Entropy production in collisions of relativistic heavy ions — a signal for quark-gluon plasma phase transition?

Entropy production in the compression stage of heavy ion collisions is discussed within three distinct macroscopic models (i.e. generalized RHTA, geometrical overlap model and three-fluid hydrodynamics). We find that within these models ∼80% or more of the experimentally observed final-state entropy is created in the early stage. It is thus likely followed by a nearly isentropic expansion. We employ an equation of state with a first-order phase transition. For low net baryon density, the entropy density exhibits a jump at the phase boundary. However, the excitation function of the specific entropy per net baryon, SA, does not reflect this jump. This is due to the fact that for final states (of the compression) in the mixed phase, the baryon density ϱB increases with s, but not the temperature T. Calculations within the three-fluid model show that a large fraction of the entropy is produced by nuclear shock waves in the projectile and target. With increasing beam energy, this fraction of SA decreases. At s=20 A GeV it is on the order of the entropy of the newly produced particles around midrapidity. Hadron ratios are calculated for the entropy values produced initially at beam energies from 2 to 200 A GeV.