  High transverse momentum (P_T) gamma-hadron correlations are currently being
regarded as the 'golden channel' for the study of the medium produced in
ultrarelativistic heavy ion collisions by means of hard probes. This is due to
several reasons, all linked to the fact that because of the smallness of the
electromagnetic coupling alpha, the photon does not substantially interact with
the medium and is expected to escape unmodified. Thus, a high P_T photon
indicates a hard process in the collision independent of the position of the
hard vertex. In contrast, there may not be a clear signal for a hard process
involving strongly interacting partons if the production vertex is deep in the
medium as both partons undergo substantial final state interaction. Equally
important, if photon production by fragmentation can be separated
experimentally, the photon provides almost full knowledge of the initial
kinematics. In the present paper, these properties are used to demonstrate a
distinguishing feature between two assumptions made in modelling the
medium-modifications of strongly interacting high P_T processes: Loss of energy
into the medium vs. medium modification of the partonic shower. Is it shown
that gamma-hadron correlations provide a very clean signature to distinguish
the two scenarios.
