Direct photon production in Pb-Pb collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}$ = 2.76 TeV

Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{{}_{\mathrm{N}\mathrm{N}}}}=2.76$ TeV was studied in the transverse momentum range $0.9<p_{\mathrm{T}}<14$ GeV$\mathrm{/}c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^{+}{e}^{-}$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_{\mathrm{T}}\gtrsim 5$ GeV$\mathrm{/}c$. Direct photon spectra down to $p_$\mathrm{T}$ \approx 1$GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_$\mathrm{T}$ < 2.1$GeV$/c$is$2.6\sigma$forthe0-20$p_$\mathrm{T}$$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^$\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}$\pm 41^$\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}$)$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.