The origin of cosmic rays (CRs) with up to 10^20 eV energies is one of the most important questions in astrophysics to date. One way to track down CR sources and study CR propagation is to look for the products of cosmic ray interactions interactions, in particular high-energy (TeV-PeV) neutrinos. These neutrinos, indeed, are characteristic of hadronic interactions and thus would be an unambiguous signature of the presence of cosmic rays.

High-energy particles and relativistic sources in astrophysics
Where do cosmic rays, neutrinos and high-energy photons come from? How is
radiation emitted from black hole environments, blazars, gamma-ray bursts and other
relativistic sources? What process gives rise to the electromagnetic counterparts of
gravitational wave events? All these questions shape the rapidly-developing fields of
high-energy astrophysics and multi-messenger astrophysics. At the heart of these