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Aspen Winter Conference, January 2006. 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

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event generator status

Aspen Winter Conference, January 2006

Event Generator Status

Peter Skands

  • Matching (who’s doing it, how, and where?)
  • New ideas and C++
traditional event generators
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

modern event generators
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

 Entering era of precision event generators for hadron colliders

Better understanding of PS uncertainties – À LA ERROR PDF’S?


Better understanding of the underlying event and non-perturbative effects - ESPECIALLY IN THE BUSY ENVIRONMENT OFFERED BY LHC

Event Generator Status



  • 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


n exclusive

n+1 exclusive

n+2 inclusive

  • “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

sherpa ariadne patriot

S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063

L. L¨onnblad, JHEP05 (2002) 046

  • 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++?)

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

Event Generator Status


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

Nason’s approach:

Generate 1st shower emission separately  easier matching

Avoid negative weights + explicit study of ZZ production

JHEP 0411(2004)040

JHEP 0608(2006)077

Event Generator Status

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


virtual numerical collider with interlinked antennae

Giele, Kosower, PS : in progress


Dipoles – a dual description of QCD



Event Generator Status

vincia style matching
VINCIA-style matching

Giele, Kosower, PS : in progress

  • 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)

Event Generator Status

h gg example
Hgg: Example

Giele, Kosower, PS : in progress

Systematically improved by matching

First Branching ~ first order in perturbation theory

VINCIA 0.008


“soft” |A|2

VINCIA 0.008


“soft” |A|2



Expect public code and long writeup before summer

Next: Quarks and ISR  hadron collider event generator



VINCIA 0.008


“hard” |A|2

VINCIA 0.008


“hard” |A|2

Later: plug-in for Pythia 8 ?



Event Generator Status

c players
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

pythia 8

Basic generator already there

Includes a few processes (+ full Pythia6 library), new pT-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!


Spring 2007: QED showers, LHAPDF, interleaved FSR, beam remnants, colour reconnections  useful

Fall-Winter 2007: resonance decays, GUI, official release?

Event Generator Status

the qcd landscape
The (QCD) Landscape


bbar from tbar decay

pbar beam remnant

p beam remnant

qbar from W

q from W

q from W

b from t decay




In reality, this all happens on top of each other.

(only possible exception: long-lived colour singlet)

D. B. Leinweber, hep-lat/0004025

Event Generator Status

new physics in the underlying event
New physics in the Underlying Event?

PS, D. Wicke : preliminary


<Δ>UE/CR ~ 0.4 GeV

ΔPS ~ 0.75 GeV

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

  • 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

Event Generator Status

beyond the standard model
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!

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

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

Ellwanger, Hugonie, Moretti, Pukhov, … : in progress

PS et al, in hep-ph/0602198


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


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

Event Generator Status

the generator outlook
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 toshowers
  • 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