Sphicas, P (reprint author), CERN, European Org Nucl Res, CH-1211 Geneva, Switzerland.
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 mu b(-1) recorded by the CMS experiment at the LHC. For events containing an isolated photon with transverse momentum p(T)(gamma) > 60 GeV/c and an associated jet with p(T)(Jet) > 30 GeV/c, the photon + jet P-T imbalance is studied as a function of collision centrality and compared to pp data and PYFHIA calculations at the same collision energy. Using the 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 p(T)(Jet)/p(T)(gamma) pi. relative to that in the PYFHIA reference is observed. Furthermore, significantly more p(T)(gamma) > 60 GeV/c photons in PbPb are observed not to have an associated p(T)(Jet) > 30 GeV/c jet, compared to the reference. However, no significant broadening of the photon + jet azimuthal correlation is observed. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.