First observation and measurement of the branching fraction for the decay B s 0 → D s ∗ ∓ K ±

The first observation of the B s 0 → D s ∗ ∓ K ± 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 B s 0 → D s ∗ − π + decays is measured to be ℬ(B_s^0→D_s^∗^∓K^±)/ℬ(B_s^0→D_s^∗^-π^+)=0.068± 0.005_-0.002^+0.003, where the first uncertainty is statistical and the second is systematic. Using a recent measurement of ℬ(B_s^0→D_s^∗^-π^+) , the absolute branching fraction of B s 0 → D s ∗ ∓ K ± is measured as ℬ(B_s^0→D_s^∗^∓K^±)=(16.3± 1.2(stat)_-0.5^+0.7(syst)± 4.8(norm))× 10^-5, where the third uncertainty is due to the uncertainty on the branching fraction of the normalisation channel.