Momentum spectra of charmonium produced in a quark-gluon plasma

Abstract

We calculate rapidity and transverse momentum distributions of charmonium formed in high-energy heavy ion collisions from incoherent recombination of charm quarks. The results are very sensitive to the corresponding distributions of the charm quarks, and thus can serve as a probe of the state of matter produced in the heavy ion collision. At one extreme, we generate charm pair momenta directly from pQCD amplitudes, which are appropriate if one can neglect the interaction of the quarks with the medium. At the other extreme, we generate momenta of charm quarks in thermal equilibrium with the expanding medium, appropriate for an extremely strong interaction. Explicit predictions are made for $J/\ensuremath{\psi}$ formation in Au-Au interactions at the BNL Relativistic Heavy Ion Collider. We find that for the case in that charm quark momenta are unchanged from the pQCD production calculation, both the rapidity and transverse momentum spectra of the formed $J/\ensuremath{\psi}$ are substantially narrower than would be anticipated in scenarios that do not include the in-medium formation. In particular, the average transverse momentum of the $J/\ensuremath{\psi}$ will exhibit a nonmonotonic behavior in the progression from pp to pA to AA interactions.