# Beam Energy and Centrality Dependence of Direct-Photon Emission from Ultrarelativistic Heavy-Ion Collisions

10 July 2019

Abstract: The PHENIX collaboration presents first measurements of low-momentum ($0.4<{p}_{T}<3\text{}\text{}\mathrm{GeV}/c$) direct-photon yields from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=39$ and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200$. Analyzing the photon yield as a function of the experimental observable $d{N}_{\mathrm{ch}}/d\eta$ reveals that the low-momentum ($>1\text{}\text{}\mathrm{GeV}/c$) direct-photon yield $d{N}_{\gamma }^{\mathrm{dir}}/d\eta$ is a smooth function of $d{N}_{\mathrm{ch}}/d\eta$ and can be well described as proportional to $\left(d{N}_{\mathrm{ch}}/d\eta {\right)}^{\alpha }$ with $\alpha \approx 1.25$. This scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and the Large Hadron Collider, for centrality selected samples, as well as for different $A+A$ collision systems. At a given beam energy, the scaling also holds for high ${p}_{T}$ ($>5\text{}\text{}\mathrm{GeV}/c$), but when results from different collision energies are compared, an additional $\sqrt{{s}_{NN}}$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.

Published in: Physical Review Letters 123 (2019)