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

Abstract

The absolute branching fraction of $\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu }$ is reported for the first time based on an $e^{+}{e}^{-}$ annihilation sample of ten billion $J/\psi$ events collected with the BESIII detector at $\sqrt{s}=3.097$ GeV. The branching fraction is determined to be $\mathcal{B}(\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu })=[1.48\pm 0.21(\mathrm{stat})\pm 0.08(\mathrm{syst})]\times 10^{-4}$, which is a significant improvement in precision over the previous indirect measurements. Combining this result with the world average of $\mathcal{B}(\Lambda \to p{e}^{-}{\bar{\nu }}_e)$, we obtain the ratio, $\frac{\Gamma (\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu })}{\Gamma (\Lambda \to p{e}^{-}{\bar{\nu }}_e)}$, 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 $\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu }$ and $\bar{\Lambda }\to \bar{p}{\mu }^{+}{\nu }_{\mu }$ is also determined, and no evidence for $CP$ violation is found.

Comments: 8 pages, 2figures