Radio Detection of High Energy Neutrinos
Ultra-high energy (UHE) neutrino astrophysics sits at the crossroads of particle physics, astronomy, and astrophysics. Through neutrino astrophysics, we can uniquely explore the structure and evolution of the universe at the highest energies at cosmic distances and test our understanding of particle physics at energies greater than those available at particle colliders. The detection of UHE neutrinos would shed light on the nature of the astrophysical sources that produce the highest energy particles in the universe. Astrophysical sources almost certainly produce UHE neutrinos in hadronic processes. Also, neutrinos above 10 eV should be produced through the GZK effect, where extragalactic cosmic rays above 10 eV interact with the cosmic microwave background within tens of Mpc of their source. It was first pointed out by Berezinsky and Zatsepin that this cosmogenic neutrino flux, sometimes called “BZ” neutrinos, could be observable. These neutrinos would point close to the cosmic ray production site both because of the close proximity of the interaction to the source and because the decay products are Lorentz boosted along the line of sight. The latter effect constrains the direction the most strongly (1 MeV/1 EeV = 10−12 whereas 100 Mpc/1 Gpc = 0.1). Since cosmic rays do not follow straight paths in magnetic fields and get attenuated above the GZK threshold, and high energy photons (E> 10 eV) are attenuated by the cosmic infrared background, neutrinos offer the unique ability to point to the location of the highest energy cosmic accelerators in aHere, we loosely define the UHE regime to be the energy range near 1018 − 1021 eV. A few radio experiments have sensitivities that reach to lower energies, while others have energy thresholds at higher energies.