Charmonium Suppression in Lead-Lead Collisions: Is There a Break in the $J/ψ$ Cross-Section?

In the framework of a model based on nuclear absorption plus comover interaction, we compute the $E_{T}$ distribution of the $J/\psi$ in $PbPb$ collisions at SPS and compare it with available NA50 data. For $E_T \lsim \ 100$ GeV the 1996 data have small oscillations around the theoretical curve but show no break at any value of $E_{T}$. Our analysis suggests that the break observed in the ratio of $J/\psi$ over Drell-Yan cross-sections at $E_{T} \sim$ 55 GeV is due to oscillations in the Drell-Yan cross-section rather than to a genuine break in the $J/\psi$ one. It further suggests that signals of new physics should rather be searched in the region $E_T \gsim 100$ GeV. The $E_{T}$ dependence of the $J/\psi$ transverse momentum has been computed. At large $E_T$ it turns out to be much flatter in the comover approach than in a phase transition framework. Estimates of the $J/\psi$ suppression at RHIC and LHC energies are also given.