Muon decay asymmetry and the standard model

The asymmetry of the positron in polarised muon decay provides confirmation that the weak interaction maximally violates parity. Since 1957, the quantity $P_\mu \xi$ has been measured with increasing precision, where $P_\mu$ is the polarisation of the muon, and $\xi$ is a parameter describing the asymmetry. So far, the results have been consistent with the standard model's $(V-A)$ interaction. The TRIUMF Weak Interaction Symmetry Test (TWIST) is close to completing a measurement of $P_\mu \xi$ at the level of $0.1\%$. An order of magnitude improvement over pre-TWIST results is anticipated. The experiment uses highly polarised muons from pion decay, and is limited in precision by a knowledge of how much the muons depolarise before decaying. At this level, the experiment is sensitive to physics beyond the standard model, such as the weak decay of right-handed muons, and left-right symmetric models where a $(V+A)$ current is introduced to conserve parity at higher energies. The physics motivation and experiment will be described, including the methods that have reduced the dominant systematic uncertainties.