PYTHIA: Past and Present (for future: see yesterday) P e t e r S k - PowerPoint PPT Presentation

S t o c k h o l m , A p r 2 5 2 0 1 2 PYTHIA: Past and Present (for future: see yesterday) P e t e r S k a n d s ( C E R N )

PYTHIA 8 Current Status Ambition Ready and tuned for Min-Bias & Cleaner code UE (+ diffraction improved over Pythia 6) More user-friendly Improved shower model + Easy interfacing interfaces to POWHEG and CKKW-L Physics Improvements Better interfaces to (B)SM generators via LHEF and semi- internal processes Team Members Contributors Stefan Ask Marc Montull Bertrand Bellenot Richard Corke Sparsh Navin Lisa Carloni Stephen Mrenna MSTW , CTEQ, H1: PDFs Tomas Kasemets Stefan Prestel DELPHI, LHCb: D/B BRs Mikhail Kirsanov Torbjorn Sjostrand + several bug reports & fixes Ben Lloyd Peter Skands P . Skands - PYTHIA 2

Key differences between PYTHIA 8 and PYTHIA 6 P . Skands - PYTHIA 3

Key differences between PYTHIA 8 and PYTHIA 6 New features, not found in 6.4 Up-to-date decay data and PDFs Underlying Event Interleaved MI + ISR + FSR Richer mix of underlying-event processes ( γ , J/ ψ , DY, . . . ) Possibility for two selected hard interactions in same event Alow parton rescattering Possibility to use one PDF set for hard process and another for rest Hard scattering in diffractive systems New SM and BSM processes P . Skands - PYTHIA 3

Key differences between PYTHIA 8 and PYTHIA 6 New features, not found Old features definitely in 6.4 removed Up-to-date decay data and PDFs Independent fragmentation Underlying Event Mass-ordered showers Interleaved MI + ISR + FSR Richer mix of underlying-event processes ( γ , J/ ψ , DY, . . . ) Possibility for two selected hard interactions in same event Alow parton rescattering Possibility to use one PDF set for hard process and another for rest Hard scattering in diffractive systems New SM and BSM processes P . Skands - PYTHIA 3

Key differences between PYTHIA 8 and PYTHIA 6 New features, not found Old features definitely in 6.4 removed Up-to-date decay data and PDFs Independent fragmentation Underlying Event Mass-ordered showers Interleaved MI + ISR + FSR Richer mix of underlying-event processes Features omitted so far ( γ , J/ ψ , DY, . . . ) Possibility for two selected hard interactions ep, γ p and γγ beams in same event Alow parton rescattering Some matrix elements, in Possibility to use one PDF set for hard particular Technicolor, partly process and another for rest SUSY Hard scattering in diffractive systems New SM and BSM processes P . Skands - PYTHIA 3

Key differences between PYTHIA 8 and PYTHIA 6 New features, not found Old features definitely in 6.4 removed Up-to-date decay data and PDFs Independent fragmentation Underlying Event Mass-ordered showers Interleaved MI + ISR + FSR Richer mix of underlying-event processes Features omitted so far ( γ , J/ ψ , DY, . . . ) Possibility for two selected hard interactions ep, γ p and γγ beams in same event Alow parton rescattering Some matrix elements, in Possibility to use one PDF set for hard particular Technicolor, partly process and another for rest SUSY Hard scattering in diffractive systems SUSY with NMFV and/or CPV (not fully validated) New SM and BSM processes Large Extra Dimensions, Unparticles Hidden Valley scenario with hidden radiation P . Skands - PYTHIA 3

Physics (1/3) Perturbative Resonance Decays Angular correlations often included (on a process- by-process basis - no generic formalism) User implementations (semi-internal resonance) P . Skands - PYTHIA 4

Physics (1/3) Hard Physics Standard Model almost all 2 → 1 almost all 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) P . Skands - PYTHIA 4

Physics (1/3) Hard Physics External Input Standard Model Les Houches Accord and LHEF (e.g., from MadGraph, almost all 2 → 1 CompHEP, AlpGen,…) almost all 2 → 2 User implementations (semi- A few 2 → 3 internal process) BSM: a bit of everything (see Inheriting from PYTHIA’s 2 → 2 base documentation) 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) P . Skands - PYTHIA 4

Physics (2/3) [T. Kasemets, arXiv:1002.4376] P . Skands - PYTHIA 5

Physics (2/3) Parton Distributions Internal (faster than LHAPDF) The standard CTEQ and MSTW LO sets, plus a few NLO ones New generation: 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’) P . Skands - PYTHIA 5

Physics (2/3) Parton Distributions Internal (faster than LHAPDF) The standard CTEQ and MSTW LO sets, plus a few NLO ones New generation: 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] P . Skands - PYTHIA 5

Physics (2/3) Parton Distributions Matrix-Element Matching Internal (faster than LHAPDF) Automatic first-order matching for most gluon-emission processes in The standard CTEQ and MSTW LO sets, plus a resonance decays, e.g.,: few NLO ones New generation: MSTW LO*, LO**, Z → qq → qqg, CTEQ CT09MC t → bW → bWg, Interface to LHAPDF H → bb → bbg, [T. Kasemets, arXiv:1002.4376] … Can use separate PDFs for hard Automatic first-order matching for scattering and UE (to ‘stay tuned’) internal 2 → 1 color-singlet Showers processes, e.g.: Transverse-momentum ordered ISR pp → Z/W/Z’/W’+jet & FSR (new: fully interleaved) pp → H+jet More to come … Includes QCD and QED Interface to AlpGen, MadGraph, … Dipole-style recoils (partly new) via Les Houches Accords Improved high-p ⊥ behavior [R. Corke] P . Skands - PYTHIA 5

Interfaces to External MEs (MLM) B. Cooper et al., arXiv:1109.5295 [hep-ph] If using one code for MEs and another for showering Tree-level corrections use α s from Matrix-element Generator Virtual corrections use α s from Shower Generator (Sudakov) Mismatch if the two do not use same Λ QCD or α s (m Z ) ⇥ d Q 2 note: running order also � Λ 2 i ( z ) | M F | 2 . α 2 MG has a (subleading) effect Q 2 ∑ s b 0 ln P Λ 2 SG i P2011 0 1 2 3 40 60 80 100 Alp. Λ ↑ jet p [GeV] N jet PS , Alp. Λ ↑ Λ ↑ T Ratio to P2011 Ratio to P2011 PS , Alp. Λ ↓ Λ ↓ 1.2 1.5 Alp. Λ ↓ 1.1 1 1 N jets p T1 0.9 0.8 0.5 0 1 2 3 40 60 80 100 AlpGen: can set xlclu = Λ QCD since v.2.14 (default remains to inherit from PDF) Pythia 6: set common PARP(61)=PARP(72)=PARP(81) = Λ QCD in Perugia 2011 tunes Pythia 8: use TimeShower:alphaSvalue and SpaceShower:alphaSvalue P . Skands - PYTHIA 6

Scales: pT and CMW Compute e + e - → 3 jets, for arbitrary choice of μ R (e.g., μ R = m Z ) One-loop correction 2Re[M 0 M 1* ] includes a universal O( α s2 ) term from integrating quark loops over all of phase space ⇤ 1 ⇧ ⇧ s 23 ⌃ ⇧ s 13 ⌃⌃ n f 6 A 0 + gluon loops ln + ln 3 µ 2 µ 2 R R Proportional to the β function (b 0 ). Can be absorbed by using μ R4 = s 13 s 23 = p T2 s. (~ “BLM”) In an ordered shower, quark (and gluon) loops restricted by strong-ordering condition → modified to μ R = p T (but depends on ordering variable?) Additional logs induced by gluon loops can be absorbed by replacing Λ MS by Λ MC ~ 1.5 Λ MS (with mild dependence on number of flavors) Catani, Marchesini, Webber, NPB349 (1991) 635 There are obviously still order 2 uncertainties on μ R , but this is the background for the central choice made in showers P . Skands - PYTHIA 7

Physics (3/3) P . Skands - PYTHIA 8

Physics (3/3) Underlying-Event and Min-Bias Multiple parton–parton interactions Multi-parton PDFs constructed from (flavor and momentum) sum rules Combined (interleaved) evolution MI + ISR + FSR downwards in p ⊥ (partly new) Optional rescattering [R. Corke] Beam remnants colour-connected to interacting systems String junctions → variable amount of baryon transport Defaults tuned to LHC (tune 4C) Improved model of diffraction Diffractive jet production [S. Navin] P . Skands - PYTHIA 8

Physics (3/3) Underlying-Event and Hadronization Min-Bias String fragmentation Multiple parton–parton Lund symmetric fragmentation function for (u,d,s) + Bowler modification for heavy interactions quarks (c,b) Multi-parton PDFs constructed from (flavor Hadron and Particle decays and momentum) sum rules Combined (interleaved) evolution MI + ISR Usually isotropic, or: + FSR downwards in p ⊥ (partly new) User decays (DecayHandler) Optional rescattering [R. Corke] Link to external packages Beam remnants colour-connected EVTGEN for B decays to interacting systems TAUOLA for τ decays Bose-Einstein effects String junctions → variable amount of baryon transport Two-particle model (off by default) Defaults tuned to LHC (tune 4C) Improved model of diffraction Diffractive jet production [S. Navin] P . Skands - PYTHIA 8