Measurement of the muon decay parameter<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>δ</mml:mi></mml:math>

The muon decay parameter $\ensuremath{\delta}$ has been measured by the TWIST collaboration. We find $\ensuremath{\delta}=0.74964\ifmmode\pm\else\textpm\fi{}0.00066(\mathrm{\text{stat}}.)\ifmmode\pm\else\textpm\fi{}0.00112(\mathrm{\text{syst}}.)$, consistent with the standard model value of $3/4$. This result implies that the product ${P}_{\ensuremath{\mu}}\ensuremath{\xi}$ of the muon polarization in pion decay, ${P}_{\ensuremath{\mu}}$, and the muon decay parameter $\ensuremath{\xi}$ falls within the 90% confidence interval $0.9960&lt;{P}_{\ensuremath{\mu}}\ensuremath{\xi}\ensuremath{\le}\ensuremath{\xi}&lt;1.0040$. It also has implications for left-right-symmetric and other extensions of the standard model.