First observation and measurement of the branching fraction for the decay $B_s^0\to D_s^{\ast \mp }{K}^{\pm }$

Abstract

The first observation of the $B_s^0\to D_s^{\ast \mp }{K}^{\pm }$ decay is reported using 3.0$f{b}^{-1}$ of proton-proton collision data collected by the LHCb experiment. The $D_s^{\ast \mp }$ mesons are reconstructed through the decay chain $D_s^{\ast \mp }\to {\hat{I}}^3{D}_s^{\mp }(K^{\mp }{K}^{\pm }\ddot{I}{€}^{\mp })$. The branching fraction relative to that for $B_s^0\to D_s^{\ast -}\ddot{I}{€}^{+}$ is measured to be $0.068\pm 0.005_{-0.002}^{+0.003}$, where the first uncertainty is statistical and the second is systematic. Using a recent measurement of $BR(B_s^0\to D_s^{\ast -}\ddot{I}{€}^{+})$ the absolute branching fraction of $B_s^0\to D_s^{\ast \mp }{K}^{\pm }$ is measured as ( 16.3 $\pm$ 1.2 (stat) ${}_{-0.5}^{+0.7}$ (syst) $\pm$ 4.8 (norm) ) $\times$ 10${}^{-5}$, where the third uncertainty is due to the uncertainty on the branching fraction of the normalisation channel.

17 pages, 4 figures