Partonic collectivity at RHIC

The measurement of event anisotropy, often called $v_{2}$, provides a powerful tool for studying the properties of hot and dense medium created in high-energy nuclear collisions. The important discoveries of partonic collectivity and the brand-new process for hadronization - quark coalescence were obtained through a systematic analysis of the $v_{2}$ for 200 GeV Au+Au collisions at RHIC [1]. However, early dynamic information might be masked by later hadronic rescatterings. Multistrange hadrons ($\phi$, $\Xi$ and $\Omega$) with their large mass and presumably small hadronic cross sections should be less sensitive to hadronic rescattering in the later stage of the collisions and therefore a good probe of the early stage of the collision. We will present the measurement of $v_{2}$ of $\pi$, p, $K_{S}^{0}$, $\Lambda$, $\Xi$, $\phi$ and $\Omega$ in heavy ion collisions. In minimum-bias Au+Au collisions at $\sqrt{\mathrm{s}_{_{\mathrm{NN}}}}$ = 200 GeV, a significant amount of elliptic flow, almost identical to other mesons and baryons, is observed for $\phi$ and $\Omega$. Experimental observations of $p_{T}$ dependence of $v_{2}$ of identified particles at RHIC support partonic collectivity. \\[4pt] [1] B. I. Abelev et al., (STAR Collaboration), Phys. Rev. {\bf C 77}, 054901 (2008).