  We study the suppression of high-pT hadron spectra in nuclear collisions,
supplementing the perturbative QCD factorized formalism with radiative parton
energy loss. We find that the nuclear modification factor which quantifies the
degree of suppression, is almost pT-independent both for RHIC (in agreement
with data) and for the LHC. This is a consequence of the shape of the partonic
pT-spectrum in elementary collisions which implies that for the same value of
the nuclear modification factor at higher pT, an increasingly smaller fraction
of parton energy loss is needed. When the values of the time-averaged transport
coefficient exceed 5 GeV^2/fm, the nuclear modification factor gradually loses
its sensitivity to the corresponding produced energy density. This is due to
particle production in the outer corona of the medium, which remains almost
unsuppressed even for extreme densities. Thus, even for the highest
experimentally accessible transverse momentum at the LHC and in contrast to
jets, the measurement of leading partons via leading hadrons is not a
penetrating probe of the dense matter, but a rather fragile probe which
fragments for the opacities reached below the skin of the medium. Relating the
transport coefficient to the energy density produced in the collision region,
we find and discuss a phenomenon reminiscent of the opacity problem of elliptic
flow: namely, the interaction of the hard parent parton with the medium appears
to be much stronger than that expected for perturbative interactions of the
hard parton with an ideal quark gluon plasma.
