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

The first measurement of the transverse momentum ($p_T$) imbalance of isolated-photon+jet pairs in relativistic heavy ion collisions is reported. The analysis uses data from PbPb collisions at a centre-of-mass energy of $2.76\:\mathrm{TeV}$ per nucleon pair, corresponding to an integrated luminosity of $150\:\mathrm{\mu b}^{-1}$ recorded by the CMS experiment at the LHC in 2011. For events containing an isolated photon with transverse momentum $p_T > 60\:\mathrm{GeV}/c$ and an associated jet with $p_T > 30\:\mathrm{GeV}/c$, the photon+jet $p_T$ imbalance is studied as a function of collision centrality and compared to pp data and \textsc{pythia} calculations at the same centre-of-mass energy. Using the $p_T$ of the isolated photon as an estimate of the energy of the scattered parton, this measurement allows an unbiased characterization of the in-medium parton energy loss.