Observation of $B_s^0\to {\bar{D}}^0{K}_S^0$ and evidence for $B_s^0\to {\bar{D}}^{\ast 0}{K}_S^0$ decays

The first observation of the $B_s^0\to {\bar{D}}^0{K}_S^0$ decay mode and evidence for the $B_s^0\to {\bar{D}}^{\ast 0}{K}_S^0$ decay mode are reported. The data sample corresponds to an integrated luminosity of 3.0 ${\text{fb}}^{-1}$ collected in $pp$ collisions by LHCb at center-of-mass energies of 7 and 8 TeV. The branching fractions are measured to be \begin{align*} $\mathcal{B}$(B^0_s\to\bar{D}^0 \bar{K}^0) &= (4.3\pm0.5($\text{stat}$)\pm0.3($\text{syst}$)\pm0.3($\text{frag}$)\pm0.6($\text{norm}$))\times10^{-4},\ $\mathcal{B}$(B^0_s\to\bar{D}^{0} \bar{K}^0) &= (2.8\pm1.0($\text{stat}$)\pm0.3($\text{syst}$)\pm0.2($\text{frag}$)\pm0.4($\text{norm}$))\times10^{-4}, \end{align} where the uncertainties are due to contributions coming from statistical precision, systematic effects, and the precision of two external inputs, the ratio $f_s\mathrm{/}{f}_d$ and the branching fraction of $B^0\to\bar{D}^0 K^0_S$, which is used as a calibration channel.