In-Medium Formation of J/Psi as a Probe of Charm Quark Thermalization

Abstract

Abstract. Charmonium formation via charm quark in-medium recombination in heavy ion interactions at collider energies has the potential to probe some properties of the medium by utilizing the sensitivity of the recombination process to the momentum distribution of the quarks. We have examined the transverse momentum spectra of J/ψ, characterized by p T 2 , which result from the formation process in which the charm quark distributions are unchanged from their initial production in a pQCD process. This is contrasted with the case in which the charm quarks have completely come into thermal equilibrium with an expanding medium whose properties are determined by the spectra of produced light hadrons. We find that the resulting p T 2 of the formed J/ψ provide a distinct signature of the underlying charm quark spectra, and that signature is essentially independent of the detailed dynamics of the in-medium formation reaction. In addition, both of these signatures are sufficiently separated from the case in which no in-medium formation takes place. Finally, utilizing a model for the fraction of J/ψ which originate from in-medium formation, we predict the centrality behavior of these signatures.The role of J/ψ produced in high energy heavy ion collisions as a signature for color deconfinement [1] has evolved in recent years with the realization that at collider energies an additional formation mechanism may become significant [2,3]. This depends on the initial production of multiple cc pairs in sufficient numbers. Initial estimates [4] from extrapolation of fixed-target data put this number at about 10 for central Au-Au collisions at RHIC. Subsequently, measurements by the PHENIX and STAR experiments indicate even higher numbers, between 20 [5] and 40 [6], respectively. The in-medium formation picture we consider here uses competing formation and dissociation reactions in a Boltzmann equation to calculate the final J/ψ population. The absolute value of this formation was found to be very sensitive to the underlying charm quark momentum distributions [7]. In addition, there is significant dependence on details of the size and expansion profile of the deconfinement region, for which various model parameters must be introduced. The initial PHENIX data [8] suffered from low statistics, and was compatible with a fairly large region of model parameter space [9].Recent work in this area concentrated on finding a signature for in-medium J/ψ formation which is independent of the detailed dynamics and magnitude of the formation. We found that the p T spectrum of the formed J/ψ may provide such a signature [10]. Our first calculations of in-medium formation used initial charm quark momentum distributions from NLO pQCD amplitudes to generate a sample of cc pairs, supplemented by an initial-state transverse momentum kick to simulate confinement and nuclear ef-