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Directed Flow Measurements at STAR Aihong Tang for the Aihong Tang Moriond QCD March 2011 Collaboration 1 A few useful concepts in heavy ion collisions Reaction plane : Defined by the beam and the line connecting two colliding nuclei z y x Peripheral collisions : collisions in which two nuclei barely graze each other. Central collisions : head-on collisions. Aihong Tang Moriond QCD March 2011 2 Definitions: directed flow (v1), elliptic flow (v2) Picture: © UrQMD py X <px> or v1=<px/pt> atan y 2 y 2 xXZ b rapidity z x – the reaction plane px2 py2 px2 py2 anisotropy in momentum space dN 3 1 d 2N E 3 1 2v1 cos R 2v 2 cos2 R K d p 2 pt dpt dy isotropic v2 y2 x2 initial spatial anisotropy px px Z py directed elliptic v n cosn RP e in RP Correlation t o t he reaction plane " anisotropic flow " Aihong Tang Moriond QCD March 2011 It is a convention to define the sign of v1 of spectators at forward rapidity as positive. 3 Anti-flow / 3rd flow component 8 A GeV flow antiflow Brachmann, Soff, Dumitru, Stocker, Maruhn, Greiner Bravina, Rischke , PRC 61 (2000) 024909. L.P. Csernai, D. Roehrich PLB 458, 454 (1999) M.Bleicher and H.Stocker, PLB 526,309(2002) Anti-flow/3rd flow component : Flat v1 at midrapidity due to 1st order phase transition Caution : Seeing anti-flow does not necessarily mean that there is a QGP EoS. (refer to UrQMD). In following slides, anti-flow only means zero or negative slope at midrapidity, due to the fast expansion of a tilted source. Aihong Tang Moriond QCD March 2011 4 v1 from baryon stopping RQMD v2.4 AuAu 200 GeV R.Snellings, H.Sorge, S.Voloshin, F.Wang, N. Xu, PRL (84) 2803(2000) Baryon stopping + positive space-momentum correlation v1 wiggle. No QGP necessary Aihong Tang Moriond QCD March 2011 5 v1 and early thermalization Piotr Bozek and Iwona Wyskiel-Piekarska, PRC 83, 024910 (2011) Sensitive to the tilt, the initial time (not shown) and the relaxation time. Aihong Tang Moriond QCD March 2011 6 STAR Experiment EMC Barrel MRPC ToF Barrel EMC End Cap FMS BBC FPD TPC ZDC & ZDC & ZDCSMD ZDCSMD FTPC PMD Aihong Tang Moriond QCD March 2011 7 v1 at RHIC, viewed with η/ybeam Preliminary Clear patterns for spectators and “transported + produced” particles. Aihong Tang Moriond QCD March 2011 8 v1 at RHIC, viewed in η-ybeam Preliminary Phobos, PRL 97 012301 (2006) Similar if viewed in the projectile frame – Limiting Fragmentation Hypothesis (It remains as a hypothesis, unfortunately !) Aihong Tang Moriond QCD March 2011 9 v1 at RHIC, system size independence STAR, PRL 101 252301 (2008) Piotr Bozek and Iwona Wyskiel-Piekarska, PRC 81, 054902 (2010) AMPT failed to describe data. Hydro+”anti-flow” works (but failed in describing PID flow – see later). Aihong Tang Moriond QCD March 2011 10 PID v1 at RHIC Preliminary Anti-proton slope has the same sign of pions – consistent with anti-flow Kaon suffers less shadowing effect due to smaller k/p cross section, yet we found negative v1 slope for Kshort – consistent with anti-flow Aihong Tang Moriond QCD March 2011 11 PID v1 at RHIC AuAu 200 GeV Preliminary Preliminary Difference seen between v1 of protons and anti-protons in 5-30% central collisions. No difference is seen in “Hydro + anti-flow” model (not shown). Aihong Tang Moriond QCD March 2011 12 Centrality dependence of v1 slope AuAu 200 GeV Preliminary Negative v1 slope for protons is observed in 30-80% centralities. Large difference seen between v1 of protons and anti-protons in 5-30% centralities. Anti-flow alone cannot explain the data. Aihong Tang Moriond QCD March 2011 13 Proton v1 Excitation Function Preliminary Rapidity window used in STAR : [-0.6,0.6] Proton v1 in mid-central collisions at RHIC stays small Aihong Tang Moriond QCD March 2011 14 Summary • Directed flow probes the early dynamics. • Directed flow has been measured by STAR over a wide η/ybeam range at different energies. The directed flow is found to be independent of system size, and its value at forward region is confirmed to scale with η-ybeam for a wide energy range. • Anti-flow alone cannot explain the difference in v1 between protons and anti-protons. • So far no models can explain all features simultaneously. Aihong Tang Moriond QCD March 2011 15 BackUp Aihong Tang Moriond QCD March 2011 16 v1 at low energies 6 A GeV Au+Au e895, PRL 85 940 (2000) E895, PRL 86 2533 (2001) Aihong Tang Moriond QCD March 2011 17 v1 at low energies peripheral mid-central central NA49, NPA 715, 583 (2003) Aihong Tang Moriond QCD March 2011 18 Data set for PID v1 analysis • Run7 , 62 M events. Preliminary Preliminary Tracks used for TPC : No. of fit hits ∈[15, 50] Global DCA∈[0.0, 1.0] No. of fit hits/No.of possible hits ∈[0.52, 1.05] 0.1 < pT < 12.0 GeV/c | η | < 1. PID achieved by TPC dE/dx Pt cut for protons/anti-protons [0.4,1 GeV/c] Aihong Tang Moriond QCD March 2011 19 Pion v1 at RHIC Au+Au 200GeV 10%-70% RQMD: Phys. Rev. Lett. 84 (2000) 2803; UrQMD: Phys. Lett. B 526 (2002) 309; AMPT: Phys. Rev. C 81, 014904 (2010); QGSM+ Partonic recombination: Phys. Rev . C 76 (2007) 024912; hydro + tilted source: Piotr Bo˙zek and Iwona Wyskiel, arXiv:1002.4999 [nucl-th] (2010). Preliminary Hydro+tilted source describes the data the best Aihong Tang Moriond QCD March 2011 20 v1 and 1st order phase transition dv1/dy at y=0 Central Aihong Tang Moriond QCD March 2011 Peripheral 21