Elsevier

Physics Letters B

Volume 450, Issue 4, 25 March 1999, Pages 456-466
Physics Letters B

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

https://doi.org/10.1016/S0370-2693(99)00178-1Get rights and content

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.

Introduction

In ordinary nuclear matter, quarks and gluons are confined within nucleons. Above the deconfinement temperature, Tc, as predicted by non-perturbative Quantum Chromodynamics (QCD) calculations, quarks and gluons are expected to behave as free objects within a new state of deconfined matter, the Quark-Gluon Plasma (QGP). It has been suggested that the phase transition between ordinary and deconfined matter could occur in ultrarelativistic heavy ion collisions where, under certain conditions, temperature and energy density could reach the required values. Heavy ion physics offers thus a unique opportunity to test a major prediction of QCD.

The suppression of charmonium states has been predicted to be an unambiguous signature of deconfinement [1]. The formation of the cc̄ bound states would be suppressed in the QGP due to the screening of the attractive color force which binds the two quarks together. Charmonia are produced in the very early stage of the collision. Strongly bound states, like the J/ψ, are expected to interact weakly with other hadrons. As a result, and more than other signatures, J/ψ suppression carries its original message through the different stages of the reacting medium.

Based on an extensive set of experimental data obtained with proton, oxygen and sulphur beams on several targets, the NA38 and NA51 experiments have indeed observed a J/ψ suppression which increases continuously and monotonically from the lighter to the heavier interacting nuclei [2]. The same behaviour is observed in sulphur-uranium collisions as a function of the centrality of the reaction. The observed suppression exhibits a completely smooth increasing pattern which is fully consistent with ordinary nuclear absorption of the charmonium states by the nucleons of the interacting nuclei. Within the framework of the Glauber model and from a simple first order exponential fit, the data lead to a J/ψ absorption cross-section of 6.1±0.7 mb [3]which accounts for the observed suppression in a range which extends from p-p up to central S-U reactions.

The J/ψ suppression measured by experiment NA50 for Pb-Pb interactions shows a clear departure from the smooth trend exhibited by measurements with lighter projectiles [3]. The data which led to the discovery of the anomalous J/ψ suppression were collected in 1995, at the CERN SPS. The J/ψ cross-section measured in Pb-Pb at 158 GeV/c per nucleon is a factor 0.71±0.03 below the value extrapolated from the NA38/NA51 data according to pure nuclear absorption. Moreover, the observed anomaly increases with increasing centrality. Within a large statistical error, the results obtained for the most peripheral collisions have been found to be compatible with normal nuclear absorption as extrapolated from p-A and S-U reactions.

In this paper we report the results from a high statistics sample of Pb-Pb interactions, collected in 1996. The aim of this larger set of data is to increase significantly the sample of high mass muon pair events, in particular those produced in peripheral reactions. The new analysis presented hereafter is devoted to the study of J/ψ suppression as a function of centrality. It is intended to explore in detail the pattern of the observed anomaly and to compare, as accurately as possible, the most peripheral Pb-Pb results with the previously studied S-U reactions. Absolute cross-sections can be found in Ref. [4].

Section snippets

Experimental setup

The experimental setup used in 1996 was almost identical to the one used in 1995. The detector has been described in detail in Ref. [4]. Muon pairs are deflected by an air gap toroidal magnet in the pseudo-rapidity interval 2.8<ηlab<4.0 and tracked by a set of multi-wire proportional chambers. A very forward (“zero degree” or ZDC) hadronic calorimeter [5]covers rapidities higher than 6.3 and measures essentially the energy, EZDC, of the spectator (non-interacting) nucleons from the incoming Pb

The “standard” analysis

The “standard” analysis method has already been described in Ref. [4]. The J/ψ and ψ′ resonances are identified through their decays into muon pairs. The numbers of J/ψ and ψ′ events and the number of Drell-Yan dimuons are obtained from the analysis of the muon pair invariant mass spectrum, taking into account the contributions originating from both the open charm associated production and the combinatorial background due to decays. The measured quantity is BμμσJ/σDY, the ratio of J/ψ to

The “minimum bias” analysis

The statistical fluctuations of the ratio σJ/σDY are essentially due to the small number of Drell-Yan events. In order to overcome this experimental drawback, an alternative independent analysis has been developed with only the J/ψ sample of events in common with the standard analysis. The sample of Drell-Yan events is now replaced by the huge sample of minimum bias events and a new estimator of the ratio σJ/σDY is built, according to the procedure detailed hereafter.

Summary and conclusions

A high statistics data sample of Pb-Pb interactions at 158 GeV/c per nucleon incident momentum has been collected in 1996 by experiment NA50. The ratio σJ/σDY has been measured as a function of the centrality of the reaction estimated, for each event, from the measured neutral transverse energy ET. The new data have been analyzed with the same method as used for the 1995 smaller sample, except for a new selection procedure which allows to include more peripheral interactions.

The results

Acknowledgements

We would like to acknowledge the constant and efficient support of CERN and, in particular, the fundamental contributions of the PS and SL groups which have provided a Pb beam of good stability, high intensity and low emittance as required by the experiment. The fine tuning and focussing of the halo-free final beam have created the conditions which allowed to develop and use the new methods described in this paper. We are indebted to J. Gosset, from SPhN, Saclay, for pertinent remarks on our

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1

Also at UCEH, Universidade do Algarve, Faro, Portugal.

2

Also at IST, Universidade Técnica de Lisboa, Lisbon, Portugal.

3

Now at FPNT, University of Mining and Metallurgy, Cracow, Poland.

4

Now at ISN, Univ. Joseph Fourier and CNRS-IN2P3, Grenoble, France.

5

Now at UERJ, Rio de Janeiro, Brazil.

6

On leave of absence from Università di Torino/INFN, Torino, Italy.

7

On leave of absence from YerPhI, Yerevan, Armenia.

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