Observation of the decay B s 0 → η c ϕ and evidence for B s 0 → η c π + π −

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 pp , 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 pp decay mode, using the decay B s 0 → J / ψπ + π − as a normalisation channel. The measured branching fractions are [ ℬ(B_s^0→η_cϕ)=(5.01± 0.53± 0.27± 0.63)×10^-4,; ℬ(B_s^0→η_cπ^+π^-)=(1.76± 0.59± 0.12± 0.29)×10^-4, ] 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.