Neutrino mass constraints on $μ$-decay and $π^0\to ν\barν$

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 $μ$- and $π$-decays and derive model-independent limits on six of the ten parameters used to parametrize contributions to $μ$-decay that do not belong to the standard model. These upper-limits provide improved constraints on the five Michel parameters, $ρ,ξ^\prime,ξ^{\prime\prime},α,α^\prime$, that exceed PDG constraints by at least one order of magnitude. For $π^0\to ν\barν$ we find for the branching ratio: $B(π^0\toν\barν) < 10^{-10}$.

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