First Observations of anti-B0s → D+ D−, Ds+ D− and D0 anti-D0 Decays

First observations and measurements of the branching fractions of the anti-B0s → D+ D−, anti-B0s → Ds+ D− and anti-B0s → D0 anti-D0 decays are presented using 1.0 fb^−1 of data collected by the LHCb experiment. These branching fractions are normalized to those of anti-B0 → D+ D−, B0 → D− Ds+ and B− → D0 Ds−, respectively. An excess of events consistent with the decay anti-B0 → D0 anti-D0 is also seen, and its branching fraction is measured relative to that of B− → D0 Ds−. Improved measurements of the branching fractions B(anti-B0s → Ds+ Ds−) and B(B− → D0 Ds−) are reported, each relative to B(B0 → D− Ds+). The ratios of branching fractions are B(anti-B0s → D+ D−)/B(anti-B0 → D+ D−) = 1.08 ± 0.20 ± 0.10, B(anti-B0s → Ds+ D−)/B(B0 → D− Ds+) = 0.050 ± 0.008 ± 0.004, B(anti-B0s → D0 anti-D0)/B(B− → D0 Ds−) = 0.019 ± 0.003 ± 0.003, B(anti-B0 → D0 anti-D0)/B(B− → D0 Ds−) < 0.0024 at 90% CL, B(anti-B0s → Ds+ Ds−)/B(B0 → D− Ds+) = 0.56 ± 0.03 ± 0.04, B(B− → D0 Ds−)/B(B0 → D− Ds+) = 1.22 ± 0.02 ± 0.07, where the uncertainties are statistical and systematic, respectively.