1. Aachen, November 2007 Event Generators 3 Practical Topics Peter Skands CERN / Fermilab

2. Peter SkandsEvent Generator Status 2 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!

3. Peter SkandsEvent Generator Status 3Generators General-PurposeSpecialized Hard Process HERWIG PYTHIA ISAJET SHERPA A lot … Resonance DecaysHDecay, … MatchingMC@NLO, POWHEG Parton ShowersARIADNE/LDC, NLLJET Underlying EventDPMJET HadronizationNone? Ordinary DecaysTAUOLA, EvtGen, … Tip: a good place to look: “Les Houches Guidebook to Monte Carlo Generators for Hadron Collider Physics” : hep-ph/0403045 (being updated)

4. Peter SkandsEvent Generator Status 4 Hard Process ►Fixed-Order Tree-Level Matrix Elements HERWIG / PYTHIA (Many SM and BSM 2  2 + some 2  3) Including 1 st 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

5. Peter SkandsEvent Generator Status 5 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 1 st order for several important processes ►PYTHIA “Old” Q 2 -ordered parton shower (Pythia 6) + “New” p T -ordered parton-dipole shower (Pythia 6.4  8) Coherence more “brute-force” Radiation fills all of phase space  no Dead Zone Matched to 1 st order for all SM and MSSM decays plus several important hard processes (+ VINCIA antenna-dipole shower written as plug-in to Pythia-8) ►ARIADNE p T -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 Q 2 -ordered parton shower (identical to Pythia 6) + New p T -ordered Nagy-Soper shower + ? Integrated ME generator with CKKW matching + more specialized: NLLJET + LDCMC + … ?

6. Peter SkandsEvent Generator Status 6 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 …

7. Peter SkandsEvent Generator Status 7Hadronization ►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

8. Peter SkandsEvent Generator Status 8 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 ►… ?

9. Peter SkandsEvent Generator Status 9 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

10. Peter SkandsEvent Generator Status 10 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!