Observation of a threshold effect in the anomalous J/ψ suppression

Abstract

We report on a search for a phase transition from ordinary nuclear matter to a state of deconfined quarks and gluons as predicted by lattice QCD calculations. A new measurement of charmonium production in Pb-Pb interactions at 158 GeV/c per nucleon agrees with our previous results and confirms the anomalous J/ψ suppression we had already observed on a significantly smaller data sample. New event selection and analysis techniques show that, for peripheral collisions, the J/ψ cross-section per nucleon-nucleon collision agrees with the precise suppression pattern inferred from a wide range of measurements extending from p-p up to S-U collisions. As the collisions become more central, the Pb-Pb cross-section exhibits a clear departure from this normal behaviour. The onset of the anomalous J/ψ suppression reported here is the first clear observation of a threshold effect in heavy ion collisions and can be considered as a strong indication of the production of a deconfined quark-gluon phase in central Pb-Pb collisions.

Another look at anomalous J/$Psi; suppression in Pb + Pb collisions at P/A = 158 GeV/c

“…In most central collisions, the J/Ψ events are significantly suppressed with respect to what is expected from normal nuclear absorption as measured in lighter systems [2,3,4,5]. According to theoretical predictions made more than ten years ago by Matsui and Satz [6], this anomalous J/Ψ suppression could be a sign of the awaited formation of a quark-gluon plasma in nucleus-nucleus collisions at very high energy.…”

“…One idea from this new data presentation, the normalization of J/Ψ events to the inclusive, or minimum bias, E T distribution, has been used recently by the NA50 collaboration [5], making the most of the whole statistics available for J/Ψ production in their 1996 data taking. However, this new presentation is not used as such, except the first step for Drell-Yan production only.…”

“…A normalization factor had to be introduced to get the (b 2 ) e values in units of fm 2 . It has been adjusted in such a way that the dependence of (b 2 ) e upon E T agrees with the correlation between the average values of E T and b listed in NA50 publications [1,5] for successive E T slices, as fitted on the basis of a Glauber model calculation. These average values are also shown in the bottom part of Fig.…”

“…Very interesting results have been obtained recently by the NA50 experiment at CERN concerning J/Ψ production in Pb + Pb collisions at P/A = 158 GeV/c [1,2,3,4,5]. In most central collisions, the J/Ψ events are significantly suppressed with respect to what is expected from normal nuclear absorption as measured in lighter systems [2,3,4,5].…”

“…2. E T values from [5] have been divided by 0.74 to take into account the different E T scales used in the NA50 publications. After the first step, i.e.…”