Inclusive isolated photons in pp and PbPb collisions at 2.76 TeV with CMS

The CMS detector at the LHC has been used to measure the yield of isolated photons in both proton-proton and lead-lead collisions at a nucleon-nucleon center of mass energy of 2.76 TeV. Photons were detected in the pseudorapidity range vertical bar eta vertical bar < 1.44 and with transverse energies E-T between 20 and 80 GeV. The E-T spectra found using next-to-leading-order perturbative QCD calculations are in good agreement with the data. The centrality dependence of the ratio of isolated photon yields in PbPb over those in pp is well described by the number of incoherent nucleon-nucleon collisions. Therefore, the yields of isolated photons in PbPb are consistent with there being no modification of their production in the initial state and no absorption of these photons as they traverse the hot and dense medium created in these very high energy heavy ion collisions.