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Christof Roland Massachusetts Institute of Technology for the CMS Collaboration

Extracting jet fragmentation functions using photon-tagged jet events in Pb+Pb collisions with the CMS experiment. Christof Roland Massachusetts Institute of Technology for the CMS Collaboration. ECT Workshop on Parton Fragmentation Processes in the Vacuum and in the Medium

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Christof Roland Massachusetts Institute of Technology for the CMS Collaboration

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  1. Extracting jet fragmentation functions using photon-tagged jet events in Pb+Pb collisions with the CMS experiment Christof Roland Massachusetts Institute of Technology for the CMS Collaboration ECT Workshop on Parton Fragmentation Processes in the Vacuum and in the Medium Trento, Italy, 2008

  2. Ingredients: Event/Centrality selection Reaction plane determination Vertex + track finding Jet finding Photon detection All results based on GEANT-4 simulations using full reconstruction algorithms on 1-year statistics Jet Hadrons q,g q,g q,g Photon In-medium fragmentation functions Multiplicity and flow measurements characterize density, path length Jet axis provides parton direction Charged hadron tracks used to calculate z = pT(track)/ET Photon energy tags parton energy ET Measure dN/dlog(1/z)

  3. Compton Bremsstrahlung Annihilation Fragmentation Signal processes Use photon to tag parton energy Goal: Best correlation of photon and parton energy Ideal: Use leading order photon In practice: Isolated photons to select events with good correlation of photon and parton energy

  4. Compton Annihilation MC signal definition Photon Isolation: • calculate total ET (*) in cone of R=0.5, ETtot • Require (ETtot – ET)< (5GeV + 0.05 ET) and • find hadron with highest pT, pTmax • pTmax < (4.5GeV + 0.025 ET) Photon-Jet pair selection • Events where an isolated photon is emitted back-to-back with a jet are our signal events (*) excluding neutrinos and muons

  5. generator- level 172o 172o σ~13% generator-level FF using photon energy tag • Using isolated photons + “back-to-back” cut on azimuthal opening angle between the photon and the jet to suppress NLO and background events • Photon energy well correlated with parton energy (RMS ~ 13%) • Fragmentation functions using parton energy and isolated photon energy agree (better than 10%)

  6. dN/dξ Adapted from hep-ph/0506218 ξ=log(ET/pT)‏ In-medium modified frag. functions Quenching increases low-pT particles 1GeV/c 10GeV/c jet z→1 z→0 (pT→ET)‏ (pT→0)‏ Quenching reduces high-pT particles hadrons

  7. Event generation • Study two scenarios • No quenching: PYTHIA signal and QCD background (p+p) events mixed with central unquenched Pb+Pb HYDJET events • No high-pT particle suppression • Leads to high background rates • Quenching: PYQUEN signal and QCD background (p+p) events mixed with central quenched Pb+Pb HYDJET events • Suppression of high-pT particles • Energy loss radiated out of jet cone • Challenging for jet finder PYQUEN v1.2: Eur. Phys. J. C 45 (2006) 211 HYDJET v1.2: hep-ph/0312204

  8. Study for one nominal LHC Pb+Pb run “year” 106 sec, 0.5nb-1, 3.9 x 109 events Use 0-10% most central Pb+Pb dN/dη|η=0~2400 Simulate signal and background QCD (p+p) events Mix into simulated Pb+Pb events (~1000 events)‏ Signal statistics generator-level Total 122158 Total 37490

  9. Background statistics • Select all PYTHIA (PYQUEN) events with potential to create high ET ECAL supercluster • i.e. is there a hadron, charged lepton or photon with ET > 70 GeV • For each such event, check if corresponding simulated event has a supercluster with ET > 60 GeV • If yes, mix PYTHIA event with simulated central Pb+Pb background event (at digi level) • We used 1000 Pb+Pb background events • Run through reconstruction

  10. Large (mid-rapidity) acceptance (tracker and calorimetry)‏ DAQ+HLT will inspect every single Pb+Pb event Large statistics for rare probes CMS Detector  = 10 for Calorimetry  = 5 for Si tracker 2 Capabilities High-precision tracking over < 2.5 Muon identification over  < 2.5 High resolution calorimetry over < 5 Forward coverage Large bandwidth: DAQ + Trigger Very well suited for HI environment J.Phys.G34:2307-2455,2007

  11. Reconstruction • Tracking • Low pT cutoff at 1GeV/c • Efficiency (algorithmic + geometric) ~ 50-60% • Fake rate ~ few % • Jet finding • Iterative cone algorithm with underlying event subtraction (R=0.5)‏ • Performance studies on away-side jet finding (see later)‏ • Photon ID • Reconstruction of high-ET isolated photons • New for this analysis (see next slides)‏ Tracking: NIM A566 (2006) 123 Jet finding: Eur. Phys. J. 50 (2007) 117

  12. p+p all hits (selective readout)‏ Pb+Pb all hits dN/dη~2400 p+p after seed threshold (0.5/0.18 GeV)‏ Pb+Pb after seed threshold (0.5/0.18 GeV)‏ ECAL response in p+p and Pb+Pb ECAL reconstruction chain used with standard p+p settings NB: The two p+p (QCD) events are not the same.

  13. Identification 10 cluster shape variables based on ECAL 10 isolation variables based on ECAL/HCAL Track-based cut Selection Total of 21 variables grouped into 3 sets Linear discriminant analysis (Fisher) andcut optimization using TMVA Photon ID: Isolation and cluster shape cuts * Prompt photon  ECAL ECAL HCAL HCAL *) Maximum set to 10 TMVA: http://tmva.sourceforge.net

  14. WP p+p Pb+Pb Photon identification performance • Set working point to 60% signal efficiency • Leads to 96.5% background rejection • Training is done on unquenched samples only

  15. Photon identification performance Quenched Pb+Pb S/B=4.5 S/B=0.3 Before cuts: After cuts: WP=60% Seff Photon isolation and shape cuts improve S/B by factor ~15 ET >70GeV

  16. Algorithm: CMS NOTE 2006/050 Authors: O. Kodolova, I. Vardanyan, A. Nikitenko, A.Oulianov. Eur. Phys. J. C 50 (2007) 117 123 Select away-side jet with (,jet) > 1720, ||< 2 and ET > 30 GeV The energy cut reduces the false rate to 10% level Analysis does not use jet energy otherwise Jet finding efficiency rises sharply between 30-100 GeV MC jet ET Main source of systematic uncertainty in reconstructed FFs Jet finding (away-side)‏ Quenched Pb+Pb Fraction of falsely matched jets Away-side jet finding efficiency Δφap>172o, |η|<2

  17. Jet finding in PYQUEN • Quenching mechanism in PYQUEN moves energy out of R=0.5 jet cone • This lowers jet finding efficiency for given initial parton ET

  18. Obtain dN/dξ using tracks in R=0.5 cone around jet axis For ξ>3 (~pT<4GeV/c) dN/dξ dominated by underlying Pb+Pb event Estimate background using R=0.5 cone rotated in φ by 90º relative to jet Sum event-by-event backgrounds and subtract Fragmentation functions Unquenched Quenched Underlying event Underlying event

  19. Major contributions to systematic uncertainty (added in quadrature)‏ Photon selection and background contamination (15%)‏ Track finding efficiency correction (10%)‏ Wrong/fake jet matches (10%)‏ Jet finder bias (largest contribution in quenched case, see next slide)‏ Reconstructed frag. functions Unquenched Quenched No or small  dependence

  20. MC truth for found reco jets MC truth for all jets Up to ~30% Quenched Up to ~10% Unquenched Jet finder bias Jet finder bias • Systematic uncertainties • Up to 30% in quenched case • 10% for unquenched case • It has two contributions • FFs and jet finding efficiency depend on parton ET • Can be corrected with known turn-on curve (not done here)‏ • For a given parton ET, jet finding probability depends on parton fragmentation pattern • The jet finder is more likely to find a jet with few high pT particles than jets with many soft particles • MC based correction might be possible (not done here)‏ • MC studies suggest that 2) dominates

  21. Medium modification of fragmentation functions can be measured High significance for 0.2 < ξ < 5 for both, ET >70GeV and ET >100GeV ET >70GeV ET >100GeV Fragmentation function ratio Reco quenched Pb+Pb / MC unquenched p+p Shaded bands show systematic uncertainties

  22. Summary • Complete study of in-medium fragmentation functions using photon-tagged jet events • Two scenarios: Unquenched and quenched cases • Pythia and Pyquen (+ Hydjet) • Key features of the study • Full statistics expected for nominal one-year CMS Pb+Pb run at LHC • Full detector simulations of signal and background • Complete reconstruction chain • Track finding • Jet finding • Photon isolation • Underlying event subtraction • Analysis of systematic errors • Uncertainty dominated by jet finder bias • Measurement of expected strong medium modification of fragmentation functions can be done reliably in central Pb+Pb

  23. BackUp Slides

  24. Rate Estimate • HYDJET + Pythia • Using HLT-Note/Jet Note parameters: • L = 0.5 nb-1, 7.8 b inelastic PbPb x-section • => 3.9 x 109 events • 0-10% central • HYDJET default data cards used for Pythia configuration • Ncoll from HYDJET • NN = 58mb • Run Pythia/Pyquen and scale with: • Ncoll x 1/ NN x 3.9*108 (0-10% central) • -Jet rate (pT>100 GeV/c, ||<2): • 1230 (0-10%), 3680 (mb)

  25. Photon isolation: Cone energy • Use the energy content in cone around candidate direction in ECAL and HCAL • ECAL: • HCAL: R Candidate R2 = 2 + 2

  26. Photon isolation: Background subtraction • Subtract HI background • ECAL: • HCAL: Use avg outside of cone (normalized with DR/4)‏

  27. Photon isolation: Combined calorimeters • Based on cone variables form • Combine to 0-10% quenched Hydjet to be determined coefficients

  28. Photon isolation: Tracks • dRxy: R of (y+1st) nearest track with pT > (0.4*x + 0.2) GeV/c • We use • dR10 for p+p • dR41 for Pb+Pb DR

  29. Photon ID: Shape variables • Based on ECAL shape variables form • Combine to 0-10% quenched Hydjet to be determined coefficients

  30. Performance for ET >70GeV Efficiency > 60% Fake rate < 20% Transverse energy resolution: 2-5% Isolated photon reconstruction Quenched Pb+Pb ET >70GeV S/B=0.3 Before cuts: S/B=4.5 After cuts:

  31. Event Selection Summary • Pb+Pb background events • 0-10% HYDJET v1.2, 1000 events, dN/d ~ 2400 • PYTHIA (v6.411)/PYQUEN (v1.2) events • pT > 70 GeV potential trigger particle • ET > 60 GeV reconstructed supercluster • Tracks • pT > 1 GeV/c, > 8 hits, prob > 0.01, DCA/DCA_Err < 3.0 • Reconstructed events • Isolated photon with ET > 70 (100) GeV, | < 2 • Jet with ET > 30 GeV, | < 2, (,jet) > 3 • Fragmentation function • Cone-size around jet axis: 0.5

  32. Jet Finder Bias (Quenched Jets) Significant bias (up to 30%) in quenched sample with ET, > 70GeV N.b.: Bias is present in p+p, i.e. can be studied independent of our measurement in Pb+Pb Bias about 50% smaller for ET, > 100GeV

  33. Unquenched Quenched Unquenched Quenched Fragmentation function results (ET, > 70 GeV)‏ Reconstructed FF agrees with MC FF within expected uncertainty Largest deviation at small  (large z)‏

  34. Unquenched Quenched Unquenched Quenched Fragmentation functions (ET, > 100 GeV)‏ Reconstructed FF agrees with MC FF within expected uncertainty 70 vs 100 GeV: Trade-off between statistical and systematic uncertainties

  35. Unquenched Quenched Unquenched Quenched Fragmentation functions (vs z)‏ ET, > 70GeV ET, > 100GeV

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