Searching for Sources of Extremely Energetic Neutrinos

Dedicated high-energy neutrino telescopes based on optical Cherenkov 
techniques have been scanning the cosmos for about a decade.  At TeV 
scales, limits on the diffuse flux have improved by several orders of 
magnitude, eliminating the most optimistic models that tend to be 
normalized to the extragalactic x-ray or gamma-ray luminosity.  At higher 
energies, they have provided the first neutrino flux limits from point 
sources and diffusely distributed sources such as cosmogenic neutrinos 
generated by the GZK process, whose existence is relatively secure but 
predicted flux is frustratingly small. To substantially improve the 
experimental capabilities at the very highest energies, new techniques are 
required.  I will briefly discuss results from the radio-based Cherenkov 
detector ANITA, and describe a new concept called ARIANNA that promises to 
increase the sensitivity to neutrinos with energies in excess of 10^17 eV. 
Radio Cherenkov telescopes have already ruled out some of the more exotic 
predictions for neutrino intensity at extreme energies and may soon test 
more conventional GZK models.  In addition to flux measurements, these 
devices can probe for non-standard particle physics by investigating the 
neutrino cross-section.