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Jet Reconstruction at STAR in p+p and d+Au collisions

STAR. Jet Reconstruction at STAR in p+p and d+Au collisions. Thomas Henry Texas A&M University for the STAR Collaboration. Outline. Introduction Inclusive jet studies Dijet studies Dihadron studies. What is a Jet?. z= p t / p 1. Jets arise from hard partonic collisions

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Jet Reconstruction at STAR in p+p and d+Au collisions

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  1. STAR Jet Reconstruction at STARin p+p and d+Au collisions Thomas Henry Texas A&M University for the STAR Collaboration Thomas Henry STAR Collaboration

  2. Outline • Introduction • Inclusive jet studies • Dijet studies • Dihadron studies Thomas Henry STAR Collaboration

  3. What is a Jet? z= pt/p1 • Jets arise from hard partonic collisions • High pt partons fragment into a collimated spray of particles. Cone Radius Jet Thrust Axis: Et Hadron pt Leading Hadron Jt transverse momentum p1 parton In general, the cone is not bisected by the thrust axis. parton hadrons p2 Thomas Henry STAR Collaboration Thomas Henry STAR Collaboration

  4. Why is Jet Reconstruction Important? z= pt/p1 • Observables from fully reconstructed jets compare directly with pQCD theory • Reconstructed Et approximates parton p1 • Reduces fragmentation function ambiguities Cone Radius Jet Thrust Axis: Et Hadron pt Leading Hadron Jt parton p1 parton hadrons p2 Thomas Henry STAR Collaboration Thomas Henry STAR Collaboration

  5. Jets in p+p, d+Au, and Au+Au • p+p leads to Jt and intrinsic kt • d+Au leads to intrinsic kt + nuclear kt • Au+Au jets cannot be fully reconstructed due to huge multiplicity • Di- (and multi-) hadron correlations necessary • p+p and d+Au jet reconstruction calibrates these correlations Thomas Henry STAR Collaboration

  6. Jet Reconstruction Algorithms • Hard parton fragmentation products are strongly correlated in momentum • Jets reconstructed by exploiting hadron momenta correlations • Cone algorithms: capture spray of particles with geometric cone. Two strategies: • Center the cone on the seed particle • More robust for high multiplicity • Optimize the direction of the cone • Cone direction optimized for maximum energy • Kt algorithm: exploit relative pt • Add hadrons with progressively larger relative momenta • Hadrons below pt threshold not used Thomas Henry STAR Collaboration

  7. The STAR Detector • The complete f coverage and large h coverage of the TPC (with particle ID) and EMC make STAR excellent at reconstructing jets. EM Calorimeter Thomas Henry STAR Collaboration

  8. Neutral energy (which includes p0 decay photons) is measured with the STAR Electromagnetic Calorimeter For 2003 RHIC run, Barrel EMC included: 2p coverage in f; 0 < h < 1 2400 Towers (Dh x Df = 0.05 x 0.05) See posters by D. Arkhipkin, M.M. de Moura Read out in Minimum Bias Events Also used as trigger to select events likely to contain jets “High tower” trigger with ET > 2.5,4.5 GeV Other trigger topologies available but not used in 2003. The STAR EMC Thomas Henry STAR Collaboration

  9. From Observed Energy to Jet Energy • Measure both charged tracks and neutral energy • Correct for double counting of charged energy • Charged particle tracking efficiency ~.9 • EMC geometric acceptance ~.94 • Long lived neutrals (n, KL, …) lost • Soft particles may fall outside jet cone • Total correction factors from Pythia • 1/~0.8 for minbias • 1/~0.86 for high tower Need verification Thomas Henry STAR Collaboration

  10. Jet Et s = 200 GeV pt Et Jt p+p High Tower p+p MinBias • p+p collisions; Rcone=.7 • MinBias • High tower triggered • Corrected <Et> = 11.3 ± 0.7sysGeV STAR Preliminary Raw Per Event Yield pt Et p1 Jt Observed (uncorr.) Jet Et (GeV) Thomas Henry STAR Collaboration

  11. Toward a Jet Fragmentation Function s = 200 GeV pt Et Jt STAR Preliminary • Pythia-GEANT is pythia run through the STAR detector simulator • Agree at low z • High z deviations under study slope ~ 8 p+p MinBias pythia-GEANT Raw Per Event Yield z= pt/p1 p1 pt/(Uncorrected) Et (~z) Thomas Henry STAR Collaboration

  12. Jet Jt s = 200 GeV pt Et Jt pt > 0.6 STAR Preliminary • Primarily sensitive to jet axis direction • Low pt kinematic limit: “Seagull Effect” • RMS Jt • 490 ± 50sysMeV/c, pt>0.6 • 615 ± 60sysMeV/c, pt>2.0 Relative 1/JtdN/dJt pt > 2.0 pp MinBias pythia-GEANT Jt in GeV/c Agree within 4% Thomas Henry STAR Collaboration Thomas Henry STAR Collaboration

  13. DiJet Df Distribution kt= <kt2> (per parton) = <Et> sin sDf p1 p2 Df • Df = p in leading order • Gluon radiation broadens Df • Df is a probe of kt Thomas Henry STAR Collaboration

  14. DiJet Reconstruction • Trigger Jet • Reconstructed from EMC and TPC • Includes high tower trigger • Determines energy scale and first thrust axis • 0.2 < h < 0.65 • Et > 5 GeV Trigger Jet High tower trigger parton • Away Jet • Charged particles only • Determines second thrust axis • -0.5 < h < 0.5 • Et > 4 GeV parton Away Jet Thomas Henry STAR Collaboration Thomas Henry STAR Collaboration

  15. p+p DiJet Df Distribution s = 200 GeV 2 kt=<pt>pair (GeV/c) 1.6* 10-3 • Dijet high tower corrected <Et>= 13.0±0.7Sys GeV • kt=2.3±0.4 ± GeV/c STAR Preliminary 0.67 1.11 L. Apanasevich et al., Phys. Rev. D59, 074007 0.03 0.05 Df=0.23±0.02± p+p STAR Preliminary Raw Per Event Yield 0 s (GeV) 0 p 2p Df (radians) Thomas Henry STAR Collaboration

  16. Nuclear kt in d+Au s = 200 GeV 1.4 * 10-3 STAR Preliminary R=.35 Cdf p+p d+Au • ktobs2 = ktintrin2 + ktnucl2 • d+Au vs p+p: Nuclear kt = 2.8±1.2±1.0 GeV/c Df=0.22 ± 0.02 ± .06 Df=0.31 ± 0.05 ± .06 Raw Per Event Yield 0 p 2p 0 Df (radians) Thomas Henry STAR Collaboration

  17. Dihadron Correlations • Au+Au: jet reconstruction fails • Resort to dihadron correlations: • extract Jt and kt*<z> • Strategy: • Calibrate d+Au dihadron with d+Au jets • Compare Au+Au dihadron with d+Au dihadron p1 p2 Df Thomas Henry STAR Collaboration

  18. High Tower – Charged Hadron Correlation Functions(variation with trigger energy) See poster by S. Chattopadhyay 4.5 < Ettrig < 6.5 GeV/c 6.5 < Ettrig < 8.5 GeV/c ptassoc > 2.0 GeV/c Preliminary results: Jt = 500 ± 40 ± 150MeV/c kt= 1.9 ± 0.2 ± 0.3 GeV/c /<z> <z> needed for trigger hadron Will measure this to very high Ettrig in Au+Au collisions during current RHIC run Thomas Henry STAR Collaboration

  19. DiHadrons and DiJets • Jt • p+p jets: 615 ± 60sys MeV/c • d+Au dihadrons: 500 ± 40 ± 150 MeV/c •  Consistent between inclusive jets and dihadrons • kt • p+p dijets: intrinsic kt = 2.3 ± 0.4 GeV/c • d+Au dijets: nuclear kt = 2.8 ± 1.2 ± 1.0 GeV/c • d+Au dihadrons: total kt = (1.9 ± 0.2 ± 0.3)/<z> GeV/c • Uncertainties are conservative +0.67 -1.11 Thomas Henry STAR Collaboration

  20. Conclusions • Fully reconstructed jets in p+p and d+Au at RHIC • p+p: Jt and intrinsic kt • d+Au: nuclear kt • Pythia provides a good description of Jt • Future: • Jet and dijet cross sections in p+p and d+Au • Rd+Au for jets/partons Thomas Henry STAR Collaboration

  21. Thomas Henry STAR Collaboration

  22. STAR-PHENIX Comparison 2 kt=<pt>pair (GeV/c) PHENIX STAR L. Apanasevich et.al., Phys. Rev. D59 074007 STAR Preliminary J. Rak, PHENIX, arXiv:nucl-ex/0306031 v3 <pt>pair (GeV/c) s (GeV) s (GeV) • Jt=615±60Sys MeV/c • kt=2.3±0.4± GeV/c • Jt=661±28 MeV/c • kt=(1.3±0.06)/<z> GeV/c 0.67 1.11 Thomas Henry STAR Collaboration

  23. Difficulty in d+Au due to High Multiplicity s = 200 GeV s = 200 GeV Cone Radius Et • d+Au jet signals at radius .7 are swamped by false jets • Smaller radius reduces measured Et • < Et(R=.35)> = .88*< Et(R=.7)> for high tower triggered events p+p : R= .35Cdf, .35, .7 STAR Preliminary p+p Pythia- GEANT Log raw per event yield Log scale STAR Preliminary 0 1 7 25 Et(R=.35)/ Et(R=.7) for identical jets Et (Uncorrected) in GeV Thomas Henry STAR Collaboration

  24. Correction for Double Counting of Charged Hadron Energy s = 200 GeV STAR Preliminary • Charged particles can leave some energy in the EMC • Either can remove 20% of Track E or 0.3 GeV per Track from the EMC hits • 20% comes from Monte Carlo • .3 GeV = MIP • The two approaches are consistent, and reduce the energy as expected .009 d+Au No subtraction 0.2 x track E 0.3 GeV/track Raw Per Event Yield Uncorrected Jet Energy in GeV Thomas Henry STAR Collaboration

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