Enhaced Direct Photon Production in Au+Au collisions at 200 GeV
measured with the PHENIX Experiment

Stefan Bathe
for the PHENIX collaboration

Experimental results from RHIC indicate the formation of dense,
rapidly thermalized partonic matter in Au+Au collisions at 200 GeV.
Such a medium is expected to emit thermal photons, providing
information on its initial temperature.  However, in the pT range from
1-3 GeV/c where thermal photons dominate the direct photon yield, the
measurement is notoriously difficult due to the large background from
hadronic decay photons.

An alternative approach to the conventional direct photon measurement
is to measure "almost real" virtual photons, which appear as low mass
e+e- pairs.  Hadronic decay photons are absent above the mass of their
parent hadrons, which is exploited to separate out direct photons.

PHENIX has measured the production of low mass e+e- pairs in p+p and
Au+Au collisions.  A highly enhanced yield above hadronic sources was
observed in Au+Au.  Treating the excess as internal conversion of
direct photons, the direct photon yield was deduced.  The direct photon
spectrum above the binary collision scaled p+p spectrum is exponential
in pT.  Its shape agrees with the thermal photon spectrum of a model
prediction for an initial temperature of the medium of 370 MeV.