Speaker
Sarah Campbell
(Iowa State University)
Description
Thermal radiation emitted by the quark-gluon plasma provides
information on the initial temperature of the collision and thermalization of the QGP. Experimentally it is difficult to separate thermal photons from the measured direct photon spectrum. In PHENIX, we measure direct photons in the central arm using two complementary methods, one with the electromagnetic calorimeter (EMC) and another with virtual photons extracted from the dielectron spectrum. Recent measurements with the EMC method have determined the direct photon spectrum out to 25 GeV and 22 GeV in p+p and Au+Au collisions respectively. At these high transverse momenta, photons emitted from hard processes dominate the direct photon spectrum and the measured QGP effects are minimal. The virtual photon method is able to measure direct photons at low pt where the thermal photons are expected to make up more of the direct photon yield. In Au+Au, the low pt virtual photon spectra lies above the T_{AA} scaled p+p spectrum while the d+Au spectrum is in agreement with the scaled p+p. This suggests enhanced thermal photon production in Au+Au and no corresponding cold nuclear matter production or suppression in d+Au. PHENIX's direct photon measurements in p+p, d+Au, Cu+Cu and Au+Au will be presented. The elliptic flow and extrapolated temperature of the Au+Au direct photons will also be discussed.
Author
Sarah Campbell
(Iowa State University)
