As far as I know the usual extension of the standard model to include a massive neutrino implies a violation of the conservation of quantum numbers, this means the lepton number (since the neutrino is uncharged). During its propagation a massive left-handed neutrino can be regarded as a massive right-handed antineutrino with an appropiate Lorentz boost (as explained here). In that case, how can neutrino-antineutrino annihilation processes be explained? Do actually neutrinos and antineutrinos annihilate? It seams to me that there could exist an observer that regards the antineutrino as a neutrino, but still the neutrino as a neutrino.