The Measurements Of High-p_T P~0Spectra And Di-electron Elliptic Flow At RHIC

The theory of Big Bang predicates that Quark Gluon Plasma(QGP) will appear in a few tens of microseconds after the Big Bang. Lattic QCD calculations predict that QGP will appear in the high temperature and small baryon density. So far, the Relativistic Heavy Ion Collider(RHIC) is the highest energy heavy ion collider except LHC. It accelerates the heavy ions to very high energy and then makes them collide. A hot and dense medium (QGP) is formed. The medium evolves and cools down and particles freeze out at the final state. We can search for and study QGP through the measurements of particles created. Energetic partons will lose energy when they pass through the hot and dense medium, resulting in less production for hadrons at high transverse momentum. This is called jet quenching, which is one of the characteristic of QGP. The ρ0is a good probe due to its short lifetime1.3fm which is nearly the same as the lifetime of the hot and dense medium created in heavy ion collisions. The measurements of spectra and nuclear modification factor(RAA) of ρ0will help us understand the characteristic of QGP created on RHIC.STAR collaboration has done ρ0measurements in the last10years. But the measurements are limited in the low ρ0region. The measurements at high-pT are very important for understanding the energy loss mechanisms of partons in the hot and dense medium. We use the new method to reconstruct the ρ0, extending its pT reach from2GeV/c to15GeV/c. We have measured the spectra of ρ0in200GeV p+p collisions and in200GeV Au+Au collisions for different centralities. The RAA(ρ0) is presented in200GeV Au+Au collisions for different centralities. The spectra of identified particles at high pT in200GeV p+p collisions can provide more stringent constraints on the quark and gluon fragmentation functions by comparing theoretical calculations with experimental data in the same kinematics in p+p collisions. We observe that the RAA decreases from peripheral to central collisions. For pT>8GeV/c, a common suppression pattern is observed in central collisions(0-12%) for different mesons(ρ0,π±,KS0), despite the differences in quark flavor composition and mass. A model for jet conversion in the hot and dense medium overpredicts the KS0enhancement at high pT.Di-leptons are also ideal probes to study the hot and dense medium. Once produced, they are not affected by the strong interaction and can be produced in the whole evolution. Therefore they can probe the whole evolution of the collision. In the intermediate mass range(1.1<Mu<2.9GeV/c), the di-lepton spectra are related to the thermal radiation of the QGP and heavy-flavor decays. In the low mass range(Mu<1.1GeV/c), we can study the vector meson in-medium properties through their di-lepton decays, the observable of possible chiral symmetry restoration. In the low and intermediate mass region, the measurements of di-electron elliptic flow help us understand the properties of medium from hadron-gas dominated to QGP dominated.In the years2010and2011, the STAR detector collected more than one billion minimum bias events for Au+Au collision at√sNN=200GeV. The2π azimuthal coverage of the newly installed time-of-flight (TOF) subdetector enables the el-liptic flow measurements of di-electrons. We have measured the mass dependence of elliptic flow of di-electrons from low to intermediate mass range. We also mea-sure the transverse momentum differential elliptic flow of di-electrons in different sub-mass regions in Au+Au collisions at√sNN=200GeV. The mass and pT dependence of elliptic flow of di-electron have been simulated. They are found to be consistent with the measured results. In addition, we report the elliptic flow of di-electron as a function of mass with STAR acceptance corrected and compare the results with theoretical model calculations.