Chemical freezeout in relativistic A + A collisions: is it close to the quark-gluon plasma?

Abstract

Preliminary experimental data for particle number ratios in the collisions of Au + Au at the BNL AGS (11 A GeV/c) and Pb + Pb at the CERN SPS (160 A GeV/c) are analysed in a thermodynamically consistent hadron gas model with excluded volume. Large values of temperature, T = 140-185 MeV, and baryonic chemical potential, MeV, close to the boundary of the quark-gluon plasma phase are found from fitting the data. This seems to indicate that the energy density at the chemical freezeout is tremendous, which would be indeed the case for point-like hadrons. However, a self-consistent treatment of the van der Waals excluded volume reveals much smaller energy densities which are far below a lowest limit estimate of the quark-gluon plasma energy density. This is consistent with a large energy density change (`latent heat') during hadron gas transition into quark-gluon plasma.