Observation of Bs0 -> Ds(*)+ Ds(*)- using e+e- collisions and a determination of the Bs-Bsbar width difference Delta Gamma_s

We have made the first observation of B-s(0) -> D-s(()()+) D-s(()()-) decays using 23.6 fb(-1) of data recorded by the Belle experiment running on the Y(5S) resonance. The branching fractions are measured to be B(B-s(0) -> Ds+Ds-) = (1.03(-0.32-0.25)(+0.39+0.26))% B(B-s(0) -> DsD-+/-(s)-/+) = (2.75(-0.71)(+0.83) +/- 0.69)%, and B(B-s(0) -> D-s(+) D-s*(-)) = (3.08(-1.04-0.86)(+1.22+0.85))%; the sum is B[B-s(0) -> D-s(()()+) D-s(()()-)] = (6.85(-1.30-1.80)(+1.53+1.79))%. Assuming B-s(0) -> D-s(()()+) D-s(()()-) saturates decays to CP-even final states, the branching fraction determines the ratio Delta Gamma(s)/cos phi, where Delta Gamma(s) is the difference in widths between the two B-s-(B) over bar (s) mass eigenstates, and phi is a CP-violating weak phase. Taking CP violation to be negligibly small, we obtain Delta Gamma(s)/Delta Gamma(s) = 0.147(-0.030)(+0.036)(stat)(-0.041)(+0.042)(syst), where Gamma(s) is the mean decay width.