Ultrahigh-energy Cosmic Neutrinos

Abstract

Many astrophysical objects in the Universe are expected to produce cosmic neutrinos with very high energies (1 TeV ≲ Eν ≲ 1 PeV), ultrahigh energies (Eν ≳ 1 PeV) or extremely high energies (Eν ≳ 100 EeV) 1. Such energetic neutrinos may serve as a unique cosmic messenger and provide us with useful information about the cosmos that cannot be extracted from the measurements of cosmic rays and gamma rays. The burning questions in neutrino astronomy include where ultrahigh-energy (UHE) cosmic neutrinos originate from and how they can be detected. In this chapter we shall first describe some possible sources of UHE cosmic neutrinos and then outline a few possible ways to detect them. The flavor distribution and oscillations of UHE cosmic neutrinos, together with their sensitivities to new physics, will also be discussed. We shall finally highlight the importance of multi-messenger astronomy by illustrating the interplay between UHE cosmic neutrinos and cosmic rays, gamma rays or gravitational waves.