J/ψ suppression in heavy ion collisions at the CERN Super Proton Synchrotron

We reexamine the production of $J/\psi$ and other charmonium states for a variety of target-projectile choices at the CERN Super Proton Synchrotron, in particular for the interesting comparison between S+U at $200\hspace{0.5em}\mathrm{GeV}/c$ and Pb+Pb at $158\hspace{0.5em}\mathrm{GeV}/c$ as observed in the experiments NA38 and NA50, respectively. For this study we use a newly constructed cascade code LUCIFER II, which yields acceptable descriptions of both hard and soft processes, specifically Drell-Yan and meson production. This code divides the ion-ion collision into an initial phase involving hard interactions of the original nucleons and no soft energy loss, followed after the meson formation time by a “normal” low energy cascade among the secondary particles. The modeling of the charmonium states differs from that of earlier workers in its unified treatment of the hidden charm meson spectrum, which is introduced from the outset as a set of coupled states $\{\psi ,{\chi }^i,{\psi }^{\prime }\}.$ The result is a description of the NA38 and NA50 data in terms of a conventional, hadronic picture. The apparently anomalous suppression found in the most massive Pb+Pb system arises in the present simulation from three sources: destruction in the initial nucleon-nucleon cascade phase, use of coupled channels to exploit the larger breakup in the less bound ${\chi }^i$ and ${\psi }^{\prime }$ states, and comover interaction in the final low energy phase.