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.Comment: 26 pages, LaTeX, 9 postscript figures, Submitted to Acta Physica
Polonica
