First observation of $B^{+} \to D_s^{+}K^{+}K^{-}$ decays and a search for $B^{+} \to D_s^{+}\phi$ decays

A search for B$^{+}$ → D$_{s}^{+}$ K$^{+}$K$^{−}$ decays is performed using pp collision data corresponding to an integrated luminosity of 4.8 fb$^{−1}$, collected at centre-of-mass energies of 7, 8 and 13 TeV with the LHCb experiment. A significant signal is observed for the first time and the branching fraction is determined to be $\mathcal{B}(B^+ \to D_{s}^+ K^+ K^-) = (7.1\pm 0.5\pm 0.6\pm 0.7) \times 10^{-6}$ where the first uncertainty is statistical, the second systematic and the third due to the uncertainty on the branching fraction of the normalisation mode $B^+ \to D^{+}_s \bar{D}^0$. A search is also performed for the pure annihilation decay $B^+ \to D_{s}^+ \phi$. No significant signal is observed and a limit of $\mathcal{B}(B^+ \to D_{s}^+ \phi) < 4.9 \times 10^{-7} (4.2 \times 10^{-7})$ is set on the branching fraction at 95% (90%) confidence level