Charm in nuclear reactions in sqrt(s)=17 and 19 GeV

Abstract

Consequences resulting from the D Dbar excess derived indirectly by the NA50 experiment in S+U and Pb+Pb collisions at sqrt(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/Psi dissociation, stronger than the absorption seen in any other elementary hadron. The J/Psi in central Pb+Pb collisions could originate dominantly from c cbar pair coalescence out of a hadronizing quark and gluon environment. Furthermore, the J/Psi appears to be suppressed in S+U collisions at sqrt(s)=19 GeV, as opposed to current interpretations. A significant change in the (J/Psi)/D Dbar ratio as well as in the number density of kaons is observed above energy density approx. 1 GeV/fm^3, suggesting a change of phase at this energy density, and underlining the importance of direct open charm measurements.