  Hard pQCD processes taking place in ultrarelativistic heavy-ion collisions
are a well-calibrated probe. It is believed that the interaction with the
surrounding medium of outgoing partons from a hard vertex is capable of
revealing details of the medium. We demonstrate that correlation measurements
of hard photon-hadron back-to-back coincidences are a tool suitable to extract
such tomographic information. Introducing the concept of averaged evergy loss
probability distributions, we first argue that almost no information about
details of the medium evolution is reflected in the nuclear suppression factor
R_AA. Thus, a wide variety of jet quenching scenarios and geometries are
compatible with the measured data. This problem can be overcome by a
gamma-hadron correlation measurement. We show that averaged probability
distributions for quarks are accessible experimentally and sketch an analysis
procedure capable of distinguishing different energy loss scenarios leading to
the same nuclear suppression factor.
