Measurement of the Michel parameter rho in muon decay

The TWIST Collaboration has measured the Michel parameter rho in normal muon decay,u+ -> e+VeVu. In the standard model of particle physics,rho = 3/4. Deviations from this value require mixing of left- and right-handed muon and electron couplings in the muon-decay Lagrangian. We find rho = 0:75080+-0:00032(stat.)+-0:00097(syst.) +- 0:00023, where the last uncertainty represents the dependence of rho on the Michel parameter n. This result sets new limits on the WL - WR mixing angle in left-right symmetric models.