Measurement of the Michel Parameter $\ddot{I}$ in Muon Decay

Abstract

The TWIST Collaboration has measured the Michel parameter $\ddot{I}$ in normal muon decay, $\hat{I}{¼}^{+}\to e^{+}\hat{I}{½}_e\bar{\hat{I}}{½}_{\hat{I}}¼$. In the Standard Model, $\ddot{I}$ = 3/4. Deviations from this value require mixing of left- and right-handed muon and electron couplings in the muon-decay Lagrangian. We find $\ddot{I}$ = 0.75080 $\pm$ 0.00044(stat.) $\pm$ 0.00093(syst.) $\pm$ 0.00023, where the last uncertainty represents the dependence of $\ddot{I}$ on the Michel parameter $\hat{I}\cdot$. This result sets new limits on the $W_L-W_R$ mixing angle in left-right symmetric models.

4 pages, 3 figures, submitted to PRL