Charmonium production in PbPb interactions at 158 GeV/c per nucleon

The NA50 collaboration has measured J/ψ, ψ′ and Drell Yan pair production in Pbsingle bondPb 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. A anomalous J/ψ 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 Pbsingle bondPb sample itself, showing a discontinuity around an ET value (the neutral transverse electromagnetic energy) of 50 GeV. The ψ′ is also suppressed relative to Drell-Yan, with a pattern very similar to the one observed in Ssingle bondU collisions. Finally, the pT distributions of dimuons are presented. The average pT2 of the J/ψ in Pbsingle bondPb collisions does not increase any more with ET above 100 GeV.