Dielectron Production In(（SNN）^1/2)=200GeV P+p And Au+Au Collisions At RHIC

The mission of a ultra-relativistic heavy ion program is to study the structure of nuclear matter with the QCD degrees of freedom including QCD matter at finite tem-perature and density in the laboratory and search for the QCD critical point and first-order phase boundary. Many experimental evidences have been found to demonstrate the formation of a strongly-coupled Quark-Gluon Plasma (sQGP). These evidences are mostly from hadron measurements in high energy heavy ion collisions at RHIC and LHC. Dileptons as an electromagnetic probe, escape the interacting system without suf-fering further strong interactions after their production. In addition, dilepton can be produced on the various stages of entire system evolution. They are therefore expected to be outstanding probes to study the property of the medium created in high energy heavy ion collisions.Traditionally, due to different physics of interest, dilepton kinematic phase space is divided into3mass regions. In the Low Mass Region-LMR (Mll<Mφ), dileptons are produced via multiple hadron-hadron scattering by coupling to vector mesons. In the Intermediate Mass Region-IMR (Mφ<Mll<MJ/ψ), dilepton production is domi-nated by contributions from the thermal radiation and semilepton decay of the correlated charmed meson. Drell-Yan process and the decay heavy quarkonia contribute mainly for the dilepton source in the High mass Region-HMR (Mll> MJ/ψ).In this thesis, I will report the measurement on dielectron production in p+p and Au+Au collisions at√SNN=200GeV for the STAR experiment. The large data set collected during year2010-2012, as well as the completion of barrel Time-Of-Flight (TOF) installation in year2010, provides electron/position samples with high statistics and purity which makes it possible for dielectron measurement at the STAR experiment. The dielectron results from200GeV p+p collisions have greatly improved (-7times more) statistics comparing to the previous published result from STAR [1]. The results from Au+Au collision at√SNN=200GeV are compared with hadronic decay cocktail and model calculations. We observed an enhancement factor of1.66±0.06(stat.)±0.24(sys.)±0.33(cocktail) in mass region0.3-0.76GeV/c2when comparing to the hadronic cocktail without p contribution. This result is extracted from the200GeV minimum bias Au+Au collisions and we do not observe strong centrality nor transverse momentum dependence. In the most top10%central collisions, this enhancement is smaller than the PHENIX results [2,3]. The model calculation based on in-medium broadening of p spectra function can reproduce the enhancement. I will also report the study of possible modification of the correlated charm and the measurements of ω and φ production through their dielectron decay channels.