1 Low energy scan and collective flow at 9 GeV Jiayun Chen, Feng Liu, Shusu Shi, Kejun Wu Weihai, Aug. 9,2009

2 Outline Motivation Collectivity from STAR at 9.2GeV MC Simulation at 9 GeV Summary and Outlook

3 Motivation  RHIC beam energy scan program : Search for critical point Draw the QCD phase boundary. QCD Phase diagram Cross over At RHIC: (1)p T -NQ scaling (2)partonic collectivity (3)deconfinement hot and dense matter with partonic collectivity has been formed at RHIC QM09 : SS Shi (STAR Collaboration)

4 Access to large range of   and T Beam Energy Scan (BES) at RHIC + SPS + FAIR RHIC: advantage of collider mode ! At fixed target geometry: detector acceptance changes with energy track density at mid-y increases fast with energy -> technical difficulties in tracking Motivation

9.2 GeV Collider Acceptance √s NN 6 GeV 17 GeV √s NN = 9.2 GeV Au+Au Collisions at RHIC Fix-target Mode NA49 Collider Mode STAR

6 RHIC run 10 (fall 2009)  s NN [p ft ] GeV [GeV/c]  B [MeV] [Hz] Days/ Mevent # events# beam days 4.6 [9.6]570395M [18.8]470745M [27.9] M [37.7] M [71.0] M [161]220> M [391]150> M1.5 (1) Large energy range accessible (2) Collider geometry (acceptance won’t change with  S, track density varies slowly) (3) STAR detectors well suited (large acceptance), tested & understood STAR PAC 2007 Strawman proposal: Note: CERN (starting in 2010): 10, 20, 30, 40, 80, 158 GeV/c

7 STAR TPC image of 9 GeV Au+Au, taken on June 7, 2007 (run , ev.44), figure from Jeff Langraf 2001: 19.6 GeV Au+Au 2004: 22.4 GeV Cu+Cu 2007: 9 GeV Au+Au observed apparent rates of collisions surprisingly high (?!) to do: (1) understand background (2) optimize triggering STAR experience with Low Energy RHIC running Collectivity from STAR at 9.2GeV

8 STAR Experiment and Collisions at E cm = 9.2 GeV Excellent Particle Identification Collisions recorded in STAR TPC Analysis based on ~ 3000 good events collected at ~ 0.7 Hz in year 2008 PID will further strengthen with the completion of ToF

9 Azimuthal Anisotropy - Directed Flow v 1 vs. η show different trend between the high and low energy because that the spectator rapidity decreases with incident energy QM09 Poster : Jiayun Chen (STAR Collaboration)

11 Azimuthal Anisotropy - Elliptic Flow QM08 Lokesh Kumar for STAR With TOF, the pt region will be extended to a higher value. Important to perform the v 2 scaling analysis.

v 1 in AMPT low energy: –the default AMPT with low-NTMAX consistent with the STAR results. –The melting AMPT seems difficult to describe the data. QM09 Poster : Jiayun Chen

13 MC Simulation at 9 GeV V 2 of all Charged Hadrons  About 3231k, 862k, 4370k and 4507k events are used for minibias calculations at UrQMD v2.3, RQMD v2.4, AMPT v2.1 with string melting and default.  v 2 : AMPT with melting > AMPT default > UrQMD >RQMD. Partonic reactions enhance hadrons v 2 !  v 2 value at AMPT with melting is about equal to the default at center rapidity, but much larger v 2 at high rapidity area – connection to the observed RIDGE: Early partonic interactions are important!

 Only 3k good events for experimental data.  Difficult to say which MC model is best suitable. MC vs. Experimental data

15 v 2 NQ Scaling in AMPT Only for AMPT with string melting  like hydrodynamic behavior mass ordering at p T <1.2GeV/c  Obvious hadrons type dependence: NQ Scaling at p T >1.2GeV/c  Crossing and subsequent splitting between meson and baryon at p T ~1.2GeV/c Why is the v 2 NQ scaling presented in the AMPT with string melting?

Difference for two AMPT versions  Quark coalescence mechanism leads to v 2 NCQ scaling Zi-Wei Lin,Che Ming Ko,etc., Phys.Rev.C 72,064901(2005), “Multiphase transport model for relativistic heavy ion collisions ”

17 Default AMPT The breaking of v 2 NQ scaling. The partons cross section almost doesn’t affect v 2 value. AMPT with string melting The excellent v 2 NQ scaling. Large partons cross section leads to strong v 2. The strength of the final hadron v 2 is directly related to the partons cross section! Partons Cross Section vs. Hadrons v 2 The broken NQ scaling behavior maybe indicates the phase transition from dominant partonic to hadronic matter!

18 v 2 in RQMD and UrQMD The v 2 NQ scaling may not be the unique feature of quark coalescence ! Hadronic interactions -> rough v 2 NQ scaling  The same v 2 NQ scaling as AMPT with SM. may be statistical fluctuation?  (no light quark component) is very important for studying the medium properties. dominant partonic matter dominant hadronic matter v 2 from KKbar fusion will not obey the v 2 NQ scaling.

Additive Quark Model cross section only depends on the quark-content of the colliding hadrons Color Strings and ropes -excitation and -fragmentation denotes the hyper-surface where hadrons are emitted. In the low pt region, frequent rescatterings among hadrons can lead to hydrodynamic-like mass ordering. In the higher pt region (pt>1.5GeV/c), particles early freeze out and lack the hydrodynamics development, and the details of the interaction cross- sections are most important. elliptic flow: Where does v 2 NCQ Scaling Come from? Y.Lu,F.Liu,N.Xu.etc., J. Phys. G: Nucl. Part. Phys. 32 (2006) 1121–1129, K. Goulianos, Phys.Rep.101,169(1983), “Diffractive interactions of hadrons at high energies ” S.A.Bass, M.Belkacem,etc., Nucl-th/ , “Microscopic Models for Ultrarelativistic Heavy Ion Collisions” The hadronic cross sections in UrQMD can been parameterized by AQM.

20 Summary and Outlook STAR will measure : yields and particle ratios T vs   particle spactra (p t, rapidity, …), strangeness production (K/ , multistrange, …), fluctuations and correlations flow (v 1,v 2,v 4, …) with charged and identified particles, HBT radii, … Search for : - disappearance of partonic activities - onset of critical phenomena: fluctuations, correlations 1) turn on and off signature of de-confinement (QGP) 2) High statistics is required for v 2 in the further experiments. The unique RHIC energy scan program will map the QCD diagram in  s NN =5-50 GeV, (corresponding to μ B ~ MeV)

Thank you

22  s NN (GeV) for Au+Au # weeks in 25-cryoweek scenario# weeks in 30-cryoweek scenario

23 Azimuthal Anisotropy - Directed Flow v 1 vs. η show different trend between the high and low energy because that the spectator rapidity decreases with incident energy QM09 Poster : Jiayun Chen (STAR Collaboration)

Difference for two AMPT versions  Quark coalescence mechanism leads to v 2 NCQ scaling

Energy scan of v 1 at RHIC energy Centrality dependence: –high energy: clearly dependence –lower energy: seems to be weaker V 1 (y) of charged particle from AMPT seems consistent with the RHIC data in sharp –high energy: melting AMPT –low energy: default AMPT The directed flow wiggle from peripheral to central: –high energy: more clearly transformation –low energy: weak effect by centrality but clearly wiggle The direction of v 1 seems consistent in different energy in mid-rapidity.

Motivation flow antiflow Brachmann, Soff, Dumitru, et. al., PRC 61 (2000) L.P. Csernai, D. Roehrich PLB 458, 454 (1999) M.Bleicher and H.Stocker, PLB 526,309(2002) Anti-flow/3rd flow component, with QGP  v 1 flat at middle rapidity. Directed flow (v 1 ) and phase transition