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Prompt photon identification via isolation cut, model comparisons: HERWIG vs PYTHIA

Prompt photon identification via isolation cut, model comparisons: HERWIG vs PYTHIA. Gustavo Conesa & Amaya Casanova. Introduction. Photons will be produced in pp and PbPb collisions at ultra-relativistic energies at the LHC.

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Prompt photon identification via isolation cut, model comparisons: HERWIG vs PYTHIA

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  1. Prompt photon identification via isolation cut, model comparisons: HERWIG vs PYTHIA Gustavo Conesa & Amaya Casanova

  2. Introduction • Photons will be produced in pp and PbPb collisions at ultra-relativistic energies at the LHC. • Photons do not interact strongly with the nuclear matter produced in HI collisions, thus they provide information on: • the initial phases of the collision (QCD predictions) • medium modifications via correlations with jets • medium modifications via identification of decaying particles (p0, , etc.) • There are different photon sources: • Prompt photons: Production not modified in HI, good tool to study medium modifications with correlated jets. • Fragmentation photons: Belong to a jet, production suppressed in HI. • Thermal photons: Produced by thermalized QGP. • Jet conversion photons: Partons interact with QGP and produce new prompt photon-jet events. • Decay photons: From hadrons suppressed by the medium.

  3. Introduction • We are interested here in prompt photons. In order to measure them we need: • Have a good understanding of their production in “simple” pp collisions • Compare different QCD generators to see their production: PYTHIA vs HERWIG • Need to reject the other photon sources in the event, specially: • Decay photons: Main photon source in the event, pi0 • Fragmentation photons: Similar amount than prompt photons. • These sources are produced together with the jet: They are accompanied by other particles • Isolation cuts discriminate between prompt photons and other sources.

  4. Similar prediction for prompt photons. Varies a 20% at 10 GeV/c to nothing at 100 GeV/c A factor 2 less fragmentation (Final State Radiation, FSR) photons in HERWIG. Similar prediction than for jets. Direct photons production in pp@14 TeV : HERWIG vs PYTHIA -1 <  < 1, 0º <  < 360º Generator level

  5. Prompt photons production in pp@14 TeV • Generated prompt  entering EMCAL acceptance • Corrected (worst case) by • conversion = 0.5 • reconstruction= 0.9 • PID=0.7 • Right axis: LHC pp 14 TeV annual statistics: • L = 1030 mb • T=107 s Will be able to measure prompt  up to 145-180 GeV. With good counting rate for correlations up to 45-60 GeV. Pythia Generator level

  6. Prompt photons production in PbPb@5.5 TeV • Generated prompt  entering EMCAL acceptance • Corrected (worst case) by • conversion = 0.5 • reconstruction= 0.9 • PID=0.7 • Right axis: LHC pp 14 TeV annual statistics: • L = 0.5·1027 mb • T=106 s Will be able to measure prompt  up to 125-155 GeV. With good counting rate for correlations up to 40-55 GeV. Pythia Generator level

  7. Direct photons and 0 production, photon NLO comparison -1 <  < 1, 0º <  < 360º • The sum of prompt and fragmentation photons tends to agree with NLO calculations, • Although NLO seems a bit higher • Fragmentation photon generated statistic still too low. • Higher production of 0 than direct photons as expected- • Difference between generators due to some resonance particles considered final state. pp@14TeV Generator level

  8. TPC IP   R candidate EMCal Isolation cut method G. Conesa et al., NIM A 580 (2007) 1446, ALICE-INT-2009-002 • Prompt g are likely to be produced isolated. • Two parameters define g isolation: • Cone size • pT threshold,candidate isolated if: • no particle in cone with pT > pT thres • pT sum in cone, SpT < SpTthres • pT sum in cone, SpT <  pTcandidate (ε = fraction of photon pT) (See Raphaelle’s talk )

  9. Isolation efficiency pp@14TeV PbPb@5.5TeV With larger cone size prompt gamma and decay gamma efficiencydecrease. With larger pT cut prompt gamma and decay gamma efficiencyincrease. Pythia Generator level

  10. Signal to Background pp@14TeV PbPb@5.5TeV With larger cone size the Signal to Background increase. With larger pT cut the Signal to Background decreases. Pythia Generator level

  11. IC parameter selection pT of the candidate at wich S/B begins to be larger than 1 for different cone sizes and pT threshold pp@14TeV PbPb@5.5TeV Best isolation parameter for pp are pT ~ 0.5-1GeV and R ~ 0.4-0.5 Best isolation parameter for PbPb are pT ~ 2GeV and R ~ 0.3 Pythia Generator level

  12. Isolation of differents particles: Generators comparison -1 < trigger< 1, 0º < trigger< 360º • Prompt photons: Particles with low hadronic activity around them • Isolated if there are no particles in a cone of size R=0.5 and pT> 1 GeV/c around the trigger particle. • Isolated hadrons mean that they are the leading jet particle with most of the jet energy. • A “mono-particle” jet. • As expected, prompt photons are isolated • More isolated in HERWIG, less underlying event around the photons • Hadrons are not isolated, fragmentation photons are ~50% • Like for prompt photons, hadrons are a more isolated in HERWIG.

  13. Isolation cutsIsolation rejection as a function of z pT trigger > 20 GeV/c -1 < trigger< 1, 0º < trigger < 360º • Isolation rejection for  and fragmentation as a function of the fraction of the momentum of the jet to which they belong. • Isolation cuts rejects low z particles from the jet. • PYTHIA and HERWIG show similar trend. • If pT fragment / pT jet > 0.5, jet fragment is isolated. • Neutral  are more rejected than charged  in both generators.

  14. EMCal g g p0 p0 l20 l20 Particle identification with the calorimeters EMCal • Different particles produce showers of different shapes. 3 regions for p0/g discrimination E < 10 GeV p0 produces 2 clusters : identified via invariant mass 10 < E < 40 GeV p0 produces one cluster, identified via shower shape E > 40 GeV p0 and g only separated with Isolation Cuts g identified as g l0 l1 g identified as p0 p0 as p0 p0 as g

  15. Direct photon identification in EMCal:Prompt photon / jet clusters with and without PIDEvents reconstructed, ALICE full material considered PbPb @ √5.5 TeV, qhat=0 PbPb @ √5.5 TeV, qhat=50 pp @ √14 TeV With PID : Clusters selected with PID l20< 0.25 With PID prompt g to jet clusters (decay g, pi0, hadrons) ratio increases significantly but it is not enough.

  16. Direct photon identification in EMCal:Isolation Cut : Prompt photon / jet clusters Events reconstructed, ALICE full material considered Ratio isolated clusters ing-jet / isolated clusters in jet-jet Clusters selected with PIDl20< 0.25 pp @ √14 TeV PbPb @ √5.5 TeV PbPb @ √5.5 TeV, qhat=50 G. Conesa Proceedings of High Pt Physics at LHC in Tokaj (2008) Prompt photons signal larger than jet-jet clusters background for pT larger than around 15 GeV/c for pp and quenched PbPb events

  17. Conclusions • Particle production in Herwig and Pythia: • Prompt photons: Similar prediction. • Fragmentation photons ~50% less produced in Herwig. • NLO comparison • Direct photon predictions and NLO are compatible. • Isolation Cuts • Best isolation parameter for pp are pT~ 0.5-1GeV and R ~ 0.4-0.5 and for PbPb pT~ 2GeV and R ~ 0.3 • When we use full reconstructed events and we identified the particles using IC and PID the S/B is larger than 1 at ~20 GeV • Particles are more isolated in HERWIG than in PYTHIA • More underlying event in PYTHIA • The isolation cuts select jet leading particles with most of the jet energy.

  18. END

  19. Isolation Cut: pp collision Efficiency: isolated gamma/ total gamma Signal/Background R=0.2 pTth=0.5 GeV/c R=0.2 pTth=4 GeV/c

  20. Isolation Cut: PbPb collision Efficiency: isolated gamma/ total gamma Signal/Background R=0.2 pTth=2 GeV/c R=0.2 pTth=4 GeV/c

  21. Jet production -2 <  < 2, 0º <  < 360º • Jets reconstructed with UA cone algorithm (default JETAN) • E seed = 4 GeV • Cone size R=1 • Jet with highest energy in the event selected. • Between 20% and 50% more jets in PYTHIA for Ejet > 30 GeV • The higher the energy the better the agreement for E jet >100 GeV

  22. -jet fragmentation function pT > 20 GeV, -1.5 < jet hadrons< 1.5, 0º < jet hadrons< 360º • Jets reconstructed with UA cone algorithm (default JETAN) • E seed = 4 GeV • Cone size R=1 • Fragmentation function constructed collecting all particles in a cone centered in the reconstructed jet axis. • R = 1 • pT cut = 0.5 GeV/c • Underlying event not subtracted. • Similar FF, although a 5-10% smaller for HERWIG at small z. • more important difference at high z but low statistic.

  23. Charged particles production -1 <  < 1, 0º <  < 360º • Charged pions production is similar in both generators, a bit larger with HERWIG. • Protons and antiprotons production is up to a factor 2 larger in HERWIG.

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