Study of η − η^′ mixing from measurement of B _(s) ⁰ → J/ψη^(′) decay rates

Abstract: A study of B and B^s meson decays into J/ψ η and J/ψ η^′ final states is performed using a data set of proton-proton collisions at centre-of-mass energies of 7 and 8 TeV, collected by the LCHb experiment and corresponding to 3.0 fb⁻¹ of integrated luminosity. The decay B⁰ → J/ψ η^′ is observed for the first time. The following ratios of branching fractions are measured: (Formula Presented) where the third uncertainty is related to the present knowledge of f_s/f_d, the ratio between the probabilities for a b quark to form a B^s or a B⁰ meson. The branching fraction ratios are used to determine the parameters of η − η^′ meson mixing. In addition, the first evidence for the decay B^s → ψ(2S)η^′ is reported, and the relative branching fraction is measured, (Formula Presented) where the third uncertainty is due to the limited knowledge of the branching fractions of J/ψ and ψ(2S) mesons.