Observation of the decay $B_{s}^{0} \to \eta_{c} \phi$ and evidence for $B_{s}^{0} \to \eta_{c} \pi^{+} \pi^{-} $

A study of B$_{s}^{0}$ → η$_{c}$ ϕ and B$_{s}^{0}$ → η$_{c}$ π$^{+}$ π$^{−}$ decays is performed using pp collision data corresponding to an integrated luminosity of 3.0 fb$^{−1}$, collected with the LHCb detector in Run 1 of the LHC. The observation of the decay B$_{s}^{0}$ → η$_{c}$ ϕ is reported, where the η$_{c}$ meson is reconstructed in the $ p\overline{p} $ , K$^{+}$ K$^{−}$ π$^{+}$ π$^{−}$, π$^{+}$ π$^{−}$ π$^{+}$ π$^{−}$ and K$^{+}$ K$^{−}$ K$^{+}$ K$^{−}$ decay modes and the ϕ(1020) in the K$^{+}$ K$^{−}$ decay mode. The decay B$_{s}^{0}$ → J/ψϕ is used as a normalisation channel. Evidence is also reported for the decay B$_{s}^{0}$ → η$_{c}$ π$^{+}$ π$^{−}$, where the η$_{c}$ meson is reconstructed in the $ p\overline{p} $ decay mode, using the decay B$_{s}^{0}$ → J/ψπ$^{+}$ π$^{−}$ as a normalisation channel. The measured branching fractions are $ \begin{array}{l}\mathrm{\mathcal{B}}\left({B}_s^0\to {\eta}_c\phi \right)=\left(5.01\pm 0.53\pm 0.27\pm 0.63\right)\times {10}^{-4},\hfill \\ {}\mathrm{\mathcal{B}}\left({B}_s^0\to {\eta}_c{\pi}^{+}{\pi}^{-}\right)=\left(1.76\pm 0.59\pm 0.12\pm 0.29\right)\times {10}^{-4},\hfill \end{array} $ where in each case the first uncertainty is statistical, the second systematic and the third uncertainty is due to the limited knowledge of the external branching fractions.