Direct photon production at RHIC

Abstract

Direct photon yields in Au+Au collisions at sNN = 200 GeV have been measured by the PHENIX experiment at RHIC, and we report recent results. Direct photons are produced in the initial hard partonic scatterings, are not affected by the strong interaction, and can be described by perturbative QCD. Consequently, they are offer a powerful probe of the nature of the produced medium. We have reported that the nuclear modification factor RAA of direct photon yield is one in the high-pT region above 5 GeV/c [1]. This means that scaling works well at high pT, and it indicates that the high-pT direct photon yield is hard-scattering dominant. At low-pT region of 1< pT <3 GeV/c, the yield of direct photons has been recently measured with the internal conversion method. At low pT, the large excess of direct photon yield is observed, and the excess of the yield is larger than that in d+Au collisions. Furthermore, we have measured the azimuthal anisotropy ν2 of direct photons, which can provide a glimpse of the early space-time evolution in the collision. The result of our measurements clearly shows that the ν2 of direct photon yield has positive value at low pT, while it is zero at high pT.