Global analysis of muon decay measurements

We have performed a global analysis of muon decay measurements to establish model-independent limits on the space-time structure of the muon decay matrix element. We find limits on the scalar, vector, and tensor coupling of right- and left-handed muons to right- and left-handed electrons. The limits on those terms that involve the decay of right-handed muons to left-handed electrons are more restrictive than in previous global analyses, while the limits on the other nonstandard model interactions are comparable. The value of the Michel parameter $\eta$ found in the global analysis is $-0.0036\pm 0.0069$, slightly more precise than the value found in a more restrictive analysis of a recent measurement. This has implications for the Fermi coupling constant $G_F$.