Measurement of $B^0 \to J/\psi \eta^{(}{}'{}^{)}$ and Constraint on the   $\eta-\eta'$ Mixing Angle
  We measure the branching fractions of $B^{0} \to J/\psi \eta^{(}{}'{}^{)}$ decays with the complete Belle data sample of $772 \times 10^{6}$ $B\bar{B}$ events collected at the $\Upsilon(4S)$ 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 $\eta-\eta'$ mixing angle is constrained to be less than $ 42.2^{\circ}$ at 90% confidence level. 
M.-C. Chang, Y.-C. Duh, J.-Y. Lin, I. Adachi, K. Adamczyk, H. Aihara,   D. M. Asner, T. Aushev, A. M. Bakich, V. Bhardwaj, B...
hep-ex