First direct measurement of the absolute branching fraction of ${\Sigma }^{+}\to \Lambda {e}^{+}{\nu }_{e}$

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 $\left(10087±44\right)×{10}^{6}\text{}\text{}$ $J/\psi $ events collected with the BESIII detector at $\sqrt{s}=3.097\text{}\text{}\mathrm{GeV}$. The branching fraction is determined to be $B\left({\Sigma }^{+}\to \Lambda {e}^{+}{\nu }_{e}\right)=\left[2.93±\phantom{\rule{0ex}{0ex}}0.74\left(\mathrm{stat}\right)±0.13\left(\mathrm{syst}\right)\right]×{10}^{-5}$, 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 $B\left({\Sigma }^{-}\to \Lambda {e}^{-}{\overline{\nu }}_{e}\right)$ and the lifetimes of ${\Sigma }^{±}$, the ratio, $\frac{\Gamma \left({\Sigma }^{-}\to \Lambda {e}^{-}{\overline{\nu }}_{e}\right)}{\Gamma \left({\Sigma }^{+}\to \Lambda {e}^{+}{\nu }_{e}\right)}$, is determined to be $1.06±0.28$, which is within 1.8 standard deviations of the value expected in the absence of second-class currents.