Status of Radio and Acoustic Detection of Ultra-High Energy Cosmic Neutrinos and a Proposal on Reporting Results

Abstract

Neutrino astronomy offers the possibility to perform extra-galactic observations well beyond the photon absorption cutoff above 50 TeV. Based on observations of cosmic rays, we already know that astrophysical sources produce particles with at least a million times more energy than this photon cutoff. Once discovered, either the nature of the sources themselves or the cross sections of ultra-high energy neutrinos with terrestrial matter may reveal exotic physical processes that are inaccessible to modern accelerators. Some of these processes may be due to as-yet unknown physics at the grand unification scale or beyond. Neutrino telescopes based on optical techniques currently operating and under construction have apertures measured in several km^3-sr. Radio and acoustic detection techniques have been demonstrated in laboratory experiments and are currently used for instrumentation of apertures 10 to 10,000 times larger than optical techniques for neutrinos above 10 PeV. I discuss the status of current and proposed neutrino telescope projects based on these techniques. These telescopes have already ruled out some of the more exotic predictions for neutrino intensity. The upcoming generation of radio-based and acoustic-based detectors should be sensitive to cosmic neutrinos above 1 EeV originating through the so-called GZK process. A comparison of different neutrino telescopes using a common aperture variable shows how they are complementary in the trade-off of volume versus threshold. I include a proposal for how neutrino telescopes should report their sensitivities to facilitate direct comparisons among them and to allow testing of neutrino brightness models that appear even after publication of the experimental results.