Heavy Flavor Measurements at STAR

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Heavy Flavor Measurements at STAR

• Haibin Zhang
• Brookhaven National Laboratory
• for the STAR Collaboration

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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 pp
  to AuAu

• Its important to measure charm total
  cross-section in AuAu and compare to that in pp
  and dAu

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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
  (lt3-5 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

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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

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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)

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Detailed Measurements

• D0?K? in dAu and AuAu (0ltpTlt3 GeV/c, ylt0.5)

• H. Zhang, QM2005

• STAR paper, PRL 94 (2005) 062301

• Heavy-flavor decayed single electron spectra in
  pp, dAu and AuAu

• Measured by Time-of-Flight Detector (1ltpTlt4
  GeV/c, -1lt?lt0)

• H. Zhang, QM2005

• STAR paper, PRL 94 (2005) 062301

• Measured by Barrel EMC Detector (1.2ltpTlt10
  GeV/c, 0lt?lt1)

• J. Bielcik, QM2005 and this conference

• Charm decayed muon spectra in AuAu
  (0.17ltpTlt0.25 GeV/c, -1lt?lt0)

• C. Zhong, this conference

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Combined Fit
D0, e? , ?? combined fit
Power-law function with parameters dN/dy, ltpTgt
and n to describe the D0 spectrum
Generate D0?e decay kinematics according to the
above parameters
Vary (dN/dy, ltpTgt, 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
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Charm Total Cross Section
Charm total cross section per NN interaction
1.4 ? 0.2(stat.) ? 0.4(sys.) mb in 200 GeV
minbias dAu
1.26 ? 0.09 ? 0.23 mb in 200 GeV minbias AuAu
STAR Preliminary
1.33 ? 0.06 ? 0.18 mb in 200 GeV 0-12 AuAu
Charm total cross section follows Nbin scaling
from dAu to minbias AuAu to central AuAu
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!!!
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Discrepancy at RHIC?
PHENIX, PRL 94, 082301(2005)
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Blast-Wave Fit Charm Freeze-Out
STAR Preliminary
STAR Preliminary
Blast-wave fit combining D0, muons, and electrons
at pTlt2 GeV/c
Charm hadrons may freeze-out earlier Tgt140 MeV
Charm hadron collective velocity lt?Tgt less than
that of ? and ? - charm flow?
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RAA of Non-Photonic Electrons
PHENIX, PRL96
J. Bielcik, this conference

STAR Highlight talk

• Evidence of large heavy quark energy loss!

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Nuclear Modification Factor - EMC

• Theories currently do not describe the data

• Only charm contribution would describe the RAA
  but not the pp spectra

• However, the amount of beauty contributions to
  electrons is still uncertain!!

• We need to measure RAA from Ds directly to
  clarify

J. Bielcik, this conference
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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 AuAu events
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Summary

• Charm total cross section per NN collision
  follows Nbin scaling from dAu to minbias
  AuAu to central AuAu ? charm produced via
  initial parton fusion

• Blast-wave fit to charm spectra ? small lt?gt,
  large Tfo ? charm hadrons may freeze-out earlier

• Strong suppression of non-photonic electron RAA
  at high pT observed in central AuAu collisions ?
  Challenge to existing energy loss models

• Charm transverse momentum distribution has been
  modified by the hot and dense medium in central
  AuAu collisions!!!