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 resu lts on the study of high energy density nuclear matter. Through the study o f hadron yields the velocity of the fireball expansion and the freeze-out t emperature of the system have been established. Chemical freeze-out tempera ture of 170 MeV and freeze-out temperature of 120 MeV, together with an ene rgy density of 3 GeV/fm(3) are over the minimum values predicted for the oc curence of phase transition. Evidence for deconfinement of quarks and gluon s from the J/psi suppression pattern measured in Pb-Pb collisions has been reported by the NA50 experiment. The enhancement of multistrange hyperon pr oduction observed by WA97 proves that strangeness is equilibrated on a very short timescale, a behaviour expected in case of transition to a deconfine d state. All the experiments agree in concluding that at least in central P b-Pb interactions a state with many of the characteristics of the theoretic ally predicted Quark-Gluon Plasma (QGP) has been created.