle ϕand transverse momentum p⊥ as the fundamental coordinates. This choice is discussed in
the introduction to cluster finding above, with the proviso that the theoretically preferred
true rapidity has to be replaced by pseudorapidity, to make contact with the real-life
detector coordinate system.
A fix detector grid is assumed, with the pseudorapidity range |η| < ηmax and the fullazimuthal range each divided into a number of equally large bins, giving a rectangulargrid. The particles of an event impinge on this detector grid. For each cell in (η,ϕ) space,
the transverse energy (normally ≈ p⊥ ) which enters that cell is summed up to give a total
cell E⊥ flow.
Clearly the model remains very primitive in a number of respects, compared with areal detector. There is no magnetic field allowed for, i.e. also charged particles move instraight tracks. The dimensions of the detector are not specified; hence the positions of
the primary vertex and any secondary vertices are neglected when determining which cella particle belongs
q to. The rest mass of particles is not taken into account, i.e. what is used
is really p⊥ = p2x + p2y , while in a real detector some particles would decay or annihilate,
and then deposit additional amounts of energy.
To take into account the energy resolution of the detector, it is possible to smear theE⊥ contents, bin by√bin. This is done according to a Gaussian, with a width assumed
proportional to the E⊥ of the bin. The Gaussian is cut off at zero and at some predetermined multiple of the unsmeared E⊥ , by default twice it. Alternatively, the smearing439

may be performed in E rather than in E⊥ . To find the E, it is assumed that the fullenergy of a cell is situated at its center, so that one can translate back and forth with
E = E⊥ cosh ηcenter .
The cell with largest E⊥ is taken as a jet initiator if its E⊥ is above some threshold.
A candidate jet is defined to consist of all cells which are within some given radius R
in the (η,ϕ) plane, i.e. which have (η − ηinitiator )2 + (ϕ − ϕinitiator )2 < R2 . Coordinates
are always given with respect to the center of the cell. If the summed E⊥ of the jet isabove the required minimum jet energy, the candidate jet is accepted, and all its cells are
removed from further consideration. If not, the candidate is rejected. The sequence is
now repeated with the remaining cell of highest E⊥ , and so on until no single cell fulfils
the jet initiator condition.
The number of jets reconstructed can thus vary from none to a maximum given bypurely geometrical considerations, i.e. how many circles of radius R are needed to cover
the allowed (η,ϕ) plane. Normally only a fraction of the particles are assigned to jets.
One could consider to iterate the jet assignment process, using the E⊥ -weighted centerof a jet to draw a new circle of radius R. In the current algorithm there is no such iteration
step. For an ideal jet assignment it would also be necessary to improve the treatment
when two jet circles partially overlap.
A final technical note. A natural implementation of a cell finding algorithm is basedon having a two-dimensional array of E⊥ values, with dimensions to match the detector
grid. Very often most of the cells would then be empty, in particular for low-multiplicity
events in fine-grained calorimeters. Our implementation is somewhat atypical, since cells
are only reserved space (contents and position) when they are shown to be non-empty.
This means that all non-empty cells have to be looped over to find which are within therequired distance R of a potential jet initiator. The algorithm is therefore faster than theordinary kind if the average cell occupancy is low, but slower if it is high.
Simple cone algorithms like the one above are robust, but may be less easy to understand in the context of a pure parton-level calculation. Therefore a Snowmass accord[Hut92] jet definition was introduced that would allow a clean parton-level definition of thejet topology. Unfortunately that accord was not specific enough to be useful for hadronlevel studies, and each collaboration supplemented the Snowmass concepts with their own
additional specifications. With time various limitations have been found [Bla00, Ell01],
such as problems with infrared safety and the splitting/merging of jets, that have marredcomparisons between theory and experiment. This appears to be a never-ending story,
with the optimal jet algorithm not yet in sight.

15.4

Event Statistics

All the event-analysis routines above are defined on an event-by-event basis. Once found,the quantities are about equally often used to define inclusive distributions as to selectspecific classes of events for continued study. For instance, the thrust routine might beused either to find the inclusive T distribution or to select events with T < 0.9. Other
measures, although still defined for the individual event, only make sense to discuss interms of averages over many events. A small set of such measures is found in PYTABU.
This routine has to be called once after each event to accumulate statistics, and once inthe end to print the final tables. Of course, among the wealth of possibilities imaginable,
the ones collected here are only a small sample, selected because the authors at somepoint have found a use for them.
15.4.1

Multiplicities

Three options are available to collect information on multiplicities in events. One gives theflavour content of the final state in hard interaction processes, e.g. the relative composition440

of dd/uu/ss/cc/bb in e+ e− annihilation events. Additionally it gives the total parton
multiplicity distribution at the end of parton showering. Another gives the inclusive rateof all the different particles produced in events, either as intermediate resonances or as
final-state particles. The number is subdivided into particles produced from fragmentation(primary particles) and those produced in decays (secondary particles).
The third option tabulates the rate of exclusive final states, after all allowed decays
have occurred. Since only events with up to 8 final-state particles are analysed, this isclearly not intended for the study of complete high-energy events. Rather the main application is for an analysis of the decay modes of a single particle. For instance, the decay
data for D mesons is given in terms of channels that also contain unstable particles, suchas ρ and η, which decay further. Therefore a given final state may receive contributions
from several tabulated decay channels; e.g. Kππ from K∗ π and Kρ, and so on.
15.4.2

Energy-Energy Correlation

The Energy-Energy Correlation is defined by [Bas78]
EEC(θ) =X 2Ei Ej
i<j

2
Evis

δ(θ − θij ) ,(300)

and its Asymmetry by
EECA(θ) = EEC(π − θ) − EEC(θ) .

(301)

Here θij is the opening angle between the two particles i and j, with energies Ei and
Ej . In principle, normalization should be to Ecm , but if not all particles are detected
it is convenient to normalize to the total visible energy Evis . Taking into account the
autocorrelation term i = j, the total EEC in an event then is unity. The δ function
peak is smeared out by the finite bin width ∆θ in the histogram, i.e., it is replaced by acontribution 1/∆θ to the bin which contains θij .
The formulae above refer to an individual event, and are to be averaged over all events
to suppress statistical fluctuations, and obtain smooth functions of θ.
15.4.3

Factorial moments

Factorial moments may be used to search for intermittency in events [Bia86]. The whole
field was much studied in the late eighties and early nineties, and a host of different
measures have been proposed. We only implement one of the original prescriptions.
To calculate the factorial moments, the full rapidity (or pseudorapidity) and azimuthal
ranges are subdivided into bins of successively smaller size, and the multiplicity distributions in bins is studied. The program calculates pseudorapidity with respect to the zaxis; if desired, one could first find an event axis, e.g. the sphericity or thrust axis, andsubsequently rotate the event to align this axis with the z direction.
The full rapidity range |y| < ymax (or pseudorapidity range |η| < ηmax ) and azimuthal
range 0 < ϕ < 2π are subdivided into my and mϕ equally large bins. In fact, thewhole analysis is performed thrice: once with mϕ = 1 and the y (or η) range graduallydivided into 1, 2, 4, 8, 16, 32, 64, 128, 256 and 512 bins, once with my = 1 and the ϕrange subdivided as above, and finally once with my = mϕ according to the same binary
sequence. Given the multiplicity nj in bin j, the i:th factorial moment is defined by
Fi = (my mϕ )i−1

X nj (nj − 1) · · · (nj − i + 1)jn(n − 1) · · · (n − i + 1)

441

.

(302)

P

Here n = j nj is the total multiplicity of the event within the allowed y (or η) limits.
The calculation is performed for the second through the fifth moments, i.e. F2 throughF5 .
The Fi as given here are defined for the individual event, and have to be averaged overmany events to give a reasonably smooth behaviour. If particle production is uniform
and uncorrelated according to Poisson statistics, one expects hFi i ≡ 1 for all moments
and all bin sizes. If, on the other hand, particles are locally clustered, factorial moments
should increase when bins are made smaller, down to the characteristic dimensions of theclustering.

15.5

Routines and Common-Block Variables

The six routines PYSPHE, PYTHRU, PYCLUS, PYCELL, PYJMAS and PYFOWO give you the possibility to find some global event shape properties. The routine PYTABU performs a statisticalanalysis of a number of different quantities like particle content, factorial moments andthe energy–energy correlation.
Note that, by default, all remaining partons/particles except neutrinos (and someother weakly interacting particles) are used in the analysis. Neutrinos may be included
with MSTU(41) = 1. Also note that axes determined are stored in PYJETS, but are notproper four-vectors and, as a general rule (with some exceptions), should therefore not be
rotated or boosted.
CALL PYSPHE(SPH,APL)
Purpose: to diagonalize the momentum tensor, i.e. find the eigenvalues λ1 > λ2 > λ3 ,with sum unity, and the corresponding eigenvectors.
Momentum power dependence is given by PARU(41); default corresponds tosphericity, while PARU(41) = 1. gives measures linear in momenta. Whichparticles (or partons) are used in the analysis is determined by the MSTU(41)value.
SPH : 32 (λ2 + λ3 ), i.e. sphericity (for PARU(41) = 2.).
= -1. : analysis not performed because event contained less than two particles(or two exactly back-to-back particles, in which case the two transverse
directions would be undefined).
3
APL : 2 λ3 , i.e. aplanarity (for PARU(41) = 2.).
= -1. : as SPH = -1.
Remark: the lines N + 1 through N + 3 (N - 2 through N for MSTU(43) = 2) in PYJETS
will, after a call, contain the following information:
K(N+i,1) = 31;
K(N+i,2) = 95;
K(N+i,3) : i, the axis number, i = 1, 2, 3;K(N+i,4), K(N+i,5) = 0;P(N+i,1) - P(N+i,3) : the i’th eigenvector, x, y and z components;
P(N+i,4) : λi , the i’th eigenvalue;
P(N+i,5) = 0;
V(N+i,1) - V(N+i,5) = 0.
Also, the number of particles used in the analysis is given in MSTU(62).
CALL PYTHRU(THR,OBL)
Purpose: to find the thrust, major and minor axes and corresponding projected momentum quantities, in particular thrust and oblateness. The performance of the442

program is affected by MSTU(44), MSTU(45), PARU(42) and PARU(48). In particular, PARU(42) gives the momentum dependence, with the default value = 1
corresponding to linear dependence. Which particles (or partons) are used in theanalysis is determined by the MSTU(41) value.
THR : thrust (for PARU(42) = 1.).
= -1. : analysis not performed because event contained less than two particles.
= -2. : remaining space in PYJETS (partly used as working area) not large enoughto allow analysis.
OBL : oblateness (for PARU(42) = 1.).
= -1., -2. : as for THR.
Remark: the lines N + 1 through N + 3 (N - 2 through N for MSTU(43) = 2) in PYJETS
will, after a call, contain the following information:
K(N+i,1) = 31;
K(N+i,2) = 96;
K(N+i,3) : i, the axis number, i = 1, 2, 3;K(N+i,4), K(N+i,5) = 0;P(N+i,1) - P(N+i,3) : the thrust, major and minor axis, respectively, fori = 1, 2 and 3;
P(N+i,4) : corresponding thrust, major and minor value;
P(N+i,5) = 0;
V(N+i,1) - V(N+i,5) = 0.
Also, the number of particles used in the analysis is given in MSTU(62).
CALL PYCLUS(NJET)
Purpose: to reconstruct an arbitrary number of jets using a cluster analysis method
based on particle momenta.
Three different distance measures are available, see section 15.3. The choice iscontrolled by MSTU(46). The distance scale djoin , above which two clusters may
not be joined, is normally given by PARU(44). In general, djoin may be varied
to describe different ‘jet-resolution powers’; the default value, 2.5 GeV, is fairly
well suited for e+ e− physics at 30–40 GeV. With the alternative mass distancemeasure, PARU(44) can be used to set the absolute maximum cluster mass, or
2
PARU(45) to set the scaled one, i.e. in y = m2 /Ecm
, where Ecm is the total invariant mass of the particles being considered.
It is possible to continue the cluster search from the configuration already found,with a new higher djoin scale, by selecting MSTU(48) properly. In MSTU(47) one
can also require a minimum number of jets to be reconstructed; combined withan artificially large djoin this can be used to reconstruct a predetermined numberof jets.
Which particles (or partons) are used in the analysis is determined by
the MSTU(41) value, whereas assumptions about particle masses is given by
MSTU(42). The parameters PARU(43) and PARU(48) regulate more technical details (for events at high energies and large multiplicities, however, the choice ofa larger PARU(43) may be necessary to obtain reasonable reconstruction times).
NJET : the number of clusters reconstructed.
= -1 : analysis not performed because event contained less than MSTU(47) (normally 1) particles, or analysis failed to reconstruct the requested number ofjets.
= -2 : remaining space in PYJETS (partly used as working area) not large enoughto allow analysis.
Remark: if the analysis does not fail, further information is found in MSTU(61) -443

MSTU(63) and PARU(61) - PARU(63). In particular, PARU(61) contains the invariant mass for the system analysed, i.e. the number used in determining the2denominator of y = m2 /Ecm
. PARU(62) gives the generalized thrust, i.e. the sumof (absolute values of) cluster momenta divided by the sum of particle momenta(roughly the same as multicity [Bra79]). PARU(63) gives the minimum distanced (in p⊥ or m) between two clusters in the final cluster configuration, 0 in caseof only one cluster.
Further, the lines N + 1 through N + NJET (N - NJET + 1 through N for
MSTU(43) = 2) in PYJETS will, after a call, contain the following information:
K(N+i,1) = 31;
K(N+i,2) = 97;
K(N+i,3) : i, the jet number, with the jets arranged in falling order of absolute
momentum;
K(N+i,4) : the number of particles assigned to jet i;
K(N+i,5) = 0;P(N+i,1) - P(N+i,5) : momentum, energy and invariant mass of jet i;
V(N+i,1) - V(N+i,5) = 0.
Also, for a particle which was used in the analysis, K(I,4)= i, where I is theparticle number and i the number of the jet it has been assigned to. Undecayed particles not used then have K(I,4) = 0. An exception is made for lines
with K(I,1) = 3 (which anyhow are not normally interesting for cluster search),where the colour-flow information stored in K(I,4) is left intact.
MSTU(3) is only set equal to the number of jets for positive NJET and MSTU(43)= 1.
CALL PYCELL(NJET)
Purpose: to provide a simpler cluster routine more in line with what is currently usedin the study of high-p⊥ collider events.
A detector is assumed to stretch in pseudorapidity between -PARU(51) and+PARU(51) and be segmented in MSTU(51) equally large η (pseudorapidity)
bins and MSTU(52) ϕ (azimuthal) bins. Transverse energy E⊥ for undecayed
entries are summed up in each bin. For MSTU(53) non-zero, the energy issmeared by calorimetric resolution effects, cell by cell. This is done according to a Gaussian √distribution; if MSTU(53) = 1 the standard deviation for theE⊥ , if MSTU(53) = 2 the standard deviation for the E is
E⊥ is PARU(55)×√
PARU(55)× E, E⊥ and E expressed in GeV. The Gaussian is cut off at 0 andat a factor PARU(56) times the correct E⊥ or E. Cells with an E⊥ below a given
threshold PARU(58) are removed from further consideration; by default PARU(58)= 0. and thus all cells are kept.
All bins with E⊥ >PARU(52) are taken to be possible initiators of jets, and aretried in falling E⊥ sequence to check
whether the total E⊥ summed over cells noq
more distant than PARU(54) in (∆η)2 + (∆ϕ)2 exceeds PARU(53). If so, thesecells define one jet, and are removed from further consideration. Contrary to
PYCLUS, not all particles need be assigned to jets. Which particles (or partons)
are used in the analysis is determined by the MSTU(41) value.
NJET : the number of jets reconstructed (may be 0).
= -2 : remaining space in PYJETS (partly used as working area) not large enoughto allow analysis.
Remark: the lines N + 1 through N + NJET (N - NJET + 1 through N for MSTU(43) =2) in PYJETS will, after a call, contain the following information:
K(N+i,1) = 31;444

K(N+i,2) = 98;
K(N+i,3) : i, the jet number, with the jets arranged in falling order in E⊥ ;K(N+i,4) : the number of particles assigned to jet i;
K(N+i,5) = 0;
V(N+i,1) - V(N+i,5) = 0.
Further, for MSTU(54) = 1
P(N+i,1), P(N+i,2) = position in η and ϕ of the center of the jet initiator cell,i.e. geometrical center of jet;P(N+i,3), P(N+i,4) = position in η and ϕ of the E⊥ -weighted center of the jet,i.e. the center of gravity of the jet;P(N+i,5) = sum E⊥ of the jet;while for MSTU(54) = 2
P(N+i,1) - P(N+i,5) : the jet momentum vector, constructed from thesummed E⊥ and the η and ϕ of the E⊥ -weighted center of the jet as(px , py , pz , E, m) = E⊥ (cos ϕ, sin ϕ, sinh η, cosh η, 0);and for MSTU(54) = 3
P(N+i,1) - P(N+i,5) : the jet momentum vector, constructed by adding vectorially the momentum of each cell assigned to the jet, assuming that all theE⊥ was deposited at the center of the cell, and with the jet mass in P(N+i,5)
calculated from the summed E and p as m2 = E 2 − p2x − p2y − p2z .
Also, the number of particles used in the analysis is given in MSTU(62), and thenumber of cells hit in MSTU(63).
MSTU(3) is only set equal to the number of jets for positive NJET and MSTU(43)= 1.
CALL PYJMAS(PMH,PML)
Purpose: to reconstruct high and low jet mass of an event. A simplified algorithm isused, wherein a preliminary division of the event into two hemispheres is donetransversely to the sphericity axis. Then one particle at a time is reassigned tothe other hemisphere if that reduces the sum of squares of the two jet masses,
m2H + m2L . The procedure is stopped when no further significant change (seePARU(48)) is obtained. Often, the original assignment is retained as it is. Whichparticles (or partons) are used in the analysis is determined by the MSTU(41)value, whereas assumptions about particle masses is given by MSTU(42).
PMH : heavy jet mass (in GeV).
= -2. : remaining space in PYJETS (partly used as working area) not large enoughto allow analysis.
PML : light jet mass (in GeV).
= -2. : as for PMH = -2.
Remark: After a successful call, MSTU(62) contains the number of particles used in theanalysis, and PARU(61) the invariant mass of the system analysed. The latter
number is helpful in constructing scaled jet masses.
CALL PYFOWO(H10,H20,H30,H40)
Purpose: to do an event analysis in terms of the Fox-Wolfram moments. The moments
Hi are normalized to the lowest one, H0 . Which particles (or partons) are usedin the analysis is determined by the MSTU(41) value.
H10 : H1 /H0 . Is = 0 if momentum is balanced.
H20 : H2 /H0 .
H30 : H3 /H0 .
445

H40 : H4 /H0 .
Remark: the number of particles used in the analysis is given in MSTU(62).
CALL PYTABU(MTABU)
Purpose: to provide a number of event-analysis options which can be be used on each new
event, with accumulated statistics to be written out on request. When errors arequoted, these refer to the uncertainty in the average value for the event sample
as a whole, rather than to the spread of the individual events, i.e. errors decrease
like one over the square root of the number of events analysed. For a correct use
of PYTABU, it is not permissible to freely mix generation and analysis of different
classes of events, since only one set of statistics counters exists. A single run may
still contain sequential ‘subruns’, between which statistics is reset. Whenever an
event is analysed, the number of particles/partons used is given in MSTU(62).
MTABU : determines which action is to be taken. Generally, a last digit equal to 0 indicates that the statistics counters for this option is to be reset; since the counters
are reset (by DATA statements) at the beginning of a run, this is not used normally. Last digit 1 leads to an analysis of current event with respect to thedesired properties. Note that the resulting action may depend on how the event
generated has been rotated, boosted or edited before this call. The statistics
accumulated is output in tabular form with last digit 2, while it is dumped in
the PYJETS common block for last digit 3. The latter option may be useful forinterfacing to graphics output.
Warning: this routine cannot be used on weighted events, i.e. in the statistics calculation
all events are assumed to come with the same weight.
= 10 : statistics on parton multiplicity is reset.
= 11 : the parton content of the current event is analysed, classified according to
the flavour content of the hard interaction and the total number of partons.
The flavour content is assumed given in MSTU(161) and MSTU(162); these
are automatically set e.g. in PYEEVT and PYEVNT calls. Main application isfor e+ e− annihilation events.
= 12 : gives a table on parton multiplicity distribution.
= 13 : stores the parton multiplicity distribution of events in PYJETS, using thefollowing format:
N = total number of different channels found;
K(I,1) = 32;
K(I,2) = 99;
K(I,3), K(I,4) = the two flavours of the flavour content;
K(I,5) = total number of events found with flavour content of K(I,3) andK(I,4);
P(I,1) - P(I,5) = relative probability to find given flavour content and a
total of 1, 2, 3, 4 or 5 partons, respectively;
V(I,1) - V(I,5) = relative probability to find given flavour content and a
total of 6–7, 8–10, 11–15, 16–25 or above 25 partons, respectively.
In addition, MSTU(3) = 1 andK(N+1,1) = 32;
K(N+1,2) = 99;
K(N+1,5) = number of events analysed.
= 20 : statistics on particle content is reset.
= 21 : the particle/parton content of the current event is analysed, also for particles
which have subsequently decayed and partons which have fragmented (unless
this has been made impossible by a preceding PYEDIT call). Particles are446

= 22 :
= 23 := 30 := 31 := 32 := 33 :

subdivided into primary and secondary ones, the main principle being thatprimary particles are those produced in the fragmentation of a string, whilesecondary come from decay of other particles.
gives a table of particle content in events.
stores particle content in events in PYJETS, using the following format:
N = number of different particle species found;
K(I,1) = 32;
K(I,2) = 99;
K(I,3) = particle KF code;
K(I,5) = total number of particles and antiparticles of this species;
P(I,1) = average number of primary particles per event;P(I,2) = average number of secondary particles per event;P(I,3) = average number of primary antiparticles per event;P(I,4) = average number of secondary antiparticles per event;P(I,5) = average total number of particles or antiparticles per event.
In addition, MSTU(3) = 1 andK(N+1,1) = 32;
K(N+1,2) = 99;
K(N+1,5) = number of events analysed;
P(N+1,1) = average primary multiplicity per event;P(N+1,2) = average final multiplicity per event;P(N+1,3) = average charged multiplicity per event.
statistics on factorial moments is reset.
analyses the factorial moments of the multiplicity distribution in different
bins of rapidity and azimuth. Which particles (or partons) are used in theanalysis is determined by the MSTU(41) value. The selection between usageof true rapidity, pion rapidity or pseudorapidity is regulated by MSTU(42).
The z axis is assumed to be event axis; if this is not desirable find an
event axis e.g. with PYSPHE or PYTHRU and use PYEDIT(31). Maximum(pion-, pseudo-) rapidity, which sets the limit for the rapidity plateau or theexperimental acceptance, is given by PARU(57).
prints a table of the first four factorial moments for various bins of pseudorapidity and azimuth. The moments are properly normalized so that theywould be unity (up to statistical fluctuations) for uniform and uncorrelatedparticle production according to Poisson statistics, but increasing for decreasing bin size in case of ‘intermittent’ behaviour. The error on the averagevalue is based on the actual statistical sample (i.e. does not use any assumptions on the distribution to relate errors to the average values of higher
moments). Note that for small bin sizes, where the average multiplicity issmall and the factorial moment therefore only very rarely is non-vanishing,
moment values may fluctuate wildly and the errors given may be too low.
stores the factorial moments in PYJETS, using the format:
N = 30, with I = i = 1–10 corresponding to results for slicing the rapidity
range in 2i−1 bins, I = i = 11–20 to slicing the azimuth in 2i−11 bins, and I= i = 21–30 to slicing both rapidity and azimuth, each in 2i−21 bins;
K(I,1) = 32;
K(I,2) = 99;
K(I,3) = number of bins in rapidity;
K(I,4) = number of bins in azimuth;
P(I,1) = rapidity bin size;
P(I,2) - P(I,5) = hF2 i–hF5 i, i.e. mean of second, third, fourth and fifth
factorial moment;
V(I,1) = azimuthal bin size;447

= 40 :
= 41 := 42 := 43 := 50 := 51 :

V(I,2) - V(I,5) = statistical errors on hF2 i–hF5 i.
In addition, MSTU(3) = 1 andK(31,1) = 32;
K(31,2) = 99;
K(31,5) = number of events analysed.
statistics on energy–energy correlation is reset.
the energy–energy correlation EEC of the current event is analysed. Whichparticles (or partons) are used in the analysis is determined by the MSTU(41)value. Events are assumed given in their c.m. frame. The weight assigned
2
to a pair i and j is 2Ei Ej /Evis, where Evis is the sum of energies of all analysed particles in the event. Energies are determined from the momenta ofparticles, with mass determined according to the MSTU(42) value. Statistics
is accumulated for the relative angle θij , ranging between 0 and 180 degrees,
subdivided into 50 bins.
prints a table of the energy–energy correlation EEC and its asymmetry
EECA, with errors. The definition of errors is not unique. In our approacheach event is viewed as one observation, i.e. an EEC and EECA distribution
is obtained by summing over all particle pairs of an event, and then the
average and spread of this event-distribution is calculated in the standard
fashion. The quoted error is therefore inversely proportional to the squareroot of the number of events. It could have been possible to view each single
particle pair as one observation, which would have given somewhat lower
errors, but then one would also be forced to do a complicated correction
procedure to account for the pairs in an event not being uncorrelated (twohard jets separated by a given angle typically corresponds to several pairs
at about that angle). Note, however, that in our approach the squared error
on an EECA bin is smaller than the sum of the squares of the errors on thecorresponding EEC bins (as it should be). Also note that it is not possible tocombine the errors of two nearby bins by hand from the information given,since nearby bins are correlated (again a trivial consequence of the presenceof jets).
stores the EEC and EECA in PYJETS, using the format:
N = 25;K(I,1) = 32;
K(I,2) = 99;
P(I,1) = EEC for angles between I-1 and I, in units of 3.6◦ ;P(I,2) = EEC for angles between 50-I and 51-I, in units of 3.6◦ ;P(I,3) = EECA for angles between I-1 and I, in units of 3.6◦ ;P(I,4), P(I,5) : lower and upper edge of angular range of bin I, expressed in radians;
V(I,1) - V(I,3) : errors on the EEC and EECA values stored in P(I,1)- P(I,3) (see = 42 for comments);
V(I,4), V(I,5) : lower and upper edge of angular range of bin I, expressed in degrees.
In addition, MSTU(3) = 1 andK(26,1) = 32;
K(26,2) = 99;
K(26,5) = number of events analysed.
statistics on complete final states is reset.
analyses the particle content of the final state of the current event record.
During the course of the run, statistics is thus accumulated on how often
different final states appear. Only final states with up to 8 particles areanalysed, and there is only reserved space for up to 200 different final states.
448

Most high-energy events have multiplicities far above 8, so the main use
for this tool is to study the effective branching ratios obtained with a given
decay model for e.g. charm or bottom hadrons. Then PY1ENT may be used
to generate one decaying particle at a time, with a subsequent analysis by
PYTABU. Depending on at what level this studied is to be carried out, someparticle decays may be switched off, like π 0 .
= 52 : gives a list of the (at most 200) channels with up to 8 particles in the
final state, with their relative branching ratio. The ordering is according tomultiplicity, and within each multiplicity according to an ascending orderof KF codes. The KF codes of the particles belonging to a given channel are
given in descending order.
= 53 : stores the final states and branching ratios found in PYJETS, using the format:
N = number of different explicit final states found (at most 200);K(I,1) = 32;
K(I,2) = 99;
K(I,5) = multiplicity of given final state, a number between 1 and 8;
P(I,1) - P(I,5), V(I,1) - V(I,3) : the KF codes of the up to 8 particles of the given final state, converted to real numbers, with trailing zeroes
for positions not used;
V(I,5) : effective branching ratio for the given final state.
In addition, MSTU(3) = 1 andK(N+1,1) = 32;
K(N+1,2) = 99;
K(N+1,5) = number of events analysed;
V(N+1,5) = summed branching ratio for finals states not given above, either because they contained more than 8 particles or because all 200 channels
have been used up.
COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
Purpose: to give access to a number of status codes and parameters which regulate theperformance of fragmentation and event analysis routines. Most parameters aredescribed in section 14.3; here only those related to the event-analysis routines
are described.
MSTU(41) : (D = 2) partons/particles used in the event-analysis routines PYSPHE,PYTHRU, PYCLUS, PYCELL, PYJMAS, PYFOWO and PYTABU (PYTABU(11) excepted).
= 1 : all partons/particles that have not fragmented/decayed.
= 2 : ditto, with the exception of neutrinos and unknown particles. Also thelowest-lying neutralino χ01 (code 1000022), the graviton (39) and the gravitino (1000039) are treated on an equal footing with neutrinos. Other similar
but not foreseen particles would not be disregarded automatically, but would
have to be put to K(I,1) > 10 by hand.
= 3 : only charged, stable particles, plus any partons still not fragmented.
MSTU(42) : (D = 2) assumed particle masses, used in calculating energies E 2 = p2 +m2 , as subsequently used in PYCLUS, PYJMAS and PYTABU (in the latter also forpseudorapidity, pion rapidity or true rapidity selection).
= 0 : all particles are assumed massless.
= 1 : all particles, except the photon, are assumed to have the charged pion mass.
= 2 : the true masses are used.
MSTU(43) : (D = 1) storing of event-analysis information (mainly jet axes), in PYSPHE,
449

PYTHRU, PYCLUS and PYCELL.
= 1 : stored after the event proper, in positions N + 1 through N + MSTU(3). If
several of the routines are used in succession, all but the latest information
is overwritten.
= 2 : stored with the event proper, i.e. at the end of the event listing, with N
updated accordingly. If several of the routines are used in succession, all theaxes determined are available.
MSTU(44) : (D = 4) is the number of the fastest (i.e. with largest momentum) particles
used to construct the (at most) 10 most promising starting configurations for thethrust axis determination.
MSTU(45) : (D = 2) is the number of different starting configurations above, which haveto converge to the same (best) value before this is accepted as the correct thrustaxis.
MSTU(46) : (D = 1) distance measure used for the joining of clusters in PYCLUS.
= 1 : dij , i.e. approximately relative transverse momentum. Anytime two clusters
have been joined, particles are reassigned to the cluster they now are closest
to. The distance cut-off djoin is stored in PARU(44).
= 2 : distance measure as in = 1, but particles are never reassigned to new jets.
= 3 : JADE distance measure yij , but with dimensions to correspond approximately to total invariant mass. Particles may never be reassigned between
clusters. The distance cut-off mmin is stored in PARU(44).
= 4 : as = 3, but a scaled JADE distance yij is used instead of mij . The distancecut-off ymin is stored in PARU(45).
= 5 : Durham distance measure ỹij , but with dimensions to correspond approximately to transverse momentum. Particles may never be reassigned between
clusters. The distance cut-off p⊥min is stored in PARU(44).
= 6 : as = 5, but a scaled Durham distance ỹij is used instead of p⊥ij . The distancecut-off ỹmin is stored in PARU(45).
MSTU(47) : (D = 1) the minimum number of clusters to be reconstructed by PYCLUS.
MSTU(48) : (D = 0) mode of operation of the PYCLUS routine.
= 0 : the cluster search is started from scratch.
= 1 : the clusters obtained in a previous cluster search on the same event (withMSTU(48) = 0) are to be taken as the starting point for subsequent cluster
joining. For this call to have any effect, the joining scale in PARU(44) or
PARU(45) must have been changed. If the event record has been modified
after the last PYCLUS call, or if any other cluster search parameter setting
has been changed, the subsequent result is unpredictable.
MSTU(51) : (D = 25) number of pseudorapidity bins that the range between -PARU(51)and +PARU(51) is divided into to define cell size for PYCELL.
MSTU(52) : (D = 24) number of azimuthal bins, used to define the cell size for PYCELL.
MSTU(53) : (D = 0) smearing of correct energy, imposed cell-by-cell in PYCELL, to simulate calorimeter resolution effects.
= 0 : no smearing.
= 1 : the transverse energy in a cell, E⊥ , is smeared√according to a Gaussian
distribution with standard deviation PARU(55)× E⊥ , where E⊥ is given in
GeV. The Gaussian is cut off so that 0 < E⊥smeared <PARU(56)×E⊥true .
= 2 : as = 1, but it is the energy E rather than the transverse energy E⊥ that issmeared.
MSTU(54) : (D = 1) form for presentation of information about reconstructed clusters
in PYCELL, as stored in PYJETS according to the MSTU(43) value.
= 1 : the P vector in each line contains η and ϕ for the geometric origin of the jet,η and ϕ for the weighted center of the jet, and jet E⊥ , respectively.
= 2 : the P vector in each line contains a massless four-vector giving the direction450

of the jet, obtained as(px , py , pz , E, m) = E⊥ (cos ϕ, sin ϕ, sinh η, cosh η, 0),
where η and ϕ give the weighted center of a jet and E⊥ its transverse energy.
= 3 : the P vector in each line contains a massive four-vector, obtained by adding
the massless four-vectors of all cells that form part of the jet, and calculating
the jet mass from m2 = E 2 − p2x − p2y − p2z . For each cell, the total E⊥ issummed up, and then translated into a massless four-vector assuming that
all the E⊥ was deposited in the center of the cell.
MSTU(61) : (I) first entry for storage of event-analysis information in last event analysedwith PYSPHE, PYTHRU, PYCLUS or PYCELL.
MSTU(62) : (R) number of particles/partons used in the last event analysis with PYSPHE,PYTHRU, PYCLUS, PYCELL, PYJMAS, PYFOWO or PYTABU.
MSTU(63) : (R) in a PYCLUS call, the number of preclusters constructed in order to speed
up analysis (should be equal to MSTU(62) if PARU(43) = 0.). In a PYCELL call,the number of cells hit.
MSTU(161), MSTU(162) : hard flavours involved in current event, as used in an analysis
with PYTABU(11). Either or both may be set 0, to indicate the presence of one ornone hard flavours in event. Is normally set by high-level routines, like PYEEVTor PYEVNT, but can also be set by you.
PARU(41) : (D = 2.) power of momentum-dependence in PYSPHE, default corresponds
to sphericity, = 1. to linear event measures.
PARU(42) : (D = 1.) power of momentum-dependence in PYTHRU, default corresponds
to thrust.
PARU(43) : (D = 0.25 GeV) maximum distance dinit allowed in PYCLUS when forming
starting clusters used to speed up reconstruction. The meaning of the parameteris in p⊥ for MSTU(46) ≤ 2 or ≥ 5 and in m else. If = 0., no preclustering is
obtained. If chosen too large, more joining may be generated at this stage thanis desirable. The main application is at high energies, where some speedup isimperative, and the small details are not so important anyway.
PARU(44) : (D = 2.5 GeV) maximum distance djoin , below which it is allowed to join
two clusters into one in PYCLUS. Is used for MSTU(46)≤ 3 and = 5, i.e. both forp⊥ and mass distance measure.
2
PARU(45) : (D = 0.05) maximum distance yjoin = m2 /Evisor ditto with m2 → p2⊥ , below
which it is allowed to join two clusters into one in PYCLUS for MSTU(46) = 4 or
6.
PARU(48) : (D = 0.0001) convergence criterion for thrust (in PYTHRU) or generalized
thrust (in PYCLUS), or relative change of m2H + m2L (in PYJMAS), i.e. when thevalue changes by less than this amount between two iterations the process is
stopped.
PARU(51) : (D = 2.5) defines maximum absolute pseudorapidity used for detector assumed in PYCELL.
PARU(52) : (D = 1.5 GeV) gives minimum E⊥ for a cell to be considered as a potential
jet initiator by PYCELL.
PARU(53) : (D = 7.0 GeV) gives minimum summed E⊥ for a collection of cells to be
accepted as a jet.
q
PARU(54) : (D = 1.) gives the maximum distance in R = (∆η)2 + (∆ϕ)2 from cell
initiator when grouping cells to check whether they qualify as a jet.
PARU(55) : (D = 0.5) when smearing the transverse energy (or energy, see MSTU(53))√
in PYCELL, √the calorimeter cell resolution is taken to be PARU(55)× E⊥ (orPARU(55)× E) for E⊥ (or E) in GeV.
PARU(56) : (D = 2.) maximum factor of upward fluctuation in transverse energy orenergy in a given cell when calorimeter resolution is included in PYCELL (see451

MSTU(53)).
PARU(57) : (D = 3.2) maximum rapidity (or pseudorapidity or pion rapidity, dependingon MSTU(42)) used in the factorial moments analysis in PYTABU.
PARU(58) : (D = 0. GeV) in a PYCELL call, cells with a transverse energy E⊥ below
PARP(58) are removed from further consideration. This may be used to represent
a threshold in an actual calorimeter, or may be chosen just to speed up thealgorithm in a high-multiplicity environment.
PARU(61) : (I) invariant mass W of a system analysed with PYCLUS or PYJMAS, withenergies calculated according to the MSTU(42) value.
PARU(62) : (R) the generalized thrust obtained after a successful PYCLUS call, i.e. ratioof summed cluster momenta and summed particle momenta.
PARU(63) : (R) the minimum distance d between two clusters in the final cluster configuration after a successful PYCLUS call; is 0 if only one cluster left.

15.6

Histograms

The GBOOK package was written in 1979, at a time when HBOOK [Bru87] was notavailable in Fortran 77. It has been used since as a small and simple histogramming
program. For this version of Pythia the program has been updated to run together
with Pythia in double precision. Only the one-dimensional histogram part has beenretained, and subroutine names have been changed to fit Pythia conventions. These
modified routines are now distributed together with Pythia. They would not be used forfinal graphics, but may be handy for simple checks, and are extensively used to provide
free-standing examples of analysis programs, to be found on the Pythia web page.
There is a maximum of 1000 histograms at your disposal, numbered in the range 1
to 1000. Before a histogram can be filled, space must be reserved (booked) for it, andhistogram information provided. Histogram contents are stored in a common block ofdimension 20000, in the order they are booked. Each booked histogram requires NX+28
numbers, where NX is the number of x bins and the 28 include limits, under/overflow andthe title. If you run out of space, the program can be recompiled with larger dimensions.
The histograms can be manipulated with a few routines. Histogram output is ‘line printer’
style, i.e. no graphics.
CALL PYBOOK(ID,TITLE,NX,XL,XU)
Purpose: to book a one-dimensional histogram.
ID :histogram number, integer between 1 and 1000.
TITLE : histogram title, at most 60 characters.
NX :number of bins in the histogram; integer between 1 and 100.
XL, XU : lower and upper bound, respectively, on the x range covered by the histogram.
CALL PYFILL(ID,X,W)
Purpose: to fill a one-dimensional histogram.
ID :histogram number.
X :x coordinate of point.
W :weight to be added in this point.
CALL PYFACT(ID,F)
Purpose: to rescale the contents of a histogram.
452

ID :
F :

histogram number.
rescaling factor, i.e. a factor that all bin contents (including overflow etc.) are
multiplied by.
Remark: a typical rescaling factor could be f = 1/(bin size * number of events) =NX/(XU-XL) * 1/(number of events).
CALL PYOPER(ID1,OPER,ID2,ID3,F1,F2)
Purpose: this is a general-purpose routine for editing one or several histograms, which
all are assumed to have the same number of bins. Operations are carried out bin
by bin, including overflow bins etc.
OPER: gives the type of operation to be carried out, a one-character string or a
CHARACTER*1 variable.
= ’+’, ’-’, ’*’, ’/’ : add, subtract, multiply or divide the contents in ID1 andID2 and put the result in ID3. F1 and F2, if not 1D0, give factors by which
the ID1 and ID2 bin contents are multiplied before the indicated operation.
(Division with vanishing bin content will give 0.)= ’A’, ’S’, ’L’ : for ’S’ the square root of the content in ID1 is taken (result 0 for
negative bin contents) and for ’L’ the 10-logarithm is taken (a nonpositivebin content is before that replaced by 0.8 times the smallest positive bincontent). Thereafter, in all three cases, the content is multiplied by F1 andadded with F2, and the result is placed in ID3. Thus ID2 is dummy in thesecases.
= ’M’ : intended for statistical analysis, bin-by-bin mean and standard deviation ofa variable, assuming that ID1 contains accumulated weights, ID2 accumulated weight*variable and ID3 accumulated weight*variable-squared. Afterwards ID2 willqcontain the mean values (= ID2/ID1) and ID3 the standard
deviations (= ID3/ID1 − (ID2/ID1)2 ). In the end, F1 multiplies ID1 (fornormalization purposes), while F2 is dummy.
ID1, ID2, ID3 : histogram numbers, used as described above.
F1, F2 : factors or offsets, used as described above.
CALL PYHIST
Purpose: to print all histograms that have been filled, and thereafter reset their bin
contents to 0.
CALL PYPLOT(ID)
Purpose: to print out a single histogram.
ID :histogram to be printed.
CALL PYNULL(ID)
Purpose: to reset all bin contents, including overflow etc., to 0.
ID :histogram to be reset.
CALL PYDUMP(MDUMP,LFN,NHI,IHI)

453

Purpose: to dump the contents of existing histograms on an external file, from which
they could be read in to another program.
MDUMP : the action to be taken.
= 1 : dump histograms, each with the first line giving histogram number and title,
the second the number of x bins and lower and upper limit, the third thetotal number of entries and under-, inside- and overflow, and subsequent ones
the bin contents grouped five per line. If NHI = 0 all existing histograms are
dumped and IHI is dummy, else the NHI histograms with numbers IHI(1)through IHI(NHI) are dumped.
= 2 : read in histograms dumped with MDUMP = 1 and book and fill histograms
according to this information. (With modest modifications this option could
instead be used to write the info to HBOOK/HPLOT format, or whatever.)
NHI and IHI are dummy.
= 3 : dump histogram contents in column style, where the first column contains
the x values (average of respective bin) of the first histogram, and subsequentcolumns the histogram contents. All histograms dumped this way must havethe same number of x bins, but it is not checked whether the x range isalso the same. If NHI = 0 all existing histograms are dumped and IHI isdummy, else the NHI histograms with numbers IHI(1) through IHI(NHI)
are dumped. A file written this way can be read e.g. by Gnuplot [Gnu99].
LFN : the file number to which the contents should be written. You must see to it that
this file is properly opened for write (since the definition of file names is platform
dependent).
NHI : number of histograms to be dumped; if 0 then all existing histograms are dumped.
IHI : array containing histogram numbers in the first NHI positions for NHI nonzero.
COMMON/PYBINS/IHIST(4),INDX(1000),BIN(20000)
Purpose: to contain all information on histograms.
IHIST(1) : (D = 1000) maximum allowed histogram number, i.e. dimension of the INDX
array.
IHIST(2) : (D = 20000) size of histogram storage, i.e. dimension of the BIN array.
IHIST(3) : (D = 55) maximum number of lines per page assumed for printing histograms. 18 lines are reserved for title, bin contents and statistics, while the rest
can be used for the histogram proper.
IHIST(4) : internal counter for space usage in the BIN array.
INDX : gives the initial address in BIN for each histogram. If this array is expanded, also
IHIST(1) should be changed.
BIN : gives bin contents and some further histogram information for the booked histograms. If this array is expanded, also IHIST(2) should be changed.

454

16

Summary and Outlook

A complete description of the Pythia program would have to cover four aspects:
1. the basic philosophy and principles underlying the programs;
2. the detailed physics scenarios implemented, with all the necessary compromises andapproximations;
3. the structure of the implementation, including program flow, internal variable names
and programming tricks; and4. the manual, which describes how to use the programs.
Of these aspects, the first has been dealt with in reasonable detail. The second is unevenly
covered: in depth for aspects which are not discussed anywhere else, more summarily forareas where separate up-to-date papers already exist. The third is not included at all, but‘left as an exercise’ for the reader, to figure out from the code itself. The fourth, finally,should be largely covered, although many further comments could have been made, inparticular about the interplay between different parts of the programs. Still, in the end,no manual, however complete, can substitute for ‘hands on’ experience.
The Pythia program is continuously being developed. We are aware of many shortcomings, some of which hopefully will be addressed in the future. No timetable is set up
for such future changes, however. After all, this is not a professionally maintained software product, but part of a small physics research project. Very often, developments ofthe programs have come about as a direct response to the evolution of the physics stage,i.e. experimental results and studies for future accelerators. Hopefully, the program willkeep on evolving in step with the new challenges opening up.
In the future, a radically new version of the program is required. Given the decisions
by the big laboratories and collaborations to discontinue the use of Fortran and instead
adopt C++, it is natural to attempt to move also event generators in that direction. The
Pythia 7 project got going in the beginning of 1998, and was an effort to reformulate theevent generation process in object oriented language [Lön99, Ber01]. However, after the
Herwig++ [Gie04] group decided to join in the development of a common administrative
structure, much of the then Pythia 7 went into this new ThePEG [Lön05] framework,
and work on the physics aspects stalled.
Pythia 8 is a clean new start, to provide a C++ successor to Pythia 6. In a returnto the traditional Pythia spirit, it is a completely standalone generator, although some
hooks for links to other programs will be provided. Work on Pythia 8 was begun fromscratch in September 2004, with a three-year ‘road map’ to produce a successor, but
initially heavily biased towards pp/pp physics and certainly far from complete in every
respect. No attempt will be made to carry over all options of the existing code; rather
the intent is to keep only the most recent and relevant physics models. The Pythia 6
and Pythia 8 versions therefore will coexist for several years, with the former rapidly
approaching a steady state, and the latter continuing to be developed further. Thus, it
is quite likely that the current manual will be the last major edition pertaining to theFortran code, with future Pythia 6 developments only documented in update notes.

455

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T. Sjöstrand, LU TP 80-3 (1980)[Sjö82]

T. Sjöstrand, Computer Physics Commun. 27 (1982) 243[Sjö83]

T. Sjöstrand, Computer Physics Commun. 28 (1983) 229[Sjö84]

T. Sjöstrand, Phys. Lett. 142B (1984) 420, Nucl. Phys. B248 (1984) 469[Sjö84a]

T. Sjöstrand, Z. Phys. C26 (1984) 93;
M. Bengtsson, T. Sjöstrand and M. van Zijl, Phys. Lett. B179 (1986) 164[Sjö85]

T. Sjöstrand, Phys. Lett. 157B (1985) 321;
M. Bengtsson, T. Sjöstrand and M. van Zijl, Z. Phys. C32 (1986) 67[Sjö86]

T. Sjöstrand, Computer Physics Commun. 39 (1986) 347[Sjö87]

T. Sjöstrand and M. Bengtsson, Computer Physics Commun. 43 (1987) 367[Sjö87a]

T. Sjöstrand and M. van Zijl, Phys. Rev. D36 (1987) 2019

[Sjö88]

T. Sjöstrand, Int. J. Mod. Phys. A3 (1988) 751[Sjö89]

T. Sjöstrand et al., in ‘Z physics at LEP 1’, eds. G. Altarelli, R. Kleiss and
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471

[Sjö92]

T. Sjöstrand, in ‘1991 CERN School of Computing’, ed. C. Verkerk, CERN
92-02 (Geneva, 1992), p. 227[Sjö92a]

T. Sjöstrand and P.M. Zerwas, in ‘e+ e− Collisions at 500 GeV: The Physics
Potential’, ed P.M. Zerwas, DESY 92-123 (Hamburg, 1992), Part A, p. 463;
T. Sjöstrand, in ‘Proceedings of the 1992 Workshops on High-Energy Physics
with Colliding Beams’, ed. J. Rogers, SLAC Report-428 (Stanford, 1993),
Vol. 2, p. 445;
V.A. Khoze and T. Sjöstrand, Phys. Lett. B328 (1994) 466[Sjö92b]

T. Sjöstrand, in ‘Physics at HERA’, eds. W Buchmüller and G. Ingelman(DESY, Hamburg, 1992), Vol. 3, p. 1405

[Sjö92c]

T. Sjöstrand, in ‘Workshop on Photon Radiation from Quarks’, ed. S.
Cartwright, CERN 92-04 (Geneva, 1992), p. 89 and p. 103[Sjö92d]

T. Sjöstrand, CERN-TH.6488/92 (1992)[Sjö94]

T. Sjöstrand, Computer Physics Commun. 82 (1994) 74[Sjö94a]

T. Sjöstrand and V.A. Khoze, Z. Phys. C62 (1994) 281, Phys. Rev. Lett. 72(1994) 28[Sjö01]

T. Sjöstrand, P. Edén, C. Friberg, L. Lönnblad, G. Miu, S. Mrenna and E.
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T. Sjöstrand, L. Lönnblad and S. Mrenna, LU TP 01–21 [hep-ph/0108264][Sjö03]

T. Sjöstrand and P.Z. Skands, Nucl. Phys. B659 (2003) 243 [hep-ph/0212264][Sjö03a]

T. Sjöstrand, L. Lönnblad, S. Mrenna, and P. Skands, LU TP 03–38, [hepph/0308153][Sjö04]

T. Sjöstrand and P.Z. Skands, JHEP 03 (2004) 053 [hep-ph/0402078][Sjö04a]

T. Sjöstrand and P.Z. Skands, Eur. Phys. J. C39 (2005) 129 [hep-ph/0408302][Ska01]

P.Z. Skands, Master’s Thesis, Niels Bohr Inst., Copenhagen University [hepph/0108207]; Eur. Phys. J. C23 (2002) 173 [hep-ph/0110137][Ska03]

P.Z. Skands et al., JHEP 07 (2004) 036 [hep-ph/0311123];
B.C. Allanach et al., in proceedings of the Workshop on Physics at TeV
Colliders, Les Houches, France, 26 May – 6 June 2003, hep-ph/0402295;
see also http://home.fnal.gov/∼skands/slha/[Ska05]P. Skands, T. Plehn and D. Rainwater, FERMILAB-CONF-05-519-T, [hepph/0511306][Skj93]A. Skjold and P. Osland, Phys. Lett. B311 (1993) 261, Nucl. Phys. B453
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K. Myklevoll, Master’s Thesis, Department of Physics, University of Bergen(2002)[Ste81]

P.M. Stevenson, Phys. Rev. D23 (1981) 2916

[Sud56]

V.V. Sudakov, Zh.E.T.F. 30 (1956) 87 (Sov. Phys. J.E.T.P. 30 (1956) 65)

472

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UA1 Collaboration, G. Arnison et al., Phys. Lett. 123B (1983) 115;
UA1 Collaboration, C. Albajar et al., Nucl. Phys. B309 (1988) 405[UA187]

UA1 Collaboration, C. Albajar, in Physics Simulations at High Energy, eds.
V. Barger, T. Gottschalk and F. Halzen (World Scientific, Singapore, 1987),
p. 39;UA1 Collaboration, A. Di Ciaccio, in Proceedings of the XVII International
Symposium on Multiparticle Dynamics, eds. M. Markytan, W. Majerotto and
J. MacNaughton (World Scientific, Singapore, 1987), p. 679[UA584]UA5 Collaboration, G.J. Alner et al., Phys. Lett. 138B (1984) 304;
UA5 Collaboration, R.E. Ansorge et al., Z. Phys. C43 (1989) 357[Ver81]

J.A.M. Vermaseren, K.J.F. Gaemers and S.J. Oldham, Nucl. Phys. B187
(1981) 301[Web86]

B.R. Webber, Ann. Rev. Nucl. Part. Sci. 36 (1986) 253[Wei79]S. Weinberg, Phys. Rev. D19 (1979) 1277;
L. Susskind, Phys. Rev. D20 (1979) 2619

[Wol02]

S. Wolf, private communication (2002)[Wu79]

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W.A. Zajc, Phys. Rev. D35 (1987) 3396

[Zhu83]R.-y. Zhu, Ph. D. Thesis (M.I.T.), MIT-LNS Report RX-1033 (1983); Caltech
Report CALT-68-1306; in Proceedings of the 1984 DPF conference, Santa Fe,
p. 229; in Proceedings of 1985 DPF conference, Oregon, p. 552

473

Appendix A: Subprocess Summary Table
This index is intended to give a quick reference to the different physics processes implemented in the program. Further details are to be found elsewhere in the manual,
especially in section 8. A trailing ’+’ on a few SUSY processes indicates inclusion of
charge-conjugate modes as well.
No.

Subprocess

Hard QCD processes:
11 fi fj → fi fj
12 fi f i → fk f k
13 fi f i → gg
28 fi g → fi g
53 gg → fk f k
68 gg → gg
Soft QCD processes:
91 elastic scattering
92 single diffraction (XB)
93 single diffraction (AX)94 double diffraction
95 low-p⊥ production
Prompt photons:
14 fi f i → gγ
18 fi f i → γγ
29 fi g → fi γ
114 gg → γγ
115 gg → gγ
Open heavy flavour:(also fourth generation)
81 fi f i → Qk Qk
82 gg → Qk Qk
83 qi fj → Qk fl
84 gγ → Qk Qk
85 γγ → Fk Fk

No. Subprocess

No. Subprocess

Closed heavy flavour:
86 gg → J/ψg
87 gg → χ0c g
88 gg → χ1c g
89 gg → χ2c g
104 gg → χ0c
105 gg → χ2c
106 gg → J/ψγ
107 gγ → J/ψg
108 γγ → J/ψγ
(1)421 gg → cc[3 S1 ] g(8)422 gg → cc[3 S1 ] g(8)423 gg → cc[1 S0 ] g(8)424 gg → cc[3 PJ ] g(8)425 gq → q cc[3 S1 ](8)
426 gq → q cc[1 S0 ](8)427 gq → q cc[3 PJ ](8)
428 qq → g cc[3 S1 ](8)
429 qq → g cc[1 S0 ](8)
430 qq → g cc[3 PJ ](1)
431 gg → cc[3 P0 ] g(1)432 gg → cc[3 P1 ] g(1)433 gg → cc[3 P2 ] g(1)434 gq → q cc[3 P0 ]

435
436
437
438
439
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479

474

(1)

gq → q cc[3 P1 ](1)
gq → q cc[3 P2 ](1)
qq → g cc[3 P0 ](1)
qq → g cc[3 P1 ](1)
qq → g cc[3 P2 ](1)
gg → bb[3 S1 ] g(8)gg → bb[3 S1 ] g(8)gg → bb[1 S0 ] g(8)gg → bb[3 PJ ] g(8)gq → q bb[3 S1 ](8)
gq → q bb[1 S0 ](8)
gq → q bb[3 PJ ](8)
qq → g bb[3 S1 ](8)
qq → g bb[1 S0 ](8)
qq → g bb[3 PJ ](1)
gg → bb[3 P0 ] g(1)
gg → bb[3 P1 ] g(1)gg → bb[3 P2 ] g(1)gq → q bb[3 P0 ](1)
gq → q bb[3 P1 ](1)
gq → q bb[3 P2 ](1)
qq → g bb[3 P0 ](1)
qq → g bb[3 P1 ](1)
qq → g bb[3 P2 ]

No.

Subprocess

Deeply Inel. Scatt.:
10 fi fj → fk fl
99 γ ∗ q → q
Photon-induced:
33 fi γ → fi g
34 fi γ → fi γ
54 gγ → fk f k58 γγ → fk f k
131 fi γT∗ → fi g
132 fi γL∗ → fi g
133 fi γT∗ → fi γ
134 fi γL∗ → fi γ
135 gγT∗ → fi f i
136 gγL∗ → fi f i
137 γT∗ γT∗ → fi f i
138 γT∗ γL∗ → fi f i
139 γL∗ γT∗ → fi f i
140 γL∗ γL∗ → fi f i
80 qi γ → qk π ±W/Z production:
1 fi f i → γ ∗ /Z0
2 fi f j → W ±22 fi f i → Z0 Z0
23 fi f j → Z0 W±
25 fi f i → W+ W−
15 fi f i → gZ0
16 fi f j → gW±
30 fi g → fi Z0
31 fi g → fk W±

No.
19
20
35
36
69
70

Subprocess
fi f i → γZ0
fi f j → γW±
fi γ → fi Z0
fi γ → fk W±
γγ → W+ W−
γW± → Z0 W±

Light SM Higgs:
3 fi f i → h0
24 fi f i → Z0 h0
26 fi f j → W± h0
32 fi g → fi h0
102 gg → h0
103 γγ → h0
110 fi f i → γh0
111 fi f i → gh0
112 fi g → fi h0
113 gg → gh0
121 gg → Qk Qk h0
122 qi qi → Qk Qk h0
123 fi fj → fi fj h0
124 fi fj → fk fl h0
Heavy SM Higgs:
5 Z0 Z0 → h0
8 W+ W− → h0
71 Z0L Z0L → Z0L Z0L
72 Z0L Z0L → WL+ WL−
73 Z0L WL± → Z0L WL±
76 WL+ WL− → Z0L Z0L
77 WL± WL± → WL± WL±

475

No. Subprocess
BSM Neutral Higgs:
151 fi f i → H0
152 gg → H0
153 γγ → H0
171 fi f i → Z0 H0
172 fi f j → W± H0
173 fi fj → fi fj H0
174 fi fj → fk fl H0
181 gg → Qk Qk H0
182 qi qi → Qk Qk H0
183 fi f i → gH0
184 fi g → fi H0
185 gg → gH0
156 fi f i → A0
157 gg → A0
158 γγ → A0
176 fi f i → Z0 A0
177 fi f j → W± A0
178 fi fj → fi fj A0
179 fi fj → fk fl A0
186 gg → Qk Qk A0
187 qi qi → Qk Qk A0
188 fi f i → gA0
189 fi g → fi A0
190 gg → gA0

No.

Subprocess

Charged Higgs:
143 fi f j → H+
161 fi g → fk H+
401 gg → tbH+
402 qq → tbH+
Higgs pairs:
297 fi f j → H± h0
298 fi f j → H± H0
299 fi f i → A0 h0
300 fi f i → A0 H0301 fi f i → H+ H−
New gauge bosons:141 fi f i → γ/Z0 /Z0 0
142 fi f j → W0 +144 fi f j → R
Leptoquarks:
145 qi `j → LQ
162 qg → `LQ
163 gg → LQ LQ
164 qi qi → LQ LQ

No. Subprocess

No. Subprocess

Technicolor:
149 gg → ηtc
191 fi f i → ρ0tc
192 fi f j → ρ+tc
0
193 fi f i → ωtc
194 fi f i → fk f k
195 fi f j → fk f l
361 fi f i → WL+ WL−∓
362 fi f i → WL± πtc+ −363 fi f i → πtc
πtc
0364 fi f i → γπtc
365 fi f i → γπ 0 0tc
0366 fi f i → Z0 πtc
367 fi f i → Z0 π 0 0tc∓
368 fi f i → W± πtc
370 fi f j → WL± Z0L
0371 fi f j → WL± πtc± 0
372 fi f j → πtc
ZL± 0373 fi f j → πtc πtc±
374 fi f j → γπtc±
375 fi f j → Z0 πtc
0376 fi f j → W± πtc
377 fi f j → W± π 0 0tc
381 qi qj → qi qj
382 qi qi → qk qk
383 qi qi → gg
384 fi g → fi g
385 gg → qk qk
386 gg → gg
387 fi f i → Qk Qk
388 gg → Qk Qk

Compositeness:
146 eγ → e∗
147 dg → d∗
148 ug → u∗
167 qi qj → d∗ qk
168 qi qj → u∗ qk
169 qi qi → e± e∗∓
165 fi f i (→ γ ∗ /Z0 ) → fk f k
166 fi f j (→ W± ) → fk f l

476

Left–right symmetry:
341 `i `j → H±±L342 `i `j → H±±R±∓343 `i γ → H±±L e±± ∓344 `±
i γ → HR e±± ∓345 `±
i γ → HL µ±± ∓346 `±
i γ → HR µ±± ∓347 `±
i γ → HL τ±± ∓348 `±
i γ → HR τ−−
349 fi f i → H++
L HL−−
350 fi f i → H++
R HR
351 fi fj → fk fl H±±
L
352 fi fj → fk fl H±±R
353 fi f i → Z0R±
354 fi f j → WR
Extra Dimensions:391 ff → G∗
392 gg → G∗
393 qq → gG∗
394 qg → qG∗
395 gg → gG∗

No.

Subprocess

SUSY:
201 fi f i → ẽL ẽ∗L
202 fi f i → ẽR ẽ∗R
203 fi f i → ẽL ẽ∗R +204 fi f i → µ̃L µ̃∗L
205 fi f i → µ̃R µ̃∗R
206 fi f i → µ̃L µ̃∗R +207 fi f i → τ̃1 τ̃1∗
208 fi f i → τ̃2 τ̃2∗
209 fi f i → τ̃1 τ̃2∗ +210 fi f j → `˜L ν̃`∗ +211 fi f j → τ̃1 ν̃τ∗ +212 fi f j → τ̃2 ν̃τ∗ +213 fi f i → ν˜` ν˜` ∗214 fi f i → ν̃τ ν̃τ∗
216 fi f i → χ̃1 χ̃1
217 fi f i → χ̃2 χ̃2
218 fi f i → χ̃3 χ̃3219 fi f i → χ̃4 χ̃4
220 fi f i → χ̃1 χ̃2
221 fi f i → χ̃1 χ̃3
222 fi f i → χ̃1 χ̃4
223 fi f i → χ̃2 χ̃3
224 fi f i → χ̃2 χ̃4
225 fi f i → χ̃3 χ̃4∓
226 fi f i → χ̃±1 χ̃1∓
227 fi f i → χ̃±2 χ̃2∓
228 fi f i → χ̃±1 χ̃2
229 fi f j → χ̃1 χ̃±
1

No.
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
246
247
248
249
250
251
252
253
254
256
258
259
261
262

Subprocess
fi f j → χ̃2 χ̃±
1
fi f j → χ̃3 χ̃±
1
fi f j → χ̃4 χ̃±
1
fi f j → χ̃1 χ̃±
2
fi f j → χ̃2 χ̃±
2fi f j → χ̃3 χ̃±
2fi f j → χ̃4 χ̃±
2fi f i → g̃χ̃1
fi f i → g̃χ̃2
fi f i → g̃χ̃3fi f i → g̃χ̃4
fi f j → g̃χ̃±
1
fi f j → g̃χ̃±2fi f i → g̃g̃
gg → g̃g̃
fi g → q̃iL χ̃1
fi g → q̃iR χ̃1
fi g → q̃iL χ̃2fi g → q̃iR χ̃2
fi g → q̃iL χ̃3fi g → q̃iR χ̃3fi g → q̃iL χ̃4fi g → q̃iR χ̃4fi g → q̃j L χ̃±1fi g → q̃j L χ̃±2fi g → q̃iL g̃
fi g → q̃iR g̃
fi f i → t̃1 t̃∗1
fi f i → t̃2 t̃∗2

477

No.
263
264
265
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296

Subprocess
fi f i → t̃1 t̃∗2 +gg → t̃1 t̃∗1
gg → t̃2 t̃∗2
fi fj → q̃iL q̃j L
fi fj → q̃iR q̃j R
fi fj → q̃iL q̃j R +fi f j → q̃iL q̃∗j L
fi f j → q̃iR q̃∗j R
fi f j → q̃iL q̃∗j R +fi f i → q̃j L q̃∗j L
fi f i → q̃j R q̃∗j R
gg → q̃iL q̃∗i L
gg → q̃iR q̃∗i R
bqi → b̃1 q̃iL
bqi → b̃2 q̃iR
bqi → b̃1 q̃iR + b̃2 q̃iL
bqi → b̃1 q̃∗i L
bqi → b̃2 q̃∗i R
bqi → b̃1 q̃∗i R + b̃2 q̃∗i L
fi f i → b̃1 b̃∗1
fi f i → b̃2 b̃∗2
gg → b̃1 b̃∗1
gg → b̃2 b̃∗2
bb → b̃1 b̃1bb → b̃2 b̃2bb → b̃1 b̃2
bg → b̃1 g̃
bg → b̃2 g̃
bb → b̃1 b̃∗2 +Appendix B: Index of Subprograms and CommonBlock Variables
This index is not intended to be complete, but gives the page where the main description begins of a subroutine, function, block data, common block, variable or array. For
common-block variables also the name of the common block is given. When some components of an array are described in a separate place, a special reference (indented withrespect to the main one) is given for these components.
AQCDUP in HEPEUP
AQEDUP in HEPEUP
BRAT in PYDAT3
CHAF in PYDAT4CKIN in PYSUBS
COEF in PYINT2
EBMUP in HEPRUPHEPEUP common block
HEPEVT common block
HEPRUP common block
ICOL in PYINT2ICOLUP in HEPEUPIDBMUP in HEPRUPIDPRUP in HEPEUP
IDUP in HEPEUPIDWTUP in HEPRUPIMSS in PYMSSMISET in PYINT2
ISIG in PYINT3
ISTUP in HEPEUP
ITCM in PYTCSM
K in PYJETSKCHG in PYDAT2
KFDP in PYDAT3
KFIN in PYSUBSKFPR in PYINT2
LPRUP in HEPRUPMAXNUP in HEPEUPMAXPUP in HEPRUPMDCY in PYDAT3MDME in PYDAT3MINT in PYINT1MOTHUP in HEPEUP
MRPY in PYDATR
MSEL in PYSUBS

252
251
420
420
189
280
245
251
67
244
281
254
245
251
252
245
223
280
281
252
221
60
414
420
188
280
250
251
245
417
417
271
253
49
187

MSTI in PYPARSMSTJ in PYDAT1, main
MSTJ(38) - MSTJ(50)MSTJ(101) - MSTJ(121)
MSTP in PYPARS, main
MSTP(22)
MSTP(61) - MSTP(72)MSTP(81) - MSTP(95)MSTP(131) - MSTP(134)MSTU in PYDAT1, main
MSTU(1) and some more
MSTU(41) - MSTU(63)MSTU(101) - MSTU(118)MSTU(161) - MSTU(162)
MSUB in PYSUBS
MWID in PYINT4
N in PYJETSNGEN in PYINT5NPRUP in HEPRUP
NUP in HEPEUP
P in PYJETSPARF in PYDAT2PARI in PYPARSPARJ in PYDAT1, main
PARJ(80) - PARJ(90)PARJ(121) - PARJ(171)
PARJ(180) - PARJ(195)
PARP in PYPARS, main
PARP(61) - PARP(72)
PARP(78) - PARP(100)
PARP(131)
PARU in PYDAT1, main
PARU(41) - PARU(63)PARU(101) - PARU(195)
PDFGUP in HEPRUP

478

229
401
316
82
193
320
321
347
352
397
431
449
218
451
188
282
60
283
249
251
62
415
231
406
320
85
221
213
323
353
355
400
451
218
245

PDFSUP in HEPRUP
PMAS in PYDAT2
PROC in PYINT6
PUP in HEPEUP
PY1ENT subroutine
PY2ENT subroutine
PY3ENT subroutine
PY4ENT subroutine
PY2FRM subroutine
PY4FRM subroutine
PY6FRM subroutine
PY4JET subroutine
PYADSH function
PYALEM function
PYALPS function
PYANGL function
PYBINS common block
PYBOEI subroutine
PYBOOK subroutine
PYCBLS common block
PYCELL subroutine
PYCHGE functionPYCKBD function
PYCLUS subroutine
PYCOMP function
PYCTAG common block
PYCTTR subroutine
PYDAT1 common block
PYDAT2 common block
PYDAT3 common block
PYDAT4 common block
PYDATA block data
PYDATR common block
PYDCYK subroutine
PYDECY subroutine
PYDIFF subroutine
PYDISG subroutine
PYDOCU subroutine
PYDUMP subroutine
PYEDIT subroutine
PYEEVT subroutine
PYERRM subroutine

245
415
283
254
392
392
393
393
264
265
266
267
315
217
217
396
454
396
452
357
444
396
271
443
396
358
356
397
414
416
420
271
49
412
395
269
269
269
453
426
80
396

PYEVNT subroutine
PYEVOL subroutine
PYEVWT subroutine
PYEXEC subroutine
PYFACT subroutine
PYFCMP function
PYFILL subroutine
PYFOWO subroutine
PYFRAM subroutine
PYGAGA subroutine
PYGAMM function
PYGANO function
PYGBEH function
PYGDIR function
PYGGAM function
PYGIVE subroutine
PYGVMD function
PYHEPC subroutine
PYHFTH function
PYHIST subroutine
PYI3AU subroutine
PYINBM subroutine
PYINDF subroutine
PYINIT subroutine
PYINKI subroutine
PYINPR subroutine
PYINRE subroutine
PYINT1 common block
PYINT2 common block
PYINT3 common block
PYINT4 common block
PYINT5 common block
PYINT6 common block
PYINT7 common block
PYINT8 common block
PYINT9 common block
PYINTM common block
PYISJN common block
PYISMX common block
PYJETS common block
PYJMAS subroutine
PYJOIN subroutine

479

183
355
236
395
452
356
452
445
185
269
270
270
270
270
270
394
270
68
270
453
271
268
395
181
268
268
268
271
280
281
282
283
283
283
284
284
356
358
358
59
445
393

PYK functionPYKCUT subroutine
PYKFDI subroutine
PYKFIN subroutine
PYKLIM subroutine
PYKMAP subroutine
PYLIST subroutine
PYLOGO subroutine
PYMAEL function
PYMASS function
PYMAXI subroutine
PYMEMX subroutine
PYMEWT subroutine
PYMIGN subroutine
PYMIHG subroutine
PYMIHK subroutine
PYMIRM subroutine
PYMRUN function
PYMSRV common block
PYMSSM common block
PYMULT subroutine
PYNAME subroutine
PYNMES subroutine
PYNULL subroutine
PYOPER subroutine
PYOFSH subroutine
PYP function
PYPARS common block
PYPART common block
PYPCMP function
PYPDEL subroutine
PYPDFL subroutine
PYPDFU subroutine
PYPDGA subroutine
PYPDPI subroutine
PYPDPR subroutine
PYPILE subroutine
PYPLOT subroutine
PYPREP subroutine
PYONIA subroutine
PYPTDI subroutine
PYPTFS subroutine

429
185
395
412
269
269
428
396
315
396
268
316
316
355
356
356
356
217
228
223
355
396
412
453
453
269
430
193, 229
324
356
270
270
270
270
270
270
269
453
395
81
395
314

PYPTIS subroutine
PYPTMI subroutine
PYQQBH subroutine
PYR function
PYRGET subroutine
PYRSET subroutine
PYRADK subroutine
PYRAND subroutine
PYRECO subroutine
PYREMN subroutine
PYRESD subroutine
PYROBO subroutine
PYSAVE subroutine
PYSCAT subroutine
PYSHOW subroutine
PYSIGH subroutine
PYSLHA subroutine
PYSPEN function
PYSPHE subroutine
PYSPLI subroutine
PYSSMT common block
PYSSPA subroutine
PYSTAT subroutine
PYSTRF subroutine
PYSUBS common block
PYTABU subroutine
PYTAUD subroutine
PYTCSM common block
PYTEST subroutine
PYTHRU subroutine
PYTIME subroutine
PYUPDA subroutine
PYUPEV subroutine
PYUPIN subroutine
PYUPRE subroutine
PYWAUX subroutine
PYWIDT subroutine
PYX3JT subroutine
PYX4JT subroutine
PYXDIF subroutine
PYXJET subroutine
PYXKFL subroutine

480

315
356
271
48
49
49
81
269
271
355
269
426
269
269
313
269
414
271
442
270
228
315
183
395
187
446
396
221
29
442
396
413
260
260
255
271
269
81
81
81
81
81

PYXTEE subroutine
PYXTOT subroutine
PYZDIS subroutine
RMSS in PYMSSMRRPY in PYDATR
RTCM in PYTCSMRVLAM in PYMSRVRVLAMB in PYMSRVRVLAMP in PYMSRVSCALUP in HEPEUPSFMIX in PYSSMTSIGH in PYINT3
SIGT in PYINT7
SMW in PYSSMTSMZ in PYSSMTSPINUP in HEPEUP
UMIX in PYSSMTUMIXI in PYSSMTUPEVNT subroutine
UPINIT subroutine
UPVETO subroutine
V in PYJETSVCKM in PYDAT2

81
268
396
226
50
222
228
228
228
251
228
281
284
228
228
255
228
228
250
244
239
62
416

VINT in PYINT1VMIX in PYSSMTVMIXI in PYSSMTVTIMUP in HEPEUPVXPANH in PYINT9VXPANL in PYINT9VXPDGM in PYINT9VXPVMD in PYINT9WIDS in PYINT4XERRUP in HEPRUPXMAXUP in HEPRUP
XPANH in PYINT8XPANL in PYINT8
XPBEH in PYINT8
XPDIR in PYINT8
XPVMD in PYINT8XSEC in PYINT5XSECUP in HEPRUPXSFX in PYINT3XWGTUP in HEPEUPZMIX in PYSSMTZMIXI in PYSSMT

481

275
228
228
255
284
284
284
284
282
250
250
284
284
284
284
284
283
250
281
251
228
228