Neutrino Mass Constraints onμDecay andπ0→νν¯

In this Letter, we show that upper limits on the 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 Particle Data Group constraints by at least one order of magnitude. For ${\pi }^0\to \nu \overline{\nu }$ we find, for the branching ratio, $B({\pi }^0\to \nu \overline{\nu })<{10}^{-10}$.