Measurements of direct-photon production in Pb-Pb collisions at $\sqrt{\rm s_{NN}}=5.02$ TeV and $\sqrt{\rm s_{NN}}=2.76$ TeV with the ALICE experiment (2308.02401v1)

Abstract: Recent results on direct-photon measurements in Pb-Pb collisions at \fivenn from central to peripheral collisions, as well as in 0-10% central and 20-40% semicentral Pb-Pb collisions at \sqrt{ s_{NN}}=2.76 TeV with improved significance are presented. A significant direct-photon signal is measured for p_T \gtrapprox 2 GeV/c from central to peripheral Pb-Pb collisions at \sqrt{s_{NN}}=5.02 TeV which is in agreement with model calculations containing pre-equilibrium photons in the intermediate p_T range and prompt photons at high p_T. No significant direct-photon signal is measured in the low p_T interval which is also in agreement with the small thermal-photon signal predicted by state-of-the art models. On the other hand, a direct-photon signal is measured for p_T > 1 GeV/c with a significance of 3.1 ($1.0 < p_{\rm T} < 1.8$ GeV/c) and 3.4 (1.0 < p_T < 2.3 GeV/c) in 0-10% and 20-40% central Pb-Pb collisions at \sqrt{s_{NN}}=2.76 TeV, respectively. The inverse slope parameters of non-prompt photon spectra in the low p_T range, 1.1 < p_T < 2.1 GeV/c, are T_ eff = 343 \pm 32 (stat)\pm 68(sys) MeV and T_eff = 339 \pm 32 (stat)\pm 68(sys) MeV for central and semicentral collisions. In the intermediate p_T} range, 2.1 < p_T < 4.0 GeV/c, the effective temperatures are T_{eff} = 406 \pm 19 (stat)\pm 36(sys) MeV and T_eff = 458 \pm 25 (stat)\pm 40(sys) MeV, respectively. The effective temperatures T_eff in the intermediate p_T range are systematically larger compared to those obtained at low p_T (although consistent within uncertainties) which may indicate an earlier photon emission, and thus sensitivity to pre-equilibrium photon production. Direct photons at both energies are in agreement with state-of-the-art theory calculations over the complete p_T range and measured centralities.