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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 $\bar c$ 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 $J/Ψ$'s over open charm pairs exhibits nontrivial structure around $\sqrt{s} \simeq 30$ AGeV, 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.