T h e PY T H I A E ve n t G e n e r a to r P e t e r S k a n d s - PowerPoint PPT Presentation

TO O L S 2 0 1 2 , S t o c k h o l m , J u n e 2 0 1 2 T h e PY T H I A E ve n t G e n e r a to r P e t e r S k a n d s ( C E R N )

LHC is a QCD Machine Hard processes initiated by partons (quarks & gluons) Associated with initial-state QCD corrections Underlying event by QCD mechanisms (MPI, color flow) Extra QCD jets, isolation, fakes → all sensitive to QCD corrections PYTHIA P . Skands 2

LHC is a QCD Machine Hard processes initiated by partons (quarks & gluons) Associated with initial-state QCD corrections Underlying event by QCD mechanisms (MPI, color flow) Extra QCD jets, isolation, fakes → all sensitive to QCD corrections Even in BSM scenarios, production of new colored states often favored Squarks, gluinos, KK gluons, excited quarks, … + extra QCD jets … PYTHIA P . Skands 2

Monte Carlo Generators Reality is more complicated Calculate Everything ≈ solve QCD → requires compromise! Improve lowest-order perturbation theory, by including the ‘most significant’ corrections → complete events (can evaluate any observable you want) Existing Approaches PYTHIA : Successor to JETSET (begun in 1978). Originated in hadronization studies: Lund String. HERWIG : Successor to EARWIG (begun in 1984). Originated in coherence studies: angular ordering. SHERPA : Begun in 2000. Originated in “matching” of matrix elements to showers: CKKW. + MORE SPECIALIZED: ALPGEN, MADGRAPH, ARIADNE, VINCIA, WHIZARD, MC@NLO, POWHEG, … PYTHIA P . Skands 3

PYTHIA PYTHIA anno 1978 (then called JETSET) LU TP 78-18 November, 1978 A Monte Carlo Program for Quark Jet Generation T. Sjöstrand, B. Söderberg A Monte Carlo computer program is presented, that simulates the fragmentation of a fast parton into a jet of mesons. It uses an iterative scaling scheme and is compatible with the jet model of Field and Feynman. Note: Field-Feynman was an early fragmentation model Now superseded by the String (in PYTHIA) and Cluster (in HERWIG & SHERPA) models. PYTHIA P . Skands 4

PYTHIA PYTHIA anno 1978 (then called JETSET) LU TP 78-18 November, 1978 A Monte Carlo Program for Quark Jet Generation T. Sjöstrand, B. Söderberg A Monte Carlo computer program is presented, that simulates the fragmentation of a fast parton into a jet of mesons. It uses an iterative scaling scheme and is compatible with the jet model of Field and Feynman. Note: Field-Feynman was an early fragmentation model Now superseded by the String (in PYTHIA) and Cluster (in HERWIG & SHERPA) models. PYTHIA P . Skands 4

PYTHIA PYTHIA anno 2012 ~ 80,000 lines of C++ (now called PYTHIA 8) What a modern MC generator has inside: • Hard Processes (internal, semi- LU TP 07-28 (CPC 178 (2008) 852) internal, or via Les Houches events) October, 2007 • BSM (internal or via interfaces) A Brief Introduction to PYTHIA 8.1 • PDFs (internal or via interfaces) T. Sjöstrand, S. Mrenna, P. Skands • Showers (internal or inherited) The Pythia program is a standard tool • Multiple parton interactions for the generation of high-energy collisions, comprising a coherent set • Beam Remnants of physics models for the evolution • String Fragmentation from a few-body hard process to a complex multihadronic final state. It • Decays (internal or via interfaces) contains a library of hard processes and models for initial- and final-state • Examples and Tutorial parton showers, multiple parton-parton interactions, beam remnants, string • Online HTML / PHP Manual fragmentation and particle decays. It • Utilities and interfaces to also has a set of utilities and interfaces to external programs. […] external programs PYTHIA P . Skands 5

( Tr a d i t i o n a l ) M o n t e C a r l o G e n e r a t o r s Perturbative Evolution Collider Observables Based on small-angle singularity of accelerated charges (synchrotron radiation, semi-classical) Hard Hadronization Process h | M (0) H | 2 n Leading Order, o s Confrontation t r r e Infinite Lifetimes, a w P with Data o … h S Classical Altarelli-Parisi Splitting Kernels Strings Leading Logarithms, Leading Color, … + Colour coherence Factorization Scale PYTHIA P . Skands 6

PYTHIA 8 Ambition Current Status Ready and tuned to LHC data Cleaner code Better interfaces to (B)SM More user-friendly generators via LHEF and semi- internal processes Easy interfacing Improved shower model + interfaces to CKKW-L, Physics Improvements POWHEG, and VINCIA Team Members Contributors Stefan Ask Richard Corke Bertrand Bellenot Marc Montull Stephen Mrenna Lisa Carloni Sparsh Navin Stefan Prestel Tomas Kasemets MSTW , CTEQ, H1: PDFs Torbjorn Sjostrand Mikhail Kirsanov DELPHI, LHCb: D/B BRs Peter Skands Ben Lloyd + several bug reports & fixes PYTHIA P . Skands 7

Hard Processes Hard Physics Standard Model almost all 2 → 1, 2 → 2 A few 2 → 3 BSM: a bit of everything (see documentation) Perturbative Resonance Decays Angular correlations often included (on a process-by-process basis - no generic formalism) User implementations (semi-internal resonance) PYTHIA P . Skands 8

Hard Processes Hard Physics External Input Standard Model Les Houches Accord and LHEF (e.g., from MadGraph, almost all 2 → 1, 2 → 2 CalcHEP, AlpGen,…) A few 2 → 3 User implementations BSM: a bit of everything (semi-internal process) (see documentation) Inheriting from PYTHIA’s 2 → 2 base class, then modify to suit you (+ automated in MadGraph 5) Perturbative Resonance Decays Angular correlations often included (on a process-by-process basis - no generic formalism) User implementations (semi-internal resonance) PYTHIA P . Skands 8

Exotic Colors Color Epsilon Topologies Example: RPV SUSY W BNV = � 00 ijk ✏ abc U ia D jb D kc PYTHIA P . Skands 9

Exotic Colors Dipole Showers: Color Epsilon Topologies Radiation pattern obtained as if Example: RPV SUSY three radiating dipoles, but with half normal strength W BNV = � 00 ijk ✏ abc U ia D jb D kc N. Desai & PS, arXiv:1109.5852. (+Sextets → two dipoles) PYTHIA P . Skands 9

Exotic Colors Dipole Showers: Color Epsilon Topologies Radiation pattern obtained as if Example: RPV SUSY three radiating dipoles, but with half normal strength W BNV = � 00 ijk ✏ abc U ia D jb D kc N. Desai & PS, arXiv:1109.5852. (+Sextets → two dipoles) Normal q-g-qbar string PYTHIA T. Sjöstrand & PS, Nucl. Phys. B659 (2003) 243 P . Skands 9

Exotic Colors Dipole Showers: Color Epsilon Topologies Radiation pattern obtained as if Example: RPV SUSY three radiating dipoles, but with half normal strength W BNV = � 00 ijk ✏ abc U ia D jb D kc N. Desai & PS, arXiv:1109.5852. (+Sextets → two dipoles) Normal q-g-qbar string PYTHIA T. Sjöstrand & PS, Nucl. Phys. B659 (2003) 243 P . Skands 9

Hidden Valleys (Courtesy M. Strassler) Models only interesting if they can give observable consequences at the LHC! PYTHIA P . Skands 10

Hidden Valleys Carloni, Rathsman, Sjöstrand, JHEP 1104 (2011) 091 Interleaved shower in QCD, QED and HV sectors: HV U(1): add γ v emissions → HV SU(N): add g v emissions (Courtesy M. Strassler) HV particles may remain invisible, or Broken U(1): γ v → lepton pairs SU(N): hadronization in hidden sector, with full string fragmentation setup. For now assumed mass-degenerate. Flavor Off-diagonal: stable & invisible Models only interesting if they can give Flavor Diagonal, can decay back to SM observable consequences at the LHC! PYTHIA P . Skands 11

Resummation and Matching Parton Distributions Internal (faster than LHAPDF) CTEQ + MSTW LO, plus a few NLO MSTW LO*, LO**, CTEQ CT09MC Interface to LHAPDF [T. Kasemets, arXiv:1002.4376] Can use separate PDFs for hard scattering and UE (to ‘stay tuned’) PYTHIA P . Skands 12

Resummation and Matching Parton Distributions Internal (faster than LHAPDF) CTEQ + MSTW LO, plus a few NLO MSTW LO*, LO**, CTEQ CT09MC Interface to LHAPDF [T. Kasemets, arXiv:1002.4376] Can use separate PDFs for hard scattering and UE (to ‘stay tuned’) Showers Transverse-momentum ordered ISR & FSR (new: fully interleaved) Includes QCD and QED Dipole-style recoils (partly new) Improved high-p ⊥ behavior [R. Corke] PYTHIA P . Skands 12

Resummation and Matching Parton Distributions Matrix-Element Matching Internal (faster than LHAPDF) Automatic first-order matching for most gluon-emission CTEQ + MSTW LO, plus a few NLO processes in resonance decays, MSTW LO*, LO**, CTEQ CT09MC e.g.,: Interface to LHAPDF [T. Kasemets, arXiv:1002.4376] Z → qq → qqg, Can use separate PDFs for hard t → bW → bWg, scattering and UE (to ‘stay tuned’) H → bb → bbg, … Showers Automatic first-order matching for internal 2 → 1 color-singlet Transverse-momentum ordered processes, e.g.: ISR & FSR (new: fully interleaved) pp → H/Z/W/Z’/W’+jet Includes QCD and QED More to come … Dipole-style recoils (partly new) Interface to AlpGen, MadGraph, Improved high-p ⊥ behavior [R. Corke] … via Les Houches Accords PYTHIA P . Skands 12