# Presentation: 319171

## Event Generators 3Practical Topics

- Event Generators 3Practical 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

- Done!

- Done!

- Event Generator Status

- Peter Skands

## Generators

|  | General-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)

- Event Generator Status

- Peter Skands

## 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
- 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

- Interfaced
- Showers + UE + Had

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

- Event Generator Status

- Peter Skands

## 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

- + more specialized: NLLJET + LDCMC + ... ?

- Event Generator Status

- Peter Skands

## 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
- I just don’t know them well enough ...

- Event Generator Status

- Peter Skands

## 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

- Event Generator Status

- Peter Skands

## 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
- ... ?

- Event Generator Status

- Peter Skands

## 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**

- Event Generator Status

- Peter Skands

## 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

- Done!

- Done!

- Done!

- Event Generator Status

- Peter Skands