  We investigate the measurement of the distribution of away-side jet $p_T$ as
a way to probe the energy loss mechanism in heavy ion collisions, and constrain
the properties of the medium created in heavy ion collisions. We define an
observable related to the fluctuation of the energy deposited in the medium,
and show that competing models of parton energy loss give differing values and
scaling of this observable. We also argue that the scaling of this observable
with system size can be related to the medium's partonic density. We then give
a qualitative discussion of how the measurement we suggest can be performed and
used to determine the parton energy loss mechanism and possibly the system's
partonic density.
