First observation of B⁺→ D⁺_s K⁺K⁻ decays and a search for B⁺ → D⁺_s φ 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

B(B+→D+sK+K−)=(7.1±0.5±0.6±0.7)×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+→D+sD¯¯¯¯0

. A search is also performed for the pure annihilation decay B⁺ → D _s ⁺ ϕ. No significant signal is observed and a limit of

B(B+→D+sϕ)<4.9×10−7(4.2×10−7) is set on the branching fraction at 95% (90%) confidence level.