Towards jet tomography: gamma-hadron correlations
  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. 
Thorsten Renk