Direct Virtual Photon Production In Au+Au Collisions At((SNN^1/2))=200GeV At RHIC

The Big-Bang theory indicates that the Quark Gluon Plasma (QGP) is formed at the very universe in a few tens of microseconds. Lattic QCD predicts a phase transition from hadronic matter to this deconfined and locally thermalized matter at high temperature and low baryon density. Relativistic Heavy Ion Col-lider (RHIC) at Brookhaven National Laboratory (BNL) provides an opportunity to study this strongly coupled QGP. The research of the behavior and proper-ty of QGP is a very interesting topic for physicist. This medium is expected to emit thermal radiation which is in the form of direct photons and dileptons. As electroweak probes, which do not suffer strong interaction, direct photon and lep-ton will traverse the hot and dense medium created by heavy ion collisions with minimal interactions once they are produced. They are believed to bring the in-formation from all the evolution steps. The research on these probes will provide the most direct and pure information.In this thesis, the first measurement on direct virtual photon from Solenoidal Tracker at RHIC (STAR) is reported. This measurement is also the first high transverse momentum(pT) result on direct photon via virtual photon method. This analysis is based on the data of sNN=200GeV Au+Au collisions taken from year2010run and year2011run (Run10and Runll). The invariant yield of direct virtual photon and the fraction of direct virtual photon versus inclusive photon are presented. This direct photon measured by virtual photon method is called direct virtual photon. Its production in the range of1<pT<10GeV/c is derived from the di-electron continuum in the low di-electron invariant mass region. The fully installed Time-of-Flight Detector (TOF) in2010provides clean electron identification from low to intermediate pT which enables the di-electron measurement. From the direct virtual photon invariant yield spectra, a significant en-hancement compared to the prediction based on p+p results is observed for1<pr<4GeV/c. The invariant yield is consistent with model prediction which includes the contributions from QGP, hadron gas and primordial produc-tion. In the high pT range, the invariant yield is consistent with the Ncoll scaled p+p results. This indicates a negligible contribution from thermal radiation. The study on the direct virtual photon provides two kinematic ranges:the low PT range where the thermal matter can be studied, the high pT range where the hard parton scattering from primordial step can be studied. The inverse slope pa-rameter which is related to the initial temperature is obtained from the invariant yield spectra.