New method for studying neutrino mixing and mass differences
  Neutrino masses and mixing can be investigated by studying the behavior of a radioactive bare nucleus which decays by emitting an electron into the open atomic K shell BEFORE and DURING its weak decay by neutrino emission. The initial nuclear state has a momentum spread required by Heisenberg. The wave packet contains pairs of components with different momenta which can produce neutrinos in two mass eigenstates with exactly the same energy and different momenta. These neutrino amplitudes mix to produce a single electron-neutrino state with the same energy. Since there is no information on which mass eigenstates produced the neutrino this is a typical quantum mechanics "two-slit" or "which path" experiment. A transition between the same initial and a final states can go via two paths with a phase difference producing interference and oscillations. Here the two paths are in momentum space   A new oscillation phenomenon providing information about neutrino mixing is obtained by following the nucleus before and during the decay. The analysis starts with Stodolsky's proof that interference between states having different energies cannot be observed in realistic experiments. Results then follow from simple rigorous quantum mechanics without the hand waving and loopholes which have confused many previous neutrino oscillation investigations. 
Harry J. Lipkin