Question

Why don't other massive particles change of flavor?

Answer 1

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:

να=∑iUαiνiνα=∑iUαiνi

αα for flavor eigenstates and ii for mass eigenstates. At a later time the state of the neutrino |ν(t)>|ν(t)> is given by a linear combination of the νiνi by their respective time evolution operator and so there is a probability to find then in different eigenstates at a later time by |<να|ν(t)>|2|<να|ν(t)>|2.