# Presentation: 565097

## Pythia Mini-Tutorial 2Advanced Topics

- Carleton U, May 2007

- Pythia Mini-Tutorial 2Advanced Topics

- Peter Skands
- Fermilab / Particle Physics Division / Theoretical Physics

- Sjöstrand, PS : NPB659(2003)243, JHEP03(2004)053, EPJC39(2005)129

## Overview

- Matching
- When needed?
- MLM, CKKW, MC@NLO, and all that
- The Underlying Event
- Models
- The ‘Old’ PYTHIA model (e.g. Tune A): impact parameter dependence
- The ‘Intermediate’ PYTHIA model: fancy beam remnants
- The ‘New’ PYTHIA model: interleaved evolution
- Not an expert on HERWIG/JIMMY, but I can still tell you as much as I know
- What is still not there?
- Tuning
- Early constraints from LHC
- The Event Generator Outlook
- The move to C++

- Event Generator Status

- Peter Skands

## Matching

- Fixed Order Matrix Elements and Parton Shower Resummations

## A Problem

- The best of both worlds? We want:
- A description which accurately predicts hard additional jets
- + jet structure and the effects of multiple soft emissions
- How to do it?
- Compute emission rates by parton showering (PS)?
- Misses relevant terms for hard jets, rates only correct for strongly ordered emissions pT1  >> pT2 >> pT3 ...
- (common misconception that showers are soft, but that need not be the case. They can err on *either side* of the right answer.)
- Unknown contributions from higher logarithmic orders
- Compute emission rates with matrix elements (ME)?
- Misses relevant terms for soft/collinear emissions, rates only correct for well-separated individual partons
- Quickly becomes intractable beyond one loop and a handfull of legs
- Unknown contributions from higher fixed orders

- Event Generator Status

- Peter Skands

## Double Counting

- Combine different multiplicites  *inclusive *sample?
- In practice – Combine
- [X]ME + showering
- [X + 1 jet]ME + showering
- ...
-  Double Counting:
- [X]ME + showering produces some X + jet configurations
- The result is X + jet in the shower approximation
- If we now add the *complete* [X + jet]ME *as well*
- the total rate of X+jet  is now _approximate + exact_ ~ double !!
- some configurations are generated *twice*.
- and the total inclusive cross section is also not well defined
- When going to X, X+j, X+2j, X+3j, etc, this problem gets worse

****

- Event Generator Status

- Peter Skands

## Matching

- Matching of up to one hard additional jet, for specific processes
- PYTHIA-style (reweight shower: MEX+jet = w*PS)
- HERWIG-style (add separate X+jet events: w = MEX+jet-PS)
- MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO)
- Matching of generic (multijet) topologies (at tree level)
- ALPGEN-style (MLM)
- SHERPA-style (CKKW)
- ARIADNE-style (Lönnblad-CKKW)
- PATRIOT-style (Mrenna & Richardson)
- Brand new approaches (still in the oven)
- Refinements of MC@NLO (Frixione, Nason, Oleari)
- CKKW-style at NLO + “Quantum Monte Carlo” (Nagy, Soper)
- SCET approach (based on SCET – Bauer, Tackmann; Alwall, Mrenna, Schwarz)
- VINCIA (based on QCD antennae – Giele, Kosower, PS)

- ME: Matrix Element
- PS: Parton Shower

- Event Generator Status

- Peter Skands

## MC@NLO

- Nason’s approach:
- Generate 1st shower emission separately  easier matching
- Avoid negative weights + explicit study of ZZ production

- Frixione, Nason, Webber, JHEP 0206(2002)029 and 0308(2003)007

- JHEP 0411(2004)040

- JHEP 0608(2006)077

- MC@NLO in comparison
- Superior precision for total cross section
- Equivalent to tree-level matching for event shapes (differences higher order)
- Inferior to multi-jet matching for multijet topologies
- So far has been using HERWIG parton shower  complicated subtractions

- Event Generator Status

- Peter Skands

## SHERPA and ARIADNE

- The CKKW algorithm
- _Slices_ phase space – two regions:
- uses matrix elements to describe the distribution of particles with a phase-space separation pT > pTcut
- uses parton showers to describe particles with a smaller separation
- [W]ME |pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
- [W + j]ME|pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
- ...
- Where Wveto < 1 is there to get rid of the double counting
- Wveto is : the probability that no parton shower emission happened above pTcut (this is called the Sudakov factor, the no-emission probability)
- SHERPA uses an approximate analytical formula
- Lönnblad’s ARIADNE-style is to run a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above pTcut
- This gets rid of double counting since those events that *would* have caused it are precisely those which *do* branch above pTcut

- L. L ̈onnblad, JHEP05 (2002) 046

- Event Generator Status

- Peter Skands

## Matched Mix of W+0,1,2,3,4 jets

- S. Mrenna, P. Richardson, JHEP0405 (2004) 040

- Matching can also be done with PYTHIA, HERWIG, but so far not automated

- Event Generator Status

- Peter Skands

## ALPGEN

- “MLM” matching (Mangano)
- Simpler but similar in spirit to CKKW
- First generate events the “stupid” way:
- [W]ME + showering
- [W + jet]ME + showering
- ...
- a set of fully showered events, *with* double counting. To get rid of the excess, accept/reject each event based on:
- (cone-)cluster showered event  njets
- match partons from the ME to the clustered jets
- If all partons are matched, keep event. Else discard it.
- Roughly equivalent to the pseudoshower approach above
- Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed

- Event Generator Status

- Peter Skands

## New Approaches – Why Bother?

- Used to think it was impossible! 
- But complicated  much work needed for each process 
- “Only” gets first jet right (rest is PS) 
- Hardwired to HERWIG
- CKKW & MLM:
- Best approach when multiple hard jets important.
- Relatively straightforward (but still very time-consuming)
- Retains LO normalization 
- Dependence on matching scale 

** ****All constructed to use existing showers (HW or PY) ******** hard to trace analytically**
**Not easy to control theoretical uncertainty on exponentiated part ******

- For more, see theory seminar tomorrow

- Event Generator Status

- Peter Skands

## The Underlying Event

- Towards a complete picture of hadron collisions

## Domain of fixed order and parton shower calculations: hard partonic scattering, and bremsstrahlung associated with it.

But hadrons are not elementary 
+ QCD diverges at low pT 
 multiple perturbative parton-parton collisions should occur

Normally omitted in explicit perturbative expansions

+ Remnants from the incoming beams
+ additional (non-perturbative / collective) phenomena?
Bose-Einstein Correlations
Non-perturbative gluon exchanges / colour reconnections ?
String-string interactions / collective multi-string effects ?
Interactions with “background” vacuum / with remnants / with active medium?

- But hadrons are not elementary
- + QCD diverges at low pT
-  multiple _perturbative_ parton-parton collisions should occur
- Normally omitted in explicit perturbative expansions
- + Remnants from the incoming beams
- + additional (non-perturbative / collective) phenomena?
- Bose-Einstein Correlations
- Non-perturbative gluon exchanges / colour reconnections ?
- String-string interactions / collective multi-string effects ?
- Interactions with “background” vacuum / with remnants / with active medium?

- e.g. 44, 3 3, 32

- Additional Sources of Particle Production

- Event Generator Status

- Peter Skands

## Classic Example: Number of tracks

- UA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias events

- Simple physics models ~ Poisson
- Can ‘tune’ to get _average_ right, but much too small _fluctuations_
-   inadequate physics model

- Morale (will return to the models later):
- It is not possible to ‘tune’ anything better than the underlying physics model allows
- Failure of a physically motivated model usually points to more physics

- Event Generator Status

- Peter Skands

## Multiple Interactions  Balancing Minijets

- Look for additional balancing jet pairs “under” the hard interaction.
- Several studies performed, most recently by Rick Field at CDF  ‘lumpiness’ in the underlying event.

-  (Run I)

- angle between 2 ‘best-balancing’ pairs

- CDF, PRD 56 (1997) 3811

- Event Generator Status

- Peter Skands

## Basic Physics

- Sjöstrand and van Zijl (1987):
- First serious model for the underlying event
- Based on resummation of _perturbative_ QCD 22 scatterings at successively smaller scales  multiple parton-parton interactions
- Dependence on impact parameter crucial to explain Nch distributions.
- Peripheral collisions  little matter overlap  few interactions. Central collisions  many
- Nch Poissonian for each impact parameter convolution with impact parameter profile  wider than Poissonian!
- Colour correlations also essential
- Determine between which partons hadronizing strings form (each string  log(mstring) hadrons)
- Important ambiguity: what determines how strings form *between* the different interactions?

- UA5
**N****ch**
- 540 GeV

- Event Generator Status

- Peter Skands

## Underlying Event and Colour

- In PYTHIA (up to 6.2), some “theoretically sensible” default values for the colour correlation parameters had been chosen
- Rick Field (CDF) noted that the default model produced too soft charged-particle spectra.

- (The same is seen at RHIC:)
- For ‘Tune A’ etc, Rick noted that <pT> increased when he increased the colour correlation parameters
- Virtually all ‘tunes’ now used by the Tevatron and LHC experiments employ these more ‘extreme’ correlations
- Tune A, and hence its more extreme colour correlations are now the default in PYTHIA

- M. Heinz (STAR), nucl-ex/0606020; nucl-ex/0607033

**STAR**
**pp @ 200GeV**

- (will return to this tomorrow...)

- Event Generator Status

- Peter Skands

## The ‘Intermediate’ Model

- Meanwhile in Lund: Sjöstrand and PS (2003):
- Further developments on the multiple-interactions idea
- First serious attempt at constructing multi-parton densitities
- If sea quark kicked out, “companion” antiquark introduced in remnant (distribution derived from gluon PDF and gluon splitting kernel)
- If valence quark kicked out, remaining valence content reduced
- Introduction of “string junctions” to represent beam baryon number
- Detailed hadronization model for junction fragmentation  can address baryon number flow separately from valence quarks

- Sjöstrand & PS  : Nucl.Phys.B659(2003)243, JHEP03(2004)053

- Event Generator Status

- Peter Skands

## The ‘New’ Model

- Sjöstrand and PS (2005):
- ‘Interleaved’ evolution of multiple interactions and parton showers

- Sjöstrand & PS : JHEP03(2004)053, EPJC39(2005)129

- multiparton
- PDFs derived
- from sum rules

- Beam remnants
- Fermi motion /
- primordial kT

- Fixed order
- matrix elements

- pT-ordered
- parton shower
- (matched to ME
- for W/Z/H/G + jet)

- perturbative
- “intertwining”?

- NB: Tune A still default since more thoroughly tested. To use new models, see e.g. PYTUNE (Pythia6.408+)

- Event Generator Status

- Peter Skands

## A First Study

- Using Tevatron min-bias as constraint
- Those were the distributions that started it all
- High-multiplicity tail should be somewhat similar to top  less extrapolation required
- Why not use LEP? Again, since the extrapolation might not be valid.
- No UE in ee, no beam remnants, less strings, no ‘bags’ in initial state.
- The comparison would still be interesting and should be included in a future study
- As a baseline, all models were tuned to describe Nch and <pT>(Nch)

- No CR

- Field’s Tunes & new models

- Improved Description of Min-Bias
- Effect _Still_ largely uncertain
- Worthwhile to look at top etc

- Tevatron Run II min-bias

- PYTHIA 6.408

- PYTHIA 6.408

- Event Generator Status

- Peter Skands

## (Beam Remnants and Multiple Interactions)

- Event Generator Status

- Peter Skands

## The Generator Outlook

- The C++ Monte Carlos

## C++ Players

**HERWIG++: complete reimplementation**
- Improved parton shower and decay algorithms
- Eventually to include CKKW-style matching (?)
- B.R. Webber; S. Gieseke, D. Grellscheid, A. Ribon, P. Richardson, M. Seymour, P. Stephens, . . .
**SHERPA: complete implementation, has CKKW**
- ME generator + wrappers to / adaptations of PYTHIA, HERWIG parton showers, underlying event, hadronization
- F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter
**PYTHIA8: selective reimplementation**
- Improved parton shower and underlying event, limited number of hard subprocesses
- Many obsolete features not carried over  simpler, less parameters
- T. Sjöstrand, S. Mrenna, P. Skands
- (+ various more specialized packages)

- Event Generator Status

- Peter Skands

## PYTHIA 8

**Basic generator already there**
**Includes a few processes (+ full Pythia6 library), new p****T****-ordered showers, new UE, Les Houches interfaces, and more**
**You are invited to try it out **
- Click _/future/_ on the Pythia homepage, download pythia8080.tgz, follow instructions in readme (./configure, ./make, and have fun)
- Still not advised for production runs
- If you have suggestions, *now* is the time!
_**Timeline:**_
- Spring 2007: QED showers, LHAPDF, interleaved FSR, beam remnants, colour reconnections  useful
- Fall-Winter 2007: resonance decays, GUI, **official release?**

- Event Generator Status

- Peter Skands

## The Generator Outlook

- Generators in state of continuous development:
- Better & more user-friendly general-purpose matrix element calculators+integrators
- Improved parton showers and improved matching to matrix elements
- Improved models for underlying events / minimum bias
- Upgrades of hadronization and decays
- Moving to C++
- Data needed to constrain models & rule out crazy ideas
- New methods  could QCD become a precision science?
- Important for virtually all other measurements + can shed light on fundamental & interesting aspects of QCD (e.g. string interactions)

- Event Generator Status

- Peter Skands