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

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