  $\gamma +$jet events provide a tomographic measurement of the medium formed
in heavy ion collisions at LHC energies. Tagging events with a well identified
high $p_{T}$ direct photon and measuring the correlation distribution of
hadrons emitted oppositely to the photon, allows us to determine, with a good
approximation, both the jet fragmentation function and the back-to-back
azimuthal misalignement of the direct photon and the jet. Comparing these two
observables measured in $pp$ collisions with the ones measured in $AA$
collisions will reveal the modifications of the jet structure induced by the
medium formed in $AA$ collisions and consequently will infer the medium
properties.
