Branching fraction measurement of the p^- decay

A bstract A measurement of the branching fraction for the decay p^- 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 B (p^-_) = (1. 462 0. 016 0. 100 0. 011) 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^ e= (p^{-_) } (p{e^-ₑ) }=0. 175 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.