Physics of the STAR Experiment at RHIC

Abstract

The primary motivation for studying relativistic heavy ion collisions is to gain an understanding of the equation of state of nuclear, hadronic and partonic matter, commonly referred to as nuclear matter. This endeavor is of cross-disciplinary interest to nuclear physics, astrophysics, cosmology and particle physics. Displayed in Fig. 1 is a schematic phase diagram of nuclear matter. The behavior of nuclear matter as a function of temperature and density (or pressure), shown in Fig. 1, is governed by its equation of state. Conventional nuclear physics is concerned primarily with the lower left portion of the diagram at low temperatures and near normal nuclear matter density. Here normal nuclei exist and at low excitation a liquid-gas phase transition is expected to occur. This is the focus of experimental studies using low energy heavy ions. At somewhat higher excitation, nucleons are excited into baryonic resonance states, along with accompanying particle production and hadronic resonance formation. In heavy ion collisions, such excitation is expected to create hadronic resonance matter. This region is presently accessible in heavy ion studies at the AGS accelerator facility at Brookhaven National Laboratory and at the SPS accelerator facility at CERN. As seen in the diagram, there is a possibility that some part of these collisions traverse the transition region into the quark-gluon plasma regime. Formation of a quark-gluon plasma, a deconfined state of quarks and gluons,¹ is the major focus of relativistic heavy ion experiments at higher energies. For this purpose the Relativistic Heavy Ion Collider (RHIC)² and associated experiments are presently under construction at Brookhaven for operation in 1999, and operation with heavy ions is also being planned for the LHC in 2005. As seen in the phase diagram, the anticipated temperature and density trajectories at RHIC (and for LHC heavy ions) lie close to that of the early universe, while those at the AGS and SPS occur at higher baryon densities.