Extremely High Energy Cosmic Neutrinos and Relic Neutrinos

I review the essentials of ultrahigh-energy neutrino interactions, show how neutral-current detection and flavor tagging can enhance the scientific potential of neutrino telescopes, and sketch new studies on neutrino encounters with dark matter relics and on gravitational lensing of neutrinos. 1. Neutrino Observatories: Expectations An early goal of the next generation of neutrino telescopes will be to detect the flux of cosmic neutrinos that we believe will begin to show itself above the atmosphericneutrino background at energies of a few TeV. A short summary of the science program of these instruments is to prospect for cosmic-neutrino sources, to characterize the sources, to study neutrino properties, and to be sensitive to new phenomena in particle physics. The expected sources include active galactic nuclei (AGN) at typical distances of roughly 100 Mpc. If neutrinos are produced there in the decay of pions created in pp or pγ collisions, then we anticipate—at the source—equal numbers of neutrinos and antineutrinos, with a flavor mix 2γ + 2νμ + 2ν̄μ + 1νe + 1ν̄e, provided that all pions and their daughter muons decay. I denote this standard flux at the source by Φstd = {φe = 13 , φμ = 2 3 , φτ = 0}. We expect that a neutrino observatory with an instrumented volume of 1 km will be able to survey the cosmic-neutrino flux over a broad range of energies, principally by detecting the charged-current interaction (νμ, ν̄μ)N → (μ−, μ) + anything. Important open questions are whether we can achieve efficient, calibrated (νe, ν̄e) and (ντ , ν̄τ ) detection, and whether we can record and determine the energy of neutralcurrent events. One of my aims in this talk will be to illustrate how adding these capabilities will enhance the scientific potential of neutrino observatories. The cross section for deeply inelastic scattering on an isoscalar nucleon may be