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.