Precise measurement of parity violation in polarized muon decay

TL;DR

This paper reports a highly precise measurement of parity violation in polarized muon decay, confirming the Standard Model predictions and constraining new physics models with improved accuracy.

Contribution

The TWIST collaboration achieved a sevenfold increase in precision for measuring muon decay asymmetry parameters, providing tighter limits on physics beyond the Standard Model.

Findings

Measured $P_^ $ with unprecedented precision

Results agree with Standard Model predictions

Set new constraints on left-right symmetric models

Abstract

We present a new high precision measurement of parity violation in the weak interaction, using polarized muon decay. The TWIST collaboration has measured $P_\mu^\pi \xi$, where $P_\mu^\pi$ is the polarization of the muon in pion decay and $\xi$ describes the intrinsic asymmetry in muon decay. We find $P_\mu^\pi \xi = 1.00084 \pm 0.00029\,(\textrm{stat.})_{-0.00063}^{+0.00165}\,(\textrm{syst.})$, in good agreement with the standard model prediction of $P_\mu^\pi=\xi=1$. Our result is a factor of 7 more precise than the pre-TWIST value, setting new limits in left-right symmetric electroweak extensions to the standard model.