Observation of the Decay B0s → anti-D0 K+ K−

The first observation of the B0s → anti-D0 K+ K− decay is reported, together with the most precise branching fraction measurement of the mode B0 → anti-D0 K+ K−. The results are obtained from an analysis of pp collision data corresponding to an integrated luminosity of 3.0 fb^−1. The data were collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. The branching fraction of the B0 → anti-D0 K+ K− decay is measured relative to that of the decay B0 → anti-D0 π+ π− to be B(B0 → anti-D0 K+ K−) / B(B0 → anti-D0 π+ π−) = (6.9 ± 0.4 ± 0.3)%, where the first uncertainty is statistical and the second is systematic. The measured branching fraction of the B0s → anti-D0 K+ K− decay mode relative to that of the corresponding B0 decay is B(B0s → anti-D0 K+ K−) / B(B0 → anti-D0 K+ K−) = (93.0 ± 8.9 ± 6.9)%. Using the known branching fraction of B0 → anti-D0 π+ π−, the values of B(B0 → anti-D0 K+ K−) = (6.1 ± 0.4 ± 0.3 ± 0.3) × 10^−5 and B(B0s → anti-D0 K+ K−) = (5.7 ± 0.5 ± 0.4 ± 0.5) × 10^−5 are obtained, where the third uncertainties arise from the branching fraction of the decay modes B0 → anti-D0 π+ π− and B0 → anti-D0 K+ K−, respectively.