Bruno Pontecorvo, Izbrannye trudy, ed. S. M. Bilenkii, Moskva, <> (1997) t.I, s.175.
Recently the question was discussed whether there exist other "mixed'' neutral particles beside the K0 mesons, i.e., particles that differ from the corresponding antiparticles, with the transitions between particle and antiparticle states not being strictly forbidden. It was noted that the neutrino might be such a mixed particle, and consequently there exists the possibility of real neutrino antineutrino transitions in vacuum, provided that
lepton (neutrino) charge is not conserved. In the present note we make a more detailed study of this possibility, in which interest has been renewed owing to recent experiments dealing with inverse beta processes. (Extracted from the introductory part of the paper.).
Related references More (earlier) information appears in B. Pontecorvo, Zh. Eksp. Teor. Fiz. 33 (1957) 549;
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Proposal for the possibility of neutrino-antineutrino oscillations.