  We find that the current radiative energy loss kernels obtained from the
opacity expansion dramatically violate the collinear approximation used in
their derivation. By keeping only the lowest order in collinearity terms,
models based on the opacity expansion have ~50% systematic uncertainty in the
calculation of pi^0 R_AA in 0-5% most central RHIC collisions resulting in a
systematic uncertainty of ~200% in the extracted medium density. Surprisingly,
the inclusion of a thermal gluon mass on the order of the Debye screening scale
affects R_AA at only about the 5% level due to non-intuitive coherence effects.
For some observables such as R_AA, the effect of these uncertainties decreases
with increasing jet energy; for others, such as the average number of radiated
gluons, the effect is energy independent. We note that it is likely that the
differences reported in the extracted values of medium parameters such as qhat
by various jet energy loss models will fall within this collinear approximation
systematic uncertainty; it is imperative for the quantitative extraction of
medium parameters or the possible falsification of the hypothesis of weak
coupling between the hard probes and soft modes of the quark gluon plasma
medium that future radiative energy loss research push beyond the lowest order
collinear approximation.
