Direct Photons at RHIC

One of the most direct probes of the QGP is the radiation of thermal photons, emerging directly from the thermalized system and providing, in principle, the temperature in its hottest phases [1]. However, photons are largely produced in hadronic decays, as well as in QCD processes, such as annihilation and Compton, that occur before thermalization. Thermally produced photons must be distinguished from those produced primordially in QCD interactions and those from final state decays of hadrons. Thus it is necessary to first determine the yield of photons that are produced in excess of the photons from known hadronic decay sources (the background).