UC Irvine

Ultrahigh energy cosmic neutrinos are incisive probes of both astrophysical sources and new TeV-scale physics. Such neutrinos would create extensive air showers deep in the atmosphere. The absence of such showers implies upper limits on incoming neutrino fluxes and cross sections. Combining the exposures of the Akeno Giant Air Shower Array, the largest existing ground array, with the exposure of the Fly’s Eye fluorescence detector integrated over all its operating epochs, we derive 95% confidence level bounds that substantially improve existing limits. We begin with model-independent bounds on astrophysical fluxes, assuming standard model cross sections, and model-independent bounds on new physics cross sections, assuming a conservative cosmogenic flux. We then derive model-dependent constraints on new components of neutrino flux for several assumed power spectra, and we update bounds on the fundamental Planck scale (Formula presented) in extra dimension scenarios from black hole production. For large numbers of extra dimensions, we find (Formula presented) for (Formula presented) comparable to or exceeding the most stringent constraints to date. © 2002 The American Physical Society.