  Recently the possibility was widely discussed that a large fraction of the
highest energy cosmic rays may be decay products of Z bosons which were
produced in the resonant annihilation of ultrahigh energy cosmic neutrinos on
cosmological relic neutrinos. If one takes this so-called Z-burst scenario
seriously, one may infer the mass of the heaviest relic neutrino as well as the
necessary ultrahigh energy cosmic neutrino flux from a comparison of the
predicted Z-burst spectrum with the observed cosmic ray spectrum.
