On the derivation of the neutrino oscillation length formula

Abstract

Forsaking the traditionnal hand-waving in the treatment of the motion, we show that the ultra-relativistic approximation and the equality of kinematical variables are unnecessary ingredients in the derivation of the oscillation length using plane waves, at least in a two flavor world. It ensues that the formula is valid as it is in the non relativistic regime, provided one uses the correct variable which is found to be momentum, not energy, and that the precise production kinematics is irrelevant. Consequences for the more complete treatments are briefly evoked.