Charm in nuclear reactions at √ s = 17 and 19 GeV 1

Consequences resulting from the DD excess derived indirectly by the NA50 experiment in S+U and Pb+Pb collisions at √ s=19, 17 GeV, relevant for the identification of the QCD phase transition in these collisions, are discussed. The dependence of open and closed charm yields in Pb+Pb collisions on the number of participating nucleons (N) indicates non thermal charm production and J/Ψ dissociation, stronger than the absorption seen in any other elementary hadron. The J/Ψ in central Pb+Pb collisions could originate dominantly from cc pair coalescence out of a hadronizing quark and gluon environment. Furthermore, the J/Ψ appears to be suppressed in S+U collisions at √ s=19 GeV, as opposed to current interpretations. A significant change in the (J/Ψ)/DD ratio as well as in the number density of kaons is observed above energy density ǫ ∼ 1 GeV/fm, suggesting a change of phase at this energy density, and underlining the importance of direct open charm measurements.