J. Astruc

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.

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

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.

Anomalous suppression in Pb+Pb collisions at 158×A GeV/c

Abstract Cross sections for J ψ,ψ′ and Drell-Yan production in Pb+Pb collisions at 158×A GeV/c are presented and compared with results obtained by the NA38 and NA51 collaborations. The Pb+Pb data have been collected by the NA50 collaboration using the NA38 dimuon spectrometer. The Drell-Yan mechanism is found to scale as (A projectile · B target ) in p+B target and A projectile + B target collisions including Pb+Pb collisions. Regarding J ψ , an anomalous suppression is observed in Pb+Pb collisions with respect to the suppression observed in p+B target , O+B target and S+U collisions. The cross section ratios ψ′ ( J ψ ) are similar in Pb+Pb and S+U collisions.

Anomalous J / psi suppression in Pb + Pb collisions at 158-A-GeV/c

Abstract Cross sections for J ψ,ψ′ and Drell-Yan production in Pb+Pb collisions at 158×A GeV/c are presented and compared with results obtained by the NA38 and NA51 collaborations. The Pb+Pb data have been collected by the NA50 collaboration using the NA38 dimuon spectrometer. The Drell-Yan mechanism is found to scale as (A projectile · B target ) in p+B target and A projectile + B target collisions including Pb+Pb collisions. Regarding J ψ , an anomalous suppression is observed in Pb+Pb collisions with respect to the suppression observed in p+B target , O+B target and S+U collisions. The cross section ratios ψ′ ( J ψ ) are similar in Pb+Pb and S+U collisions.

Charmonium production in PbPb interactions at 158 GeV/c per nucleon

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.

Anomalous J/ψ suppression in 158 GeV/c Pb-Pb collisions at the CERN SPS

Heavy ion production of J/ψ mesons has been studied at CERN for a decade, in search for the Quark-Gluon Plasma predicted by lattice-QCD. The suppression observed very early in O-Cu, O-U and S-U reactions has been shown to be eventually explained by nuclear absorption, and follow the same exponential behavior as the proton-nucleus reactions. The new Pb-Pb data deviate from this behavior very strongly, indicating the onset of a new regime in J/ψ suppression.

Charmonium production in Pb Pb interactions at 158-GeV/c per nucleon

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.

Anomalous suppression in PbPb 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.