The measurement of direct photons from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3\mathrm{Gev}/c$ is presented by the PHENIX collaboration at the BNLRelativistic 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 $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from $\mathrm{Pb}+\mathrm{Pb}$ 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 $\mathrm{Gev}/c$. The spectra have a local inverse slope $T_{\mathrm{eff}}$ increasing with $p_T$ of $0.174\pm 0.018\mathrm{Gev}/c$ in the range $0.4<p_T<1.3\mathrm{Gev}/c$ and increasing to $0.289\pm 0.024\mathrm{Gev}/c$ for $0.9<p_T<2.1\mathrm{Gev}/c$. 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.

• Received 24 March 2022
• Accepted 21 October 2022

DOI:https://doi.org/10.1103/PhysRevC.107.024914