  UHE cosmic neutrino interaction with the cosmic neutrino background (CnuB) is
expected to produce absorption dips in the UHE neutrino flux at energies above
the threshold for Z-boson resonant production. The observation of these dips
would constitute an evidence for the existence of the CnuB; they could also be
used to determine the value of the relic neutrino masses as well as some
features of the population of UHE neutrino sources. After breafly discussing
the current prospects for relic neutrino spectroscopy, we present a calculation
of the UHE neutrino transmission probability based on finite-temperature field
theory which takes into account the thermal motion of the relic neutrinos. We
then compare our results with the approximate expressions existing in the
literature and discuss the influence of thermal effects on the absorption dips
in the context of realistic UHE neutrino fluxes and favoured neutrino mass
schemes.
