Precise measurement of parity violation in polarized muon decay

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.