Abstract

We propose to study J/Psi production in relation with open charm production in nuclear reactions. It appears that a suppression of the J/Psi over Drell Yan (DY) ratio has been observed by the CERN experiment NA50 in Pb + Pb collisions at roots = 17 GeV above the initial energy density epsilon similar to 2.3 GeV fm(-3), which was not seen in S + U collisions at roots = 19 GeV at any epsilon. In our view a clear interpretation of these results has not been achieved. The same experiment has measured an excess in mu (+)mu (-), which can be interpreted as resulting from D (D) over bar decay. We demonstrate that the suppression of the J/Psi does appear in S + U collisions, as well as in Pb + Pb collisions at a lower epsilon similar to 1 GeV fm(-3), if the J/Psi is normalized to D (D) over bar, instead of DY. This underlines the importance of direct open charm measurements for the interpretation of charmonia suppression. Furthermore we study the dependence of the J/Psi and D (D) over bar on the number of participating nucleons (N(p)). The results indicate non-thermal charm production as expected, and J/Psi dissociation which is stronger than the absorption seen in other hadrons. We find that the J/Psi in central Pb + Pb collisions is compatible with dominant production from c (c) over bar pair coalescence out of a hadronizing quark and gluon environment. A significant change in the J/Psi) /D (D) over bar ratio as well as in the number density of kaons is observed simultaneously above the initial energy density epsilon similar to 1 GeV fm(-3), suggesting a change of phase associated with this epsilon.