Results from heavy ions experiments at the CERN SPS: from hadronic to deconfined matter

The CERN heavy ion program has collected a huge amount of experimental results on the study of high energy density nuclear matter. Through the study of hadron yields the velocity of the fireball expansion and the freeze-out temperature of the system have been established. Chemical freeze-out temperature of 170 MeV and freeze-out temperature of 120 MeV, together with an energy density of 3 GeV/fm/sup 3/ are over the minimum values predicted for the occurrence of phase transition. Evidence for deconfinement of quarks and gluons from the J/ psi suppression pattern measured in Pb-Pb collisions has been reported by the NA50 experiment. The enhancement of multistrange hyperon production observed by WA97 proves that strangeness is equilibrated on a very short timescale, a behaviour expected in case of transition to a deconfined state. All the experiments agree in concluding that at least in central Pb-Pb interactions a state with many of the characteristics of the theoretically predicted Quark-Gluon Plasma (QGP) has been created. (32 refs).