Charmonium Production and Corona Effect

We study the centrality dependence to be expected if only charmonium production in the corona survives in high energy nuclear collisions, with full suppression in the hot, deconfined core. To eliminate cold nuclear matter effects as far as possible, we consider the ratio of charmonium to open charm production. The centrality dependence of this ratio is found to follow a universal geometric form, applicable to both LHC and RHIC in collisions at both central and forward rapidities. The behavior of charmonium production in high energy nuclear collisions was proposed quite some time ago as probe of color deconfinement [1]. The basic idea was that color screening in the hot primary medium prevents the binding of cc̄ pairs produced in the early stages of nucleon-nucleon collisions. As a result, the ratio of hidden to open charm production should vanish when the energy densities of the medium produced in the collision are sufficiently high. Since the observed J/ψ production rate is partially due to feed-down from higher mass excited states, the suppression should occur in a sequential fashion, first for the decay products from ψ and χc production, then for directly produced J/ψ(1S) [2–4]. In subsequent work [5–7], it was argued that, at the hadronisation point, statistical combination of the charm quarks present in the deconfined medium could lead to regeneration of charmonia. Even if the “primary” charmonium states produced in individual nucleonnucleon collisions are dissociated through color screening, the abundance of charm production at high collision energy could provide enough charm quarks to result in “secondary” charmonium formation through the binding of combinatorial pairs of c and c̄ quarks produced in different initial nucleon-nucleon collisions. Since the charm production rate grows with collision energy and centrality, this would imply a corresponding increase in the ratio of hidden to open charm. In Fig. 1, we schematically compare the statistical recombination prediction to that of the sequential color screening scenario. The ratio of hidden to open charm in nuclear collisions is normalized to the pp value, scaled by the number of binary nucleon-nucleon collisions. The increase of this ratio above unity in central collisions for high charm production rates is a crucial prediction of the statistical recombination model. On the other hand, the sequential suppression scenario predicts