B. Chaurand

First Measurement of the

We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360,000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi- -->rho -->mu+mu- annihilation. The associated space-time averaged spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.

\rho

Abstract The ratio of cross-sections for muon pair production through the Drell-Yan process in p − p and p − d reactions has been measured at y ≈ 0, with 450 GeVc incident protons. The asymmetry A DY = δ pp − δ pn δ pp +δ pn amounts to −0.09 ± 0.02 ± 0.025. The ratio u d of the nucleon sea structure functions derived from this measurement amounts to 0.51 ± 0.04 ± 0.05 at x = 0.18 and suggests that isospin symmetry is broken in the light quark sea of the nucleon.

Spectral Function in High-Energy Nuclear Collisions

Abstract The study of oxygen-uranium reactions at 200 GeV/nucleon shows a significant transverse energy dependence of the yield of J Ψ s relative to muon pairs produced in the mass continuum. This feature, observed for the first time, is in agreement with predictions from quark-gluon plasma formation, although alternative explanations by hadronic effects cannot be ruled out at this stage.

Study of the isospin symmetry breaking in the light quark sea of the nucleon from the Drell-Yan process

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.

The production of j/

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.

\psi

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.

in 200 GeV/nucleon oxygen-uranium interactions

Abstract.The production of

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

{\rm J}/\psi

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

and

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

\psi^\prime