Aachen, November 2007

Event Generators 3Practical Topics

Peter Skands

CERN / Fermilab

Peter Skands Event Generator Status 2

Master PlanMaster Plan► Lecture 1: Fundamental Topics

• Fundamentals of Generators, Parton Showers, and Hadronization

► Lecture 2: Advanced Topics

• Hadron Collisions and the Underlying Event

• Matching

► Lecture 3: Practical Topics + Open Q & A

• Overview of Event Generator Landscape

• Overview of useful parameters in PYTHIA

• Open Question-and-Answer Session

• Beer

Done!

Done!

Peter Skands Event Generator Status 3

GeneratorsGeneratorsGeneral-Purpose Specialized

Hard Process

HERWIG

PYTHIA

ISAJET

SHERPA

A lot …

Resonance Decays HDecay, …

Matching MC@NLO, POWHEG

Parton Showers ARIADNE/LDC, NLLJET

Underlying Event DPMJET

Hadronization None?

Ordinary Decays TAUOLA, EvtGen, …

Tip: a good place to look: “Les Houches Guidebook to Monte Carlo Generators for Hadron Collider Physics” : hep-ph/0403045 (being updated)

Peter Skands Event Generator Status 4

Hard ProcessHard Process► Fixed-Order Tree-Level Matrix Elements

• HERWIG / PYTHIA (Many SM and BSM 2 2 + some 2 3)• Including 1st order matching in many cases

• ALPGEN (SM) : Specific 2 n processes• Matching relatively straightforward

• AcerMC (SM) : Specific 2 n processes (heavy quarks)

• HELAC/PHEGAS (SM) : Generic 2 n processes (large n)

• VECBOS (SM) : Z/W + jets

• MadGraph (SM+BSM) : Generic 2 n processes • Automated matching

• “Easy” to implement new BSM models

• CalcHEP/CompHEP/LanHEP (SM+BSM) : Generic 2 n processes• Easy to implement new BSM models

• SHERPA (SM+MSSM+ADD) : Generic 2 n processes• No hadronization interfaced to PYTHIA

• WHIZARD (SM + MSSM) : Generic 2 n processes

Inte

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edS

how

ers

+ U

E +

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+ Many specific programs for NLO processes (e.g., MCFM), but not possible to interface to general-purpose generators

Peter Skands Event Generator Status 5

Parton ShowersParton Showers► HERWIG

• Angular ordered parton shower = coherence by construction (+ improvements in HERWIG++)

• Probably technically most sophisticated

• Dead Zone in hard region with no radiation

• Matched to 1st order for several important processes

► PYTHIA• “Old” Q2-ordered parton shower (Pythia 6) + “New” pT-ordered parton-dipole shower (Pythia 6.4 8)

• Coherence more “brute-force”

• Radiation fills all of phase space no Dead Zone

• Matched to 1st order for all SM and MSSM decays plus several important hard processes

• (+ VINCIA antenna-dipole shower written as plug-in to Pythia-8)

► ARIADNE• pT-ordered dipole shower = coherence by construction

• Probably most elegant

• Can still do matching etc, now automated for several important processes

• Qualitatively different from other approaches

► SHERPA• Q2-ordered parton shower (identical to Pythia 6) + New pT-ordered Nagy-Soper shower + ?

• Integrated ME generator with CKKW matching

+ more specialized: NLLJET + LDCMC + … ?

Peter Skands Event Generator Status 6

Underlying EventUnderlying Event► HERWIG

• Fit to UA5 minimum-bias data = reference “soft” underlying-event “model”

► HERWIG+JIMMY• MPI-based add-on ( HERWIG++)

► PYTHIA• “Old” MPI-based model (Tune A, DW, DWT, Rome, etc)

• “New” MPI-based model (Pythia 6.4 Pythia 8) • includes showers off MPI, evolution of MPI interleaved with showers, more sophisticated beam

remnant treatment, flavour correlated multi-parton densities, …• Pythia 8: double Drell-Yan, studies of rescattering, …

► SHERPA• “Old” PYTHIA model + showers off MPI

► ISAJET, DPMJET, PHOJET• I just don’t know them well enough …

Peter Skands Event Generator Status 7

HadronizationHadronization► HERWIG

• Cluster Hadronization: • Based on ideas of pre-confinement

• At end of shower, all gluons split into qqbar pairs

• Colour-connected quark pairs then form “clusters”

• Small clusters decay isotropically to two hadrons each

• Large clusters are split into smaller clusters along “string-like” axis

► PYTHIA

• String Hadronization• Based on ideas of linear confinement

• At end of shower, colour-connected partons form “string” pieces, with quarks being endpoints and gluons being transverse “kinks”. (And “string junctions” being the antisymmetric colour tensor carrying baryon number)

• String breaks governed by tunneling factors = area law

• Small strings clusters

Peter Skands Event Generator Status 8

Particle DecaysParticle Decays► EvtGen

• Specific for B decays

• Includes up-to-date decay tables for even the most exotic modes

• E.g., used by BaBar

► TAUOLA

• Specific for tau decays

• Includes polarization

► … ?

Peter Skands Event Generator Status 9

Les Houches GuidebookLes Houches Guidebook► Now being updated for Les Houches 2007 writeups

► Focusses on Generators for Hadron Colliders

► Only programs which are publically available and documented

► Only programs which include Standard Model physics• Programs which include BSM physics can mention that but we don’t include

only-BSM programs

► Only matrix element and NLO programs which can be interfaced to general purpose event generators.

• There are too many NLO parton-level generators to include them all.

► Aim to have the text written in plain language at a level that can be understood by graduate students

• Including suitable few-page introductions to each “class” of generators

► The previous edition (2004) can be found at: hep-ph/0403045

Peter Skands Event Generator Status 10

Master PlanMaster Plan► Lecture 1: Fundamental Topics

• Fundamentals of Generators, Parton Showers, and Hadronization

► Lecture 2: Advanced Topics

• Hadron Collisions and the Underlying Event

• Matching

► Lecture 3: Practical Topics + Open Q & A

• Overview of Event Generator Landscape

• Overview of useful parameters in PYTHIA

• Open Question-and-Answer Session

• Beer

Done!

Done!

Done!