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

TL;DR

This study measures direct-photon production in Pb-Pb collisions at 5.02 and 2.76 TeV, finding signals consistent with theoretical models and providing insights into photon emission stages in heavy-ion collisions.

Contribution

It provides new measurements of direct photons across various centralities and energies, with improved significance and detailed temperature analysis, enhancing understanding of photon production mechanisms.

Findings

Significant direct-photon signals observed at high p_T in both energies.

Effective temperatures suggest earlier photon emission at intermediate p_T.

Results agree with state-of-the-art theoretical models.

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.