An Anatomy of Neutrino Oscillations
  We ponder about neutrino oscillations; a particle without a clear identity, a neutrino of a given flavor in fact does not satisfy the Dirac equation (which is used to define the mass eigen-states and mass eigen-values). This alters the basic treatment of neutrino oscillations, in that the Dirac spinors are defined as the mass eigen-states while the flavor states can only be given as linear combinations of the mass eigen-states (of Dirac equations). Even though neutrino masses are tiny, the impacts of several neutrino masses in a single reaction or a single decay, such as possible violation of the energy-momentum conservation, should not be overlooked.   Among those sources of oscillating neutrinos, we point out that the ultra high energy cosmic rays (UHECR's) such as the proton of energy $10^{18}\, eV$ or higher, used to think of being rather stable, can capture, in the matter media, an electron to convert into an electron-like neutrino and a spectator neutron. This would be the most important neutrino source of the UHECR origin. 
W-Y. Pauchy Hwang
hep-ph