  The J/psi suppression observed by the NA50 experiment is one of the most
striking signatures for quark gluon plasma formation in Pb-Pb collisions at 158
AGeV. The J/psi production has been studied as a function of the centrality of
the collision estimated via the forward energy EZDC released in a zero degree
calorimeter (ZDC). The study of the correlation between the number of
participant nucleons in the collisions, Npart, and EZDC allows to check whether
the J/psi suppression pattern vs. EZDC is compatible with a sudden J/psi
suppression mechanism expressed as a function of Npart.
