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

The absolute branching fraction of $\mathrm{\Lambda }\to p{\mu }^{-}{\bar{\nu }}_{\mu }$ is reported for the first time based on an $e^{+}{e}^{-}$ annihilation sample of ten billion $J\mathrm{/}\psi$ events collected with the BESIII detector at $\sqrt{s}=3.097$ GeV. The branching fraction is determined to be ${\mathcal B}(\Lambda \to p\mu^- \bar{\nu}{\mu}) = [1.48\pm0.21(\rm stat) \pm 0.08(\rm syst)]\times 10^{-4}$, which is a significant improvement in precision over the previous indirect measurements. Combining this result with the world average of $\mathcal{B}(\mathrm{\Lambda }\to p{e}^{-}{\bar{\nu }}_e)$, we obtain the ratio, $\frac{\Gamma(\Lambda \to p \mu^- \bar{\nu}{\mu})}{\Gamma(\Lambda \to p e^- \bar{\nu}{e})}$,tobe$0.178 \pm 0.028$, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of $\mathrm{\Lambda }\to p{\mu }^{-}{\bar{\nu }}_{\mu }$ and $\bar{\Lambda} \to \bar{p} \mu^+ \nu{\mu}$isalsodetermined,andnoevidencefor$CP$ violation is found.