Results from the NA50 experiment on J/Ψ suppression in Pb-Pb collisions at the CERN SPS

The NA50 experiment has measured, at the CERN SPS, the dimuon production in Pb-Pb collisions at 158 GeV/c per nucleon, following the program of the previous NA38 and NA51 experiments, performed with proton and light ion beams. The J/Ψ production has been studied by NA50 as a function of the centrality of the collisions. The results of the analyses performed on three data samples, collected from 1995 to 1998, show that for peripheral collisions the J/Ψ cross section can be understood in terms of ordinary nuclear absorption, consistently with the extrapolation of previous measurements extending from p-p to S-U. On the contrary, the central events show an anomalous J/Ψ suppression, and the pattern of J/Ψ yield versus centrality features two discontinuities that cannot be described by conventional models based on hadronic scenarios. These results can be considered as a strong indication of the creation of a new state of matter in central Pb-Pb collisions at the SPS.