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

This paper describes a measurement of the Λb0 → 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/ψKS0 decays, B(Λb0 → J/ψΛ) / B(B0 → J/ψKS0) = 0.750 ± 0.005 ± 0.022 ± 0.005 ± 0.062, yielding B(Λb0 → 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 Λb0 baryon and B0 meson hadronisation fractions. In addition, the isospin asymmetry between the rates of B0 → J/ψKS0 and B+ → J/ψK+ decays is measured to be AI = −0.0135 ± 0.0004 ± 0.0133, where the first uncertainty is statistical and the second systematic.