Asymmetric Collision of Concepts: Why Eigenstates Alone are Not Enough   for Neutrino Flavor Oscillations
  The symmetry of the problem of the apparent deficit in upward-going atmospheric muon neutrinos reveals two possible, nonexclusive kinds of solution: Nonlinearity in distance or nonlinearity in angle of observation.   Nonlinearity in distance leads to the most popular theory for the atmospheric problem, neutrino flavor oscillations. If the observed deficit is caused by oscillations and not, say, flavor-changing or other weak-force scattering, neutrinos must be massive. But, if flavor oscillations occur in vacuum, all oscillating neutrinos must have exactly equal mass. Theories of oscillation in matter such as the Mikheyev-Smirnov-Wolfenstein (MSW) effect do not work in vacuum. This is the conceptual conflict of kinematics versus vacuum oscillations. Flavor-changing oscillations like those of the Cabibbo-Kobayashi-Maskawa (CKM) quark theory become possible in vacuum if freely propagating neutrinos may be associated with local substructure.   Nonlinearity in angle of observation leads to a simple prediction of an excess of horizontal muon flavor. This and other angle-based effects should be observable at Super-Kamiokande or other instruments which can measure atmospheric flux by flavor. 
John Michael Williams