Jaroslav Bielčík for STAR collaboration Czech Technical University & NPI ASCR Prague 34-th International Conference on High Energy Physics 2008 Philadelphia, PA, USA The study of heavy flavors via non-photonic electrons in STAR

Outline Motivation for heavy flavor physics Spectra – Charm mesons: D 0 – Non-photonic electrons Energy loss Summary

3 Probing of dense nuclear matter with jets p+p Collision Au+Au Collision partons interact with medium gluon radiation/energy loss measuring high-p T particles in Au+Au vs. p+p to extract the properties of medium Average number of NN collisions in AA collision nuclear modification factor R AA : No “Effect” of nuclear matter: R AA = 1 at higher momenta where hard processes dominate Suppression: R AA < 1

4 Direct photons : not suppressed, do not interact binary scaling works Hadron yields : strongly suppressed in central Au+Au at 200 GeV Hadron suppression in central Au+Au Large energy loss of light partons in the formed nuclear matter Energy loss depends on properties of medium (gluon densities, size) properties of “probe” (color charge, mass)

5 Heavy quarks as a probe p+p data:  baseline of heavy ion measurements  test of pQCD calculations Due to their large mass heavy quarks are primarily produced by gluon fusion in early stage of collision  production rates calculable by pQCD M. Gyulassy and Z. Lin, PRC 51, 2177 (1995) heavy ion data: Studying energy loss of heavy quarks  independent way to extract properties of the medium parton hot and dense medium light M.Djordjevic PRL 94 (2004) ENERGY LOSS dead-cone effect: Dokshitzer and Kharzeev, PLB 519, 199 (2001)

Open heavy flavor Direct: reconstruction of all decay products Indirect: charm and beauty via electrons c  e + + anything (B.R.: 9.6%) b  e + + anything (B.R.: 10.9%) issue of photonic background charm (and beauty) via muons c   + + anything (B.R.: 9.5%)

7 STAR detector TPC (tracking,p,dEdx) |  | < 1.5  p/p = 2-4%  dE/dx /dEdx = 8% BEMC (energy, trigger) |  | < 1 dE/E ~ 16%/  E Shower maximum detector Most relevant for presented results:

8 STAR arXiv: Direct D-meson reconstruction at STAR D0D0 Phys. Rev. Lett. 94 (2005) K  invariant mass distribution in d+Au, Au+Au, Cu+Cu at 200 GeV collisions No topological analysis => only p T <3.3 GeV/c

9 Measurement of charm cross section STAR charm cross section: combined fit of muons, D 0 and low p T electrons  90% of total kinematic range covered Low p T muons constrain charm cross-section STAR arXiv: AuAu 200 GeV

10 High-tower EMC trigger => high p T electrons FONLL scaled by ~5, describes shape of p+p spectra well suggesting bottom contribution High p T non-photonic electrons STAR Phys. Rev. Lett. 98 (2007) STAR Phys. Rev. Lett. 98 (2007) PHENIX Phys. Rev. Lett. 97 (2006)

11 Charm cross-section at 200 GeV STAR data in various systems  observation of binary scaling Consistent with NLO calculation  however uncertainties are huge STAR x PHENIX results ~ factor 2 difference

d+Au: no suppression expected slight enhancement expected (Cronin effect) Peripheral Au+Au: no suppression expected Semi-Central Au+Au: very little suppression expected Non-photonic electrons suppression STAR hadrons p T > 6 GeV/c Central Au+Au: little suppression expected ?! Nuclear modification factor STAR Phys. Rev. Lett. 98 (2007) PHENIX Phys.Rev.Lett.98 (2007) Non-photonic electrons suppression similar to hadrons p T (NPE) < p T (D  NPE)

preliminary –R AA ~ for p T > 3 GeV/c Non-photonic electrons in Cu+Cu 200GeV – ~ 82 for 0-54% Cu+Cu 200 GeV:

* Consistency with Au+Au 200GeV results Consistent with  ± R AA in Cu+Cu 200 GeV Consistent with Au+Au 200 GeV data for similar N part R AA for  ±, Cu+Cu 200 GeV R AA for non-photonic e ± STAR: PRL 98 (2007) PHENIX: PRL 98 (2007) Non-photonic e ± Cu+Cu 200 GeV 0-54% STAR preliminary

Radiative energy loss Radiative energy loss alone in medium with reasonable parameters does not describe the data Djordjevic, Phys. Lett. B (2006) Armesto, Phys. Lett. B (2006) What are the other sources of energy loss ? parameters of medium in models extracted from hadron data STAR Phys. Rev. Lett. 98 (2007) PHENIX Phys.Rev.Lett.98 (2007)

Role of collisional energy loss Wicks, nucl-th/ van Hess, Phys. Rev. C (2006) Collisional/elastic energy loss may be important for heavy quarks Still not good agreement at high-pT STAR Phys. Rev. Lett. 98 (2007) PHENIX Phys.Rev.Lett.98 (2007)

Charm alone? Since the suppression of b quark electrons is smaller – charm alone agrees better What is b contribution? STAR Phys. Rev. Lett. 98 (2007) PHENIX Phys.Rev.Lett.98 (2007)

18 Relative bottom contribution Good agreement among different analyses. Relative b/(b+c) ratio consistent with FONLL. (b  e)/(c  e+b  e) Difficult to interpret suppression without the knowledge of bottom vs. charm relative ratio Data shows non-zero bottom contribution

19 Conclusions Heavy flavor is an important tool to understand HI physics at RHIC STAR results are interesting and challenging charm cross section Binary scaling in charm production produced in initial phase FONLL is consistent with data STAR/PHENIX discrepancy RUN 8 data on tape non-photonic electrons Strong high-p T suppression in Au+Au large energy loss of c b contribution consistent with FONLL important b contribution Cu+Cu and Au+Au results consistent heavy quark energy loss not understood How much is b suppressed? Not clear yet. also large uncertainties

Bottom suppression from data No experimental evidence that bottom is suppressed more than predicted T.Ullrich Hard Probes 2008

21 STAR PHENIX PHOBOS BRAHMS RHIC has been exploring nuclear matter at extreme conditions over the last few years STAR Relativistic Heavy Ion Collider RHIC site in BNL on Long Island Lattice QCD predicts a phase transition from hadronic matter to a deconfined state, the Quark-Gluon Plasma Colliding systems: p  +p , d+Au, Cu+Cu, Au+Au Energies √s NN = 20, 62, 130, 200GeV

PRL 98, (2007) l at  B = 0 l  + P = Ts l then l  /s = ( )/4  l transport models l Rapp & van Hees (PRC 71, (2005)) –diffusion coefficient required for simultaneous fit of R AA and v 2 –D HQ x2  T ~ 4-6 Estimating  /s l Moore & Teaney (PRC 71, (2005)) –difficulties to describe R AA and v 2 simultaneously –calculate perturbatively (and argue that plausible also non-perturbatively) –D HQ / (  /(  +P)) ~ 6 (for N f = 3)

S. Gavin and M. Abdel-Aziz: PRL 97:162302, 2006 p T fluctuations STAR Comparison with other estimates R. Lacey et al.: PRL 98:092301, 2007 v 2 PHENIX & STAR H.-J. Drescher et al.: arXiv: v 2 PHOBOS conjectured quantum limit l estimates of  /s based on flow and fluctuation data l indicate small value as well l close to conjectured limit l significantly below  /s of helium (  s ~ 9)

Charm ~ y

Uncertainty of c/b relative contribution FONLL: Large uncertainty on c/b crossing 3 to 9 GeV/c Beauty predicted to be significant above 4-5 GeV/c

Low-p T (p T < 0.25 GeV/c) muons can be measured with TPC + ToF - this helps to constrain charm cross-section Separate different muon contributions using MC simulations: -K and  decay -charm decay -DCA (distance of closest approach) distribution is very different m inv 2 (GeV 2 /c 4 )   0.17 < p T < 0.21 GeV/c 0-12% Au+Au Muon measurement TPC+TOF Inclusive   from charm  from  / K (simu.) Signal+bg. fit to data (STAR), Hard Probes 2006 m 2 =(p/  /  ) 2

Extraction of the cross-section Inelastic cross-section in p+p (UA5) Conversion to full rapidity (Pythia) Ratio obtained from e + e - collisions Number of binary collisions (Glauber) STAR Preliminary:

hadrons electrons 1.TPC: dE/dx for p > 1.5 GeV/c Only primary tracks (reduces effective radiation length) Electrons can be discriminated well from hadrons up to 8 GeV/c Allows to determine the remaining hadron contamination after EMC 2.EMC : a)Tower E ⇒ p/E~1 for e - b)Shower Max Detector Hadrons/Electron shower develop different shape 85-90% purity of electrons (p T dependent) electrons  Kp d all p>1.5 GeV/c 2 p/E SMD Electron ID in STAR – EMC

Photonic electrons background  Background : Mainly from  conv and    Dalitz  Rejection strategy: For every electron candidate  Combinations with all TPC electron candidates  M e+e- <0.14 GeV/c 2 flagged photonic  Correct for primary electrons misidentified as background  Correct for background rejection efficiency ~50-60% for central Au+Au Inclusive/Photonic:  Excess over photonic electrons observed for all system and centralities => non-photonic signal

 CC : comparison with other measurements

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) Combined Fit