First Observation and Measurement of the Branching Fraction for the Decay $B^0_S \rightarrow D{_S}^* K$

The Bs -> Ds* K and Bs -> Ds* \pi decays are of paramount interest as they can be used, combined with Bs -> Ds K and Bd -> D* \pi, to measure the weak phase \gamma. We present the first observation of the Bs -> Ds* K decay, where Ds* are reconstructed through the decay chain Ds* -> Ds (-> K K \pi) \gamma, using an integrated luminosity of 3.0 fb^{-1} recorded by the LHCb experiment.