THE HADRON TO QUARK-GLUON TRANSITION IN RELATIVISTIC HEAVY-ION COLLISIONS

In this paper we construct a scenario for the QCD transition from the hadron phase to the quark/gluon phase using physical models for these phases. The hadron phase is modeled by a spectrum of hadrons with mass es which drop (with a common scaling factor) towards zero at chiral sy mmetry restoration. The number of hadronic effective degrees of freedo m is limited by the number of microscopic degrees of freedom in the qu ark/gluon phase. This limitation can be imposed either by fiat or thro ugh the introduction of a temperature-dependent excluded volume. Given that the number of degrees of freedom in hadrons and in quarks and gl uons are roughly equal, the QCD phase transition is inhibited by the b ag constant. The only phase transition seen in lattice-gauge calculati ons, once low-mass quarks are included, is the restoration of chiral s ymmetry which occurs at the relatively low temperature of approximatel y 150 MeV. At present, lattice gauge calculations do not have the reso lution to determine the properties of the higher hadronic states accur ately. They do, however, demonstrate that chiral restoration takes pla ce in the (rho, A1), (N(1/2 +)), (N(1/2 -)) and (pi, sigma) systems by yielding ''screening masses'' for chiral partners which are distinct for T < T(chi)SR and identical for T > T(chi)SR. Further, within numer ical accuracy, these ''screening masses'' are consistent with pure the rmal energies and show no evidence of remaining bare masses once chira l symmetry is restored. These, and other lattice-gauge results, will b e discussed in the light of our scenario. We shall also consider the c onsequences of our picture for relativistic heavy-ion experiments.