l - ex / 0 51 10 42 v 1 2 2 N ov 2 00 5 Direct photons in 200 GeV p + p , d + Au , and Au + Au from PHENIX

Direct photons are a unique probe of the hot and dense matter created at RHIC: they allow access to the initial, thermalized state of the collision. Their measurement, however, is challenging. One has to cope with a large background from hadronic decays. Direct photon measurements in p + p constrain the hard-scattering contribution in Au+Au. A comparison to d+Au allows to quantify contributions from initial-state effects. In heavy ion collisions, thermal direct photons constrain the temperature of the collision system in its hottest phase while hard direct photons serve as a crucial baseline for the interpretation of the high-pT hadron suppression observed earlier [ 1].