  Heavy ion collisions at RHIC and at the LHC give access to the medium-induced
suppression patterns of heavy-flavored single inclusive hadron spectra at high
transverse momentum. This opens novel opportunities for a detailed
characterization of the medium produced in the collision. In this note, we
point out that the capacity of a QCD medium to absorb the recoil of a partonic
projectile is an independent signature, which may differ for different media at
the same density. In particular, while the mass hierarchy (i.e., the projectile
mass dependence) of radiative energy loss depends solely on a property of the
projectile, the mass hierarchy of collisional energy loss depends significantly
on properties of the medium. By varying these properties in a class of models,
we find that the mass hierarchy of collisional parton energy loss can be
modified considerably and can even be inverted, compared to that of radiative
parton energy loss. This may help to disentangle the relative strengths of
radiative and collisional contributions to jet quenching, and it may be
employed to constrain properties of the produced QCD medium beyond its density.
