Latest Results from NA50 on $J/\psi$ Suppression in $Pb-Pb$ Collisions

The main goal of the NA50 experiment is to study the J/$\psi$ suppression pattern in Pb-Pb interactions, at 158 GeV/c per nucleon at the CERN SPS. We present here the results from the 1996 (final) and 1998 (preliminary) data taking periods. They confirm and extend our previous observation that the J/$\psi$ is anomalously suppressed from peripheral to central collisions. With new event selection procedures and different analysis techniques, we observe that in peripheral collisions the J/$\psi$ cross section per nucleon-nucleon collision agrees with the pattern inferred from a wide range of measurements with lighter systems, from pp up to S-U. When the collisions become more central a clear departure from this behaviour is observed. The 1996 data show a sudden drop in the J/$\psi$ production yield for $E_ {T}$ values above 40 GeV, where $E_ {T}$ is the neutral transverse electromagnetic energy released in the collision and measured in the EM calorimeter. The 1998 data provide a big improvement in the study of the most central region, where a second change in the pattern becomes visible. (14 refs).