A systematic study of J/psi suppression in cold nuclear matter

Understanding the effects of cold nuclear matter on J/psi production is a key requirement in order to interpret the J/psi suppression reported in heavy-ion collisions. Based on a Glauber model, the J/psi-nucleon inelastic cross section is determined from a statistical analysis of J/psi world data on nuclear targets. The global fit of all data gives sigmaJ/psiN = 3.4 +/- 0.2 mb, significantly smaller than previous estimates, yet the chi2 of the fit is pretty large, chi2/ndf = 1.5. A similar value, sigmaJ/psiN = 3.5 +/- 0.2 +/- 1.7 mb, is obtained when the De Florian-Sassot modifications of the nuclear parton densities are included in the analysis, although we emphasize that the present uncertainties on gluon (anti)shadowing do not allow for a precise determination of sigmaJ/psiN. Finally, the possible energy dependence of the J/psi-nucleon inelastic cross section is investigated within this framework. No significant energy dependence of the J/psi-N interaction is observed

A Proposal to Upgrade the Silicon Strip Detector

The STAR Silicon Strip Detector (SSD) was built by a collaboration of Nantes, Strasbourg and Warsaw collaborators. It is a beautiful detector; it can provide 500 mu m scale pointing resolution at the vertex when working in combination with the TPC. It was first used in Run 4, when half the SSD was installed in an engineering run. The full detector was installed for Run 5 (the Cu-Cu run) and the operation and performance of the detector was very successful. However, in preparation for Run 6, two noisy ladders (out of 20) were replaced and this required that the SSD be removed from the STAR detector. The re-installation of the SSD was not fully successful and so for the next two Runs, 6 and 7, the SSD suffered a cooling system failure that allowed a large fraction of the ladders to overheat and become noisy, or fail. (The cause of the SSD cooling failure was rather trivial but the SSD could not be removed betweens Runs 6 and 7 due to the inability of the STAR detector to roll along its tracks at that time.

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

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.Parmi les signatures proposées pour l'observation du plasma de quarks et de gluons (PQG), l'une des plus prometteuses est la suppression des résonances quark-antiquark de saveurs lourdes par effet d'écran de couleur. Les premières mesures auprès du SPS du CERN ont montré une "suppression anormale" de la production du J/psi (résonance ccbar) dans les collisions centrales. La mesure de cette production dans différents systèmes et à différentes énergies de collisions pourrait permettre de conclure quant à la formation du PQG. Cette recherche se poursuit maintenant auprès de l'accélérateur RHIC, dont l'énergie dans le centre de masse est dix fois supérieure à celle obtenue au SPS.Le travail exposé dans cette thèse porte sur l'étude de la production du J/psi mesuré via son mode de déintégration e! n paires de muons, par l'expéience PHENIX dans les collisions Au-Au à 200 GeV par paire de nucléons. L'étude de la production du J/psi en fonction de la centralité des collisions montre une suppression d'un facteur 3 dans les collisions les plus centrales par rapport à la simple proportionalité avec le nombre de collisions binaires nucléon-nucléon.Pour comparer les résultats obtenus dans les collisions or-or à ceux des collisions proton-proton, les "effets nucléaires froids" doivent être pris en compte. Ils ne suffisent pas à expliquer la suppression observée, suggèrant l'existence de mécanismes supplémentaires. Les modèles qui prennent en compte la recombinaison du J/psi semblent mieux reproduire cette suppression. Néanmoins, l'ensemble de ces résultats est compatible avec le scénario selon lequel seules les résonances chi_c et psi' subissent l'écrantage de couleur