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Heavy Flavor Measurements at STAR

Heavy Flavor Measurements at STAR. Haibin Zhang Brookhaven National Laboratory for the STAR Collaboration. Motivation – Charm Production Mechanism. Our final goal is to understand the properties of the hot and dense matter produced in heavy ion collisions.

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Heavy Flavor Measurements at STAR

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  1. Heavy Flavor Measurements at STAR Haibin Zhang Brookhaven National Laboratory for the STAR Collaboration Haibin Zhang

  2. Motivation – Charm Production Mechanism • Our final goal is to understand the properties of the hot and dense matter produced in heavy ion collisions • Charm can provide a unique tool to study important properties of the new matter • However, we have to understand the charm production mechanism first: initial parton fusion, flavor excitation, etc. • Theorists believe charm is mainly produced in initial collisions via gluon fusion in relativistic heavy ion collisions (M. Gyulassy & Z. Lin, PRC 51 (1995) 2177) charm total cross-section should follow Nbin scaling from p+p to Au+Au • It’s important to measure charm total cross-section in Au+Au and compare to that in p+p and d+Au Haibin Zhang

  3. Motivation – Charm vs. Thermalization • Charm (Moore and Teney, PRC 71(2005) 064904) or “charm resonance” (Hees and Rapp, PRC 71(2005) 034907)interact with the medium via scattering: • Its phase space shape may be changed at low pT (<3-5 GeV/c) • Charm could pick up elliptic flow from the medium • Measurements of charm pT spectra and elliptic flow may give us hint that the partonic matter could be thermalized Haibin Zhang

  4. light (M.DjordjevicPRL 94 (2004)) Motivation – Charm Energy Loss • In 2001, Dokshitzer and Kharzeev proposed “dead cone” effect  charm quark small energy loss • Recent: Heavy quark energy loss in medium, e.g.: Armesto et al, PRD 71, 054027,2005;M. Djordjevic et al., PRL 94, 112301, 2005. • Mechanisms other than gluon emission may play an important role for heavy quark energy loss • Heavy quarks will be important to understand the Energy Loss mechanisms and the competition between them Haibin Zhang

  5. What STAR Measures • Hadronic decay channels:D0Kp (B.R.: 3.8%) • Semileptonic channels: • c  ℓ+ + anything (B.R.: 9.6%) • D0  e+ + anything(B.R.: 6.87%) • D0 + + anything(B.R.: 6.5%) Haibin Zhang

  6. STAR Main Detector Haibin Zhang

  7. D0 Measurement Technique Event mixing technique Select K and  tracks from PID by energy loss in TPC Combine all pairs from same event  Signal+Background • Combine pairs from different events Background • Signal = same event spectra – mixed event spectra • More details about this technique can be found at • PRC 71 (2005) 064902 and PRL 94 (2005) 062301 Haibin Zhang

  8. D0 Signal QM05 nucl-ex/0510063 PRL 94 (2005) 062301 Haibin Zhang

  9. p  e K e |1/–1| < 0.03  Electron ID - TOF • TOF measures particle velocity • TPC measures particle energy loss • The cut |1/-1|<0.03 with TOF excludes kaons and protons • TPC dE/dx further separates the electron and pion bands Haibin Zhang

  10. Electron ID - EMC • Charged tracks selected by TPC • EMC Tower hits association with TPC tracks required • Momentum/Energy ratio is cut to be around one for electron candidates • Shower size measured by Shower Max Detector (SMD) • Small shower size for hadrons • Large shower size for electrons • Both inclusive electron yield and hadron contamination obtained from Gaussian fit electrons Haibin Zhang

  11. Dominant source at low pT Photonic Background • For each tagged e+(e-), we select a partner e-(e+) identified only with the TPC and calculate the invariant mass of the pair. γ conversion π0Dalitz decay η Dalitz decay Kaon decay vector meson decays • Combinatorial background reconstructed by track rotating or like-sign technique. • Photonic background is subtracted in a statistical manner: Nphotonic = (un_like – rotating)/bkgrd_eff STAR Preliminary Haibin Zhang

  12. 0-12% Au+Au STAR Preliminary 0.25<pT<0.27 GeV/c 0.17<pT<0.21 GeV/c 0.21<pT<0.25 GeV/c Muon ID – TPC + TOF 0.17<pT<0.21 GeV/c 0-12% Au+Au STAR Preliminary p m m2 (GeV2/c4) Muon and pion bands slightly separated at low momentum in TPC TOF can further help to identify muons in mass2 distribution Backgrounds are mainly from ,K+ decays, can be subtracted from DCA distributions  charm decayed muons!! Haibin Zhang

  13. EMC non-photonic electron spectra are measured in p+p, d+Au, Au+Au 0-5%, 10-40%, 40-80% Non-Photonic Electron Spectra • TOF non-photonic electron spectra are measured in p+p, d+Au, Au+Au minbias, 0-12%, 0-20%, 20-40%, 40-80% STAR Preliminary • Non-photonic electron spectra measured by TOF and EMC are consistent with each other by proper Nbin scaling Haibin Zhang

  14. Combined Fit D0, e , combined fit Power-law function with parameters dN/dy, <pT> and n to describe the D0 spectrum Generate D0e decay kinematics according to the above parameters Vary (dN/dy, <pT>, n) to get the min. 2 by comparing power-law to D0 data and the decayed e shape to e and  data Spectra difference between e and  ~5% (included into sys. error) Advantage: D0 and  constrain low pT e constrains higher pT Haibin Zhang

  15. Charm Total Cross Section Charm total cross section per NN interaction 1.4  0.2(stat.)  0.4(sys.) mb in 200 GeV minbias d+Au 1.26  0.09  0.23 mb in 200 GeV minbias Au+Au STAR Preliminary 1.33  0.06  0.18 mb in 200 GeV 0-12% Au+Au Charm total cross section follows Nbin scaling from d+Au to minbias Au+Au to central Au+Au considering errors Supports conjecture that charm is exclusively produced in initial scattering However, the total cross section is a factor of ~5 larger than NLO predictions!!! Haibin Zhang

  16. Blast-Wave Fit – Charm Freeze-Out STAR Preliminary STAR Preliminary Blast-wave fit combining D0, muons, and electrons at pT<2 GeV/c Charm hadrons may freeze-out earlier – T>140 MeV Charm hadron collective velocity <T> less than that of  and  - charm flow? Haibin Zhang

  17. Nuclear Modification Factor - TOF STAR Preliminary • TOF non-photonic electron spectra suppressed in 0-12% central Au+Au Haibin Zhang

  18. STAR: Phys. Rev. Lett. 91 (2003) 172302 Nuclear Modification Factor - EMC • RdAu is above/consistent with unity • RAA suppression up to ~0.6 in 40-80% • Suppression up to ~0.5 in 10-40% • Strong suppression up to ~0.2 in 0-5% centrality at high pT (4-8 GeV/c) • Charm high pT suppression is as strong as light hadrons!!! • Careful with comparison of (decay) electrons and hadrons – only sensible when RAA flat at high-pT Haibin Zhang

  19. Nuclear Modification Factor - EMC • Charm high pT suppression is as strong as light hadrons!!! • Theories currently do not describe the data • Only charm contribution would describe the RAA but not the p+p spectra • However, the amount of beauty contributions to electrons is still uncertain!! • We need to measure RAA from Ds directly to clarify Haibin Zhang

  20. 0-80% 12M events STAR Preliminary Subtracted spectrum • An upper limit for  production is estimated from triggered data samples in 200 GeV Au+Au collisions • Detector upgrade: a full coverage (||<1 and 0<<2) TOF will be installed  greatly improve the electron identification ability to help the quarkonium measurements Quarkonium Measurements p+p STAR Preliminary • J/ signals observed in Au+Au and p+p 200 GeV collisions  more work needed to reach physics conclusions Haibin Zhang

  21. Detector Upgrate – Heavy Flavor Tracker A silicon detector, can provide a ~50m DCA resolution to reconstruct secondary decay vertices of charm hadrons Simulation with 1.43M central Au+Au events Haibin Zhang

  22. Summary • Charm total cross section per NN collision follows Nbin scaling from d+Au to minbias Au+Au to central Au+Au  charm produced via initial parton fusion • Blast-wave fit to charm spectra  small <>, large Tfo  charm hadrons may freeze-out earlier • Strong suppression of non-photonic electron RAA at high pTobserved in central Au+Au collisions  Challenge to existing energy loss models • Charm transverse momentum distribution has been modified by the hot and dense medium in central Au+Au collisions!!! Haibin Zhang

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