The dependence of the anomalous J/ψ suppression on the number of participant nucleons

The observation of an anomalous J/ψ suppression in Pb–Pb collisions by the NA50 Collaboration can be considered as the most striking indication for the deconfinement of quarks and gluons at SPS energies. In this Letter, we determine the J/ψ suppression pattern as a function of the forward hadronic energy EZDC measured in a Zero Degree Calorimeter (ZDC). The direct connection between EZDC and the geometry of the collision allows us to calculate, within a Glauber approach, the precise relation between the number of participant nucleons Npart and EZDC. Then, we check if the experimental data can be better explained by a sudden or a smooth onset of the anomalous J/ψ suppression as a function of the number of participants.