Identified baryon and meson distributions at large transverse momenta in 200 GeV Au+Au collisions

1. Identified baryon and meson distributions at large transverse momenta in 200 GeV Au+Au collisions LijuanRuan (for the STAR Collaboration) (Lawrence Berkeley National Laboratory) Outline: • Motivation • Detector and Analysis Technique • Physics Results •  charged  & p spectra up to 12 GeV/c from STAR • RCP, particle ratios • Summary and Outlook Hard Probes 2006, Lijuan Ruan

2. p+p collisions and pQCD (AKK) NLO pQCD calculations with AKK FF are consistent with pion data at high pT (> 2 GeV/c) NLO pQCD calculations only with AKK FF (Nucl.Phys.B725:181-206,2005) consistent with the p+pbar data AKK differ from KKP, in the way the light flavor FF are obtained OPAL Collaboration : Eur. Phys. J. C 17 (2000) 207 STAR Collaboration, PLB 637 (161) 2006 Hard Probes 2006, Lijuan Ruan

3. Particle production in Au+Au collisions PHENIX: Phys. Rev. Lett. 91 (2003) 172301 STAR: Phys. Rev. Lett. 92 (2004) 052302 • p(pbar)/ ratio ~ 1 at pT~2 GeV/c. • Baryons (p, ) increase faster than mesons (, Ks, K) from peripheral to central collisions. consistent with the coalescence of constituent quarks at hadronization from a collective partonic system at intermediate pT. R.J. Fries, et al., Phys. Rev. Lett. 90 202303 (2003); R. C. Hwa, et al., Phys. Rev. C 70, 024905 (2004); V. Greco, et al., Phys. Rev. C 68, 034904(2003). • The high pT PID spectra will be presented to address the transition between coalescence and jet fragmentation. Hard Probes 2006, Lijuan Ruan

4. Color charge dependence of energy loss • In Au+Au collisions at RHIC: Fragmentation + Energy loss at pT > 6 GeV/c. • Color charge dependence of energy loss: dE/dx(g) ≈ 9/4 dE/dx(q). • Gluon jet contribution to protons is significantly larger than to pions at high pT in p+p collisions at RHIC. pbar vs  Hard Probes 2006, Lijuan Ruan

5. Color charge dependence of energy loss X.N. Wang: PRC58(2321)1998 • PID spectra, centrality dependence of pbar/p and pbar/ ratios, address the color charge dependence of energy lossTo further understand how the gluon jet/quark jet interact with the medium created in Au+Au collisions. Hard Probes 2006, Lijuan Ruan

6. STAR detector: TPC & MRPC-TOF • A new technology (TOF) ---- • Multi-gap Resistive Plate Chamber • adopted from CERN-Alice. • Good timing resolution, (, K) ~1.6 GeV/c , p ~ 3 GeV/c • 2. One tray (1/200 acceptance) for now, 120 trays in the future Time Projection Chamber • Tracking • Ionization energy loss (dE/dx) • Coverage -1<<1 Hard Probes 2006, Lijuan Ruan

7. TPC TOF Particle identification ( ,p) ~12 GeV/c M. Shao et al., NIMA 558, (419) 2006 Hard Probes 2006, Lijuan Ruan

8. Spectra central STAR Preliminary peripheral STAR: nucl-ex/0606003 Hard Probes 2006, Lijuan Ruan

9. Nuclear modification factors RCP STAR Preliminary In central Au+Au collisions: • At 1.5<pT<6 GeV/c, RCP(p+pbar) > RCP() , not consistent with binary scaling for proton production. • At pT>6 GeV/c, RCP(p+pbar) ~ RCP() . Jet fragmentation dominates over this pT region in Au+Au collisions. I. Vitev, hep-ph/0603010. STAR: nucl-ex/0606003 Hard Probes 2006, Lijuan Ruan

10. Anti-particle to particle ratios STAR Preliminary STAR: nucl-ex/0606003 • -/+ are consistent with flat at unity in all pT, no significant centrality dependence. • pbar/p ratio: no significant centrality dependence, not consistent with the jet quenching prediction (X.N. Wang, PRC 58 (2321) 1998). Hard Probes 2006, Lijuan Ruan

11. Proton over pion ratios STAR Preliminary STAR: nucl-ex/0606003 At pT>5 GeV/c at RHIC at 200 GeV: p(pbar)/ are larger than the ratios from quark jet.  gluon jet plays a dominant role for protons production at RHIC. p(pbar)/ (Au+Au) ~ p(pbar)/ (d+Au, p+p).No centrality dependence of p(pbar)/ ratios. No clear color charge dependence of energy loss. We expect pbar/ (Au+Au) < pbar/ (d+Au, p+p). (X.N. Wang, PRC 58 (2321) 1998) R.J. Fries, et al., PRL 90 202303 (2003); R. C. Hwa, et al., PRC 70, 024905 (2004); DELPHI Collaboration, Eur. Phy. J. C 5, 585 (1998), Eur. Phy. J. C 17, 207 (2000). Hard Probes 2006, Lijuan Ruan

12. Summary • The spectra of and p (pbar) from 200 GeV Au+Au collisions are presented up to pT=12 GeV/c. • In central Au+Au collisions, at intermediate pT, RCP(p+pbar) > RCP() ; RCP(p+pbar) decreasing trend with pT, not consistent with binary scaling; the p(pbar)/ ratio~1 at pT 2~3 GeV/c. • In central Au+Au collisions, at high pT, RCP(p+pbar) ~ RCP(), p(pbar)/ and pbar/p ratios approach the ratios in p+p and d+Au collisions. • Indicate that the partonic sources of protons and pions have similar energy loss. No clear color charge dependence of energy loss was observed. Hard Probes 2006, Lijuan Ruan

13. Discussion: energy loss in QCD matter • With the addition of collisional energy loss in the recent calculation, the gluon jet quenching is a factor of 3 higher than quark jet quenching. (S. Wicks et al.,nucl-th/0512076). • Look into how gluon/quark jets interact with the medium. (Color Charge exchange: W. Liu, X.N. Wang, B. Zhang) • Proper fragmentation functions are required. • Systematically study the color charge dependence of energy loss  measure p(pbar)/, pbar/p ratios in different energy by changing the g/q relative contribution to particle production in the future. STAR Preliminary Hard Probes 2006, Lijuan Ruan