Hadronic Signatures of Deconfinement in relativistic nuclear collisions

Abstract

We describe the remarkable accomplishments of the current heavy ion Pb--Pb collision experiments involving strange particle production, carried out at 158A GeV at CERN--SPS. These experimental results, together with the earlier 200A GeV S-induced reactions, imply that, at central rapidity, a novel mechanism of strangeness production arises, accompanied by excess entropy formation. We argue that: These results are consistent with the formation of a space-time localized, highly excited, dense state of matter; The freeze-out properties of strange hadrons are suggestive of the formation of a color-deconfined, thermally and nearly chemically equilibrated phase, which provides at present the only comprehensive framework to describe all experimental data; The matter fireball is undergoing a transverse expansion with nearly the velocity of sound of relativistic matter; longitudinal expansion is not in the scaling regime. We present a first analysis of the recent Pb-Pb results and discuss several alternative reaction scenarios. We evaluate quantitatively strangeness production in the deconfined quark-gluon phase and obtain yields in agreement with the experimental observations made in 200A GeV S--W and 158A GeV Pb-Pb interactions. We also present a qualitative discussion of J/Psi results consistent with our understanding of strange particle results.