First observation and measurement of the branching fraction for the decay B⁰_s → D^∗∓_sK^±

The first observation of the B⁰_s → D^∗∓_sK^± decay is reported using 3.0 fb⁻¹ of proton-proton collision data collected by the LHCb experiment. The D^∗∓_s mesons are reconstructed through the decay chain D^∗∓_s → γD^∓_s (K^∓K^±π^∓). The branching fraction relative to that for B⁰_s → D^∗−_sπ⁺ decays is measured to be B(B⁰_s → D^∗∓_sK^±)/B(B⁰_s → D^∗−_sπ⁺) = 0.068±0.005^+0.003_−0.002, where the first uncertainty is statistical and the second is systematic. Using a recent measurement of B(B⁰_s → D^∗−_sπ⁺), the absolute branching fraction of B⁰_s → D^∗∓_sK^± is measured as B(B⁰_s → D^∗∓_sK^±) = (16.3±1.2(stat)^+0.7_−0.5(syst)±4.8(norm))×10⁻⁵, where the third uncertainty is due to the uncertainty on the branching fraction of the normalisation channel.