Precision muon decay measurements and improved constraints on the weak interaction

Abstract

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=0.74977±0. 00012(stat)±0.00023(syst) and δ=0.75049±0.00021(stat) ±0.00027(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. © 2012 American Physical Society.