PHENIX measurements of low momentum direct photon radiation

Abstract

The versatility of RHIC allowed the PHENIX collaboration to measure low momentum direct photons from small systems, such as p+p, p+A, d+Au at $\sqrt{s_{NN}} = $200 GeV as well as from large A+A systems, such as Au+Au and Cu+Cu at 200 GeV and Au+Au at 62.4 GeV and 39 GeV. In these measurements PHENIX has discovered a large excess over the scaled p+p yield of direct photons in A+A collisions, and a non-zero excess over the scaled p+p yield in central p+A collisions. Another PHENIX discovery is that at low-$p_{T}$ the integrated yield of direct photons, $dN_{\gamma}/dy$, from large systems follows a universal scaling as a function of the charged-particle multiplicity, $(dN_{ch}/d\eta)^{\alpha}$, with $\alpha = 1.25$. The observed scaling properties of direct photons from these systems show that the photon production yield increases faster than the charged-particle multiplicity.