Charmonium Suppression and Regeneration from SPS to RHIC

The production of charmonia is investigated for heavy-ion collisions from SPS to RHIC energies. Our approach incorporates two sources of $J/\Psi$ yield: (i) a direct contribution arising from early (hard) parton-parton collisions, subject to subsequent nuclear absorption, quark-gluon plasma and hadronic dissociation, and (ii) statistical production at the hadronization transition by coalescence of $c$ and $\bar{c}$ quarks. Within an expanding thermal fireball framework, the model reproduces $J/\Psi$ centrality dependencies observed at the SPS in Pb-Pb and S-U collisions reasonably well. The study of the $\Psi'/\Psi$ ratio at SPS points at the importance of the hadronic phase for $\Psi'$ interactions, possibly related to effects of chiral symmetry restoration. Predictions are given for the centrality dependence of the $N_{J/\Psi}/N_{c\bar{c}}$ ratio at full RHIC energy. We also calculate the excitation function of this ratio. The latter exhibits a characteristic minimum structure signalling the transition from the standard $J/\Psi$ suppression scenario prevailing at SPS to dominantly thermal regeneration at collider energies.