C. Soave

Anomalous J/psi suppression in Pb-Pb interactions at 158 GeV/c per nucleon

Abstract The Drell-Yan and J/t) cross-sections measured in Pb-Pb collisions are compared with the values extrapolated from the results obtained in proton and light ion induced reactions. While the Drell-Yan production exhibits the normal expected behaviour, the yield of J/e in Pb-Pb interactions is abnormally low, as it lies 9 standard deviations below the expected value. Moreover, the departure from the expected behaviour increases significantly from peripheral to central collisions. 0 1997 Elsevier Science B.V. 1. Introduction It is now a well established prediction from lattice QCD calculations [l] that, under extreme energy density conditions, ordinary hadronic matter will un- dergo a phase transition to the so-called Quark-Gluon Plasma (QGP). The main feature of this phase of matter is that the elementary constituents of hadrons are decon- fined, i.e. that quarks and gluons do not form any more individual hadrons and are free to move within the medium. From the very beginning and before the start up of the experimental search in the field, it was pointed out [2] that deconfinement would induce color screening which in turn would prevent J/+ formation, making of “J/I+!I suppression” one of the gold-plated signatures for QGP experimental evi- dence. Starting from 1986 on, high energy ion beams (0, S and finally Pb) became available at CERN. They made possible the search for QGP formation in ultrarelativistic nucleus-nucleus collisions where it was believed that the required conditions could be reached at best. Experiment NA38 developed an extensive program studying charmonium in p-A and A-B reactions with incident proton, oxygen and sul- fur ions on different nuclear targets. The results obtained for J/I) production [3] led to a solid sys- tematic experimental reference. They triggered a considerable amount of theoretical work [4] and can be understood, according to some authors, in an overall coherent framework [5]. In this letter, we compare the Pb-Pb values mea- sured by the NA50 experiment, as reported in the preceding letter [6], with the values previously ob- tained with lighter incident ions. 2.

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.

J/psi and Drell-Yan cross-sections in Pb-Pb interactions at 158 GeV/c per nucleon

Abstract First results are reported on J ψ and Drell-Yan cross-sections in PbPb reactions at 158 GeV/ c per nucleon. The ratio of cross-sections σ J/ψ /σ DY is studied as a function of the impact parameter of the collision estimated from the measured transverse energy.

Transverse momentum distributions of J/ψ, ψ′, Drell–Yan and continuum dimuons produced in Pb–Pb interactions at the SPS

Muon pairs produced in Pb-Pb interactions at 158~GeV/

Performances of zero degree calorimeters for the ALICE experiment

c

Intermediate mass dimuons in NA38/NA50

per nucleon are used to study the transverse momentum distributions of the \jpsi, \psip\ and dimuons in the mass continuum. In particular, the dependence of these distributions on the centrality of the Pb-Pb collision is investigated in detail.

Study of resistive plate chambers for the alice dimuon spectrometer

Abstract Results on the performances of two hadron calorimeter prototypes for the ALICE experiment are presented. The two prototypes are made of brass and copper as passive material and PMMA fibres, arranged in a spaghetti calorimeter geometry, as active material. The calorimeters were tested at the CERN SPS, using hadron and positron beams of variable momentum in the range 50– 200 GeV /c . The main features of the detectors, such as linearity, energy resolution and uniformity of response with respect to the beam impact point on their front face, are presented. The experimental results are compared to Monte Carlo simulations.

CHARMONIUM PRODUCTION IN PB-PB INTERACTIONS AT 158 GEV/C PER NUCLEON

The NA38/NA50 experiments have measured, at the CERN SPS, the dimuon production in proton-nucleus and nucleus-nucleus collisions. In this paper it is shown that the mass continuum between the and the J/ψ can be satisfactorily described, after having removed the combinatorial background due to uncorrelated π and K decays, as a sum of two contributions, namely the Drell-Yan process and the semi-leptonic decay of pairs of charmed mesons, whose mass shape in the acceptance of the experiment has been evaluated using PYTHIA. However, in order to describe the A-B (namely S-U and Pb-Pb) mass spectra, the dimuon yield from open charm decays, which in p-A collisions is found to be consistent with direct open charm measurements from other CERN and FNAL experiments, has to be enhanced with respect to a linear extrapolation of p-A results. The size of the enhancement smoothly increases from peripheral S-U to central Pb-Pb interactions, reaching a factor ~3 in central Pb-Pb collisions. The pT distributions of the events in the mass continuum are also compatible with the hypothesis of open charm enhancement in A-B collisions.

Results on open charm from NA50

Abstract The trigger system of the Alice dimuon spectrometer is based on RPC detectors. We present experimental results of a beam test about rate capability, time resolution and cluster size of single gap RPC detectors operating both in ‘streamer mode’ and in ‘avalanche mode’. We have compared the performances of small chambers (50×50 cm 2 ) built with bakelite of different resistivity, from ∼ 3 · 10 9 Ωcm to ∼ 3 · 10 11 Ωcm. For the low resistivity RPC we obtained a rate capability of several hundreds of Hz/cm 2 when it is operating in ‘streamer mode’, and of several thousands of Hz/cm 2 when it is operating in ‘avalanche mode’.

Latest results from NA50 on J/ψ suppression in Pb-Pb collisions

Abstract The NA50 collaboration has measured J/ψ , ψ′ and Drell Yan pair production in Pb Pb interactions at 158 GeV/ c per nucleon at the CERN SPS. Final results from the 1995 run and preliminary ones from the higher statistics 1996 run are presented. A anomalous J/ψ suppression (relative to the Drell-Yan process) has been observed with respect to the suppression pattern established in experiments NA38 and NA51 with proton, oxygen and sulfur beams. The 1996 data allow a detailed study of the suppression pattern in the Pb Pb sample itself, showing a discontinuity around an E T value (the neutral transverse electromagnetic energy) of 50 GeV. The ψ′ is also suppressed relative to Drell-Yan, with a pattern very similar to the one observed in S U collisions. Finally, the p T distributions of dimuons are presented. The average p T 2 of the J/ψ in Pb Pb collisions does not increase any more with E T above 100 GeV.