System Size and Collision Energy Dependence of $v_2$ for Identified Charged Hadrons at RHIC-PHENIX

Abstract

The transverse momentum ($p_{\rm T}$) and centrality dependence of the azimuthal anisotropy ($v_{2}$) are measured for charged hadrons in different energies and collision species by the PHENIX experiment at RHIC. We find that $v_{2}$ divided by the participant eccentricity of the initial geometry proportionally increases with the number of participants to the 1/3 power except at small $N_{\rm part}$ in Cu+Cu at $\sqrt{s_{\rm NN}}$ = 62.4 GeV. Taking the eccentricity and quark number scaling into account, there is a universal scaling for $v_{2}$ with different energies and collision sizes. The results indicate that $v_{2}$ is determined by more than just the geometrical eccentricity. It also depends on the size of collision. We also report that both the behaviors of $v_{2}$ and $p_{\rm T}$ distributions can be understood from the thermal nature of produced particles based on hydro-dynamical behavior.