Observation of the $varXi^{-}_{b}to J/psivarLambda K^{-}$ decay

The observation of the decay $varXi^{-}_{b}to J/psivarLambda K^{-}$ is reported, using a data sample corresponding to an integrated luminosity of $3~mathrm{fb}^{-1}$, collected by the LHCb detector in $pp$ collisions at centre-of-mass energies of $7$ and $8~mathrm{TeV}$. The production rate of $varXi_{b}^{-}$ baryons detected in the decay $varXi_{b}^{-}to J/psivarLambda K^{-}$ is measured relative to that of $varLambda_{b}^{0}$ baryons using the decay $varLambda_{b}^{0}to J/psi varLambda$. Integrated over the $b$-baryon transverse momentum $p_{rm T}<25~mathrm{GeV/}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/psivarLambda K^{-})}{mathcal{B}(varLambda_{b}^{0}to J/psi varLambda)}=(4.19pm 0.29~(mathrm{stat})pm0.15~(mathrm{syst}))times 10^{-2}, end{equation*}where $f_{varXi_{b}^{-}}$ and $f_{varLambda_{b}^{0}}$ are the fragmentation fractions of $btovarXi_{b}^{-}$ and $btovarLambda_{b}^{0}$ transitions, and $mathcal{B}$ represents the branching fraction of the corresponding $b$-baryon decay. The mass difference between $varXi_{b}^{-}$ and $varLambda_{b}^{0}$ baryons is measured to be begin{equation*} M(varXi_{b}^{-})-M(varLambda_{b}^{0})=177.08pm0.47~(mathrm{stat})pm0.16~(mathrm{syst} )~mathrm{MeV/}c^{2}. end{equation*}