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, $d{N}_{\gamma }/dy$, from large systems shows a behavior of universal scaling as a function of the charged-particle multiplicity, $(d{N}_{ch}/d\eta {)}^{\alpha }$, with $\alpha =1.25$, which means that the photon production yield increases faster than the charged-particle multiplicity.

Proceedings for the XIIIth Workshop on Particle Correlations and Femtoscopy (WPCF 2018). arXiv admin note: substantial text overlap with arXiv:1812.01841