Thermal versus direct J/Ψ production in ultrarelativistic heavy-ion collisions

The production of J/Ψ mesons in central collisions of heavy nuclei is investigated as a function of collision energy. Two contributions are considered simultaneously: early (hard) production coupled with subsequent suppression in a quark–gluon plasma, as well as thermal recombination of primordially produced c and c̄ quarks at the hadronization transition. Whereas the former still constitutes the major fraction of the observed J/Ψ abundance at SpS energies, the latter dominates the yield at RHIC. The resulting excitation function for the number of J/Ψʹs over open charm pairs exhibits nontrivial structure around s≃30 A GeV, evolving into a significant rise towards maximal RHIC energy. We study this feature within different (thermal) scenarios for J/Ψ suppression, including parton-induced quasifree destruction as a novel mechanism.