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Heavy Flavor Measurements at STAR

This research aims to investigate heavy flavor phenomena, particularly focusing on charm production mechanisms and interactions with the hot and dense matter created in heavy ion collisions. The study delves into charm's role in thermalization and energy loss processes, utilizing measurements of charm total cross-section, pT spectra, and elliptic flow to gain insights into partonic matter properties. The STAR Collaboration at Brookhaven National Laboratory employs innovative techniques for D0 measurement, electron and muon identification, and detection of non-photonic electrons. Through a combined fit approach, this study analyzes D0, e±, and μ± spectra to understand the behaviors of heavy quarks and their decays in various collision scenarios.

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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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