Measurements of soft and intermediate $p_T$ photons from hot and dense matter at RHIC-PHENIX

Abstract

The measurements of direct photons in $1.0 < p_{T} < 5.0 $GeV/$c$ for p+p and Au+Au collisions have been made through the virtual photon method at PHENIX. The fraction of the virtual direct photon component to the inclusive $e^{+}e^{-}$ yield is determined by a shape analysis using the $e^{+}e^{-}$ mass spectra in $m_{ee} < 300 $MeV/$c^{2}$ for $1.0 < p_{T} < 5.0 $GeV/$c$. The real direct photon spectra in p+p and Au+Au collisions are obtained from the virtual direct photon fractions, and a significant excess over a binary-scaled p+p result is seen in Au+Au collisions. Hydrodynamical models which reproduce the Au+Au result indicate the initial temperature of the matter is higher than the critical temperature of QGP. The d+Au data taken in 2008 are promising to evaluate the contribution of the nuclear effects because of its large statistics.