Charm and strangeness in nuclear reactions at √s ≤ 19 GeV

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