The anomalous suppression of J/~b production in Pb-Pb collisions at 158A GeV observed by the NA50 Collaboration can be explained as due to the occurrence of a new phase of strong J/¢ absorption, which sets in when the local energy density exceeds about 3.4 GeV/fm 3. The peculiar behavior of the ¢//¢ ratio in p-A and nucleus-nucleus collisions can be understood as due to approximately equal ¢-N and ¢1-N absorption cross sections, but greater absorption cross sections for ¢1 than J/¢ with regard to absorption by soft particles and matter in the new phase.
1. Introduction
The interaction between a c quark and a ~ antiquark is screened in the qnark-gluon plasma. The screening length decreases as the temperature increases. Above a critical temperature, the screening length becomes so short that a quasi-bound c~ system, which evolves otherwise into a J/¢ in free space, cannot form a bound state in the plasma and will be removed from J/¢ production. This led to the suggestion of Matsui and Satz to use the suppression of J/~b production to probe the presence of the plasma [1] (see e.g.
[2] for an introduction). A comparison of the suppression of J/¢ and ¢' has also been suggested to study deconfinement [3]. While the suppression of Y/¢ and ¢' has been observed in p-A, O-A, and S-U collisions at 200A GeV [4-7], the phenomenon can be explained by absorption models without assuming the occurrence of the plasma [8][9][10][11][12][13][14].
Recent data from NA38 [6,7] and NA50 [15,16] reveal that in Pb-Pb collisions at 158A
GeV, or/¢ is anomalously suppressed relative to expectations based on experimental systematics from p-A, O-A, and S-U collisions. The NA38 and NA50 Collaborations also found that ¢~/%b is nearly independent of the mass number in p-A collisions but depends on mass numbers and centrality in nucleus-nucleus collisions. What are the underlying physical processes which lead to the anomalous d/¢ suppression in Pb-Pb collisions and the peculiar behavior of ¢//¢?
2. ABSORPTION OF d/¢ AND ¢1 IN NUCLEUS-NUCLEUS COLLISIONS
We envisage that in p-A and A-B (nucleus-nucleus) collisions, the reaction of the parton of a projectile nucleon and the patton of a target nucleon can produce a quasi-bound c~ system which later evolves into a J/¢ or ¢' in free space. Because J/~b and ¢' are