Observation of Bs0→Ds(*)+Ds(*)- Using e+e- Collisions and a Determination of the Bs-B¯s Width Difference ΔΓs

We have made the first observation of B_s⁰→D_s^(*)+D_s^(*)- decays using 23.6fb^-1 of data recorded by the Belle experiment running on the Υ(5S) resonance. The branching fractions are measured to be B(B_s⁰→D_s⁺D_s^-)=(1.03_-0.32-0.25^+0.39+0.26)%, B(B_s⁰→D_s^*±D_s^∓)=(2.75_-0.71^+0.83±0.69)%, and B(B_s⁰→D_s^*+D_s^*-)=(3.08_-1.04-0.86^+1.22+0.85)%; the sum is B[B_s⁰→D_s^(*)+D_s^(*)-]=(6.85_-1.30-1.80^+1.53+1.79)%. Assuming B_s⁰→D_s^(*)+D_s^(*)- saturates decays to CP-even final states, the branching fraction determines the ratio ΔΓ_s/cos⁡φ, where ΔΓ_s is the difference in widths between the two B_s-B¯_s mass eigenstates, and φ is a CP-violating weak phase. Taking CP violation to be negligibly small, we obtain ΔΓ_s/Γ_s=0.147_-0.030^+0.036(stat)_-0.041^+0.042(syst), where Γ_s is the mean decay width.