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

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: ℬ B 0 → J / ψ η ′ ℬ B s 0 → J / ψ η ′ = 2.28 ± 0.65 stat ± 0.10 syst ± 0.13 fs / f d × 1 0 − 2 , ℬ B 0 → J / ψ η ℬ B s 0 → J / ψ η = 1.85 ± 0.61 stat ± 0.09 syst ± 0.11 fs / f d × 1 0 − 2 , 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, ℬ B s 0 → ψ 2 S η ′ ℬ B s 0 → J / ψ η ′ = 38.7 ± 9.0 stat ± 1.3 syst ± 0.9 ℬ × 1 0 − 2 , where the third uncertainty is due to the limited knowledge of the branching fractions of J/ψ and ψ(2S) mesons.