First observation of the decay $B_s^0\to \varphi {\bar{K}}^{\ast 0}$

The first observation of the decay $B_s^0\to \varphi {\bar{K}}^{\ast 0}$ is reported. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0 fb${}^{-1}$ of $pp$ collisions at $\sqrt{s}=7TeV$, collected with the LHCb detector. A yield of $30\pm 6$ $B_s^0 \rightarrow (K^+K^-)(K^-\pi^+)$decaysisfoundinthemasswindows$1012.5 < M(K^+K^-) < 1026.5 MeV/c^2$and$746 < M(K^-\pi^+)< 1046 MeV/c^2$. The signal yield is found to be dominated by $B_s^0\to \varphi {\bar{K}}^{\ast 0}$ decays, and the corresponding branching fraction is measured to be ${\cal B}(B_s^0 \rightarrow \phi \bar{K}^{*0}) = (1.10 \pm 0.24 (stat) \pm 0.14 (syst) \pm 0.08 (f_d/f_s)) \times 10^{-6}$, where the uncertainties are statistical, systematic and from the ratio of fragmentation fractions $f_d\mathrm{/}{f}_s$ which accounts for the different production rate of $B^0$ and $B_s^0$ mesons. The significance of $B_s^0 \rightarrow \phi \bar{K}^{*0}$ signal is 6.1 standard deviations. The fraction of longitudinal polarization in $B_s^0\to \varphi {\bar{K}}^{\ast 0}$ decays is found to be $f_0=0.51\pm 0.15(stat)\pm 0.07(syst)$.