Results on Photon Production in Au+Au Collisions at RHIC

The aim of this analysis is to search for direct photons, i.e. photons that are not produced in hadron decays such as π → γγ or η → γγ. Direct photons are generally subdivided into prompt photons from initial hard parton scatterings and thermal photons from a possible quark-gluon plasma and from the hadron gas. Prompt photons are produced before a possible quark-gluon plasma has formed. Above a transverse momentum of pT ≈ 3 − 5 GeV prompt photons are expected to be the dominant source of direct photons in Au+Au collisions at RHIC. In p+p reactions direct photons are used to obtain information about the gluon distribution in the proton since their production in p+p is dominated by quark-gluon Compton scattering (gq → γq). By measuring direct photons in Au+Au at RHIC one is sensitive to modifications of the parton distribution functions in the nucleus (nuclear shadowing) [ 1]. The agreement between perturbative QCD calculations and experimental direct photon data in p+p and p+A can be improved if a momentum component kT of the initial partons transverse to the beam axis is introduced in the calculations [ 2]. In nucleus-nucleus collisions one expects a stronger kT than in p+p due to e.g. multiple soft scatterings of the incoming nucleons. As demonstrated in [ 3] the magnitude of this intrinsic kT can be constrained by measurement of direct photons in nuclear collisions. Thermal photons are produced over almost the entire evolution of a nucleus-nucleus collision: in a possible quark-gluon plasma phase and in the later hadron gas phase. The strongest contribution from thermal photons is expected roughly in the range pT ≈ 1−5 GeV. Theoretically, the entire hydrodynamical evolution of a nucleus-nucleus reaction needs to be modeled to describe thermal photon production. A comparison of these models with data constrains the initial temperature of the reaction system. In order to obtain information about the existence of a quark-gluon plasma one needs to compare models with and without phase transition to the data [ 4].