Observation of the decay ${\bar{B}}_s^0\to \psi (2S)K^{+}{\pi }^{-}$

The decay ${\bar{B}}_s^0\to \psi (2S)K^{+}{\pi }^{-}$ is observed using a data set corresponding to an integrated luminosity of $3.0f{b}^{-1}$ collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV. The branching fraction relative to the $B^0\to \psi (2S)K^{+}{\pi }^{-}$ decay mode is measured to be \begin{equation} \frac{{\cal B}(\overline{B}^0_s \rightarrow \psi(2S) K^+ \pi^-)}{{\cal B}(B^0 \rightarrow \psi(2S) K^+ \pi^-)} = 5.38 \pm 0.36 (stat) \pm 0.22 (syst) \pm 0.31 , (f_s/f_d) , %,\nonumber \end{equation} where $f_s\mathrm{/}{f}_d$ indicates the uncertainty due to the ratio of probabilities for a $b$ quark to hadronise into a $B_s^0$ or $B^0$ meson. Using an amplitude analysis, the fraction of decays proceeding via an intermediate $K^{\ast }(892{)}^0$ meson is measured to be $0.645 \pm 0.049 (stat) \pm 0.049 (syst)$anditslongitudinalpolarisationfractionis$0.524 \pm 0.056 (stat) \pm 0.029 (syst)$. The relative branching fraction for this component is determined to be \begin{equation} \frac{{\cal B}(\overline{B}^0_s \rightarrow \psi(2S) K^(892)^0)}{{\cal B}(B^0 \rightarrow \psi(2S) K^(892)^0)} = 5.58 \pm 0.57 (stat) \pm 0.40 (syst) \pm 0.32 , (f_s/f_d) , %. \nonumber \end{equation} In addition, the mass splitting between the $B_s^0$ and $B^0$ mesons is measured as \begin{equation} M(B^0_s) - M(B^0) = 87.45 \pm 0.44 (stat) \pm 0.07 (syst) MeV/c^2. \nonumber \end{equation}