Branching fraction measurement of the Λ → pμ⁻ν¯_μ decay

A measurement of the branching fraction for the decay Λ → pμ⁻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 B(Λ → pμ⁻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^μe = (Γ(Λ → pμ⁻ν¯_μ)) / (Γ(Λ → pe⁻ν¯_e)) = 0.175 ± 0.012, agrees with the Standard Model prediction.