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

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 TeV2.76 TeV per nucleon pair corresponding to an integrated luminosity of 150 μb−1150 μb−1 recorded by the CMS experiment at the LHC. For events containing an isolated photon with transverse momentum View the MathML sourcepTγ>60 GeV/c and an associated jet with View the MathML sourcepTJet>30 GeV/c, the photon + jet pTpT imbalance is studied as a function of collision centrality and compared to pp data and pythia calculations at the same collision energy. Using the View the MathML sourcepTγ 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 View the MathML sourcepTJet/pTγ relative to that in the pythia reference is observed. Furthermore, significantly more View the MathML sourcepTγ>60 GeV/c photons in PbPb are observed not to have an associated View the MathML sourcepTJet>30 GeV/c jet, compared to the reference. However, no significant broadening of the photon + jet azimuthal correlation is observed.