J/ψ Production In Au+Au From Rhic At (（sNN）^1/2)=39Gev,62.4Gev And200Gev

The Standard Model of particle physics is very successful in describing a large amount of experimental results. It has been strongly supported by the prediction and subsequent discovery of the charm, bottom, and top quarks, and the Z, W±, and Higgs bosons. The Quantum Chromodynamics (QCD) describes the strong interaction between quarks and gluons in the Standard Model. Due to the lattice QCD calculations, a phase transition has been predicted from hadronic matter to a deconfined and locally thermalized Quark Gluon Plasma (QGP) at high temperature and energy density. The QGP state is also thought to have existed before the universe cooled and free quarks and gluons combined into hadrons. The Relativistic Heavy Ion Collider (RHIC) was built to create the QGP in laboratory through the high energy heavy ion collisions to test the predictions of QCD and understand the properties of the deconfined medium.Charm (c) quarks have been proposed as ideal probes of the hot and dense medium created in heavy ion collisions, cause they are produced primarily in the initial hard scattering of the collision due to their large mass. The suppression of the charm anti-charm bound state J/ψ(1S) in relativistic heavy ion collisions has been suggested as a smoking gun signature of the formation of a quark gluon plasma, and is expected to arise form the color screening of the cc binding poten-tial in the deconfined medium. In addition to the screening effect, other mecha-nisms, such as the cold nuclear matter effect and charm quark recombination, are likely to contribute to the observed modification of J/ψ production in heavy-ion collisions. Measurements of J/ψ invariant yields in different collision energies, and centralities can shed new light on the interplay of these mechanisms for J/ψ production and medium properties.In this thesis, we report on new measurements of J/ψ production at midra- pidity (|y|<1.0) from the Beam Energy Scan program (Au+Au collisions at (?)=39GeV,62.4GeV) and200GeV taken in2010and2011by STAR with full Time-of-Flight detector and Barrel Electromagnetic Calorimeter detector in operation. This is the first J/ψ production measurements at STAR for39GeV and62.4GeV. With TPC, TOF and BEMC, one can get good electron identification. Thus the signal-to-background ratio is good. As expected, the J/ψ production in-crease with the increasing colliding energy. Centrality and transverse momentum dependence of J/ψ invariant yields and the nuclear modification factor (RAA) are also presented and compared to those for Au+Au collisions at (?)=200GeV and to model calculations. Significant production suppression is observed at these three energy points at central collisions. And the suppression pattern and magni-tude at these energies are similar within uncertainties. No energy dependence is indicated at these energies within uncertainties. The model calculations are con-sistent with our measurements, which indicate that the regeneration contribution is sizeable at RHIC and the energy dependence of J/ψ suppression is small due to the two competing mechanisms (screening and regeneration).