Studies of jet quenching using isolated-photon + jet correlations in PbPb and pp collisions at √sN N = 2.76 TeV

Abstract

Results from the first study of isolated-photon + jet correlations in relativistic heavy ion collisions are reported. The analysis uses data from PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair corresponding to an integrated luminosity of 150 μb−1 recorded by the CMS experiment at the LHC. For events containing an isolated photon with transverse momentum pγ T > 60 GeV/c and an associated jet with pJet T > 30 GeV/c, the photon + jet pT imbalance is studied as a function of collision centrality and compared to pp data and pythia calculations at the same collision energy. Using the pγ T of the isolated photon as an estimate of the momentum of the associated parton at production, this measurement allows an unbiased characterisation of the in-medium parton energy loss. For more central PbPb collisions, a significant decrease in the ratio pJet T /pγ T relative to that in the pythia reference is observed. Furthermore, significantly more pγ T > 60 GeV/c photons in PbPb are observed not to have an associated pJet T > 30 GeV/c jet, compared to the reference. However, no significant broadening of the photon + jet azimuthal correlation is observed.