Measurement of the rate for pion beta decay

The conserved-vector-current hypothesis, together with measured nuclear beta-decay rates, predicts a value of the rate for the decay ${\pi }^{+}$→${\pi }^0$ $e^{+}$ν of 0.4027±0.0018 ${\mathrm{s}}^{\mathrm{-}1}$. Using a decay-in-flight technique we have made the most precise measurement to date of this rate, obtaining the value 0.394±0.015 ${\mathrm{s}}^{\mathrm{-}1}$, in good agreement with the prediction. This differs slightly from the value previously reported in W. K. McFarlane et al., Phys. Rev. Lett. 51, 249 (1983).