Astroparticle Physics

90 6 Primary Cosmic RaysFig. 6.11Momentum spectrum of single-ringelectron-like events inSuper-Kamiokande. The solid linerepresents the Monte Carloexpectation {9}Fig. 6.12Momentum spectrum of single-ringmuon-like events inSuper-Kamiokande. The solid linerepresents the Monte Carloexpectation. The cutoff arround 10GeV originates from the conditionthat the muon tracks must becontained in the detector {9}neutrino oscillationseigenstatesof the weak interactionmass eigenstatesAfter careful checks of the experimental results and investigationsof possible systematic effects the general opinionprevails that the deficit of muon neutrinos can only beexplained by neutrino oscillations.Mixed particle states are known from the quark sector(see Chap. 2). Similarly, it is conceivable that in the leptonsector the eigenstates of weak interactions ν e , ν µ ,andν τ are superpositions of mass eigenstates ν 1 ,ν 2 ,andν 3 .A muon neutrino ν µ born in a pion decay could be transformedduring the propagation from the source to the observationin the detector into a different neutrino flavour. If themuon neutrino in reality was a mixture of two different masseigenstates ν 1 and ν 2 , these two states would propagate atdifferent velocities if their masses were not identical andso the mass components get out of phase with each other.This could possibly result in a different neutrino flavour atthe detector. If, however, all neutrinos were massless, theywould all propagate precisely at the velocity of light, andthe mass eigenstates can never get out of phase with eachother.