Charged current neutrino cross section and tau energy loss at ultrahigh energies

We evaluate both the tau-lepton energy loss produced by photonuclear interactions and the neutrino charged current cross section at ultrahigh energies, relevant to neutrino bounds with Earth-skimming tau neutrinos, using different theoretical and phenomenological models for nucleon and nucleus structure functions. The theoretical uncertainty is estimated by taking different extrapolations of the structure function F₂ to very low values of x, in the low and moderate Q² range for the tau-lepton interaction and at high Q² for the neutrino-nucleus inelastic cross section. It is at these extremely low values of x where nuclear shadowing and parton saturation effects are unknown and could be stronger than usually considered. For tau and neutrino energies E=10⁹ GeV we find uncertainties of a factor 4 for the tau energy loss and of a factor 2 for the charged current neutrino-nucleus cross section.