T. Chambon

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

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.

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/

J/ψ and ψ′ production in p, O and S induced reactions at SPS energies

c

Study of J/ψ production in p-U, O-U and S-U interactions at 200 GeV per nucleon

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.

J/ψ, ψ′ and Drell-Yan production in S-U interactions at 200 GeV per nucleon

Abstract The production of the J/ ψ and ψ ′ charmonia states has been studied, through their dimuon decay, in proton, Oxygen and Sulphur induced reactions, by the NA38 experiment at the CERN SPS. The proton data was collected with beams of 200 and 450 GeV, while the ion beams had an energy of 200 GeV per incident nucleon. The J/ ψ production cross-section per nucleon-nucleon collision exhibits a remarkably continuous pattern, as a function of the product of the mass numbers of the interacting nuclei, from pp up to S-U reactions. The same pattern is observed within S-U collisions, as a function of the collision centrality. While in p-A interactions both charmonia states exhibit the same A-dependence, in S-U collisions the ψ ′ production is very strongly suppressed.

J/ψ, ψ′ and Drell-Yan production in pp and pd interactions at 450 GeV/c

Abstract In a search for quark-gluon plasma formation, the production of J/ψ and muon pairs in the mass continuum region is studied in oxygen-uranium and sulphur-uranium interactions. The yield of J/ψ relative to the continuum is measured to be a decreasing function of the neutral transverse energy produced in the collision, i.e. of the energy density. A comparison is made with proton-uranium reactions.

Charmonia production in 450-GeV/c proton induced reactions

Abstract A detailed study of J/ ψ , ψ ′ and Drell-Yan production in S-U collisions has been performed by experiment NA38 at the CERN SPS. This paper presents production cross sections and their centrality dependence, based on the largest sample of S-U events collected by the experiment.