J/ψ suppression in heavy ion collisions at the CERN SPS

Abstract

We reexamine the production of J/ψ and other charmonium states for a variety of target-projectile choices at the SPS, in particular for the interesting comparison between S+U at 200 GeV/c and Pb+Pb at 158 GeV/c as observed in the experiments NA38 and NA50 respectively. For this study we use a newly constructed cascade code LUCIFER II, which yields acceptable descriptions of both hard and soft processes, specifically Drell-Yan and meson production. This code divides the ion-ion collision into an initial phase involving hard interactions of the original nucleons and no soft energy loss, followed after the meson formation time by a 'normal' low energy cascade among the secondary particles. The modeling of the charmonium states differs from that of earlier workers in its unified treatment of the hidden charm meson spectrum, which is introduced from the outset as a set of coupled states ψ, χ i , ψ ′ . The result is a description of the NA38 and NA50 data in terms of a conventional, hadronic picture. The apparently anomalous suppression found in the most massive Pb+Pb system arises in the present simulation from three sources: destruction in the initial nucleon-nucleon cascade phase, use of coupled channels to exploit the larger breakup in the less bound χ i and ψ ′ states, and comover interaction in the final low energy phase.

Evidence for deconfinement of quarks and gluons from the J/ψ suppression pattern measured in Pb-Pb collisions at the CERN-SPS

“…The steady decrease of the J/ψ suppression pattern, seen in the most central Pb-Pb collisions collected in 1998, is in clear disagreement with the presently existing conventional hadronic models [12,13,14,15]. Indeed, the main characteristic of these models is that they predict a smooth decrease of the J/ψ production yield, from p-p to Pb-Pb collisions, tending to a saturation of the J/ψ suppression in the most central collisions.…”

“…This is clearly illustrated in Fig. 6, where our data are compared to the curves calculated by models which assume that the charmonia states are absorbed by interactions with comoving hadrons [12,13,14,15].…”

Charmonium absorption by nucleons

“…However, the long term puzzle of the problem is the strength of the hadronic dissociation of the J/Ψ meson. The various calculations [2,3,[5][6][7]9,10] of J/Ψ production from heavy ion collisions in practice adopt different dissociation cross sections. For instance, the cross section for J/Ψ+N dissociation used in these models ranges from 3.0 up to 6.7 mb when dealing with the same experimental data on J/Ψ production from A+A collisions.…”

Dilepton signals in heavy-ion collisions at the CERN SPS

“…The left panel of Fig. 11 shows that the observed suppression pattern is in clear disagreement with the predictions of the "conventional" models [17][18][19][20], that attribute the disappearance of the J/ψ mesons to interactions with "comoving" hadrons. A first departure from the conventional curves is seen for collisions that release around 40 GeV of neutral transverse energy in the electromagnetic calorimeter of NA50.…”