First direct measurement of the absolute branching fraction of Σ+ → Λe+υe

The first direct measurement of the absolute branching fraction of Σ+ →Λ⁢𝑒+⁢𝜈𝑒 is reported based on an 𝑒+⁢𝑒− annihilation sample of (10087±44)×106   𝐽/𝜓 events collected with the BESIII detector at √𝑠 =3.097  GeV. The branching fraction is determined to be ℬ⁡(Σ+ →Λ⁢𝑒+⁢𝜈𝑒) =[2.93 ± 0.74⁢(stat) ±0.13⁢(syst)] ×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⁡(Σ− →Λ⁢𝑒−⁢¯𝜈𝑒) and the lifetimes of Σ±, the ratio, Γ⁡(Σ−→Λ⁢𝑒−⁢¯𝜈𝑒)/Γ⁡(Σ+→Λ⁢𝑒+⁢𝜈𝑒), 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.