Study of bulk properties at high energy nuclear collisions?the search for the partonic equation of state at RHIC

Abstract

We discuss recent results from RHIC. Issues of energy loss and partonic collectivity from Au + Au collisions at √ s NN = 200 GeV are the focus of this paper. We propose a path toward the understanding of the partonic Equation of State in high energy nuclear collisions. IntroductionThe purpose of the heavy ion program at Brookhaven National Laboratory is to probe strongly interacting matter under extreme conditions, i.e. at high densities and temperatures. Naturally the search for the existence of a new form of matter -the quark-gluon plasma (QGP) -is the experimental focus of the program.Lattice QCD calculations [1,2] predict the transition between QGP and hadronic state at T c ≈ 150 − 180 MeV with vanishing baryon density 1 . The energy density at the transition point is determined to be c ≈ 0.7 − 1.0 GeV/fm 3 . Under the same condition, chiral symmetry restoration also happens [1]. Therefore experimentally one would search for signatures of both QGP formation and in-medium effects of hadron properties.In high energy nuclear collisions, the term flow has two important aspects: (i) collectivity of produced hadrons and (ii) the local thermalization among these hadron [3]. As long as there are interactions among constituents, collectivity of the matter will be developed provided there is inhomogeneity of matter density. When the interactions last long enough the system will approach local equilibrium and hence develop flow. At the early stage of the collision, both density of the matter and gradient are large, therefore we expect the development of partonic collectivity -the collective motion of partons. The issues of thermalization can be addressed by studying heavy-flavor (c−, b-quarks) collectivity, because the collisions that generate the collective motion for heavy-quarks will likely lead to thermalization among the light quarks (u−, d−, s-quarks). It is important to note that collectivity is cumulative and it is not affected by the details of the hadronization process.Many exciting results emerged from the RHIC experiments since its first started in the year of 2000. Using high transverse momentum hadrons we demonstrated the interactions at early stage of the collisions. These interactions are most likely to occur among the 1 Including finite baryon density does not affect general properties of the phase transition in Lattice results [4] 2 partons. At RHIC, much stronger collective flow has been observed, especially for the early developing v 2 , compared to results from lower energy heavy ion collisions. This might imply that rescattering effects arise from the partonic stage at RHIC. This report will summarize the recent results from experiments at RHIC. It will be divided into three parts: (i) observations at the intermediate transverse momentum region (3 ≤ p T ≤ 10 GeV/c); (ii) bulk properties of the matter produced at RHIC; (iii) evidence of partonic collectivity at RHIC. Finally, the report will be ended with a brief summary and outlook. Energy loss -Interactions at early partonic stageThe measurements...