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$J/Ψ$ suppression in nuclear collisions at SPS energies

We have carried out a first calculation of $J/Ψ$ 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 $J/Ψ$'s 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 $D\bar{D}$ threshold, we find that data on $J/Ψ$ suppression from both proton-nucleus and nucleus-nucleus collisions (including Pb + Pb) can be explained without assuming the formation of a quark-gluon plasma in these collisions. Our microscopic studies thus confirm the suggestion of Gavin et al. that $J/Ψ$ suppression observed in nuclear collisions is largely due to absorption by comovers, i.e., the produced mesons.