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Etude de la production du J/psi dans les collisions or-or à 200 GeV par paire de nucléons dans l'expérience PHENIX.

Abstract : One of the most promising signature of Quark Gluon Plasma formation is the heavy quarkonium suppression due to color screening effect. First experiments at the SPS (CERN) have measured an « anomalous suppression » of the J/ψ yields (c¯c state) in central Pb+Pb collisions. However, measurements at different collision energies and with different ions are mandatory to conclude about the discovery of a new state of nuclear matter. Experiments dedicated to heavy ion studies at CERN have completed data taking. Currently, RHIC facility allows to study heavy ions at ten times higher a collision energy, before the start of the LHC with 30 times higher incident energies. This thesis describes the J/ψ production measured in the dimuon decay channel by the PHENIX experiment studying Au+Au collisions at 200 GeV in the center of mass. The J/ψ yield measured in the most central collisions is suppressed by a factor of 3 as compared to the yield expected assuming binary scaling. Within the error bars, the suppression does not affect the J/ψ's rapidity distribution. However, a broadening of the transverse momentum distribution is observed as compared to the distribution measured in p+p collisions. In order to understand this suppression, « cold nuclear effects », namely nuclear absorption and shadowing, are to be taken into account. These effects can describe neither the suppression amplitude nor the suppression pattern, suggesting that other mechanims are involved. Predictions from different models which reproduce the suppression observed by NA50, can hardly describe the PHENIX measurements and over-estimate the suppression at RHIC. Comparisons with predictions from models including recombination of charm quarks give a reasonnable description of the suppression amplitude as a function of centrality. However, these predictions are not in good agreement with the observed rapidity and transverse momentum distributions. Finally, one possible scenario is that the temperature at RHIC is not high enough to reach direct J/ψ melting and that the measured suppression is due to the sequential disappearance from higher mass resonnances (χc and ψ′). In this case, direct J/ψ should be suppressed at LHC temperatures. Hence, J/ψ measurements at LHC are crucial for a better understanding of quarkonium production in heavy ions collisions.
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Vi-Nham Tram. Etude de la production du J/psi dans les collisions or-or à 200 GeV par paire de nucléons dans l'expérience PHENIX.. Physique des Hautes Energies - Expérience [hep-ex]. Ecole Polytechnique X, 2006. Français. ⟨pastel-00001917⟩

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