Precision muon decay measurements and improved constraints on the weak interaction

TL;DR

The TWIST Collaboration measured muon decay parameters  ho and  delta with unprecedented precision, confirming Standard Model predictions and tightening constraints on potential right-handed weak interactions.

Contribution

This work provides the most precise measurements of  ho and  delta, and updates global constraints on weak interactions involving right-handed particles.

Findings

Measured  ho and  delta with over 10 times improved precision.

Results are consistent with Standard Model predictions.

Enhanced constraints on right-handed weak interactions.

Abstract

The TWIST Collaboration has completed its measurement of the three muon decay parameters \rho, \delta, and P_\mu\xi. This paper describes our determination of \rho, which governs the shape of the overall momentum spectrum, and \delta, which controls the momentum dependence of the parity-violating decay asymmetry. The results are \rho=0.74977\pm 0.00012(stat.)\pm 0.00023(syst.) and \delta = 0.75049\pm 0.00021(stat.)\pm 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 \rho, \delta, and P_\mu\xi 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.