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

The collaboration
2022.

Abstract (data 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. A significant direct photon yield is observed in both the systems. A universal scaling is observed when the direct photon $p_T$ spectra for the different center of mass energies and for different centrality selection at $\sqrt{s_{NN}} = 62.4$ GeV is scaled with $(dN_{ch}/d\eta)^{\alpha}$ with $\alpha=1.21 \pm 0.04$. This scaling also holds true for direct photon spectra from Au$+$Au collisions at $\sqrt{s_{NN}} = 200$ GeV measured by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{NN}} = 2760$ GeV published by ALICE. It is also demonstrated that the scaling power $\alpha$ seem 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/$c$. They are concave with an increasing local inverse slope $T_{eff}$ that is $0.174\pm0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increases to $0.289\pm0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of low $p_T$ direct photon production from $\sqrt{s_{NN}} = 39$ GeV to $2760$ GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and that the possible differences in the space time evolution do not alter direct photon emission significantly.

• #### Figure8acd

10.17182/hepdata.133218.v1/t1

$R_{\gamma}$ for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV...

• #### Figure8b

10.17182/hepdata.133218.v1/t2

$R_{\gamma}$ for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV...

• #### Figure9acd

10.17182/hepdata.133218.v1/t3

Direct photon spectra for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For...

• #### Figure9b

10.17182/hepdata.133218.v1/t4

Direct photon spectra for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For...

• #### Figure12

10.17182/hepdata.133218.v1/t5

Inverse slopes, $T_{eff}$ , obtained from fitting the combined data from central collisions shown in Fig. 11 is compared to...

• #### Figure13a

10.17182/hepdata.133218.v1/t6

Integrated invariant direct-photon yields vs. charged particle multiplicity for $p_{T}$ integrated from (a) 0.4 GeV/c, (b) 1.0 GeV/c, (c) 1.5...

• #### Figure13b

10.17182/hepdata.133218.v1/t7

Integrated invariant direct-photon yields vs. charged particle multiplicity for $p_{T}$ integrated from (a) 0.4 GeV/c, (b) 1.0 GeV/c, (c) 1.5...

• #### Figure13c

10.17182/hepdata.133218.v1/t8

Integrated invariant direct-photon yields vs. charged particle multiplicity for $p_{T}$ integrated from (a) 0.4 GeV/c, (b) 1.0 GeV/c, (c) 1.5...

• #### Figure13d

10.17182/hepdata.133218.v1/t9

Integrated invariant direct-photon yields vs. charged particle multiplicity for $p_{T}$ integrated from (a) 0.4 GeV/c, (b) 1.0 GeV/c, (c) 1.5...

• #### Figure15a

10.17182/hepdata.133218.v1/t10

Integrated direct-photon yields from A+A collisions for $p_{T,min}$ of $5$ GeV/c (a) and $8$ GeV/c. The representation is the same...

• #### Figure15b

10.17182/hepdata.133218.v1/t11

Integrated direct-photon yields from A+A collisions for $p_{T,min}$ of $5$ GeV/c (a) and $8$ GeV/c. The representation is the same...

• #### Figure16

10.17182/hepdata.133218.v1/t12

The $\alpha$ values extracted using fits to integrated direct photon yields. The dashed line gives the average $\alpha$ value for...