Quantum mixing (or particle mixing) is a quantum-mechanical phenomena in which one elementary particle changes into another, in a weak interaction. I suspect it is considered different than decaying if the changes form a loop, i.e., eventually, it returns back to the same particle, a type of oscillation. Quarks are considered to undergo this phenomena (CKM mixing). A mixing of leptons is behind neutrino oscillation (the Mikheyev-Smirnov-Wolfenstein effect is a phenomenon that modifies this oscillation, which applies to energetic solar neutrinos). In supersymmetric models, neutralinos are the result of quantum mixing, in this case, treating the resulting "loop" as a single type of particle.