Extremely High Energy Neutrinos and Their Detection

We discuss in some detail the production of extremely high energy (EHE) neutrinos with energies above 1018 eV. The most certain process for producing such neutrinos results from photopion production by EHE cosmic rays in the cosmic background photon Ðeld. However, using assumptions for the EHE cosmic-ray source evolution that are consistent with results from the deep QSO survey in the radio and X-ray range, the resultant Ñux of neutrinos from this process is not strong enough for plausible detection. A measurable Ñux of EHE neutrinos may be present, however, if the highest energy cosmic rays that have recently been detected well beyond 1020 eV are the result of the annihilation of topological defects which formed in the early universe. Neutrinos resulting from such decays reach energies of the grand uniÐed theory scale, and collisions of superhigh-energy neutrinos with the cosmic background neutrinos initiate neutrino cascading, which enhances the EHE neutrino Ñux at Earth. We have calculated the neutrino Ñux including this cascading e†ect for either massless or massive neutrinos, and we Ðnd that these Ñuxes are conceivably detectable by air Ñuorescence detectors now in development. The neutrino-induced showers would be recognized by their starting deep in the atmosphere. We evaluate the feasibility of detecting EHE neutrinos this way using air Ñuorescence air shower detectors and derive the expected event rate. Other processes for producing deeply penetrating air showers constitute a negligible background. Subject headings : cosmic microwave background È cosmic rays È cosmic strings È early universe È elementary particles È instrumentation : detectors