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BruceW

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my question seems pretty simple, but I couldn't find any answers. OK, so the neutrino flavour eigenstates are different to the neutrino mass eigenstates. And this is why neutrino oscillation is possible. But for the charged leptons (the electron, muon and tauon) the mass eigenstates are the same as the flavour eigenstates. I understand that this is a good thing, because experimentally we don't observe oscillations between the electron muon and tauon. But I was hoping for some theoretical reason for why the mass eigenstates must be the same as the flavour eigenstates for the charged leptons. Otherwise, it just seems like a weird coincidence that the mixing matrix for the charged leptons is diagonal, while the neutrino mixing matrix is not diagonal.

I was thinking maybe it has something to do with the fact that the electron, muon and tauon all have charge (and maybe this somehow means their mixing matrix must be diagonal). But I can't think of why this would be true. Or maybe the mixing matrix for the charged leptons is not diagonal, but because the masses are so great, the effect of oscillations of the charged leptons is negligible anyway. (This is compared to the neutrinos, which have much smaller masses, and we can observe their oscillations between different flavours).

Anyway, thanks for reading, hopefully someone out there knows the answer :)