Measurement of Direct Photons in pp and Pb–Pb Collisions with ALICE☆

By M. Wilde, for the Alice collaboration. 2012

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Abstract The measurement of the direct photon transverse momentum spectrum in Pb–Pb collisions at s N N = 2.76 TeV with data taken by the ALICE experiment is presented. The measurement shows a clear direct-photon signal for 0–40% most central collisions below 4 GeV/c that can not be described by next-to-leading-order perturbative QCD (NLO pQCD) calculations. Above this value of p T the result is in agreement with pQCD predictions. The low p T signal is expected to have thermal photon contributions. The inverse slope parameter of an exponential fit is extracted as T LHC = 304 ± 51 syst + stat MeV . For a baseline measurement the analysis is performed for proton-proton collisions at s = 7 TeV and for peripheral (40–80%) Pb–Pb collisions. Both results show no low p T direct-photon signal and are in agreement with pQCD calculations.

Subfields: Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/9deee782ed9661375df85b8ba848325569989c57