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

A bstract A measurement of the branching fraction for the decay $$ \Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu } $$ Λ → p μ − ν ¯ μ is presented using 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 fb − 1 . The result is obtained using Λ → pμ − decays as a normalisation channel. The measured branching fraction is $$ \mathcal{B}\left(\Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu}\right)=\left(1.462\pm 0.016\pm 0.100\pm 0.011\right)\times {10}^{-4} $$ B Λ → p μ − ν ¯ μ = 1.462 ± 0.016 ± 0.100 ± 0.011 × 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 → u quark transitions. It is consistent with previous measurements, and the extracted lepton flavour universality test observable, $$ {R}^{\mu e}=\frac{\Gamma \left(\Lambda \to p{\mu}^{-}{\overline{\nu}}_{\mu}\right)}{\Gamma \left(\Lambda \to p{e}^{-}{\overline{\nu}}_e\right)}=0.175\pm 0.012 $$ R μe = Γ Λ → p μ − ν ¯ μ Γ Λ → p e − ν ¯ e = 0.175 ± 0.012 , agrees with the Standard Model prediction.