Measurement of the Branching Ratio:R=[(π+→π0+e++νe)(π+→μ++νμ)]

Abstract

The branching ratio for pion beta decay [$R=\frac{({\pi }^{+}\to {\pi }^0+e^{+}+{\nu }_e)}{({\pi }^{+}\to {\mu }^{+}+{\nu }_{\mu })}$] has been measured with a system of spark chambers and scintillation counters. The high spatial resolution of the spark chambers was used to measure the position of the stopping pion as well as the angle between the nearly back-to-back gamma rays arising from the decay of the neutral pion. A total of 95×${10}^9$ positive pions was stopped in a central polystyrene-plate spark chamber. After measurement, computation, and background subtraction, the experiment yielded 35±3 pion beta-decay events. These were used with the over-all efficiency of the apparatus (about 3.3%) to compute a branching ratio $R=(1.10\mathrm{}\pm 0.26)\times {10}^{-8\mathrm{}}$. The result is in agreement with the predictions of the conserved-vector-current theory.