Direct Photons in PHENIX

Stefan Bathe for the PHENIX collaboration

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
nuclear collision. Their measurement, however, is
challenging. One has to cope with a large background
from hadronic decays as well as disentangle the contributions from
thermal radiation and hard scattering.

Direct photon measurements in p+p collisions provide a superb test of
pQCD as well as allow quantification of the hard-scattering
contribution to the observed signal in Au+Au collisions.
With its high-resolution, highly-segmented electromagnetic calorimeter
PHENIX has an excellent capability to measure photons. PHENIX has made
precision measurements of neutral pion and eta mesons, the principal
contributors to the direct photon background from hadronic decay, up to
transverse momenta of 15 GeV/c.  These measurements will be shown and
discussed as a function of collision energy and system size. With this
crucial measurement of the direct photon background, PHENIX has extracted
direct photons in p+p and Au+Au collisions at sqrt(s_NN) = 200 GeV.
The results will be presented and compared to theoretical calculations.