Measurement of Direct Photons in \sqrt{s_NN} = 200 GeV p+p, d+Au, and
Au+Au Collisions with the PHENIX Experiment at RHIC

Stefan Bathe for the PHENIX collaboration

Due to their weak coupling with the dense matter created at RHIC
direct photons are a unique observable: they allow access to the
initial state of the collision. By this virtue, the PHENIX discovery
of a large photon excess over the meson-decay background in central
Au+Au collisions at high pT confirms the final-state nature of the
high-pT hadron suppression previously observed.

We present a systematic study of direct photon production in p+p,
d+Au, and Au+Au collisions. Direct photon measurements in p+p provide
a superb test of pQCD. Comparison with d+Au permits the extraction of
initial-state effects in the Au nucleus. The high-statistics Au+Au
data set acquired in 2004 allows a direct photon measurement beyond
pT=10 GeV/c and also at intermediate pT where thermal photons and
photons from jet-plasma interactions are expected to be
important. Additional information on the origin of direct photons will
be gained through the measurement of direct photon elliptic flow.