Charmonium Suppression in Nuclear Collisions

Has the quark gluon plasma been discovered at the CERN SPS? Experiment NA50 has reported an abrupt decrease in ψ production in Pb+Pb collisions at 158 GeV per nucleon [1]. Specifically, the collaboration presented a striking ‘threshold effect’ in the ψ–to–continuum ratio by plotting it as a function of a calculated quantity, the mean path length of the ψ through the nuclear medium, L, as shown in fig. 1a. This apparent threshold has sparked considerable excitement as it may signal deconfinement in the heavy Pb+Pb system [2]. In this talk I report on work with Ramona Vogt in ref. [3] comparing Pb results to predictions [4, 5] using a hadronic model of charmonium suppression. We first demonstrate that the behavior in the NA50 plot, fig. 1a, is not a threshold effect but, rather, reflects the approach to the geometrical limit of L as the collisions become increasingly central. When plotted as a function of the measured neutral transverse energy ET as in fig. 1b, the data varies smoothly as in S+U measurements in fig. 3b below [1, 6, 7, 8, 9]. The difference between S+U and Pb+Pb data lies strictly in the relative magnitude. To assess this magnitude, we compare ψ and ψ data to expectations based on the hadronic comover model [4, 5]. The curves in fig. 1 represent our calculations using parameters fixed earlier in Ref. [5]. Our result is essentially the same as the Pb+Pb prediction in [4]. Our primary intention is to demonstrate that there is no evidence for a strong discontinuity between pA, S+U and Pb+Pb data. However, to quote Maurice Goldhaber, “. . . absence of evidence is not evidence of absence.” Our secondary goal is to show that our model predictions agree with the new Pb+Pb data. The consistency of these predictions is evident from the agreement of our old pA and S+U calculations with more recent NA38 and NA51 data. Nevertheless, the significance