First observation and measurement of the branching fraction for the decay Bs0 → Ds∗∓K±

The first observation of the Bs0 → Ds∗∓K± decay is reported using 3.0 fb−1 of proton-proton collision data collected by the LHCb experiment. The Ds∗∓ mesons are reconstructed through the decay chain Ds∗∓ → γDs∓(K∓K±π∓). The branching fraction relative to that for Bs0 → Ds∗−π+ decays is measured to be ℬBs0→Ds∗∓K±/ℬBs0→Ds∗−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 ℬBs0→Ds∗−π+, the absolute branching fraction of Bs0 → Ds∗∓K± is measured as ℬBs0→Ds∗∓K±=16.3±1.2stat−0.5+0.7syst±4.8norm×10−5, where the third uncertainty is due to the uncertainty on the branching fraction of the normalisation channel.