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Open Charm Measurement at

Open Charm Measurement at. STAR. Yifei Zhang. University of Science & Technology of China Lawrence Berkeley National Lab. Outline: Introduction Measurements / problems Heavy flavor in the future Summary. STAR Regional Meeting, Pusan, May 6-9, 2009. Motivation - Heavy quark masses.

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Open Charm Measurement at

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  1. Open Charm Measurement at STAR Yifei Zhang University of Science & Technology of China Lawrence Berkeley National Lab • Outline: • Introduction • Measurements / problems • Heavy flavor in the future • Summary STAR Regional Meeting, Pusan, May 6-9, 2009

  2. Motivation - Heavy quark masses Mb 4.8 GeV >> Tc, LQCD, Muds Mc 1.5 GeV • Higgs mass: electro-weak symmetry breaking. (current quark mass) • QCD mass: Chiral symmetry breaking. (constituent quark mass) • Heavy quark masses are not modified by QCD vacuum. Strong interactions do not affect them. • Important tool for studying properties of the hot-dense matter created at RHIC energy. X.Zhu, et al., PLB 647 (2007) 366 September 23, 2014 Yifei ZhangUSTC/LBNL 2

  3. Probe medium properties Charm quarks are believed to be mainly produced by initial gluon fusions => Probe the medium properties at early stage. Charm collectivity => Light flavor thermalization. High dense / strong interacting matter September 23, 2014 Yifei ZhangUSTC/LBNL 3

  4. Heavy quark e-loss M.Djordjevic PRL 94 (2004) S. Wicks et al.NPA 784 (2007) 426 • Dead cone effect predicts energy loss b < c < q. • Additional elastic energy loss for charm and bottom? • Separate RAA(ce) & RAA(be)? => heavy quark energy loss mechanism, heavy quark interaction with medium. September 23, 2014 Yifei ZhangUSTC/LBNL 4

  5. STAR Detector TPC – p, tracking Large acceptance Mid-rapidity || < 1 Full barrel coverage 0 <  < 2p TOF – PID 2 trays in Run3 and Run4 ~ /30 5 trays in Run8 ~ /12 -1 <  < 0 September 23, 2014 Yifei ZhangUSTC/LBNL 5

  6. Open charm measurement at STAR Three independent measurements: Direct D0 reconstruction (D0 K, B.R. = 3.83%) muon from charm semileptonic decay (c  + + X, B.R. = 8.2%) electron from heavy quark semileptonic decay (c  e+ + X, B.R. = 10.3%) Physics: Charm cross section Charmed hadron freeze out and flow properties Heavy flavor energy loss Separation between charm and bottom September 23, 2014 Yifei ZhangUSTC/LBNL 6

  7. Direct D0 reconstruction The invariant mass spectrum of D0 mesons -- pairing each oppositely charged kaon and pion in the same event. Kaon and pion were identified by the dE/dx measured in theTPC. Combinatorial background – reconstructed using event-mixing technique. Invariant mass distr. from same event and mix event. Invariant mass distr. After mix-event b.g. subtracted. September 23, 2014 Yifei ZhangUSTC/LBNL 7

  8. Final spectra and combined fit Black curves: power-law D0 pT spectrum and electron spectra decay from charmed hadron power-law distributions. Red curves: blast-wave D0 pT spectrum and electron spectra decay from charmed hadron blast-wave distribution. STAR Preliminary Freeze-out/flow parameters and cross-sections are extracted from the combined fit. See C. Zhong ‘s talk for the details of heavy flavor lepton measurements. September 23, 2014 Yifei ZhangUSTC/LBNL 8

  9. Charm cross-section The cross sections follow number of binary collisions scaling within errors => evidence of  charm production at initial stage. PRL 94 (2005) Systematical errors dominant. Huge uncertainties of theory calculations. STAR data are about a factor of 2 higher than PHENIX. September 23, 2014 Yifei ZhangUSTC/LBNL 9

  10. Inconsistency between STAR & PHENIX Systematically a factor of 2 higher than PHENIX. STAR low pT muon and high pT electron can be fitted by one decay curve. Can NOT fit STAR low pT muon and PHENIX electron simultaneously. September 23, 2014 Yifei ZhangUSTC/LBNL 10

  11. Consistency at STAR Au+Au minbias 0-80% at 200 GeV The cross sections from combined fit with +e (w/o D0) and D0+e (w/o ) are consistent with D0++e. Combining all these three measurements reduces errors, covering ~90% of kinematics. September 23, 2014 Yifei ZhangUSTC/LBNL 11

  12. Consistency at STAR STAR TOF data are consistent with STAR EMC data in different collision systems. September 23, 2014 Yifei ZhangUSTC/LBNL 12

  13. BEMC TOF trays TPX + TOF 1 sector installed Consistency at STAR • Run8 absence of inner tracking detectors, background of gamma conversion reduced by a factor of ~ 10. • The sum of e/ from gamma conversion and Dalitz decays of light mesons and NPE is consistent with inclusive e/ for both Run3 and Run8. September 23, 2014 Yifei ZhangUSTC/LBNL 13

  14. D0 STAR Preliminary Charm freeze out and flow STAR Preliminary p and  RAA are rescaled to follow binary scaling. Deviation from proton and  suggests charmed hadron freeze-out and flow are different from light hadrons. Tfo = 220 MeV (best fit) – freeze out earlier <t> = 0.23 (best fit), <0.42 (1-). favor moderate or small radial flow. -- dense medium, light flavor thermalization. September 23, 2014 Yifei ZhangUSTC/LBNL 14

  15. Heavy flavor energy loss • Strong suppression at high pT has been observed. • The suppression level (~0.3) is similar as light hadrons. • Many models try to explain the data. But the heavy flavor energy loss mechanism is still not clear. • Bottom contributes at high pT. It is crucial to separate charm and bottom for understanding the heavy flavor energy loss mechanism. STAR, PRL 98 (2007) 192301 September 23, 2014 Yifei ZhangUSTC/LBNL 15

  16. Correlation: bottom contribution 9 1 pT [GeV/c] • NPE-h correlation, agree with FONLL calculation. • RAA(eB) < 0.9, bottom suppressed. • Large systematic errors for both theory and data. • Strong model dependent for data, but it is the best attempt to separate bottom. • Inner vertex detector will provide better constraint. September 23, 2014 Yifei ZhangUSTC/LBNL 16

  17. Which charm cross section is correct? STAR or PHENIX? Does charm indeed freeze out earlier and favor smaller flow? Why NPE has similar suppression as light hadrons? What is the energy loss mechanisms of heavy flavor? What is bottom and what is charm? ...... To solve above puzzles / problems: We need inner vertex detector That is the mission of the STAR Heavy Flavor Tracker ...

  18. Inner Tracking Detectors • SSD Silicon Strip Detector, existing single layer detector, double side trips. • IST 500 m x 1cm strips along beam direction, 24 liquid cooled ladders equipped with 6 silicon strip-pad sensors. • PIXEL double layers, 18.4x18.4 m pixel pitch, 2 cm x 20 cm each ladder. Deliver ultimate pointing resolution, hit density for 1st layer ~ 60 cm.-2 September 23, 2014 Yifei ZhangUSTC/LBNL 18

  19. Hadronic channels D0 c STAR HFT has the capability to reconstruct the displaced vertex of D0K (B.R.=3.8%, c=123 m) and c Kp (B.R.=5.0%, c=59.9 m) September 23, 2014 Yifei ZhangUSTC/LBNL 19

  20. D0 reconstruction (b) (a) • D0 efficiency vs pT • D0 significance vs pT • D0 mass at 3.0 < pT < 3.5 GeV/c before background subtraction. (c) September 23, 2014 Yifei ZhangUSTC/LBNL 20

  21. Error estimate of D0 v2 and Rcp Assuming D0 v2 distribution from quark coalescence. 500M Au+Au m.b. events at 200 GeV. Charm collectivity => Medium properties, light flavor thermalization. Assuming D0 Rcp distribution as charged hadron. 500M Au+Au m.b. events at 200 GeV. Charm energy loss => Energy loss mechanisms, QCD in dense medium. September 23, 2014 Yifei ZhangUSTC/LBNL 21

  22. Electron dca and spectra Electrons: PID by TPC+TOF, || < 0.9, 2 PXL hits required. Photonic background can be removed from its small invariant mass character combining a pair of electrons. Other background is small. Normalized by the F.R. and B.R., and total electron yield was normalized to STAR measured NPE spectrum. (Be) / NPE ratio was normalized to fit STAR measured data (from e-h correlation). HFT has the capability to distinguish the different source of electrons. Yifei ZhangUSTC/LBNL

  23. c reconstruction Good D0 pT distribution measurement as reference. The unique charmed baryon. c / D0 ratio => enhancement? September 23, 2014 Yifei ZhangUSTC/LBNL 23

  24. Summary • Three independent measurements D0, , e are measured by STAR covering ~90% kinematics. • Charm cross section was found to follow number of binary collisions scaling, indicating charm mainly produced at initial stage. But discrepancy between STAR and PHENIX has not been understood yet. • Charmed hadron freeze-out and flow properties are different from light hadrons. • Large suppression of NPE at high pT was observed and similar as light hadrons. But the energy loss mechanism of heavy flavor is still unknown. • e-h correlation was tried to separate charm and bottom, but with large sys. error and model dependent. • STAR has bright future to measure D mesons topologically and constrains electron from charm and bottom decays separately. 고맙습니다 !! September 23, 2014 Yifei ZhangUSTC/LBNL 24

  25. Low pT  in 200 GeV Au+Au collisions e-print ArXiv: 0805.0364 STAR Preliminary STAR Preliminary TPC + TOF PID STAR Preliminary 0.16 < pT < 0.21 GeV/c Low pT  constrains dN/dy No photonic background. Signal: excess above the bg. September 23, 2014 Yifei ZhangUSTC/LBNL 25

  26. |1-1/| < 0.03 TPC+TOF Electron identification p K TPC p e  K  e Inclusive electron was separated from hadrons by TOF velocity cut. Raw yields were extracted from fit to the projected dE/dx distributions. September 23, 2014 Yifei ZhangUSTC/LBNL 26

  27. Inclusive electron and photonic B.G. Method: J. Adams, et al. PRC 70 (2004) 044902 inclusive / photonic ratio increases, the excess above 1 indicates the signal. September 23, 2014 Yifei ZhangUSTC/LBNL 27

  28. Additional Capability -- Semi-leptonic Channels B.R. = Branching Ratio F.R. = Fragmentation Ratio The distance of closest approach to primary vertex (dca): Due to larger c, B e has broader distribution than D  e Dca of D+ e is more close to that of B  e dca September 23, 2014 Yifei ZhangUSTC/LBNL 28

  29. (Be)/NPE and v2 (Be)/NPE ratio can be directly measured from spectra. The statistical errors are estimated for 100M Au+Au central 200 GeV events. We will have high pT electron trigger (EMC HT) in the future, high pT statistics will not be a problem. Assuming D meson v2 from quark coalescence, using decay form factor to generate D e v2 distributions from efficiency and dca distribution we can estimate the error for 500M AuAu minbias events. September 23, 2014 Yifei ZhangUSTC/LBNL 29

  30. Measure v2 from dca B e v2 and D  e v2 can be measured from different dca cuts. For example: r  v2(B) + (1-r)  v2(D) = v2(NPE) v2(NPE) is the total non-photonic electron v2 after dca selection. v2(B) is B e v2 v2(D) is D e v2 September 23, 2014 Yifei ZhangUSTC/LBNL 30

  31. Error estimate for v2 Assuming D meson v2, using decay form factor to generate D e v2 distributions. r  v2(B) + (1-r)  v2(D) = v2(NPE) v2(D) is D e v2 v2(B) is B e v2 Heavy quark collectivity Study charm and bottom separately to understand the mass effect of such heavy quarks. Probe medium properties. September 23, 2014 Yifei ZhangUSTC/LBNL 31

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