$J/\psi$ and $\psi '$ production in $p$, O and S induced reactions at SPS energies

The production of the \jpsi\ and \psip\ charmonia states has been studied, through their dimuon decay, in proton, Oxygen and Sulphur induced reactions, by the NA38 experiment at the CERN SPS. The proton data was collected with beams of 200 and 450~GeV, while the ion beams had an energy of 200~GeV per incident nucleon. The \jpsi\ production cross-section per nucleon-nucleon collision exhibits a remarkably continuous pattern, as a function of the product of the mass numbers of the interacting nuclei, from pp up to S-U reactions. The same pattern is observed within S-U collisions, as a function of the collision centrality. While in p-A interactions both charmonia states exhibit the same A-dependence, in \mbox{S-U} collisions the \psip\ production is very strongly suppressed.