Observation of the ${\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}{K}^{+}$ and ${\mathrm{\Xi }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}{\pi }^{+}$ decays

The first observation of the ${\mathrm{\Xi }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}{\pi }^{+}$ decay and the most precise measurement of the branching fraction of the ${\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}{K}^{+}$ decay are reported, using proton-proton collision data from the LHCb experiment collected in 2016–2018 at a centre-of-mass energy of 13~TeV, corresponding to an integrated luminosity of 5.4~fb${}^{-1}$. Using the ${\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi \mathrm{\Lambda }$ and ${\mathrm{\Xi }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}$ decays as normalisation channels, the ratios of branching fractions are measured to be: $$\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}{K}^{+})}{\mathcal{B}({\mathrm{\Lambda }}_b^0\to J\mathrm{/}\psi \mathrm{\Lambda })}=(1.17\pm 0.14\pm 0.08)\times {10}^{-2},$$ $$\frac{\mathcal{B}({\mathrm{\Xi }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-}{\pi }^{+})}{\mathcal{B}({\mathrm{\Xi }}_b^0\to J\mathrm{/}\psi {\mathrm{\Xi }}^{-})}=(11.9\pm 1.4\pm 0.6)\times {10}^{-2},$$ where the first uncertainty is statistical and the second systematic.