First observation and measurement of the branching fraction for the decay B(formula presented.)

The first observation of the B 0s → 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 0s → D ∗s − π + decays is measured to be B(B0s→D∗s∓K±)/B(B0s→D∗s−π+)=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(B0s→D∗s−π+), the absolute branching fraction of B 0s → D ∗s ∓ K ± is measured as B(B0s→D∗s∓K±)=(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.