The standard model of the strong interaction provides that the constituent quarks and gluons of hadrons (strongly interaction particles) are confined over a distance scale of approximately 1 x 10-15 meters, the characteristic size of the elementary particles. This confinement is thought to be absolute in the sense that individual quarks and gluons are not observable in free space. In the language of Quantum Chromodynamics (QCD), the theory of the strong interaction, quarks and gluons which carry bare color charges must combine in clusters to form color singlet states, the only states which are actually observable in nature. Thus, these color singlet states correspond to the spectrum of observed elementary physics. Direct experimental study of the confinement of color is the domain of experimental Relativistic Heavy Ion Physics. While QCD specifies that observable physical states be color singlets, it does not specify the total volume over which such color singlets are established. Large nuclei, under normal conditions, consist of clusters of individual hadrons, in particular neutrons and protons in which the color remains confined inside the individual hadrons. This is apparently the ground state of QCD. Nuclear matter at sufficiently high temperature or density, however, may undergo a phase transition in which color is deconfined from the individual neutrons and protons only to be confined at a much larger volume corresponding to the volume of the entire nucleus. The resulting excited state of matter corresponds to the hypothetical state known as the quark/gluon plasma. In this state, quarks and gluons are free to move as though they were independent particles. The requirements of color confinement are still maintained outside the volume as a whole. The early universe, a few microseconds after the big bang, was probably a similar large volume of essentially free quarks and gluons. As time progressed and the universe expanded and cooled, the quark gluon plasma condensed to form the hadrons out of which all matter in the universe is made today.

Relativistic Heavy Ion Physics is the study of nucleus-nucleus collisions at high energies in order to understand the behavior of extended nuclear matter under the extreme conditions of high density and temperature. The primary goal is to reach the phase transition from ordinary nuclear matter to a quark-gluon plasma. This field of research offers the only means to study the fundamental theory of strong interactions in the high density limit and to observe directly the parameters of the predicted phase transition. It may also enable us to study the physical properties of the Quantum Chromodynamics vacuum state which reflects manifestly long-range phenomena over large distance scales, not realizable in collisions of elementary particles. Relativistic collisions of heavy ions will provide the information of the equation of state of nuclear matter at densities relevant to the interior of neutron stars, and further our understanding of the creation of the universe.

For additional information on Relativistic Heavy Ion Physics, visit the STAR website at http://www.star.bnl.gov/central/physics/