Measurement of the Branching Ratio for the DecayK+→e++νe

Abstract

The branching ratio for the ${K}_{e2}$ decay, ${K}^{+}\ensuremath{\rightarrow}{e}^{+}+{\ensuremath{\nu}}_{e}$, has been measured relative to the branching ratio for the ${K}_{\ensuremath{\mu}2}$ decay, ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}+{\ensuremath{\nu}}_{\ensuremath{\mu}}$. The measurement was made at the Rutherford High Energy Laboratory using spark-chamber techniques. Positive kaons from a 700-MeV/c separated beam stopped and decayed in the beryllium plates of a small spark chamber. Charged decay products were momentum-analyzed with a spectrometer system consisting of a bending magnet and four sonic spark chambers. Electrons were identified by a threshold gas \ifmmode \check{C}\else \v{C}\fi{}erenkov counter and by their range in a lead-aluminum spark chamber. Ten events were obtained in the ${K}_{e2}$ region of the electron momentum spectrum. We estimate a background of 0.34 events in this region caused by the ${K}_{\ensuremath{\mu}2}$ decay mode. We obtained a value for the ratio ${R}_{K}=\frac{\ensuremath{\Gamma}({{K}_{e2}}^{+})}{\ensuremath{\Gamma}({{K}_{\ensuremath{\mu}2}}^{+})}=({{1.9}_{\ensuremath{-}0.5}}^{+0.7})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. This is in good agreement with the prediction of the $V\ensuremath{-}A$ weak-interaction theory assuming muon-electron universality: ${R}_{K}=2.10\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. The predicted value includes a radiative correction of - 18.5% resulting from virtual-photon processes. From our measurement of ${R}_{K}$ we obtained an upper limit on the amount of pseudoscalar coupling contributing to the ${K}_{e2}$ decay. We find $\frac{|{f}^{P}|}{|{f}^{A}|}l2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$, where ${f}^{P}$ and $\frac{{f}^{A}}{{m}_{K}}$ are the pseudoscalar and axial-vector coupling constants.