Observation of the $B_s^0\to J\mathrm{/}\psi K_{\mathrm{S}}^0{K}^{\pm }{\pi }^{\mp }$ decay

Decays of the form $B^{0}{(s)}\rightarrow J/\psi K{{\rm S}}^{0} h^+ h^{\left(\prime\right) -}$($h^{(\prime)} = K, \pi$) are searched for in proton-proton collision data corresponding to an integrated luminosity of $1.0 , {\rm fb}^{-1}$recordedwiththeLHCbdetector\mathrm{.}Thefirstobservationofthe$B^{0}{s}\rightarrow J/\psi K{{\rm S}}^{0} K^{\pm} \pi^{\mp}$ decay is reported, with significance in excess of 10 standard deviations. The $B^0\to J\mathrm{/}\psi K_{\mathrm{S}}^0{K}^{+}{K}^{-}$ decay is also observed for the first time. The branching fraction of $B^{0}\rightarrow J/\psi K_{{\rm S}}^{0} \pi^{+} \pi^{-}$ is determined, to significantly better precision than previous measurements, using $B^0\to J\mathrm{/}\psi K_{\mathrm{S}}^0$ as a normalisation channel. Branching fractions and upper limits of the other $B_{(s)}^0\to J\mathrm{/}\psi K_{\mathrm{S}}^0{h}^{+}{h}^{(\mathrm{\prime })-}$ modes are determined relative to that of the $B^{0}\rightarrow J/\psi K_{{\rm S}}^{0} \pi^{+} \pi^{-}$ decay.