Direct Photon Production in Pb-Pb Collisions at sqrt(s_NN) = 2.76 TeV

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2.76 TeV
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Direct photon production at mid-rapidity in Pb-Pb collisions at $\snn = \unit[2.76]{TeV}$ was studied in the transverse momentum range $1 \lesssim \pT \lesssim \unit[13]{GeV}/c$. Photons were detected alternatively via conversions in the ALICE detector material with the $e^+e^-$ conversion pair reconstructed in the central tracking system or with the highly segmented electromagnetic calorimeter PHOS. The results of the two methods were combined and direct photon spectra were measured for the 0-20\%, 20-40\%, and 40-80\% most central collisions. For all three classes agreement was found with perturbative QCD calculations for $\pT \gtrsim \unit[5]{GeV}/c$. For the 20-40\% and 0-20\% centrality classes direct photon spectra could be extracted down to $\pT \approx \unit[1]{GeV}/c$. For the 0-20\% most central collisions a fit with an exponential for $0.9 < \pT < \unit[2.1]{GeV}/c$ yields an inverse slope parameter of $(296 \pm 12^\mathrm{stat}\pm 40^\mathrm{syst})\,\mathrm{MeV}$.


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