Transcript Standard Model Higgs - RWTH Aachen University

Aachen, November 2007
Event Generators 3
Practical Topics
Peter Skands
CERN / Fermilab
Master 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
Peter Skands
Event Generator Status
2
Generators
General-Purpose
A lot …
Hard Process
Resonance Decays
Matching
HERWIG
PYTHIA
Parton Showers
Underlying Event
Hadronization
Specialized
HDecay, …
MC@NLO, POWHEG
ARIADNE/LDC, NLLJET
ISAJET
SHERPA
DPMJET
None?
TAUOLA, EvtGen, …
Ordinary Decays
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
3
Hard 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
•
•
SHERPA (SM+MSSM+ADD) : Generic 2  n processes
•
•
Easy to implement new BSM models
No hadronization  interfaced to PYTHIA
WHIZARD (SM + MSSM) : Generic 2  n processes
+ Many specific programs for NLO processes (e.g., MCFM), but not
possible to interfaceEvent
to Generator
general-purpose
generators
Peter Skands
Status
4
Parton 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
Peter Skands
+ more
specialized:
NLLJET + LDCMC + … ?
Event Generator
Status
5
Underlying 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
•
Peter Skands
I just don’t know them well enough …
Event Generator Status
6
Hadronization
► 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
7
Particle 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
8
Les 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
9
Master 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
Peter Skands
Event Generator Status
10