Branching Ratio of the Electronic Mode of Positive Pion Decay

A new measurement of the branching ratio $\frac{({\pi }^{+}\to e^{+}+\nu )}{({\pi }^{+}\to {\mu }^{+}+\nu )}$ has been completed. A double-focussing magnetic spectrometer was used to observe the spectra of electrons emitted in $\pi$ decay and in $\mu$ decay. The scintillation pulses from the pion and its decay electron were recorded on a travelling-wave oscilloscope. Timing and pulse-height measurements were used to distinguish good events from accidentals. The total number of $\pi -e$ events recorded in this experiment was 1346, of which 6% were accidentals and 5% were $\pi -\mu -e$ contamination. The branching ratio obtained from an analysis of the data over the $\pi -e$ and $\mu -e$ distributions and corrected to include all decay electrons was (1.21±0.07)×${10}^{-4\mathrm{}}$. This is close to the result expected for a universal $V-A$ interaction. Kinoshita's calculation, taking into account radiative effects, gave 1.23×${10}^{-4\mathrm{}}$. Our data also gave for the mean life of $\pi$ decay ${\tau }_{\pi }=(25.6\mathrm{}\pm 0.8)\times {10}^{-9\mathrm{}}$ second.