Photon and electroweak boson production in PbPb collisions

The production of electroweak bosons (photons, W and Z particles) in PbPb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV has been measured with the CMS detector at the LHC. Direct photons are selected by applying isolation criteria, while W and Z bosons are reconstructed through their muonic decay. The production rates in PbPb data are studied as a function of the collision centrality and compared to that in pp interactions (or next-to-leading-order calculations), once normalised by the number of binary nucleon–nucleon interactions. Apart from an expected asymmetry between image and image due to the different quark content of protons and Pb nuclei, no deviation from binary-scaled production is observed. The isolated-photon energy is further used as a reference for away-side jets, which are found to be less energetic in PbPb than in pp collisions.