We present a new measurement of $J/\psi$ production in Pb-Pb collisions at 158 GeV/nucleon, from the data sample collected in year 2000 by the NA50 Collaboration, under improved experimental ...conditions with respect to previous years. With the target system placed in vacuum, the setup was better adapted to study, in particular, the most peripheral nuclear collisions with unprecedented accuracy. The analysis of this data sample shows that the ($J/\psi$)/Drell-Yan cross-sections ratio measured in the most peripheral Pb-Pb interactions is in good agreement with the nuclear absorption pattern extrapolated from the studies of proton-nucleus collisions. Furthermore, this new measurement confirms our previous observation that the ($J/\psi$)/Drell-Yan cross-sections ratio departs from the normal nuclear absorption pattern for semi-central Pb-Pb collisions and that this ratio persistently decreases up to the most central collisions.
We report a new measurement of J/ψ, ψ′ and Drell–Yan cross-sections, in the kinematical domain -0.425<ycm<0.575 and -0.5<cosθCS<0.5, performed at the CERN-SPS using 400 GeV/c incident protons on Be, ...Al, Cu, Ag, W and Pb targets. The dependence of the charmonia production cross-sections on the size of the target nucleus allows to quantify the so-called normal nuclear absorption. In the framework of the Glauber model, this new measurement is combined with results previously obtained with the same apparatus, under different experimental conditions, and leads to a precise determination of the J/ψ and ψ′ absorption cross-sections in the surrounding nuclear matter.
The analysis of the data collected by the NA50 experiment in 1998, reported in this paper, extends and clarifies the pattern of the previously observed J/
ψ anomalous suppression. This new ...measurement, besides providing a deeper understanding of the previous observations, reveals a steady significative decrease in the J/
ψ production rate up to the most central Pb-Pb collisions. It clearly rules out the presently available conventional (hadronic) models of J/
ψ suppression, which unanimously predict a saturation of the J/
ψ rate for central Pb-Pb collisions. On the contrary and together with the sharp onset of the anomalous suppression previously reported, the new observation leads to a global production rate pattern which finds its natural explanation in the framework of the formation of a deconfined state of quarks and gluons.
The CERN heavy ion program, started in 1986 with the oxigen beam, has accumulated an impressive amount of experimental results on the study of high energy density nuclear matter. Many experiments ...where devoted to the study of the lead-lead collisions at center of mass energy of about 20 GeV and all finally agreed in concluding that in the central lead-lead interactions a new state of matter as been created with many of the characteristics foreseen for the Quark-Gluon Plasma.
ψ′ production is studied in Pb–Pb collisions at 158 GeV/c per nucleon incident momentum. Absolute cross-sections are measured and production rates are investigated as a function of the centrality of ...the collision. The results are compared with those obtained for lighter colliding systems and also for the J/ψ meson produced under identical conditions.
The production of \({\rm J}/\psi\) and \(\psi^\prime\) 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 \({\rm J}/\psi\) and \(\psi^\prime\) production cross-sections have been determined for each p-A system and their dependences on the nucleus size have been studied, leading to the so-called normal nuclear absorption. Comparing the two patterns we see that the nuclear absorption is stronger for the \(\psi^\prime\) than for the \({\rm J}/\psi\). Given the high statistics of the data samples, the \(x_{\rm F}\) (or rapidity) differential cross-sections of the \({\rm J}/\psi\) and \(\psi^\prime\) states have also been studied, for each of the target nuclei.
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.
Muon pair production in p-A, S-U and Pb-Pb collisions has been studied by the NA38 and NA50 collaborations at the CERN SPS. In this paper we present an analysis of the dimuon invariant mass region ...between the \(\phi\) and the J/\(\psi\). In p-A collisions we find that, after the subtraction of the combinatorial background due to \(\pi\) and K decays, the superposition of dimuons from semi-leptonic decays of D mesons and from the Drell-Yan process reproduces well the measured mass and transverse momentum spectra. The yield of open charm dimuons required to fit our data leads to a charm production cross section in good agreement with previous measurements. A linear extrapolation of the p-A sources with the product of the mass numbers of the projectile and target nuclei, \(A\times B\), underestimates the dimuon yield measured in S-U and Pb-Pb collisions. The excess increases with the number of participant nucleons, and the ratio between the observed dimuon yield and the expected sources reaches a factor 2 for central Pb-Pb interactions. The kinematical distributions of the measured dimuon excess are compatible with those expected from the open charm contribution.
Charmonium production in p–A collisions is a unique tool for the study of the interaction of bound cc states in nuclear matter. It can provide details on the basic features of the resonance formation ...mechanism and, in particular, on its non-perturbative aspects. In this Letter, we present an experimental study of charmonia and Drell–Yan production in proton–nucleus collisions at 450 GeV/c. The results are analyzed in the framework of the Glauber model and lead to the values of the nuclear absorption cross-section σabspA for J/ψ and ψ′. Then, we compare the J/ψ absorption in proton–nucleus and sulphur–uranium interactions, using NA38 data. We obtain that, for the J/ψ, σabspA and σabsSU are compatible, showing that no sizeable additional suppression mechanism is present in S–U collisions, and confirming that the anomalous J/ψ suppression only sets in for Pb–Pb interactions.
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