Study of η − η′ Mixing from Measurement of B0(s) → J/ψ η(′) Decay Rates

A study of B0 and B0s 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 LHCb 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: B(B0 → J/ψ η′)/B(B0s → J/ψ η′) = (2.28 ± 0.65(stat) ± 0.10(syst) ± 0.13(f_s/f_d)) × 10^−2, B(B0 → J/ψ η)/B(B0s → J/ψ η) = (1.85 ± 0.61(stat) ± 0.09(syst) ± 0.11(f_s/f_d)) × 10^−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 B0s 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 B0s → ψ(2S) η′ is reported, and the relative branching fraction is measured, B(B0s → ψ(2S) η′)/B(B0s → J/ψ η′) = (38.7 ± 9.0(stat) ± 1.3(syst) ± 0.9(B)) × 10^−2, where the third uncertainty is due to the limited knowledge of the branching fractions of J/ψ and ψ(2S) mesons.