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The first direct measurement of the absolute branching fraction of $\Sigma^+\to \Lambda e^+ \nu_{e}$ is reported based on an $e^+e^-$ annihilation sampleof $(10087\pm44) \times 10^6$ $J/\psi$ events collected with the BESIIIdetector at $\sqrt{s}=3.097$ GeV. The branching fraction is determined to be${\mathcal B}(\Sigma^+ \to \Lambda e^+ \nu_{e}) = [2.93\pm0.74(\rm stat) \pm0.13(\rm syst)]\times 10^{-5}$, which is the most precise measurement obtainedin a single experiment to date and also the first result obtained at a colliderexperiment. Combining this result with the world average of ${\mathcalB}(\Sigma^- \to \Lambda e^- \bar{\nu}_{e})$ 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 expectedin the absence of second-class currents that are forbidden in the StandardModel.

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