High-p T Direct-photon Results from Phenix * Klaus Reygers for the Phenix Collaboration

Direct-photon measurements in p+p and Au+Au collisions at √ s NN = 200 GeV from the PHENIX experiment are presented. The p+p results are found to be in good agreement with next-to-leading-order (NLO) pertur-bative QCD calculations. Direct-photon yields in Au+Au collisions scale with the number of inelastic nucleon-nucleon collisions and don't exhibit the strong suppression observed for charged hadrons and neutral pions. This observation is consistent with models which attribute the suppression of high-p T hadrons to energy loss of quarks and gluons in the hot and dense medium produced in Au+Au collisions at RHIC. The virtue of direct photons is that they emerge directly from the interaction of point-like partons in processes like quark-gluon Compton scattering (q + g → q + γ) and quark-anti-quark annihilation (q + ¯ q → g + γ). Measurements of direct photons in p+p collisions therefore allow to test perturbative QCD (pQCD) without the complication of phenomenological parton-to-hadron fragmentation functions which are needed in the description of high-p T hadron production. Moreover, direct-photon measurements in p+p collisions provide information about the gluon distribution of the proton due to the large contribution from quark-gluon Compton scattering. This is especially interesting for fractional gluon momenta x Bjorken 0.1 because this range is not well constrained by other processes like Deep-Inelastic Scattering and Drell-Yan. However, direct-photon data from p + p and p + ¯ p collisions are not generally used in global QCD fits for the determination of parton distribution functions. One reason for this are discrepancies between * Presented at the PHOTON2005 International Conference on the Structure and Interactions of the Photon (1)