  The energy loss of high p_T partons propagating through a hot and dense
medium is regarded as a valuable tool to probe the medium created in
ultrarelativistic heavy-ion collisions. The angular correlation pattern of
hadrons associated with a hard trigger in the region of p_T ~1-2 GeV which
exhibits a dip in the expected position of the away side jet has given rise to
the idea that energy is lost predominantly to propagating collective modes
('Mach cones'). Recent measurements by the STAR collaboration have shown that
for a high p_T > 8 GeV trigger the angular pattern of associate hadrons for p_T
> 4 GeV shows the emergence of the expected away side peak. These di-jet events
suggest that the away side parton may emerge occasionally without substantial
energy loss. Since in such a back-to-back configuration one of the partons may
travel through the central region of the fireball, the average in-medium
pathlength is substantial and the expected energy loss is not only sensitive to
the initial geometry of matter but also to the change of geometry due to
expansion. We show that radiative energy loss is able to explain the dijet
events provided that the expansion of the medium is taken into account.
