Precision muon decay measurements and improved constraints on the weak interaction

The TWIST Collaboration has completed its measurement of the three muon decay parameters ρ, δ, and P_μξ. This paper describes our determination of ρ, which governs the shape of the overall momentum spectrum, and δ, which controls the momentum dependence of the parity-violating decay asymmetry. The results are ρ=0.749 77±0.000 12(stat)±0.000 23(syst) and δ=0.750 49±0.000 21(stat)±0.000 27(syst). These are consistent with the value of $3/4$ given for both parameters in the standard model, and each is over a factor of 10 more precise than the measurements published prior to TWIST. Our final results on ρ, δ, and P_μξ have been incorporated into a new global analysis of all available muon decay data, resulting in improved model-independent constraints on the possible weak interactions of right-handed particles.