Observation of the decay $B_s^0\to J\mathrm{/}\psi \eta$ and Evidence for $B_s^0\to J\mathrm{/}\psi {\eta }^{\mathrm{\prime }}$

We report the first observation of $B_s^0\to J\mathrm{/}\psi \eta$ and evidence for $B_s^0\to J\mathrm{/}\psi {\eta }^{\mathrm{\prime }}$. These results are obtained from $23.6{\mathrm{f}\mathrm{b}}^{-1}$ of data collected at the $\mathrm{{\rm Y}}(5S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We measure the branching fractions $$\mathcal{B}$(B_s^0\to J/\psi\eta)= (3.32 \pm 0.87($\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}$)^{+0.32}{-0.28}($\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}$) \pm 0.42(f_s))\times 10^{-4}}$withasignificanceof$7.3 \sigma$,and$$\mathcal{B}$(B_s^0\to J/\psi\eta’)= (3.1 \pm 1.2($\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}$)^{+0.5}{-0.6}($\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}$) \pm 0.38(f_s))\times 10^{-4}$withasignificanceof$3.8 \sigma$.