  An evanescent wave is a non-propagating wave with an imaginary wave vector.
In this study, we prove that these are solutions of the tachyon-like Klein
Gordon equation, and that in the tunneling of ultrarelativistic half integer
spin particles they describe superluminal states arising from interactions
between a particle and barrier. These states decay as a particle emerges from
the opposite side of a potential barrier, conserving the same initial energy
but not necessarily the same mass. The obtained theory is applied to the
neutrino, to explain flavor oscillations during free flight and determine the
conditions that maximize the probability of their occurrence.
