Chapter 7 CHARM AND BEAUTY IN MEDIA

Quarkonium in media is a topic which is central to the ultrarelativistic heavy ion program. In recent years this subject has been among the focal points of discussion at meetings such as “Quark Matter”. (“The Quark Matter” series is traditionally the main forum of the high energy heavy ion community). Indeed, in these collisions — “little bangs” — one hopes to recreate matter as it was at the very beginning of the universe: a hot system with deconfined quarks and gluons and no chiral symmetry breaking. This fascinating possibility calls for a number of theoretical and phenomenological studies. The QCD phase diagram and the mechanisms of chiral symmetry restoration, screening, and deconfinement at high temperature and baryon density need to be understood. A theory of the initial conditions must be developed and the equilibration of the plasma, if any, must be assessed in real experiments. It is also necessary to identify the thermodynamical region which is being explored and to study nonequilibrium effects. Finally, observables must be defined which provide physical signatures in real experiments. Quarkonium plays a very important role in these phenomena. Indeed, quarkonium suppression was long ago suggested as a signal of deconfinement [1]. Due to their small size, quarkonia can, in principle, survive the deconfinement phase transition. However, because of colour screening, no bound state can exist at temperatures ̧ÉÈü ̧uf when the screening radius, Š é }wf Ñ ̧ Ò , becomes smaller than the typical bound-state size [1]. Later it was realized that dramatic changes in the gluon momentum distributions at the deconfinement phase transition result in a sharp increase in the quarkonium dissociation rates [2–4]. Both the magnitude [5] and the energy dependence [6] of charmonium dissociation by gluons result in significant suppression of the ê ê states even for ̧ ‰ ̧ f but higher than the deconfinement transition temperature, ̧1è . Moreover, close to ̧uf the thermal activation mechanism is expected to dominate [7,8]. The relative importance of gluon dissociation and thermal activation is governed by the ratio of the quarkonium binding energy r Ñ ̧ Ò and the temperature ̧ , ~ Ñ ̧ Ò w r Ñ ̧ Ò é ̧ [9]. At ~ Ñ ̧ Ò x Š thermal activation dominates while for ~ Ñ ̧ Ò Á Š the dominant mechanism is “ionization” by gluons. Dissociation due to colour screening was studied using potential models with different parameterizations of the heavy quark potential [10–13] to determine ̧œf . All these studies predicted that excited charmonium states ( Œ è , * 2 ) will essentially dissolve at ̧cè while the ground state é * will dissociate at Š Ý Š – Š Ý o ̧gè . Some potential models also predicted a strong change in the binding energy, see e.g., Ref. [12]. Recently, charmonium properties were investigated using lattice calculations [14, 15] which indicate that the ground states exist with essentially unchanged properties at temperatures around Š ÝÔå ̧ è . Lattice investigations suggest that at low temperatures, ̧ ‰ Š ÝÔå ̧ è , screening is not efficient and therefore gluon dissociation may be the appropriate source of quarkonium suppression. One should also keep in mind that non-equilibrium effects in the very early stages of a heavy-ion collision, when the energy density is highest, should be considered for quarkonium suppression. Not much is known about