Neutrinos are very special particles that only interact through weak interaction. In this way they travel cosmological distances in straight lines, because the are neutral and so they are not affected by the galactic magnetic fields, and also due to lack of interactions, the universe is transparent to them. This give us a privileged messenger that reach the Earth almost unaffected.

In the Standard Model they are considered massless particles, however, the discovery of the neutrino flavour oscillation lead to the fact that they have non-zero mass, and so it provided the first clear indication of the need to go beyond the Standard Model.

The neutrino oscillations occur due to the fact that the interaction eigenstates, in which the neutrinos are created and detected, i.e. electron, muon, and tau neutrinos, are a superposition of mass eigenstates, which due to the different masses oscillates at different speeds during the propagation.

At Earth, the detected flux can be divided in several energy ranges. The low-energy spectrum, which includes the cosmic neutrino background (which similarly to CMB is a relic from the early universe, but even older than CMB) and the neutrinos mainly produced in nuclear processes at the sun or supernovas. The high-energy spectrum, from a few 106 eV to 1014 eV are mainly produced from the interaction of CR with the atmosphere. The very-high-energy range, from 1014 eV to 1016 eV, are thought to be produced on extreme energetic objects in the universe, like supernova remnants, Gamma Ray Bursts or Active Galactic Nuclei. Finally, the spectrum above 1016 eV, i.e. the ultra-high-energy range or cosmogenic neutrinos, are produced from the interaction of the ultra-high-energy cosmic rays with the diffuse extragalactic background radiation.

Above high energy range, neutrinos are very difficult to detect, so it is required extraordinarily large detectors. In fact, the neutrinos are not detected directly, but identifying the Cherenkov light produced by the charged lepton produced in the interaction of the neutrino with the matter nuclei (ν+N→l+N).

In addition, the study of the detected flux on Earth of the most energetic ranges provide a good window to check new physic theories, because in absence of new physics the flux of neutrino only suffers an energy shift due to the universe expansion, but, for instance, considering LIV the neutrino becomes unstable leading to a cutoff in the detected spectrum.