Study of photon-tagged jet events in high-energy heavy ion collisions with CMS

The energy loss of fast partons traversing the strongly interacting matter produced in high-energy nuclear collisions is one of the most interesting observables to probe the nature of the produced medium. The collisional and radiative energy loss of the partons will modify their fragmentation functions depending on the path length of the partons in the medium and the medium density. Pb+Pb collisions at $\sqrt{s_{\scriptscriptstyle{{\rm NN}}}}=5.5$~TeV at the LHC will allow us to perform the first detailed measurements of the in-medium modifications of fragmentation functions of parton initiated jets, using the $\gamma$-jet channel. Since the photon does not strongly interact with the medium, the initial transverse energy of the fragmenting parton can be related to the photon transverse energy in back-to-back $\gamma$-jet events. This in turn allows for precision studies of the jet fragmentation function, exploiting the CMS track reconstruction capabilities and the large rapidity coverage of the CMS calorimeters.