Precise measurement of parity violation in polarized muon decay

Understand

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.