A bstract This paper describes a measurement of the b⁰ J/ 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/ Kₒ⁰ B 0 → J / ψ K S 0 decays, B ({b⁰ J/) }B ({B⁰ J/ Kₒ⁰) }=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 (b⁰ J/) = (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 b⁰ Λ b 0 baryon and B 0 meson hadronisation fractions. In addition, the isospin asymmetry between the rates of B⁰ J/ Kₒ⁰ B 0 → J / ψ K S 0 and B + → J/ψK + decays is measured to be A_{\
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