Improved measurement of (B)over-bar0 → Ds- D+ and search for (B)over-bar0 → Ds+ Ds- at Belle

We reconstruct (B) over bar (0)-> Ds-D+ decays using a sample of 449x10(6) B (B) over bar pairs recorded by the Belle experiment, and measure the branching fraction to be B((B) over bar (0)-> Ds-D+)=[7.5 +/- 0.2(stat)+/- 0.8(syst)+/- 0.8(B(')s)]x10(-3). A search for the related decay (B) over bar (0)-> Ds+Ds- is also performed. Since we observe no statistically significant signal, an upper limit on the branching fraction is set at 3.6x10(-5) (90% C.L.).