JΨ suppression in nuclear collisions at SPS energies

We have carried out a first calculation of JΨ production from nuclear collisions within the covariant transport approach HSD, which has been very successful in describing both hadronic and electromagnetic observables from heavy-ion collisions at intermediate and high energies. The production of JΨ is based on the Lund string fragmentation model, while its interactions with hadrons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the J/Ψ-baryon cross sections and 3 mb for the J/Ψ - meson cross sections above the DD threshold, we find that data on JΨ suppression from both proton-nucleus and nucleus-nucleus collisions (including Pb+Pb) can be described without assuming the formation of a quark-gluon plasma in these collisions. In particular, mesons are found to absorb half the JΨʹs in nucleus-nucleus collisions. The smaller absorption by nucleons compared to that found in the Glauber model analysis is due to the finite hadron formation time included in the transport approach. However, an appreciable volume of the hadronic matter in which JΨʹs are absorbed is found to have an energy density which may be above that for the transition to a quark-gluon plasma.