Observation of $B^0 \to \bar{D}^0 K^+ K^-$ and evidence of $B^0_s \to \bar{D}^0 K^+ K^-$

The first observation of the decay $B^{0} \to \overline{D}^{0}K^{+}K^{-}$ is reported from an analysis of $0.62 \ \mathrm{fb}^{-1}$ of $pp$ collision data collected with the LHCb detector. Its branching fraction is measured relative to that of the topologically similar decay $B^{0} \to \overline{D}^{0}\pi^{+}\pi^{-}$ to be $\frac{ {\cal B}\left(B^{0} \to \overline{D}^{0}K^{+}K^{-}\right)}{ {\cal B}\left(B^{0} \to \overline{D}^{0}\pi^{+}\pi^{-}\right)} = 0.056 \pm 0.011 \pm 0.007$, where the first uncertainty is statistical and the second is systematic. The significance of the signal is $5.8\,\sigma$. Evidence, with $3.8\,\sigma$ significance, for $B^{0}_{s} \to \overline{D}^{0}K^{+}K^{-}$ decays is also presented. The relative branching fraction is measured to be $\frac{ {\cal B}\left(B^{0}_{s} \to \overline{D}^{0}K^{+}K^{-}\right)}{ {\cal B}\left(B^{0} \to \overline{D}^{0}K^{+}K^{-}\right)} = 0.90 \pm 0.27 \pm 0.20$. These channels are of interest to study the mechanisms behind hadronic B decays, and open new possibilities for CP violation analyses with larger data sets.