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

Abstract: The first observation of the B _s(formula presented.)$$\mathrm{\mathcal{B}}\left({B}_s^0\to{D_s^{\ast}}^{\mp}{K}^{\pm}\right)=\left(16.3\pm1.2{\left(\mathrm{stat}\right)}_{-0.5}^{+0.7}\left(\mathrm{syst}\right)\pm 4.8\left(\mathrm{norm}\right)\right)\times 1{0}^{-5}, $$where the third uncertainty is due to the uncertainty on the branching fraction of the normalisation channel.[Figure not available: see fulltext.]