Branching fraction measurement of the $\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu }$ decay

A measurement of the branching fraction for the decay $\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu }$ is presented using $\mathit{\text{pp}}$ collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV. The analysis is based on data recorded between 2016 and 2018, corresponding to an integrated luminosity of $5.4{\text{fb}}^{-1}$. The result is obtained using $\Lambda \to p{\pi }^{-}$ decays as a normalisation channel. The measured branching fraction is $B(\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu })=(1.462\pm 0.016\pm 0.100\pm 0.011)\times {10}^{-4}$, where the uncertainties are statistical, systematic, and due to the limited knowledge of the normalisation mode branching fraction, respectively. This result improves the precision of the branching fraction measurement by a factor of two compared to the previous best measurement and sets a more stringent bound on lepton flavour universality in $s\to u$ quark transitions. It is consistent with previous measurements, and the extracted lepton flavour universality test observable, $R^{\mu e}=\frac{\mathrm{\Gamma }(\Lambda \to p{\mu }^{-}{\bar{\nu }}_{\mu })}{\mathrm{\Gamma }(\Lambda \to p{e}^{-}{\bar{\nu }}_e)}=0.175\pm 0.012$, agrees with the Standard Model prediction.