J PSI-SUPPRESSION, PSI'-SUPPRESSION AND CHI-SUPPRESSION IN NUCLEUS-NUCLEUS COLLISIONS AT SPS ENERGIES/

A quantum-mechanical treatment for the evolution of a c - (c) over bar pair inside a quark-gluon plasma and subsequent mixed phase is presen ted. It is shown, that contrarily to the expectation of the semi-class ical approach, the psi' and J/psi suppression at SPS energies are not consistent with the existence of a plasma. An alternative explanation involving the formation of a medium with partial screening is proposed .