Direct photon production in Pb–Pb collisions at √s_NN = 2.76 TeV

Direct photon production at mid-rapidity in Pb–Pb collisions at √s_NN = 2.76 TeV was studied in the transverse momentum range 0.9 < p_T < 14 GeV/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_T ≳ 5   GeV/c . Direct photon spectra down to p_T ≈ 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_T < 2.1 GeV/c is 2.6σ for the 0–20% class. The spectrum in this p_T range and centrality class can be described by an exponential with an inverse slope parameter of ( 297±12^stat±41^syst ) MeV . State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.