# Presentation: 367773

## Event Generator Status

- Peter Skands

-  Matching (who’s doing it, how, and where?)
-  New ideas and C++

## Traditional Event Generators

- Basic aim: improve lowest order perturbation theory by including leading corrections  exclusive event samples
**sequential resonance decays**
**parton showers**
**underlying event**
**hadronization**
**hadron and tau decays**

- Helper: I’ll try to avoid acronyms, but this list will be on all slides and may help when I fail. It is ordered alphabetically.

- Event Generator Status

- Peter Skands

## Modern Event Generators

- Specialized tools for calculating higher fixed orders (and BSM processes) plus **matching techniques**
- hard subprocess (and to some extent resonance decays) increasingly handled by separate codes (LO ... NnLO)
- Need universal interfaces and standards
- [e.g. the Les Houches Accords **(Les Houches 2007: Jun 11-29, France) **]
_**Beyond fixed order**_

**MC4LHC** `06: “A standard format for Les Houches Event Files” - **hep-ph/0609017**

- Better understanding of PS uncertainties – À LA ERROR PDF’S?
- Improved PS formulations – MORE CONSISTENT, MATCHING TO NnLO, RESUMMATION OF HIGHER LOGS & SMALL-X EFFECTS (BFKL), ...
- Better understanding of the underlying event and non-perturbative effects - ESPECIALLY IN THE BUSY ENVIRONMENT OFFERED BY LHC

-  Entering era of precision event generators for hadron colliders

- Event Generator Status

- Peter Skands

## Matching

- Matching of up to one hard additional jet
- PYTHIA-style (reweight shower)
- HERWIG-style (add separate events from ME: weight = ME-PS)
- MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO)
- Matching of generic (multijet) topologies:
- 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 (Nason)
- CKKW-style at NLO (Nagy, Soper)
- SCET approach (based on SCET – Bauer, Schwarz, SEE BAUER’S TALK ON FRIDAY!)
- VINCIA (based on QCD antennae – Giele, Kosower, PS, THIS TALK)

- Event Generator Status

- Peter Skands

## ALPGEN

- “MLM” matching (Mangano)
- Simpler but similar in spirit to CKKW
- First generate events the “stupid” way:
- [Wn]ME + showering
- [Wn+1]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.
- Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed

- n inclusive

- n+1 inclusive

- n+2 inclusive

- Event Generator Status

- Peter Skands

## SHERPA, ARIADNE, PATRIOT

- The CKKW algorithm
- _Slices_ phase space :
- ME for pT > pTcut
- PS for pT < pTcut
- [Wn]ME |pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
- [Wn+1]ME|pT>pTcut * Wveto(pTcut) + showeringpT<pTcut
- ... [Wnmax] ME|pT>pTcut + showering
- Wveto < 1 is the probability that no parton shower emission happened above pTcut. Computed using clustered ‘parton shower histories’ on the ME final states. SHERPA uses an approximate analytical formula
- Lönnblad’s ARIADNE-style uses a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above pTcut
- Gets rid of double counting: those events that *would* have caused it are precisely those which *do* branch above pTcut
- A final improvement by Mrenna and Richardson was to require physical flavour and colour flows in the ‘preclustering’ step  PATRIOT database (and HERWIG++?)

- L. L ̈onnblad, JHEP05 (2002) 046

- S. Mrenna, P. Richardson, JHEP 0405(2004)040

- 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

- Event Generator Status

- Peter Skands

## VINCIA

- _VINCIA Dipole shower_
- C++ code for gluon showers – running
- Can evolve in either of 2 different shower evolution variables:
- pT-ordering (~ ARIADNE)
- Virtuality-ordering (~ Pythia 6.2 & SHERPA)
- + not hard to generalize to arbitrary IR safe
- For _each evolution variable: _
- an infinite family of radiation  functions implemented, all with correct collinear and soft behaviour (= “antenna functions”)

- First parton shower with systematic possibility of variation of shower variable and shower functon  control uncertainties
- (Future plans include also variation of kinematic map)
- To any fixed order, these variations can be absorbed by  a new type of matching to matrix elements

- Dipoles – a dual description of QCD

- 1

- 3

- 2

- virtual numerical collider with interlinked antennae

- Giele, Kosower, PS : in progress

- Event Generator Status

- Peter Skands

## Subtraction method: subtract showering off n-parton state from n+1-parton Matrix Element  subtracted (IR finite) matrix elements. 
Generate parton-level configurations, with weights given by the subtracted Matrix Elements
Shower them using the subtraction function from step 1.
Similar to HERWIG and MC@NLO approaches (with antenna subtraction instead of Catani-Seymour)  matching at NLO
Can simultaneously include arbitrarily many tree-level ME’s  multijet matching
Combines virtue of CKKW (multijet matching) with that of MC@NLO (NLO matching)
Avoids the vices of CKKW (dependence on unphysical clustering and cuts, LO) and MC@NLO (complicated, not applicable beyond one jet)

- _Subtraction method:_ subtract showering off n-parton state from n+1-parton Matrix Element  subtracted (IR finite) matrix elements.
- Generate parton-level configurations, with weights given by the subtracted Matrix Elements
- Shower them using the subtraction function from step 1.
- Similar to HERWIG and MC@NLO approaches (with antenna subtraction instead of Catani-Seymour)  _matching at NLO_
- Can simultaneously include arbitrarily many tree-level ME’s  _multijet matching_
- Combines virtue of CKKW (multijet matching) with that of MC@NLO (NLO matching)
- Avoids the vices of CKKW (dependence on unphysical clustering and cuts, LO) and MC@NLO (complicated, not applicable beyond one jet)

- VINCIA-style matching

- Giele, Kosower, PS : in progress

- Event Generator Status

- Peter Skands

## Hgg: Example

- VINCIA 0.008
- Unmatched
- “soft” |A|2

- VINCIA 0.008
- Unmatched
- “hard” |A|2

- First Branching ~ first order in perturbation theory

- VINCIA 0.008
- Matched
- “soft” |A|2

- VINCIA 0.008
- Matched
- “hard” |A|2

| Antenna Function | IR singularities plus arbitrary finite terms
|A(sa,sb;s)|2    
 (a.k.a. Radiation Function, a.k.a. Splitting Kernel) |
| --- | --- |

- Systematically improved by matching

- Expect public code and long writeup before summer

- Giele, Kosower, PS : in progress

- y12

- y23

- y23

- y23

- y23

- Later: plug-in for Pythia 8 ?

- Next: Quarks and ISR  hadron collider event generator

- y12

- Event Generator Status

- Peter Skands

## C++ Players

**HERWIG++: complete reimplementation**
- Improved PS 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
- F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter
**PYTHIA8: selective reimplementation**
- Improved PS and UE, limited number of hard subprocesses
- Many obsolete features not carried over  simpler, less parameters
- T. Sjöstrand, S. Mrenna, P. Skands

- 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 pythia8070.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 (QCD) Landscape

- D. B. Leinweber, hep-lat/0004025

- In reality, this all happens on top of each other.
- (only possible exception: long-lived colour singlet)

- Event Generator Status

- Peter Skands

## New physics in the Underlying Event?

- Given the lack of analytical solutions  impacts on hadronic precision observables poorly known
- Example: sensitivity of Tevatron top mass analysis (simplified!) to variations of:
**Colour reconnections **
- string-string interactions?
- string-vacuum (re)interactions?
- “colour annealing” model
- large strength ( >10% ) required                                                            for acceptable fits to min-bias
**Parton showers**
- Pythia: **mass-ordered** vs **pT-ordered**
**Underlying-event parameters**
- E.g. Tune A vs Tune DW, etc

- Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120

- Event Generator Status

- Peter Skands

## Beyond the Standard Model

- SUSY Les Houches Accord being extended to RPV, NMFV, CPV, NMSSM, ...   SLHA2
- Apologies for slow progress. A skeleton exists, but so far only small number of practical implementations to test viability
- General CPV and NMFV: MSSM implementation for PYTHIA8
- NMSSM:
- Les Houches BSM Tools Repository:
- Recent years  more focus on non-SUSY
- 2nd MC4BSM: Princeton, March 21-24 2007 (in conjunction with LHC Olympics)
- Teams of “writers” + teams of “testers”: sign up for the fun!

- G. Bozzi, B. Fuks, M. Klasen, PS : in progress

- Ellwanger, Hugonie, Moretti, Pukhov, ... : in progress

- http://www.ippp.dur.ac.uk/montecarlo/BSM

- http://www.phys.ufl.edu/~matchev/MC4BSM/

- B. Allanach et al, in hep-ph/0602198

- Summary of 1st  MC4BSM: J. Hubisz, PS, FERMILAB-CONF-05-082-T

- PS et al, in hep-ph/0602198

- lots of material, e.g. for warped ED: R. Contino et al, hep-ph/0612180

- Event Generator Status

- Peter Skands

## The Generator Outlook

- Generators in state of continuous development:
- Better & more user-friendly general-purpose ME calculators+integrators
- New libraries of physics processes, also to NLO
- Improved parton showers
- Better matching of matrix elements to showers
- Improved models for underlying events / minimum bias
- Upgrades of hadronization and decays
- Moving to C++

****** ****more precise, more reliable theoretical predictions**

**Ultimately, the interesting talk is the experimental one, how good is a good calculation? Compare to LEP, Tevatron, and RHIC data ******** constraints. Absolutely vital for high precision + often uncovers defects, and even hints of new phenomena...**

- Event Generator Status

- Peter Skands