Charmonium production in Pb-Pb interactions at 158 GeV/c per nucleon

The NA50 collaboration has measured J/psi, psi' and Drell-Yan pair production in Pb-Pb interactions at 158 GeV/c per nucleon at the CERN SPS. Final results from the 1995 run and preliminary ones from the higher statistics 1996 run are presented. An anomalous J/psi suppression (relative to the Drell-Yan process) has been observed with respect to the suppression pattern established in experiments NA38 and NA51 with proton, oxygen and sulfur beams. The 1996 data allow a detailed study of the suppression pattern in the Pb-Pb sample itself, showing a discontinuity around an E-T value (the neutral transverse electromagnetic energy) of 50 GeV. The psi' is also suppressed relative to Drell-Yan, with a pattern very similar to the one observed in S-U collisions. Finally, the pr distributions of dimuons are presented. The average p(T)(2) of the J/psi in Pb-Pb collisions does not increase any more with E-T above 100 GeV.