Astroparticle Physics

104 6 Primary Cosmic Raysis extremely small. It is not a surprise that the decision onthe oscillation scenario has come from observations of solarand atmospheric neutrinos and accelerator experimentswith well-defined, flavour-selected neutrino beams. Withthe experimental evidence of cosmic-ray-neutrino experiments(Davis, GALLEX, SAGE, Super-Kamiokande, SNO)and recent accelerator and reactor experiments (K2K, Kam-LAND) there is now unanimous agreement that oscillationsin the neutrino sector are an established fact.6.2.4 High-Energy Galacticand Extragalactic NeutrinosFig. 6.23Comparison of cosmic neutrinofluxes in different energy domains‘blackbody’ neutrinosThe measurement of high-energy neutrinos (≥ TeV range)represents a big experimental challenge. The arrival directionof such neutrinos, however, would directly point back tothe sources of cosmic rays. Therefore, a substantial amountof work is devoted to prototype studies for neutrino detectorsin the TeV range and the development of experimentalsetups for the measurement of galactic and extragalactichigh-energy neutrinos. The reason to restrict oneself tohigh-energy neutrinos is obvious from the inspection of Fig.6.23. The neutrino echo of the Big Bang has produced energiesbelow the meV range. About a second after the BigBang weak interactions have transformed protons into neutronsand neutrons into protons thereby producing neutrinos(p + e − → n + ν e , n → p + e − +¯ν e ). The temperature ofthese primordial neutrinos should be at 1.9 K at present time.Blackbody photons have a slightly higher temperature(2.7 K) because, in addition, electrons and positrons havetransformed their energy by annihilation into photon energy.The Big Bang neutrinos even originate from an earlier cosmologicalepoch than the blackbody photons since the universewas much earlier transparent for neutrinos. In so farthese cosmological neutrinos are very interesting as far asdetails of the creation and the development of the early universeare concerned. Unfortunately, at present it is almost inconceivablethat neutrinos of such low energies in the meVrange can be measured at all.The observation of solar (≈ MeV range) and supernovaneutrinos (≈ 10 MeV) is experimentally established.Atmospheric neutrinos represent a background for neutrinosfrom astrophysical sources. Atmospheric neutrinos originateessentially from pion and muon decays. Their productionspectra can be inferred from the measured atmospheric