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Event Generators 3 Practical Topics

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

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Event Generators 3 Practical Topics

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

  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! Done! Event Generator Status

  3. Generators 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

  4. 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

  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 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

  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 … Event Generator Status

  7. 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

  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 • … ? Event 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 Event 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! Done! Done! Event Generator Status

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