Measurements of the Rates of the Decayπ+→π0+e++νand ofπ−Capture in LiH, CH2, and CH

Abstract

We have measured the branching ratio for ${\pi }^{+}\to {\pi }^0+e^{+}+\nu$. Our method exploited the near anticollinearity of the two $\gamma$ rays resulting from the subsequent decay of the ${\pi }^0$. The detection of these $\gamma$ rays using spark chambers, together with the identification of the positron in a scintillation target detector, enabled us to identify the pion beta-decay process in a manner which was effectively free from background. On the basis of 39 events, of which 3 are attributed to background, and an independent calibration of the over-all efficiency of the apparatus, we deduce for the branching ratio (0.97±0.020)×${10}^{-8\mathrm{}}$, in agreement with previous measurements and the prediction of the conserved-vector-current hypothesis. Using the same apparatus, we have also measured $W$, the fraction of stopped ${\pi }^{-}$ mesons captured by hydrogen nuclei bound in chemical compounds. We find $W_{\mathrm{LiH}}=(40\mathrm{}\pm 4)\times {10}^{-3\mathrm{}}$, $W_{\mathrm{C}\mathrm{H}2}=(17.9\mathrm{}\pm 1.9)\times {10}^{-3\mathrm{}}$, and $W_{\mathrm{CH}}=(5.5\mathrm{}\pm 0.7)\times {10}^{-3\mathrm{}}$. Negative pion capture in ${\mathrm{H}}_2$-${\mathrm{N}}_2$ and ${\mathrm{H}}_2$-He gas mixtures has also been briefly studied.