Observation of the ${\Xi }_b^{-}\to J\mathrm{/}\psi \Lambda K^{-}$ decay

The observation of the decay ${\Xi }_b^{-}\to J\mathrm{/}\psi \Lambda K^{-}$ is reported, using a data sample corresponding to an integrated luminosity of $3{\mathrm{f}\mathrm{b}}^{-1}$, collected by the LHCb detector in $pp$ collisions at centre-of-mass energies of $7$ and $8\mathrm{T}\mathrm{e}\mathrm{V}$. The production rate of ${\Xi }_b^{-}$ baryons detected in the decay $\varXi_{b}^{-}\to J/\psi\varLambda K^{-}$ismeasuredrelativetothatof$\varLambda_{b}^{0}$baryonsusingthedecay$\varLambda_{b}^{0}\to J/\psi \varLambda$. Integrated over the $b$-baryon transverse momentum $p_{\mathrm{T}}<25\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}c$ and rapidity $2.0<y<4.5$, the measured ratio is \begin{equation*} \frac{f_{\varXi_{b}^{-}}}{f_{\varLambda_{b}^{0}}}\frac{$\mathcal{B}$(\varXi_{b}^{-}\to J/\psi\varLambda K^{-})}{$\mathcal{B}$(\varLambda_{b}^{0}\to J/\psi \varLambda)}=(4.19\pm 0.29~($\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}$)\pm0.15~($\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}$))\times 10^{-2}, \end{equation*}where $f_{{\Xi }_b^{-}}$ and $f_{{\Lambda }_b^0}$ are the fragmentation fractions of $b\to {\Xi }_b^{-}$ and $b\to {\Lambda }_b^0$ transitions, and $\mathcal{B}$ represents the branching fraction of the corresponding $b$-baryon decay. The mass difference between ${\Xi }_b^{-}$ and ${\Lambda }_b^0$ baryons is measured to be \begin{equation*} M(\varXi_{b}^{-})-M(\varLambda_{b}^{0})=177.08\pm0.47~($\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}$)\pm0.16~($\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}$ )~$\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}$c^{2}. \end{equation*}