Precision measurement of the muon decay parameters ρ and δ

The TWIST Collaboration has performed new measurements of two of the parameters that describe muon decay: ρ, which governs the shape of the overall momentum spectrum, and δ, which governs the momentum dependence of the parity-violating decay asymmetry. This analysis gives the results ρ=0.750 14±0.000 17(stat)±0.000 44(syst)±0.000 11(η), where the last uncertainty arises from the correlation between ρ and the decay parameter η, and δ=0.750 67±0.000 30(stat)±0.000 67(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_μ^πξ of 0.995 24<P_μ^πξ≤ξ<1.000 91 at 90% confidence are determined, where P_μ^π is the polarization of the muon when it is created during pion decay, and ξ 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.