Measurement of Branching Ratio forK+→e+ν

Using a $2\pi -\mathrm{s}\mathrm{r}$ magnetic spectrometer, the branching ratio for $K^{+}\to e^{+}\nu$ relative to that for $K^{+}\to {\mu }^{+}\nu$ has been measured to be (2.42 ± 0.42) × ${10}^{-5\mathrm{}}$, in good agreement with the predictions of the $V-A$ theory.