ABSTRACT
Having discussed neutrinos from stars with energies Eν < 100 MeV which have already been observed, we now want to discuss additional astrophysical neutrinos produced with much higher energy. The observation of cosmic rays with energies beyond 1020 eV, even though the details are under intense debate, by the Fly’s Eye and AGASA air shower arrays [Bir91,Aga01] supports the possibility of also observing neutrinos up to this energy range. Neutrinos mostly originate from the decay of secondaries like pions resulting from ‘beam dump’ interactions of protons with other protons or photons as in accelerator experiments. Like photons, neutrinos are not affected by a magnetic field. Even absorption which might affect photon detection is not an issue and this allows a search for hidden sources, which cannot be seen otherwise. Therefore, they are an excellent candidate for finding point sources in the sky and might help to identify the sources of cosmic rays. In addition, our view of the universe in photons is limited for energies beyond 1 TeV. The reason is the interaction of such photons with background photons γ + γBG → e+e−. The reaction has a threshold of 4Eγ EγBG ≈ (2me)2. In this way TeV photons are damped due to reactions on the IR background and PeV photons by the cosmic microwave background (see chapter 13). For additional literature see [Sok89,Gai90,Ber91,Lon92, 94,Gai95,Lea00,Sch00,Gri01,Hal02].
The search for high-energy neutrinos might be split into two lines. One is the obvious search for point sources, in the hope that a signal will shed light on the question of what the sources of cosmic rays are. The second one would be a diffuse neutrino flux like the one observed in gamma rays. It is created by pion decay, produced in cosmic-ray interactions within the galactic disc. Instead of this more-observational-motivated division, the production mechanism itself can be separated roughly in two categories, i.e. acceleration processes and annihilation in combination with the decay of heavy particles. The acceleration process can be subdivided further into those of galactic and extragalactic origin.
