γ-hadron correlations as a tool to trace the flow of energy lost from hard partons in heavy-ion collisions

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 for 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 modeling the medium modifications of strongly interacting high P-T processes: loss of energy into the medium vs medium modification of the partonic shower. It is shown that gamma-hadron correlations provide a very clean signature for distinguishing the two scenarios.