Why don't other massive particles change of flavor?

Question

I have seen that the argument of why neutrino oscillations exist is that their mass matrix is nondiagonal and complex and so one needs to transform it into diagonal by unitary rotations:$$\nu_\alpha=\sum_i U_{\alpha i}\nu_i$$ $\alpha $ for flavor eigenstates and $i$ for mass eigenstates. At a later time the state of the neutrino $|\nu(t)>$ is given by a linear combination of the $\nu_i$ by their respective time evolution operator and so there is a probability to find then in different eigenstates at a later time by $|<\nu_\alpha|\nu(t)>|^2$.

Why not using the same argument to say that leptons or quarks change of flavor as they propagate? I don't know much of this, any wisdom on the subject would be appreciated.

Answer 1

Flavor is defined by the eigenstates of the lepton mass matrix. Neutrino oscillations happen not just because their mass matrix is not diagonal, but because their mass matrix is not diagonal when the lepton mass matrix is.

The way you see neutrino oscillation is by generating neutrinos from leptons of a specific mass, and then seeing if they interact with leptons of a different mass after traveling some distance. If neutrinos generated from $511$ keV leptons always interact only with $511$ keV leptons then there is no mass oscillation.

We can imagine reversing the experiment. Use neutrinos of a specific mass to generate leptons, let the leptons travel, and then measure their interactions. If we could do this, we would see "lepton flavor oscillation." The only difference is that we can't isolate neutrinos of specific mass because they are so hard to interact with and manipulate.