  We derive model-independent constraints on chirality-changing terms in the
muon decay Lagrangian using limits on neutrino mass. We consider all
dimension-six operators invariant under the gauge symmetry of the Standard
Model which contribute to either a Dirac neutrino mass or muon decay. Taking an
upper limit on neutrino mass of 1 eV, we derive limits on the contributions of
chirality-changing operators to the Michel parameters four orders of magnitude
tighter than the current experimental constraints. We also identify two
operators which, due to their flavor structure, are not constrained by neutrino
mass. If near-future experiments find contributions to muon decay from these
operators, it could indicate interesting flavor structure in physics beyond the
SM.
