Centrality Dependence Of K_s～0 And Λ Elliptic Flow In Au+Au Collisions At √s_NN=200 GeV

The heavy ion collisions at Relativistic Heavy Ion Collider (RHIC) located at Brookhaven National Laboratory (BNL) provide a high energy density environment of nuclear matter to search for Quark-Gluon Plasma (QGP) and study its properties. The signature of QGP formation has to be identified from the measured final-state hadrons. Measurements of azimuthal anisotropy in transverse momentum distribution of produced particles can get information on the early stage of the heavy ion collisions. Prom the data collected at RHIC during year 2002, the second harmonic azimuthal anisotropy, elliptic flow v₂ of identified particles from Au + Au collisions established hydro-like mass hierarchy at low pT. This observation demonstrates the development of partonic collectivity. Further, the Number-of-Constituent-Quark scaling observed at intermediate pT suggest the system has been in the deconfined state prior to hadronization. On the other hand, charged particle 2> scaled by the eccentricity suggests possible thermalization only in the most central collisions at RHIC. The comparisons have been focused either on identified hadrons from minimum bias collisions or integrated v₂ of charged hadrons. A systematic comparisons for identified hadrons at different collision centralities are still scarce. The centrality dependence of v₂ measurement for identified particles will gain information on interplay of collectivity, NQ scaling and thermalization as a function of collision centralities.This thesis presents STAR results on the elliptic flow v₂ of K_S⁰ and (?) + (?) from (?)sNN = 200 GeV Au + Au collisions at RHIC. The high statistics data were collected at RHIC during year 2004. The detailed study of centrality dependence of v₂ over a broad transverse momentum range is presented. The results from Lee-Yang Zero method and Event Plane method are shown. Comparisons with multi-strange particlesφ, (?) + (?) andΩ+Ωare made for systematic study of identified particles.In the relatively low pT region (pT < 2 GeV/c), for a given pT, the heavier particle has smaller v₂ than the lighter particle. This mass ordering is predicted by the hydrodynamical calculations. The hydro-like mass hierarchy is observed for all centrality bins. In this pT region, a scaling with kinetic energy mT - m is observed for all measured identified hadrons in each centrality bin.In the higher pT region, v₂ deviates from hydrodynamical calculations and then saturates or decreases. Baryon saturates at higher pT with larger values of v₂ than meson. For all measured hadrons, v₂ scaled by the number of quark n_q within a given hadron follows a universal curve as a function of pT scaled by n_q. This Number-of-Quark scaling is observed for all centrality bins.n_q-scaled v₂ is divided by the participant eccentricity for different centralities to remove the initial geometry effect. We do not observe v₂(pT) scaled by the participant eccentricityεpart to be independent of centrality. As a function of collision centrality, an increase of 2>/<εpart> has been observed. This indicates a stronger collective flow in more central Au + Au collisions.For hydrodynamical calculations with assumptions of local thermalization, /<ε> shows little sensitivity to centrality bins. The rate of increase in 2>/<εpart> for data appears to slow down when Npart > 170 and approach that from the hydrodynamic model calculations. This indicates that the system created in 200 GeV Au + Au collisions reached local thermalization in central collisions when the number of participants is larger than 170. Further, the observed collectivity, mT - m scaling and number-of-quark scaling do not depend on local thermalization.