Neutrino oscillation is purely a quantum mechanical phenomenon, in which a neutrino of one flavor, say να develops a component of a neutrino of another flavor, say νβ, where α, β = e, μ, τ, and α ≠ β. This phenomenon of neutrino oscillation implies that neutrinos have definite mass and their flavor eigenstates are different from their mass eigenstates.
The idea of neutrino masses, mixing, and oscillations was first proposed in 1957–58  just after the two-component neutrino theory was confirmed by the Goldhaber et al.  experiment. Pontecorvo  conjectured the idea of neutral particle (not necessarily being elementary particle) oscillation based on the analogy with kaon regeneration, and argued that : ‘If the two- component neutrino theory should turn out to be incorrect (which at present seems to be rather improbable) and if the conservation law of neutrino charge would not apply, then in principle neutrino ↔ antineutrino transitions could take place in vacu’. At that time only one type of neutrino existed, therefore, for the two-component theory, there were only left-handed neutrinos and right-handed antineutrinos, implying oscillations between. These transitions are however not possible because of the conservation of total angular momentum.
Around the same time, Davis  conducted an experiment using reactor antineutrino beams and looked for the electron in the reaction
which showed negative results. Contrary to the negative observations, a rumor reached Pontecorvo that Davis has observed the aforementioned event before the actual publication of the work. Based on the assumed positive results, in 1958, Pontecorvo, in an another paper , concluded that: “The result of the Davis experiment was nonzero probability for the process under study, if confirmed, definitely shows that the (strict) law of neutrino charge conservation is not valid.” He suggested that the events could be due to the transitions. However, this explanation contradicted the two-component neutrino theory, where only the left-handed neutrinos or the right-handed antineutrinos participate in the weak interaction. Therefore, the suggestion of νR and similarly, by Pontecorvo implies the existence of a class of neutrinos which are noninteracting; they are called sterile neutrinos.