Neutrino mass constraints on $\mu$-decay and ${\pi }^0\to \nu \bar{\nu }$

Abstract

In this letter, we show that upper-limits on neutrino mass translate into upper-limits on the class of neutrino-matter interactions that can generate loop corrections to the neutrino mass matrix. We apply our results to $\mu$- and $\pi$-decays and derive model-independent limits on six of the ten parameters used to parametrize contributions to $\mu$-decay that do not belong to the standard model. These upper-limits provide improved constraints on the five Michel parameters, $\rho ,{\xi }^{\prime },{\xi }^{\prime \prime },\alpha ,{\alpha }^{\prime }$, that exceed PDG constraints by at least one order of magnitude. For ${\pi }^0\to \nu \bar{\nu }$ we find for the branching ratio: $B({\pi }^0\to \nu \bar{\nu })<10^{-10}$.

Comments: Accepted for publication in PRL