Branching Ratio of the Electronic Mode of Positive Pion Decay

A new measurement of the branching ratio (π⁺→e⁺+ν)(π⁺→μ⁺+ν) has been completed. A double-focussing magnetic spectrometer was used to observe the spectra of electrons emitted in π decay and in μ 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 π-e events recorded in this experiment was 1346, of which 6% were accidentals and 5% were π-μ-e contamination. The branching ratio obtained from an analysis of the data over the π-e and μ-e distributions and corrected to include all decay electrons was (1.21±{}0.07)×{}10^-4. 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. Our data also gave for the mean life of π decay τ_π=(25.6±0.8)×10^-9 second.