Measurement of the Michel Parameter $ρ$ in Muon Decay

Abstract

The TWIST Collaboration has measured the Michel parameter $ρ$ in normal muon decay, $μ^+ \to e^+ ν_e \barν_μ$. In the Standard Model, $ρ$ = 3/4. Deviations from this value require mixing of left- and right-handed muon and electron couplings in the muon-decay Lagrangian. We find $ρ$ = 0.75080 $\pm$ 0.00044(stat.) $\pm$ 0.00093(syst.) $\pm$ 0.00023, where the last uncertainty represents the dependence of $ρ$ on the Michel parameter $η$. This result sets new limits on the $W_L-W_R$ mixing angle in left-right symmetric models.