  The unexpectedly strong suppression of high p_T heavy-quarks in heavy-ion
collisions has given rise to the idea that partons propagating through a medium
in addition to energy loss by induced radiation also undergo substantial energy
loss due to elastic collisions. However, the precise magnitude of this elastic
energy loss component is highly controversial. While it is for a parton inside
a medium surprisingly difficult to define the difference between elastic and
radiative processes rigorously, the main phenomenological difference is in the
dependence of energy loss on in-medium pathlength: in a constant medium
radiative energy loss is expected to grow quadratically with pathlength,
elastic energy loss linearly. In this paper, we investigate a class of energy
loss models with such a linear pathlength dependence and demonstrate that they
are incompatible with measured data on hard hadronic back-to-back correlations
where a substantial variation of pathlength is probed. This indicates that any
elastic energy loss component has to be small.
