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P YTHIA 8 Richard Corke Department of Astronomy and Theoretical Physics Lund University June 2010 Torbj orn Sj ostrand, Stefan Ask, Stephen Mrenna, Peter Skands, Lisa Carloni Richard Corke (Lund University) TOOLS 2010, Winchester, UK

  1. P YTHIA 8 Richard Corke Department of Astronomy and Theoretical Physics Lund University June 2010 Torbj¨ orn Sj¨ ostrand, Stefan Ask, Stephen Mrenna, Peter Skands, Lisa Carloni Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 1 / 43

  2. Overview P YTHIA 1 Physics overview 2 BSM Physics 3 Running P YTHIA 8 4 Conclusions 5 Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 2 / 43

  3. P YTHIA ◮ General purpose Monte Carlo event generator ◮ Combine pQCD and models to provide link from theory (quarks, gluons) to experiment (mesons, baryons) ◮ Full problem “factorised” into different components ◮ Hard process ◮ Resonance decays ◮ Parton showers ◮ Underlying event ◮ Hadronisation ◮ Hadron decays ◮ Different parts may be handled by other external programs (e.g. Tauola) ◮ Or (with P YTHIA 8) through plugins (e.g. V INCIA ) ◮ Outputs exclusive hadronic events ◮ Analyse (e.g. FastJet) ◮ Pass to detector simulator (e.g. GEANT) ◮ ... Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 3 / 43

  4. P YTHIA Richard Corke (Lund University) Richard Corke (Lund University) TOOLS 2010, Winchester, UK P YTHIA 8 April 2009 June 2010 3 / 14 4 / 43

  5. Physics overview Further reading ◮ Latest downloads and news: ◮ http://home.thep.lu.se/˜torbjorn/Pythia.html ◮ “P YTHIA 6.4 Physics and Manual” T. Sj¨ ostrand, S. Mrenna and P . Skands, JHEP 0605:026,2006, [hep-ph/0603175]. ◮ “A Brief Introduction to P YTHIA 8.1” T. Sj¨ ostrand, S. Mrenna and P . Skands, Comput. Phys. Comm. 178 (2008) 852 [arXiv:0710.3820]. ◮ And references therein Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 5 / 43

  6. Physics overview Beams and hard processes ◮ Beams ◮ Incoming beams: pp , p¯ p , e + e − , µ + µ − ◮ P YTHIA 8: no ep , γ p or γγ beam configurations ◮ Built in parton distribution function (PDF) sets ◮ GRV94L, CTEQ5L ◮ MSTW2008 (LO and NLO), MRST LO** ◮ CTEQ6L, CTEQ6L1, CTEQ6.6, CT09MC1, CT09MC2, CT09MCS ◮ Easy to link to LHAPDF for many more W + u g d p p / p ◮ Hard Processes ◮ Built-in library of many leading-order processes ◮ SM: almost all 2 → 1 and 2 → 2, some 2 → 3 ◮ BSM: a bit of everything (more to come) ◮ External input through Les Houches Accord (LHA) and Les Houches Event Files (LHEF) Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 6 / 43

  7. Physics overview Parton showers ◮ Regions of phase space where higher-order terms are enhanced ◮ Full matrix element calculation not feasible ◮ DGLAP evolution equations; leading log approximation of QCD ◮ Sudakov form factor; shower evolution as a probabilistic process + + FSR ISR ◮ Initial state radiation performed through backwards evolution ◮ Pick a hard 2 → 2 process ◮ What is the probability that incoming parton b came from a splitting a → bc ? ◮ PDF factors enter the evolution ◮ Iterate to build up event c s W + u g d p p / p Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 7 / 43

  8. Physics overview Parton showers ◮ Still choices to make! ◮ Ordering ◮ Transverse-momentum-ordered showers ⊥ p 2 p 2 p 2 ⊥ ⊥ ⊥ y y y 2 ◮ Recoil strategy ◮ Dipole approach to recoil ◮ Each radiator parton has a recoiler partner ◮ Kinematics constructed directly after each branching ◮ All unevolved partons on mass shell Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 8 / 43

  9. Physics overview Parton showers ◮ Matching to ME for first emission in many processes ◮ Aim to provide better shower behaviour at large p ⊥ ◮ Dampen shower tail in coloured final states ◮ Also examine interfacing of POWHEG NLO generators to P YTHIA ◮ RC & T. Sj¨ ostrand, arXiv:1003.2384 [hep-ph] (b) 10 -2 POWHEG Pythia Default (Power) 10 -3 Pythia Damp, k = 2 ] Pythia Damp, k = 1 [GeV -1 Pythia Wimpy 10 -4 dP / dp ⊥ 10 -5 10 -6 10 -7 100 200 300 400 500 600 700 800 900 1000 p ⊥ [GeV] 2 2 ◮ Implementation of CKKW-L in progress (Stefan Prestel) Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 9 / 43

  10. Physics overview Underlying event ◮ Multiple parton-parton interactions ◮ QCD 2 → 2, prompt photon production, Drell Yan, Charmonium & Bottomonium ◮ Impact parameter dependence ◮ Dampened cross section in p ⊥ → 0 limit ⊥ → r r r r ∝ α 2 S ( p 2 → α 2 S ( p 2 ⊥ 0 + p 2 ⊥ ) d ˆ σ ⊥ ) d p 2 p 4 ( p 2 ⊥ 0 + p 2 ⊥ ) 2 d d ⊥ ⊥ λ ∼ 1 /p ⊥ resolved screened ◮ Interleaved p ⊥ evolution with ISR and FSR ◮ ISR and MI “compete” for beam ◮ Flavour dependent PDF effects ◮ Showering from all interactions Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 10 / 43

  11. Physics overview Underlying event ◮ Picture now a lot more messy c s W + u g d p p / p ◮ Rescattering: scattered parton allowed to interact again ◮ Same order in α s , but one PDF weight less ◮ Large background → will be tough to find direct evidence ◮ RC & T. Sj¨ ostrand, JHEP 01 (2010) 035 [arXiv:0911.1909] → → Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 11 / 43 New source of 3-jet topologies; visible effects after tuning?

  12. Physics overview Hadronisation � String fragmentation - “The Lund Model” � d E � � d p z � � d E � � d p z � � � � � � � � � � = � = � = � = κ � � � � � � � � d z d z d t d t � � � � g ( rb ) q ( r ) t q q z q ( b ) � String breaking modelled by tunnelling q � q � q q � q � q q q d = m ⊥ q / κ m ⊥ q � = 0 m ⊥ q � > 0 � Particle decays, usually isotropic Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 12 / 43

  13. Physics overview Hadronisation ◮ Everything connected by colour confinement strings ◮ Strings fragment to produce primary hadrons ◮ Unstable hadrons decay further Everything is connected by colour confinement strings Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 13 / 43

  14. Physics overview Hadronisation ◮ Everything connected by colour confinement strings ◮ Strings fragment to produce primary hadrons ◮ Unstable hadrons decay further The strings fragment to produce primary hadrons Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 13 / 43

  15. Physics overview Hadronisation ◮ Everything connected by colour confinement strings ◮ Strings fragment to produce primary hadrons ◮ Unstable hadrons decay further Many hadrons are unstable and decay further Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 13 / 43

  16. b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b Physics overview Current status ◮ Tuning with Rivet + Professor A. Buckley et al. [hep-ph/1003.0694, hep-ph/0907.2973] ◮ Tuning to e + e − data looks okay ◮ Hard physics distributions also okay ◮ But problems describing the underlying event? Transverse region charged ∑ p ⊥ density Transverse region charged particle density � N ch � /d η d φ � /d η d φ / GeV 2 CDF data 1 ’ 6142 -default’ ’ 8135 -default’ 0 . 8 1 . 5 � ∑ p track CDF data 0 . 6 T ’ 6142 -default’ 1 ’ 8135 -default’ 0 . 4 0 . 5 0 . 2 0 0 MC/data MC/data 1 . 4 1 . 4 1 . 2 1 . 2 1 1 0 . 8 0 . 8 0 . 6 0 . 6 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 p T ( leading jet ) / GeV p T ( leading jet ) / GeV ◮ Possible causes ◮ Final-state dipoles with initial-state recoil ◮ Azimuthal asymmetry of radiation Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 14 / 43

  17. b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b Physics overview Current status ◮ Initial results promising, but still much to be checked Transverse region charged particle density Transverse region charged ∑ p ⊥ density � N ch � /d η d φ � /d η d φ / GeV 2 CDF data 1 ’ 6142 -default’ ’ 8135 -default’ 0 . 8 1 . 5 ’ 8140 -default-asym’ � ∑ p track CDF data 0 . 6 T ’ 6142 -default’ 1 ’ 8135 -default’ 0 . 4 ’ 8140 -default-asym’ 0 . 5 0 . 2 0 0 MC/data MC/data 1 . 4 1 . 4 1 . 2 1 . 2 1 1 0 . 8 0 . 8 0 . 6 0 . 6 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 p T ( leading jet ) / GeV p T ( leading jet ) / GeV ◮ Go further? ◮ Compare first parton shower emission to 2 → 3 matrix elements ◮ How does 2 → 2 ⊗ PS fill the phase space? ◮ Work ongoing! Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 15 / 43

  18. BSM Physics Overview ◮ Much early focus on SM physics ◮ Emphasis on providing solid links to external programs ◮ Les Houches Accord (LHA) and Les Houches Events Files (LHEF) can be used to read in parton-level events for showering and hadronisation ◮ Easy to use P YTHIA to simulate a wide range of BSM processes in this way ◮ Important to understand what choices need to be made and what P YTHIA can and can’t do ◮ But also complemented by a library of common BSM processes Richard Corke (Lund University) TOOLS 2010, Winchester, UK June 2010 16 / 43

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