Measurement of Azimuthal Anisotropy for High p T Charged Hadrons at RHIC-PHENIX 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 sensitive probe for studying properties of the hot dense matter made by heavy ion collisions (quark-gluon plasma). Maya SHIMOMURA University of Tsukuba for the PHENIX collaboration 2006 November Quark Matter Conference ψ: azimuthal angle of reaction plane v 2 : anisotropy parameter (Elliptic Flow)  A,B : reaction plane determined for each sub sample. 1. The behavior of anisotropy can be explained by a hydro-dynamical model and initial pressure gradient at transverse momentum, pT < 2GeV/c, but not at higher p T. (Fig. 1) The theory curves in Fig. 2, 3 are calculations of energy loss models done by Ivan Vitev. [ref:Phys.Rev.Lett.86: ,2001] (BJ – assuming uniform Bjorken expanding fireball) (WoodSaxon – assuming the matter is produced w/ a binary collision density) The high-p T v 2 of both charged hadron and  0 is non zero. In non-central collisions, the reaction plane is determined in the BBC and the yield as a function of the azimuthal angle is measured. Beam axis x z Reaction plane non central collision Ｙ x (Reaction Plane) Ｙ φ Motivation If yield is (x direction)>(y direction), v2 >0. Fourier expansion of the distribution of produced particle angle, Φ, to RP v 2 is the coefficient of the second term  indicates ellipticity 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. PRL 91, 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 Large energy loss Small energy loss Reaction Plane Image of Jet Therefore, can v2 be scaled by the ellipticity of the participants from low pT to higher pT, even though the mechanisms are different? 2. The v2 as the function of pT at 62.4GeV(AuAu), 200GeV （ AuAu ） and 130GeV(AuAu) are consistent, and the v 2 decreases to ~50% RHIC values at 17.2GeV(PbPb) so that v2 looks like it is saturated at RHIC energies. (Fig.5) What we have learned is the following: PRL 94, Can the different results be observed in different systems （ AuAu, CuCu ） and/or different collision energies （ 200GeV, 62.4GeV)? Results Introduction 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). Black 200GeV Red 62.4GeV GeV/c GeV/c GeV/c ellipticity of participant 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 ( [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) Comparison between 200 and 62.4GeV Au+Au collisions Fig.7 Comparison between 200 and 62.4GeV Cu+Cu collisions GeV/c GeV/c GeV/c Black AuAu 200GeV Red AuAu 62.4GeV Green CuCu 200GeV Blue CuCu 62.4GeV 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. 黒 200GeV 赤 62.4GeV GeV/c GeV/c 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. PHENIX Collaborators √(2* ) =1/correction factor Fig.5 The jets interact with the hot dense matter created in high energy collisions, and some of the jet energy is absorbed. Fig.8 Fig.10 Fig.11 Fig.12 Fig.13 Fig.2 Fig.3 Fig.4 Fig. 6 Fig.9 Summary