The Fragility of High-pT Hadron Spectra as a Hard Probe

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.