First observation of $B_s^0\to J/\psi\eta$ and $B_s^0\to J/\psi\eta'$

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We report first observations of $B_s^0\to J/\psi\eta$ and $B_s^0\to J/\psi\eta'$.

• The results are obtained from 121.4 fb^{-1} of data collected at the $\Upsilon(5S)$ resonance with the Belle detector at the KEKB $e^+e^-$ collider.
• We obtain the branching fractions Br(B_s^0\to J/\psi\eta)= [5.10\pm 0.50(stat.)\pm 0.25(syst.)^{+1.14}_{-0.79}(N_{B_s^(*)\bar B_s^(*))] \times 10^{-4}, and Br(B_s^0\to J/\psi\eta')=[3.71\pm 0.61(stat.)\pm 0.18(syst.)^{+0.83}_{-0.57}(N_{B_s^(*)\bar B_s^(*))] \times 10^{-4}.
• The ratio of the two branching fractions is measured to be $\frac{Br(B_s\to J/\psi\eta')}{Br(B_s\to J/\psi \eta)} = 0.73\pm 0.14(stat.)\pm 0.02(syst.)$.