Measurement of Direct Photons in pp and Pb-Pb Collisions with ALICE

摘要

The measurement of the direct photon transverse momentum spectrum in Pb-Pb collisions at √s_(NN) = 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 protonproton 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.