Measurement of the branching fraction of the $\Lambda_b^0\to J/\psi\Lambda$ decay and isospin asymmetry of $B\to J/\psi K$ decays

Abstract

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$\,\text{fb}^{-1}$. The branching fraction is determined relative to that of $B^0\to J/\psi K^0_\text{S}$ decays, $\frac{\mathcal{B}(\Lambda_b^0\to J/\psi\Lambda)}{\mathcal{B}(B^0\to J/\psi K^0_\text{S}} = 0.750 \pm 0.005 \pm 0.022 \pm 0.005 \pm 0.062\,,$ yielding $\mathcal{B}(\Lambda_b^0\to J/\psi\Lambda) = (3.34 \pm 0.02 \pm 0.10 \pm 0.08 \pm 0.28)\times 10^{-4}$, 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 $B^0\to J/\psi K^0_\text{S}$ and $B^+\to J/\psi K^+$ decays is measured to be $A_{\rm I} = -0.0135 \pm 0.0004 \pm 0.0133$, where the first uncertainty is statistical and the second systematic.