Measurement of the Michel Parameterρin Muon Decay

The TWIST Collaboration has measured the Michel parameter ρ in normal muon decay, μ⁺→e⁺νₑν_μ. In the standard model, ρ=3/4. Deviations from this value imply mixing of left- and right-handed muon and electron couplings. We find ρ=0.750 80±0.000 32(stat)±0.000 97(syst)±0.000 23, where the last uncertainty represents the dependence of ρ on the Michel parameter η. This result sets new limits on the WL-WR mixing angle in left-right symmetric models.