Direct photon measurement in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with ALICE

TL;DR

This paper reports on the measurement of direct photon spectra in Pb-Pb collisions at 2.76 TeV using ALICE, revealing a thermal photon signal and comparing results with models and previous experiments.

Contribution

It introduces a combined measurement method for direct photons over a broad pT range in heavy-ion collisions at LHC energies.

Findings

Observation of a thermal photon signal with 2.6σ significance in central collisions

The direct photon spectrum fits an exponential with a slope of about 304 MeV

Results are consistent with various photon production models in heavy-ion collisions

Abstract

The ALICE experiment has measured the direct photon spectra in Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV for three different centrality selections. The measurement was performed emplying a method utilizing conversion of photons into $e^+e^-$ pairs in the detector material, and a method using the PHOS calorimeter. The two measurements were combined in order to measure direct photons over a broad transverse momentum range of $0.9 < p_{\rm T} < 14$ GeV/$c$. A direct photon signal was observed in the thermal region $0.9 < p_{\rm T} < 2.1$ GeV/$c$, with a significance of $2.6\sigma$ in the 20\% most central collisions. The corresponding direct photon spectrum can be described by an exponential with an inverse slope parameter of $(304 \pm 11^{\rm stat} \pm 40^{\rm syst})$ MeV, without subtracting the hard scattering component, which is somewhat larger than the inverse slope parameter of the direct photon spectrum at RHIC energies. Within uncertainties, the data can be described by different models for photon production in heavy-ion collisions. These proceedings provide a summary of the results published in [1] and [2].