Introduction

Neutrino oscillation is purely a quantum mechanical phenomenon, in which aneutrino of one flavor, say να develops acomponent of a neutrino of another flavor, sayνβ, whereα, β =e, μ,τ, and α ≠ β.This phenomenon of neutrino oscillation implies that neutrinos have definitemass and their flavor eigenstates are different from their masseigenstates.

The idea of neutrino masses, mixing, and oscillations was first proposed in1957–58 [179] just after the two-component neutrino theory wasconfirmed by the Goldhaber et al. [153] experiment. Pontecorvo [179]conjectured the idea of neutral particle (not necessarily being elementaryparticle) oscillation based on the analogy with kaon regeneration, andargued that : ‘If the two- component neutrino theory should turn outto be incorrect (which at present seems to be rather improbable) and if theconservation law of neutrino charge would not apply, then in principleneutrino ↔ antineutrino transitions could take placein vacu’. At that time only one type of neutrino existed, therefore,for the two-component theory, there were only left-handed neutrinos andright-handed antineutrinos, implying oscillations between. These transitionsare however not possible because of the conservation of total angularmomentum.

Around the same time, Davis [968] conducted an experiment using reactorantineutrino beams and looked for the electron in the reaction

which showed negative results. Contrary to the negative observations, a rumorreached Pontecorvo that Davis has observed the aforementioned event beforethe actual publication of the work. Based on the assumed positive results,in 1958, Pontecorvo, in an another paper [969], concluded that: “Theresult of the Davis experiment was nonzero probability for the process understudy, if confirmed, definitely shows that the (strict) law of neutrinocharge conservation is not valid.” He suggested that the events couldbe due to the transitions. However, this explanation contradicted thetwo-component neutrino theory, where only the left-handed neutrinos or theright-handed antineutrinos participate in the weak interaction. Therefore,the suggestion of νR and similarly, byPontecorvo implies the existence of a class of neutrinos which arenoninteracting; they are called sterile neutrinos.