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. Our analysis suggests that the existence of new physics (deconfinement phase transition) in the region $E_{T} \lsim 100$ GeV is unlikely and 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.