  A theoretical structure that involves neutrino mass eigenstates at non
relativistic as well as relativistic energies is presented. Using this
framework, we find that if the particle X, with mass 33.9 MeV, of the KARMEN
collaboration anomaly is identified with the third neutrino mass eigenstate,
then the present limit of 23 MeV upper bound on the tau neutrino mass implies
|U_{\tau 3}| < 0.82.
