Partonic Equation of State in Relativistic Heavy Ion Collisions

The goal of the RHIC and the future LHC programs is to identify and study the matter with partonic degrees of freedom and determine the phase structure of the QCD phase diagram [1, 2]. In high-energy nuclear collisions, due to large initial density and frequent re-scatterings, collective motion develop [3]. Depends on both the duration and the strength of the frequent scatterings, local thermalization might be reached in central collisions. Whence the equilibrium limit is reached, the collective motion approaches its maximum. Therefore, the information on the equation of state can be extract in analyzing the flow distributions. Since the strength of flow depends on the nature of the interactions, different hadrons that freezeout at different stages of the collision will carry different flow. For example, heavy flavor and multi-strange hadrons freezeout early [4] so they are powerful tools for studying evolution at the early partonic stage in high-energy nuclear collisions [5]. In this paper, we will focus on bulk production. Those who are interested in the physics issues of initial conditions and high pT phenomena are referred to Refs. [6–8] and references therein. We will first discuss the recent results on partonic collectivity observed at RHIC. Chemical freeze-out and transverse momentum distributions will be compared with model predictions. Later, we discuss the heavy flavor results. The connection between heavy flavor collectivity and light flavor thermalization will be address. A brief summary will be presented at the end.