First Measurement of the Absolute Branching Fraction of $\Lambda \to p{\mu }^{-}{\overline{\nu }}_{\mu }$

The absolute branching fraction of $\Lambda \to p{\mu }^{-}{\overline{\nu }}_{\mu }$ is reported for the first time based on an ${e}^{+}{e}^{-}$ annihilation sample of $10×{10}^{9}\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 $B\left(\Lambda \to p{\mu }^{-}{\overline{\nu }}_{\mu }\right)=\left[1.48±0.21\left(\mathrm{stat}\right)±0.08\left(\mathrm{syst}\right)\right]×{10}^{-4}$, which is improved by about 30% in precision over the previous indirect measurements. Combining this result with the world average of $B\left(\Lambda \to p{e}^{-}{\overline{\nu }}_{e}\right)$, we obtain the ratio $\left\{\left[\Gamma \left(\Lambda \to p{\mu }^{-}{\overline{\nu }}_{\mu }\right)\right]/\left[\Gamma \left(\Lambda \to p{e}^{-}{\overline{\nu }}_{e}\right)\right]\right\}$ to be $0.178±0.028$, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of $\Lambda \to p{\mu }^{-}{\overline{\nu }}_{\mu }$ and $\overline{\Lambda }\to \overline{p}{\mu }^{+}{\nu }_{\mu }$ is also determined, and no evidence for $CP$ violation is found.