Study of jet quenching using photon-jet events in PbPb collisions at 2.76 TeV with CMS

The first measurement of the transverse momentum imbalance of isolated-photon-jet pairs and the modification of fragmentation function of the inclusive jets in relativistic heavy ion collisions are reported. The analysis uses data from \PbPb\ collisions at a center-of-mass energy of 2.76 TeV per nucleon pair and corresponding to an integrated luminosity of 150~$\mu$b$^{-1}$ recorded by the CMS experiment at the LHC in 2011. For events containing an isolated photon with transverse momentum $p_T >$ 60~GeV$/$c and an associated jet with $p_T >$ 30~GeV$/$c, the photon-jet $p_T$ imbalance is studied as a function of collision centrality and compared to \pp\ data and \PYTHIA\ calculations at the same center-of-mass energy. Using the $p_T$ of the isolated photon as an estimate of the energy of the associated parton at production, this measurement allows an unbiased characterization of the in-medium parton energy loss. In addition, further study on the modification of jet fragmentation function was studied using inclusive jets with $p_T >$ 100 GeV$/$c. Both show the gradual centrality dependence of jet energy loss and fragmentation function modification.