Dissociation of a heavy quarkonium at high temperatures

We examine three different ways a heavy quarkonium can dissociate at high temperatures. The heavy quarkonium can dissociate spontaneously when it becomes unbound at a temperature above its dissociation temperature. Following the recent work of Digal, Petreczky, and Satz, we calculate the dissociation temperatures of heavy quarkonia taking into account the angular momentum selection rules and using a temperature-dependent potential inferred from lattice gauge calculations. We find that the selection rules change the dissociation temperatures substantially for charmonia but only slightly for bottomia. A quarkonium system in thermal equilibrium with the medium can dissociate by thermalization. The fraction of quarkonium lying above the dissociation threshold increases as temperature increases. A quarkonium can also dissociate by colliding with light hadrons. We evaluate the cross sections for the dissociation of $J/\psi$ and ${\rm Y}$ in collision with $\pi$ as a function of the temperature of the hadron medium, using the quark-interchange model of Barnes and Swanson. We find that as the temperature increases, the threshold energy decreases and the dissociation cross section increases.