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

Abstract

Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}=2.76$ TeV was studied in the transverse momentum range $0.9<p_{\mathrm{T}}<14$ 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 $p_{\mathrm{T}}\gtrsim 5$ GeV$/c$. 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 $2.6\sigma$ for the 0-20% class. The spectrum in this $p_{\mathrm{T}}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297\pm 12^{\mathrm{stat}}\pm 41^{\mathrm{syst}})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.

25 pages, 6 captioned figures, 2 tables, authors from page 20, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/1887

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