Observation of the decay $\overline{B_s^0}\to {\chi }_{c2}{K}^{+}{K}^{-}$ in the $\phi$ mass region

The $\overline{B_s^0}\to {\chi }_{c2}{K}^{+}{K}^{-}$ decay mode is observed and its branching fraction relative to the corresponding ${\chi }_{c1}$ decay mode, in a $\pm 15\text{MeV}\mathrm{/}{c}^2$ window around the $\varphi$ mass, is found to be $\frac{$\mathcal{B}$(\overline{B_s^0} \rightarrow \chi_{c2} K^+ K^-) }{ $\mathcal{B}$(\overline{B_s^0} \rightarrow \chi_{c1} K^+ K^-)} = (17.1 \pm 3.1 \pm 0.4 \pm 0.9)%,$ where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the branching fractions of radiative ${\chi }_c$ decays. The decay mode $\overline{B_s^0} \rightarrow \chi_{c1} K^+ K^- $allowsthe$ B_s^0$masstobemeasuredas$m(B_s^0) = 5366.83 \pm 0.25 \pm 0.27 , $\text{MeV}$/c^2,$ where the first uncertainty is statistical and the second systematic. A combination of this result with other LHCb determinations of the $B_s^0$ mass is made.