Observation of $B_s^0\to {\bar{D}}^{\ast 0}\varphi$ and search for $B^0\to {\bar{D}}^0\varphi$ decays

The first observation of the $B_s^0\to {\bar{D}}^{\ast 0}\varphi$ decay is reported, with a significance of more than seven standard deviations, from an analysis of $pp$ collision data corresponding to an integrated luminosity of 3 fb${}^{-1}$, collected with the LHCb detector at centre-of-mass energies of $7$ and $8$ TeV. The branching fraction is measured relative to that of the topologically similar decay $B^0\to {\bar{D}}^0{\pi }^{+}{\pi }^{-}$ and is found to be $$\mathcal{B}$(B_s^0 \to \overline{D}^{*0} \phi) = (3.7 \pm 0.5 \pm 0.3 \pm 0.2) \times 10^{-5}$, where the first uncertainty is statistical, the second systematic, and the third from the branching fraction of the $B^0 \to \overline{D}^0 \pi^+\pi^-$ decay. The fraction of longitudinal polarisation in this decay is measured to be $f_{\mathrm{L}}=(73\pm 15\pm 3)\mathrm{\%}$. The most precise determination of the branching fraction for the $B_s^0 \to \overline{D}^{0} \phi$decayisalsoobtained,$$\mathcal{B}$(B_s^0 \to \overline{D}^{0} \phi) = (3.0 \pm 0.3 \pm 0.2 \pm 0.2) \times 10^{-5}$. An upper limit, $$\mathcal{B}$(B^0 \to \overline{D}^{0} \phi) < 2.0 \ (2.2) \times 10^{-6}$at$90%$(95\mathrm{\%})confidencelevelisset\mathrm{.}Aconstraintonthe$\omega-\phi$mixingangle$\delta$issetat$|\delta| < 5.2^\circ~ (5.5^\circ)$at$90%$($95%$) confidence level.