Low-$p_T$ direct-photon production in Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}=39$ and 62.4 GeV

Abstract

The measurement of direct photons from Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3$ Gev/$c$ is presented by the PHENIX collaboration at the Relativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $p_T$ spectra for different center-of-mass energies and for different centrality selections at $\sqrt{s_{{}_{NN}}}=62.4$ GeV is scaled with $(d{N}_{\mathrm{ch}}/d\eta {)}^{\alpha }$ for $\alpha =1.21\pm 0.04$. This scaling also holds true for direct-photon spectra from Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{{}_{NN}}}=2760$ GeV published by ALICE. The scaling power $\alpha$ seems to be independent of $p_T$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $p_T$ of 2 GeV/$c$. The spectra have a local inverse slope $T_{\mathrm{eff}}$ increasing with $p_T$ of $0.174\pm 0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increasing to $0.289\pm 0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of low-$p_T$ direct-photon production from $\sqrt{s_{{}_{NN}}}=39$ to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.

434 authors from 78 institutions, 19 pages, 16 figures, 4 tables. v2 is version accepted for publication in Physical Review C as an Editor's Suggestion. HEPdata tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at http://www.phenix.bnl.gov/papers.html