SQM2006, 03/27/2006Haibin Zhang1 Heavy Flavor Measurements at STAR Haibin Zhang Brookhaven National Laboratory for the STAR Collaboration

SQM2006, 03/27/2006Haibin Zhang2 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 N bin 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

SQM2006, 03/27/2006Haibin Zhang3 Motivation – Charm vs. Thermalization Charm (Moore and Teney, PRC 71(2005) ) or “charm resonance” (Hees and Rapp, PRC 71(2005) ) interact with the medium via scattering:  Its phase space shape may be changed at low p T (<3-5 GeV/c)  Charm could pick up elliptic flow from the medium Measurements of charm p T spectra and elliptic flow may give us hint that the partonic matter could be thermalized

SQM2006, 03/27/2006Haibin Zhang4 light (M.Djordjevic PRL 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, , 2005; M. Djordjevic et al., PRL 94, , Heavy quarks will be important to understand the Energy Loss mechanisms and the competition between them Mechanisms other than gluon emission may play an important role for heavy quark energy loss

SQM2006, 03/27/2006Haibin Zhang5  Hadronic decay channels: D 0  K  (B.R.: 3.8%)  Semileptonic channels:  c  ℓ + + anything (B.R.: 9.6%) –D 0  e + + anything(B.R.: 6.87%) –D 0   + + anything(B.R.: 6.5%) What STAR Measures

SQM2006, 03/27/2006Haibin Zhang6 STAR Main Detector

SQM2006, 03/27/2006Haibin Zhang7 D 0 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) and PRL 94 (2005)

SQM2006, 03/27/2006Haibin Zhang8 D 0 Signal PRL 94 (2005) QM05 nucl-ex/

SQM2006, 03/27/2006Haibin Zhang9 Electron ID - TOF e  K p 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 |1/  – 1| < 0.03 e 

SQM2006, 03/27/2006Haibin Zhang10 Electron ID - EMC electrons Charged tracks selected by TPC EMC Tower hits association with TPC tracks required Shower size measured by Shower Max Detector (SMD)  Small shower size for hadrons  Large shower size for electrons Momentum/Energy ratio is cut to be around one for electron candidates Both inclusive electron yield and hadron contamination obtained from Gaussian fit

SQM2006, 03/27/2006Haibin Zhang11 γ conversion π 0 Dalitz decay η Dalitz decay Kaon decay vector meson decays Dominant source at low p T 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. 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

SQM2006, 03/27/2006Haibin Zhang12 Muon ID – TPC + TOF   0.17<p T <0.21 GeV/c Muon and pion bands slightly separated at low momentum in TPC TOF can further help to identify muons in mass 2 distribution Backgrounds are mainly from  , K    +  decays, can be subtracted from DCA distributions  charm decayed muons!! m 2 (GeV 2 /c 4 ) STAR Preliminary 0-12% Au+Au 0.17<p T <0.21 GeV/c 0.21<p T <0.25 GeV/c 0.25<p T <0.27 GeV/c STAR Preliminary 0-12% Au+Au

SQM2006, 03/27/2006Haibin Zhang13 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% Non-photonic electron spectra measured by TOF and EMC are consistent with each other by proper N bin scaling STAR Preliminary EMC non-photonic electron spectra are measured in p+p, d+Au, Au+Au 0-5%, 10-40%, 40-80%

SQM2006, 03/27/2006Haibin Zhang14 Combined Fit Power-law function with parameters dN/dy, and n to describe the D 0 spectrum D 0, e ,   combined fit Generate D 0  e decay kinematics according to the above parameters Vary (dN/dy,, n) to get the min.  2 by comparing power-law to D 0 data and the decayed e shape to e  and   data Advantage: D 0 and   constrain low p T e  constrains higher p T Spectra difference between e  and   ~5% (included into sys. error)

SQM2006, 03/27/2006Haibin Zhang15 Charm Total Cross Section 1.26  0.09  0.23 mb in 200 GeV minbias Au+Au 1.4  0.2(stat.)  0.4(sys.) mb in 200 GeV minbias d+Au Charm total cross section per NN interaction Charm total cross section follows N bin scaling from d+Au to minbias Au+Au to central Au+Au considering errors Supports conjecture that charm is exclusively produced in initial scattering 1.33  0.06  0.18 mb in 200 GeV 0-12% Au+Au However, the total cross section is a factor of ~5 larger than NLO predictions!!! STAR Preliminary

SQM2006, 03/27/2006Haibin Zhang16 Blast-Wave Fit – Charm Freeze-Out Blast-wave fit combining D 0, muons, and electrons at p T <2 GeV/c Charm hadrons may freeze-out earlier – T>140 MeV Charm hadron collective velocity less than that of  and  - charm flow? STAR Preliminary

SQM2006, 03/27/2006Haibin Zhang17 Nuclear Modification Factor - TOF TOF non-photonic electron spectra suppressed in 0-12% central Au+Au STAR Preliminary

SQM2006, 03/27/2006Haibin Zhang18 Nuclear Modification Factor - EMC R dAu is above/consistent with unity R AA 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 p T (4-8 GeV/c) Charm high p T suppression is as strong as light hadrons!!! Careful with comparison of (decay) electrons and hadrons – only sensible when R AA flat at high-p T STAR: Phys. Rev. Lett. 91 (2003)

SQM2006, 03/27/2006Haibin Zhang19 Nuclear Modification Factor - EMC Charm high p T suppression is as strong as light hadrons!!! However, the amount of beauty contributions to electrons is still uncertain!! We need to measure R AA from Ds directly to clarify Theories currently do not describe the data Only charm contribution would describe the R AA but not the p+p spectra

SQM2006, 03/27/2006Haibin Zhang20 Subtracted spectrum 0-80% 12M events STAR Preliminary Quarkonium Measurements J/  signals observed in Au+Au and p+p 200 GeV collisions  more work needed to reach physics conclusions STAR Preliminary p+p 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

SQM2006, 03/27/2006Haibin Zhang21 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

SQM2006, 03/27/2006Haibin Zhang22 Summary Charm total cross section per NN collision follows N bin scaling from d+Au to minbias Au+Au to central Au+Au  charm produced via initial parton fusion Strong suppression of non-photonic electron R AA at high p T observed 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!!! Blast-wave fit to charm spectra  small, large T fo  charm hadrons may freeze-out earlier