A. Romana

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.

The production of j/

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.

\psi

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.

in 200 GeV/nucleon oxygen-uranium interactions

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.

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

We present the angular distributions of muon pairs obtained in a high-statistics experiment using a 194-GeV/c π- impinging on a tungsten target. Our results are based on the analysis of 145,000 events with positive Feynmanx and mass above 4.07 GeV/c2, excluding the ϒ region. Simple first-order QCD relations allow us to determine the ratio of annihilation with hard-gluon emission to the sum of annihilation with hard-gluon emission and hard-gluon Compton scattering, which is found to be about 58% to 75%. We determine the parton square intrinsic transverse momenta to be of the order of 0.6 (GeV/c)2, and about 30% larger in the pion than in the nucleon. At largex1, our data agree with the higher-twist hypothesis, and support the interpretation of the relevant scale parameter as the dimuon square transverse momentum.

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

Abstract.The production of

Angular Distributions of Muon Pairs Produced by 194-{GeV}/

{\rm J}/\psi

c

and

Negative Pions

\psi^\prime

Charmonium production and nuclear absorption in p-A interactions at 450 GeV

charmonium states in proton-nucleus interactions has been investigated by the NA50 experiment, at the CERN SPS. High statistics data sets were collected with collisions induced by 450 GeV protons incident on Be, Al, Cu, Ag and W targets. The