First Direct Measurement of the Absolute Branching Fraction of ${\Sigma }^{+}\to \Lambda e^{+}{\nu }_e$

Abstract

The first direct measurement of the absolute branching fraction of ${\Sigma }^{+}\to \Lambda e^{+}{\nu }_e$ is reported based on an $e^{+}{e}^{-}$ annihilation sample of $(10087\pm 44)\times 10^6$ $J/\psi$ events collected with the BESIII detector at $\sqrt{s}=3.097$ GeV. The branching fraction is determined to be $\mathcal{B}({\Sigma }^{+}\to \Lambda e^{+}{\nu }_e)=[2.93\pm 0.74(\mathrm{stat})\pm 0.13(\mathrm{syst})]\times 10^{-5}$, which is the most precise measurement obtained in a single experiment to date and also the first result obtained at a collider experiment. Combining this result with the world average of $\mathcal{B}({\Sigma }^{-}\to \Lambda e^{-}{\bar{\nu }}_e)$ and the lifetimes of ${\Sigma }^{\pm }$, the ratio, $\frac{\Gamma ({\Sigma }^{-}\to \Lambda e^{-}{\bar{\nu }}_e)}{\Gamma ({\Sigma }^{+}\to \Lambda e^{+}{\nu }_e)}$, is determined to be $1.06\pm 0.28$, which is within 1.8 standard deviations of the value expected in the absence of second-class currents that are forbidden in the Standard Model.

Comments: 8 pages, 3 figures