JΨ, Ψ′ and ξ suppression in nucleus-nucleus collisions at SPS energies

A quantum-mechanical treatment for the evolution of a c - c pair inside a quark-gluon plasma and subsequent mixed phase is presented. It is shown, that contrarily to the expectation of the semi-classical approach, the Ψ′ and JΨ 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.