Branching fraction measurement of the Λ→pμ−ν‾μΛ→pμ−νμ​ decay

A measurement of the branching fraction for the decay Λ→pμ−ν‾μΛ→pμ−νμ​ 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−1−1. 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)×10−4B(Λ→pμ−νμ​)=(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, Rμe=Γ(Λ→pμ−ν‾μ)Γ(Λ→pe−ν‾e)=0.175±0.012Rμe=Γ(Λ→pe−νe​)Γ(Λ→pμ−νμ​)​=0.175±0.012, agrees with the Standard Model prediction.