Question

Why don't other massive particles change of flavor?

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.

Answer 2

Plug in values in the neutrino oscillation formulaexcept now use ultrarelativistic charged leptons, say e and μ. In natural units, do you see that L ≈ π E / Δm² ? For a μ with mass of 0.1 GeV and an e of relatively negligible mass, and an E of hundreds of GeVs,.