Astroparticle Physics

7.3 Cosmic Rays Underground 151N(ν µ +¯ν µ )N(ν e +¯ν e ) ≈ 2 . (7.6)This ratio, however, is modified by propagation effects likeneutrino oscillations (see Sect. 6.2: Neutrino Astronomy).neutrino-flavour ratio7.3 Cosmic Rays Underground“If your experiment needs statistics,then you ought to have done a better experiment.”Ernest RutherfordParticle composition and energy spectra of secondary cosmicrays underground are of particular importance for neutrinoastronomy. Experiments in neutrino astronomy areusually set up at large depths underground to provide a sufficientshielding against the other particles from cosmic rays.Because of the rarity of neutrino events even low fluxes ofresidual cosmic rays constitute an annoying background. Inany case it is necessary to know precisely the identity andflux of secondary cosmic rays underground to be able todistinguish a possible signal from cosmic-ray sources fromstatistical fluctuations or systematical uncertainties of the atmosphericcosmic-ray background.Long-range atmospheric muons, secondary particles locallyproduced by muons, and the interaction products createdby atmospheric neutrinos represent the important backgroundsources for neutrino astronomy.Muons suffer energy losses by ionization, direct electron–positronpair production, bremsstrahlung, and nuclearinteractions. These processes have been described in ratherdetail in Chap. 4. While the ionization energy loss at highenergies is essentially constant, the cross sections for theother energy-loss processes increase linearly with the energyof the muon,− dE = a + bE. (7.7)dxThe energy loss of muons as a function of their energy isshown in Fig. 7.17 for iron as absorber material. The energyloss of muons in rock in its dependence on the muon energywas already shown earlier (Fig. 4.3).Equation (7.7) allows to work out the range R of muonsby integration,particle compositionundergroundbackground sourcesfor neutrino astrophysicsenergy loss of muonsrange of muons