Astroparticle Physics

150 7 Secondary Cosmic RaysIn addition to muons and protons, one also finds elec-trons, positrons, and photons at sea level as a consequenceof the electromagnetic cascades in the atmosphere. A certainfraction of electrons and positrons originates from muon decays.Electrons can also be liberated by secondary interactionsof muons (‘knock-on electrons’).The few pions and kaons observed at sea level are pre-dominantly produced in local interactions.Apart from charged particles, electron and muon neu-trinos are produced in pion, kaon, and muon decays. Theyconstitute an annoying background, in particular, for neutrinoastronomy. On the other hand, the propagation of atmosphericneutrinos has provided new insights for elementaryparticle physics, such as neutrino oscillations. A comparisonof vertical and horizontal neutrino spectra (Fig. 7.16) showsa similar tendency as for muon spectra.positrons, electrons,and photons fromelectromagnetic cascadespions and kaons at sea levelproduction of ν e and ν µFig. 7.16Energy spectra of muon andelectron neutrinos for vertical andhorizontal directionsSince the parent particles of neutrinos are dominantly pi-ons and kaons and their decay probability is increased comparedto the interaction probability at inclined directions, thehorizontal neutrino spectra are also harder in comparison tothe spectra from vertical directions. Altogether, muon neutrinoswould appear to dominate, since the (π → eν) and(K → eν) decays are strongly suppressed due to helicityconservation. Therefore, pions and kaons almost exclusivelyproduce muon neutrinos only. Only in muon decay equalnumbers of electron and muon neutrinos are produced. Athigh energies also semileptonic decays of charmed mesonsconstitute a source for neutrinos.Based on these ‘classical’ considerations the integralneutrino spectra yield a neutrino-flavour ratio ofneutrino parentsdominance of ν µ