The oscillation mechanism that we’ve come up with (and that fits data from reactor experiments to astronomical ones pretty well) only works if the mass eigen st

The oscillation mechanism that we’ve come up with (and that fits data from reactor experiments to astronomical ones pretty well) only works if the mass eigen states of the neutrinos are different from their flavor eigen states, otherwise there is no mixing. We can measure the mass differences between the neutrinos pretty well through these oscillation experiments, but this also doesn’t tell us which the mass hierarchy. It could be bottom up or the other way round. In principle, one neutrino could be massless and the mass differences we’ve measured so far would still be correct. Aside from this, pretty much anything is on the table. Neutrinos being their own anti particles? Maybe. Fourth generation of neutrinos? Could be.