A model-independent analysis of the dependence of the anomalous J/psi suppression on the number of participant nucleons

A recently published experimental dependence of the J/psi to Drell-Yan ratio on the measured, by a zero degree calorimeter, forward energy E_ZDC in Pb+Pb collisions at the CERN SPS is analyzed. Using a model-independent approach it is shown that the data are at variance with an earlier published experimental dependence of the same quantity on the transverse energy of neutral hadrons E_T. The discrepancy is related to a moderate centrality region: 100 < N_p < 200 (N_p is the number of participant nucleons) and is peculiar only to the data obtained within the `minimum bias' analysis (using the `theoretical Drell-Yan'). This could result from systematic experimental errors in the minimum bias sample. A possible source of the errors is discussed.Comment: 10 pages, LaTeX, 3 PS-figures. V2: Misprints are correcte

Interpretations of J/ψJ/\psi suppression

We review the two main interpretations of $J/\psi$ suppression proposed in the literature. The phase transition (or deconfining) scenario assumes that below some critical value of the local energy density (or of some other geometrical quantity which depends both on the colliding systems and on the centrality of the collision), there is only nuclear absorption. Above this critical value the absorptive cross-section is taken to be infinite, i.e. no $J/\psi$ can survive in this hot region. In the hadronic scenario the $J/\psi$ dissociates due both to nuclear absorption and to its interactions with co-moving hadrons produced in the collision. No discontinuity exists in physical observables. We show that an equally good description of the present data is possible in either scenario.Comment: 12 pages, LaTeX, uses epsfig and ioplppt; review talk given by A. Capella at the International Symposium on Strangness in Quark Matter, Santorini (Greece), April 1997; Figs. 1 and 2 not available but can be found in Refs. 13 and 6 respectivel

Hadronic centrality dependence in nuclear collisions

The kaon number density in nucleus+nucleus and p+p reactions is investigated for the first time as a function of the initial energy density $\epsilon$ and is found to exhibit a discontinuity around $\epsilon$=1.3 GeV/fm$^3$. This suggests a higher degree of chemical equilibrium for $\epsilon >$ 1.3 GeV/fm$^3$. It can also be interpreted as reflection of the same discontinuity, appearing in the chemical freeze out temperature (T) as a function of $\epsilon$. The $N^{\alpha \sim 1}$ dependence of (u,d,s) hadrons, whith N the number of participating nucleons, also indicates a high degree of chemical equilibrium and T saturation, reached at $\epsilon >$1.3 GeV/fm$^3$. Assuming that the intermediate mass region (IMR) dimuon enhancement seen by NA50 is due to open charm ($D \bar{D}$), the following observation can be made: a) Charm is not equilibrated. b) $J/\Psi/D \bar{D}$ suppression -unlike $J/\Psi/DY$- appears also in S+A collisions, above $\epsilon$ $\sim$1 GeV/fm$^3$. c) Both charm and strangeness show a discontinuity near the same $\epsilon$. d) $J/\Psi$ could be formed mainly through $c \bar{c}$ coalescence. e) The enhancement factors of hadrons with u,d,s,c quarks may be connected in a simple way to the mass gain of these particles if they are produced out of a quark gluon plasma (QGP). We discuss these results as possible evidence for the QCD phase transition occuring near $\epsilon \sim$1.3 GeV/fm$^3$.Comment: 4 pages, 4 figures, proceedings of Vth International Conference on Strangeness in Quark Matter, 20-25 July 2000, Berkeley, California. To appear in Journal of Physics G: Nuclear and Particle Physic

Importance of reaction volume in hadronic collisions: Canonical enhancement

We study the canonical flavor enhancement arising from exact conservation of strangeness, and charm flavor. Both the theoretical motivation, and the practical consequences are explored. We argue using qualitative theoretical arguments and quantitative evaluation, that this proposal to reevaluate strangeness signature of quark--gluon plasma is not able to explain the majority of available experimental results.Comment: 14 pages including 6 figures, submitted to Journal of Physics G Presented at: Strange Quark Matter, September 2001, Frankfur

J/ψJ/\psi production in PHENIX

Heavy quarkonia production is expected to be sensitive to the formation of a quark gluon plasma (QGP). The PHENIX experiment has measured $J/\psi$ production at $\sqrt{s_{NN}}=$~200 GeV in Au+Au and Cu+Cu collisions, as well as in reference p+p and d+Au runs. $J/\psi$'s were measured both at mid ($|y|<0.35$) and forward ($1.2<|y|<2.2$) rapidity. In this letter, we present the A+A preliminary results and compare them to normal cold nuclear matter expectations derived from PHENIX d+Au and p+p measurements as well as to theoretical models including various effects (color screening, recombination, sequential melting...).Comment: 5 pages, 7 figures. To appear in the proceedings of Hot Quarks 2006: Workshop for Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions, Villasimius, Italy, 15-20 May 200

Colour Deconfinement and Quarkonium Binding

At high temperatures, strongly interacting matter becomes a plasma of deconfined quarks and gluons. In statistical QCD, deconfinement and the properties of the resulting quark-gluon plasma can be investigated by studying the in-medium behaviour of heavy quark bound states. In high energy nuclear interactions, quarkonia probe different aspects of the medium formed in the collision. We survey the results of recent charmonium production studies in SPS and RHIC experiments.Comment: 50 pages, 53 figures; revised section 6.

Centrality and sNNDependenceofthe\sqrt{s_{NN}} Dependence of the dE_{T}/d\etaand and dN_{ch}/d\eta$ in Heavy Ion Collisions at Mid-Rapidity

The PHENIX experiment at RHIC has measured transverse energy and charged particle multiplicity at mid-rapidity in Au + Au collisions at $\sqrt{s_{NN}}$ = 19.6, 130, 62.4 and 200 GeV as a function of centrality. The presented results are compared to measurements from other RHIC experiments, and experiments at lower energies. The $\sqrt{s_{NN}}$ dependence of $dE_{T}/d\eta$ and $dN_{ch}/d\eta$ per pair of participants is consistent with logarithmic scaling for the most central events. The centrality dependence of $dE_{T}/d\eta$ and $dN_{ch}/d\eta$ is similar at all measured incident energies. At RHIC energies the ratio of transverse energy per charged particle was found independent of centrality and growing slowly with $\sqrt{s_{NN}}$. A survey of comparisons between the data and available theoretical models is also presented.Comment: Proccedings of the Workshop: Focus on Multiplcity at Bari, Italy, June 17-19,2004. To be submitted to the Jornal of Physics, "Conference series". Includes: 20 Pages, 15 figures, 3 Tables, 80 Referencie

Strange Messages: Chemical and Thermal Freeze-out in Nuclear Collisions

Thermal models are commonly used to interpret heavy-ion data on particle yields and spectra and to extract the conditions of chemical and thermal freeze-out in heavy-ion collisions. I discuss the usefulness and limitations of such thermal model analyses and review the experimental and theoretical evidence for thermalization in nuclear collisions. The crucial role of correlating strangeness production data with single particle spectra and two-particle correlation measurements is pointed out. A consistent dynamical picture for the heavy-ion data from the CERN SPS involves an initial prehadronic stage with deconfined color and with an appreciable isotropic pressure component. This requires an early onset of thermalization.Comment: 15 pages, 2 figures, talk given at Strange Quark Matter '98, Padova, Italy, 20-24 July 1998, to be published in J. Phys. G 25; final version with updated reference

Dissociation rates of J/psi's with comoving mesons - thermal vs. nonequilibrium scenario

We study J/psi dissociation processes in hadronic environments. The validity of a thermal meson gas ansatz is tested by confronting it with an alternative, nonequilibrium scenario. Heavy ion collisions are simulated in the framework of the microscopic transport model UrQMD, taking into account the production of charmonium states through hard parton-parton interactions and subsequent rescattering with hadrons. The thermal gas and microscopic transport scenarios are shown to be very dissimilar. Estimates of J/psi survival probabilities based on thermal models of comover interactions in heavy ion collisions are therefore not reliable.Comment: 12 pages, 6 figure