Observation of ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{D}_s^{-}$ and ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}_c^0{D}_s^{-}$ decays

The ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}_c^{+}{D}_s^{-}$ and ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Xi }}_c^0{D}_s^{-}$ decays are observed for the first time using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=13\mathrm{T}\mathrm{e}\mathrm{V}$, corresponding to an integrated luminosity of $5.1{\mathrm{f}\mathrm{b}}^{-1}$. The relative branching fractions times the beauty-baryon production cross-sections are measured to be \begin{align*} $\mathcal{R}$\left(\frac{Ξ_b^0}{Λ_b^0}\right) \equiv \frac{σ\left(Ξ_b^0\right)}{σ\left(Λ_b^0\right)} \times \frac{$\mathcal{B}$\left(Ξ_b^0 \rightarrow Ξ_c^+ D_s^-\right)}{$\mathcal{B}$\left(Λ_b^0 \rightarrow Λ_c^0 D_s^-\right)} =(15.8\pm1.1\pm0.6\pm7.7)%, $\mathcal{R}$\left(\frac{Ξ_b^-}{Λ_b^0}\right) \equiv \frac{σ\left(Ξ_b^-\right)}{σ\left(Λ_b^0\right)} \times \frac{$\mathcal{B}$\left(Ξ_b^- \rightarrow Ξ_c^0 D_s^-\right)}{$\mathcal{B}$\left(Λ_b^0 \rightarrow Λ_c^0 D_s^-\right)} =(16.9\pm1.3\pm0.9\pm4.3)%, \end{align*} where the first uncertainties are statistical, the second systematic, and the third due to the uncertainties on the branching fractions of relevant charm-baryon decays. The masses of ${\mathrm{\Xi }}_b^0$ and ${\mathrm{\Xi }}_b^{-}$ baryons are measured to be $m_{{\mathrm{\Xi }}_b^0}=5791.12\pm 0.60\pm 0.45\pm 0.24\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$ and $m_{{\mathrm{\Xi }}_b^{-}}=5797.02\pm 0.63\pm 0.49\pm 0.29\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}{c}^2$, where the uncertainties are statistical, systematic, and those due to charm-hadron masses, respectively.