Measurement of the branching fraction of the $ {\Lambda}_b^0\to J/\psi \Lambda $ decay and isospin asymmetry of B → J/ψK decays

This paper describes a measurement of the ${\Lambda}_b^0\to J/\psi \Lambda$ 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 ${B}^0\to J/\psi {K}_{\textrm{S}}^0$ decays, $\frac{\mathcal{B}\left({\Lambda}_b^0\to J/\psi \Lambda \right)}{\mathcal{B}\left({B}^0\to J/\psi {K}_{\textrm{S}}^0\right)}=0.750\pm 0.005\pm 0.022\pm 0.005\pm 0.062,$ 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 $\mathcal{B}\left({\Lambda}_b^0\to J/\psi \Lambda \right)=\left(3.34\pm 0.02\pm 0.10\pm 0.08\pm 0.28\right)\times {10}^{-4}$ , 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 ${\Lambda}_b^0$ baryon and B 0 meson hadronisation fractions. In addition, the isospin asymmetry between the rates of ${B}^0\to J/\psi {K}_{\textrm{S}}^0$ and B + → J/ψK + decays is measured to be ${A}_{\textrm{I}}=-0.0135\pm 0.0004\pm 0.0133,$ A I = − 0.0135 ± 0.0004 ± 0.0133 , where the first uncertainty is statistical and the second systematic.