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

TL;DR

This paper reports the first direct measurement of the absolute branching fraction of $oldsymbol{ ext{Sigma}^+ o ext{Lambda} e^+ u_e}$, providing the most precise value to date and testing Standard Model predictions regarding second-class currents.

Contribution

It presents the first direct measurement of this branching fraction at a collider and improves the precision of this fundamental parameter.

Findings

Branching fraction measured as (2.93 ± 0.74 (stat) ± 0.13 (syst)) × 10^{-5}

Ratio of decay widths consistent with Standard Model expectations

Most precise single-experiment measurement of this decay mode

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 $(10087\pm44) \times 10^6$ $J/\psi$ events collected with the BESIII detector at $\sqrt{s}=3.097$ GeV. The branching fraction is determined to be ${\mathcal B}(\Sigma^+ \to \Lambda e^+ \nu_{e}) = [2.93\pm0.74(\rm stat) \pm 0.13(\rm syst)]\times 10^{-5}$, 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.