The absolute branching fraction of $\mathrm{\Lambda }\to p{\mu }^{-}{\overline{\nu }}_{\mu }$ is reported for the first time based on an $e^{+}{e}^{-}$ annihilation sample of $10\times {10}^9\mathrm{J}/\psi$ events collected with the BESIII detector at $\sqrt{s}=3.097\mathrm{GeV}$. The branching fraction is determined to be $\mathcal{B}(\mathrm{\Lambda }\to p{\mu }^{-}{\overline{\nu }}_{\mu })=[1.48\pm 0.21(\mathrm{stat})\pm 0.08(\mathrm{syst})]\times {10}^{-4}$, which is improved by about 30% in precision over the previous indirect measurements. Combining this result with the world average of $\mathcal{B}(\mathrm{\Lambda }\to p{e}^{-}{\overline{\nu }}_e)$, we obtain the ratio $\{[\mathrm{\Gamma }(\mathrm{\Lambda }\to p{\mu }^{-}{\overline{\nu }}_{\mu })]/[\mathrm{\Gamma }(\mathrm{\Lambda }\to p{e}^{-}{\overline{\nu }}_e)]\}$ to be $0.178\pm 0.028$, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of $\mathrm{\Lambda }\to p{\mu }^{-}{\overline{\nu }}_{\mu }$ and $\overline{\mathrm{\Lambda }}\to \overline{p}{\mu }^{+}{\nu }_{\mu }$ is also determined, and no evidence for $CP$ violation is found.

• Received 15 July 2021
• Accepted 16 August 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.121802

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Published by the American Physical Society