Measurement of the branching fraction of the Λ⁰_b → J/ψΛ decay and isospin asymmetry of B →J/ψK decays

This paper describes a measurement of the Λ⁰_b → J/ψΛ 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/ψK⁰_S decays,

B(Λ⁰_b → J/ψΛ) / B(B⁰ → J/ψK⁰_S) = 0.750 ± 0.005 ± 0.022 ± 0.005 ± 0.062,

yielding B(Λ⁰_b → J/ψΛ) = (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 Λ⁰_b baryon and B⁰ meson hadronisation fractions. In addition, the isospin asymmetry between the rates of B⁰ → J/ψK⁰_S and B⁺ → J/ψK⁺ decays is measured to be

A_I = −0.0135 ± 0.0004 ± 0.0133,

where the first uncertainty is statistical and the second systematic.