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 Pμπξ, where Pμπ is the polarization of the muon in pion decay and ξ describes the intrinsic asymmetry in muon decay. We find Pμπξ=1.000 84±0.000 29(stat.)-0.00063+0.00165(syst.), in good agreement with the standard model prediction of Pμπ=ξ=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.