Measurement of Azimuthal Anisotropy for High p T Charged Hadrons at RHIC-PHENIX

1. non central collision 2.0-4.0 GeV/c 1.0-2.0 GeV/c 0.2-1.0 GeV/c z φ Ｙ Beam axis Reaction plane Ｙ x x (Reaction Plane) PHENIX Collaborators Large energy loss Small energy loss Reaction Plane 1.0-2.0 GeV/c 0.2-1.0 GeV/c 2.0-4.0 GeV/c 1.0-2.0 GeV/c 0.2-1.0 GeV/c Measurement of Azimuthal Anisotropy for High pT Charged Hadronsat RHIC-PHENIX Maya SHIMOMURA University of Tsukuba for the PHENIX collaboration Introduction The azimuthal anisotropy of particle production in non-central collisions is sensitive to the early stage of high-energy heavy ions collision and, the strength of the elliptic anisotropy (v2) is a sensitiveprobe for studying properties of the hot dense matter made by heavy ion collisions (quark-gluon plasma). ψ: azimuthal angle of reaction plane v2 : anisotropy parameter (Elliptic Flow) < Elliptic Flow Measurement > If yield is (x direction)>(y direction), v2 >0. Fourier expansion of the distribution of produced particle angle, Φ, to RP < Resolution Calculation of Reaction Plane > v2is the coefficient of the second term  indicates ellipticity A,B : reaction plane determined for each sub sample. √(2*<cos(2*(ΨS –ΨN))>) =1/correction factor In non-central collisions, the reaction plane is determined in the BBC and the yield as a function of the azimuthal angle is measured. The initial geometrical anisotropy is transferred by the pressure gradients into a momentum space anisotropy  the measured v2 reflects the dense matter produced in the collisions. Image of Jet PRL 91, 182301 Motivation What we have learned is the following: 1. The behavior of anisotropy can be explained by a hydro-dynamical model and initial pressure gradientat transverse momentum, pT < 2GeV/c, but not at higher pT. (Fig. 1) At higher pT, particles are produced from jet-fragmentation made by initial collisions. Jet production occurs in the overlap region and is not related to the reaction plane. However, in the high pT region, a non zero v2 is still observed. (Fig.2,3,4) One possible explanation is that jets lose its energy in the medium. Since the collision overlap region forms an almond shape (not round) in non-central collisions, high pT partons from jets go through less medium and lose less energy in the in-plane direction compared to the out-of-plane direction. The difference in how much the jets are absorbed between the in-plane and out-of-plane directions makes the v2 positive. Fig.1 The jets interact with the hot dense matter created in high energy collisions, and some of the jet energy is absorbed. Fig.3 Fig.2 Therefore, can v2 be scaled by the ellipticity of the participants from low pT to higher pT, even though the mechanisms are different? PRL 94, 232302 2. The v2 as the function of pT at 62.4GeV(AuAu), 200GeV（AuAu） and 130GeV(AuAu) are consistent, and the v2 decreases to ~50% RHIC values at 17.2GeV(PbPb) so that v2 looks like it is saturated at RHIC energies. (Fig.5) Fig.5 Fig.4 Can the different results be observed in different systems （AuAu, CuCu） and/or different collision energies （200GeV, 62.4GeV)? The theory curves in Fig. 2, 3 are calculations of energy loss models done by Ivan Vitev. [ref:Phys.Rev.Lett.86:2537-2540,2001] (BJ – assuming uniform Bjorken expanding fireball) (WoodSaxon – assuming the matter is produced w/ a binary collision density) The high-pT v2 of both charged hadron and p0 is non zero. Results Comparison between 200 and 62.4GeV Cu+Cu collisions Comparison between 200 and 62.4GeV Au+Au collisions ellipticity of participant Black 200GeV Red 62.4GeV Fig.10 黒 200GeV 赤 62.4GeV Fig.8 Fig.7 Fig. 6 Fig.9 The v2 results of Au+Au 62.4GeV and Au+Au 200GeV data have good agreement at all centralities. This fact is consistent with past results (Fig. 6, 7). The v2 results of Cu+Cu 62.4GeV and Cu+Cu 200GeV data are consistent within errors at all centralities (Fig. 9, 10). Fig.11 v2/eccentricity vs. Npart has different slopes for different pT. This may be because the v2 production mechanisms are different between the high pT and low pT region. At low pT (02.-1.0 [GeV/c]), v2/eccentricity is almost constant at any centrality except the most central bin where the systematic errors of eccentricity are large, so that v2 can be scaled by eccentricity within errors. (Fig. 8, 11) Summary Comparison of results from these four data set v2 is not scaled by Npart but is well scaled by geometrical eccentricity in mid-central collisions. (Fig. 12, 13) （Other models of eccentricity such as participant eccentricity could work better for the most central and most peripheral collisions.） Although errors are big, the results of CuCu 62.4GeV seem not to be scaled with the same slope of others, so that there can be system dependence. Black AuAu 200GeV Red AuAu 62.4GeV Green CuCu 200GeV Blue CuCu 62.4GeV Fig.13 Fig.12 v2 scaled by eccentricity seems to saturate when the energy density of the produced matter is high enough. (Cu+Cu 62.4 GeV might not be high enough energy)  Need more study to conclude this. 2006 November Quark Matter Conference