Measurement of the branching fraction of the Λb⁰→ J/ψ Λ decay and isospin asymmetry of B → J/ψK decays

A bstract This paper describes a measurement of the Λb⁰→ J/ψ Λ Λ b 0 → J / ψ Λ branching fraction using data collected with the LHCb experiment in proton-proton collisions from 2016 to 2018. The dataset corresponds to an integrated luminosity of 5 . 4 fb − 1 . The branching fraction is determined relative to that of B⁰→ J/ψ KS⁰ B 0 → J / ψ K S 0 decays, B(Λb⁰→ J/ψ Λ )B(B⁰→ J/ψ KS⁰)=0.750± 0.005± 0.022± 0.005± 0.062, B Λ b 0 → J / ψ Λ B B 0 → J / ψ K S 0 = 0.750 ± 0.005 ± 0.022 ± 0.005 ± 0.062 , yielding B(Λb⁰→ J/ψ Λ )=(3.34± 0.02± 0.10± 0.08± 0.28)× 10⁻⁴ B Λ b 0 → J / ψ Λ = 3.34 ± 0.02 ± 0.10 ± 0.08 ± 0.28 × 10 − 4 , where the first uncertainty is statistical, the second systematic, the third due to external inputs on branching fractions and the fourth due to the ratio of Λb⁰ Λ b 0 baryon and B 0 meson hadronisation fractions. In addition, the isospin asymmetry between the rates of B⁰→ J/ψ KS⁰ B 0 → J / ψ K S 0 and B + → J/ψK + decays is measured to be $$ {A}_{\