Measurement of $B^0\to J\mathrm{/}\psi {\eta }^{(}{}^{\mathrm{\prime }}{}^{)}$ and Constraint on the $\eta -{\eta }^{\mathrm{\prime }}$ Mixing Angle

We measure the branching fractions of $B^0\to J\mathrm{/}\psi {\eta }^{(}{}^{\mathrm{\prime }}{}^{)}$ decays with the complete Belle data sample of $772\times {10}^6$ $B\bar{B}$ events collected at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The results for the branching fractions are: ${\cal B}(B^{0} \to J/\psi \eta)=(12.3 \pm ^{1.8}_{1.7} \pm 0.7) \times 10^{-6}$and${\cal B}(B^{0} \to J/\psi\eta’) < 7.4 \times 10^{-6}$ at 90% confidence level. The $\eta -{\eta }^{\mathrm{\prime }}$ mixing angle is constrained to be less than ${42.2}^{\circ }$ at 90% confidence level.