PHENIX measurements of low momentum direct photon radiation from large and small systems in (ultra)relativistic heavy ion collisions: direct photon scaling

Abstract: The PHENIX collaboration has measured low momentum direct photon radiation in Au+Au collisions at 200 GeV, 62.4 GeV and 39 GeV, in Cu+Cu at 200 GeV as well as in p+p, p+Au and d+Au at $\sqrt{s_{NN}} =$ 200 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, observed within systematic uncertainties, over the scaled p+p yield in central p+A collisions. Another finding is that at low-$p_{T}$ the integrated yield of direct photons, $dN_{\gamma}/dy$, from large systems shows a behavior of universal scaling as a function of the charged-particle multiplicity, $(dN_{ch}/d\eta)^{\alpha}$, with $\alpha = 1.25$, which means that the photon production yield increases faster than the charged-particle multiplicity.