Ultrahigh Energy Neutrinos

The ultrahigh energy neutrino cross section is well understood in the standard model for neutrino energies up to 10${}^{12}$ GeV. Test of neutrino oscillations (${\nu }_{\mu }\leftrightarrow {\nu }_{\tau }$) from extragalactic sources of neutrinos are possible with large underground detectors. Measurments of horizontal air shower event rates at neutrino energies above 10${}^{10}$ GeV will be able to constrain nonstandard model contributions to the neutrino-nucleon cross section, e.g., from mini-black hole production.