First observations of B̄s0→D+^D-, Ds+^D- and D0D^̄0 decays

First observations and measurements of the branching fractions of the B̄s0→D⁺D^-, B̄s0→Ds+D^- and B̄s0→D0D^̄0 decays are presented using 1.0 fb ^-1 of data collected by the LHCb experiment. These branching fractions are normalized to those of B^̄0→D ⁺D^-, B0→D^-Ds+ and B^-→D0Ds- , respectively. An excess of events consistent with the decay B ^̄0→D0D^̄0 is also seen, and its branching fraction is measured relative to that of B^-→D0Ds-. Improved measurements of the branching fractions B(B̄s0→Ds+Ds-) and B(B ^-→D0Ds-) are reported, each relative to B(B0→D ^-Ds+). The ratios of branching fractions are B(B̄s0→D ⁺D^-)B(B^̄0→D⁺D ^-)=1.08±0.20±0.10, B(B̄s0→Ds+D ^-)B(B0→D^-Ds+)=0.050±0.008±0.004, B(B̄s0→D0D^̄0)B(B^-→D0Ds-)=0. 019±0.003±0.003, B(B^̄0→D0D ^̄0)B(B^-→D0Ds-)<0.0024 at 90% CL, B(B̄s0→Ds+Ds-)B(B0→D^-Ds+)=0.56±0.03±0. 04, B(B^-→D0Ds-)B(B0→D^-Ds+)=1.22±0. 02±0.07, where the uncertainties are statistical and systematic, respectively.