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 B ⁰ → 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 B ⁰ 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.