A Precision Measurement of the Muon Decay Parameters $\ddot{I}$ and $\hat{I}´$

Abstract

The TWIST collaboration has performed new measurements of two of the parameters that describe muon decay: $\ddot{I}$, which governs the shape of the overall momentum spectrum, and $\hat{I}´$, which governs the momentum dependence of the parity-violating decay asymmetry. This analysis gives the results $\ddot{I}=0.75014\pm 0.00017(\text{stat})\pm 0.00044(\text{syst})\pm 0.00011(\hat{I}\cdot )$, where the last uncertainty arises from the correlation between $\ddot{I}$ and the decay parameter $\hat{I}\cdot$, and $\hat{I}´=0.75067\pm 0.00030(\text{stat})\pm 0.00067(\text{syst})$. These are consistent with the value of 3/4 given for both parameters in the Standard Model of particle physics, and are a factor of two more precise than the measurements previously published by TWIST. A new global analysis of all available muon decay data incorporating these results is presented. Improved lower and upper limits on the decay parameter $P_{\hat{I}}{¼}^{\ddot{I}}€\hat{I}¾$ of $0.99524<P_{\hat{I}}{¼}^{\ddot{I}}€\hat{I}¾\le \hat{I}¾<1.00091$ at 90% confidence are determined, where $P_{\hat{I}}{¼}^{\ddot{I}}€$ is the polarization of the muon when it is created during pion decay, and $\hat{I}¾$ governs the muon decay asymmetry. These results set new model-independent constraints on the possible weak interactions of right-handed particles. Specific implications for left-right symmetric models are discussed.

14 pages, 14 figs, accepted for publication in Phys. Rev. D