Observation of the $B_s^0\to {\eta }^{\mathrm{\prime }}{\eta }^{\mathrm{\prime }}$ decay

The first observation of the $B_s^0\to {\eta }^{\mathrm{\prime }}{\eta }^{\mathrm{\prime }}$ decay is reported. The study is based on a sample of proton-proton collisions corresponding to $3.0$ $\mathrm{f}{\mathrm{b}}^{-1}$ of integrated luminosity collected with the LHCb detector. The significance of the signal is $6.4$ standard deviations. The branching fraction is measured to be $[3.31 \pm 0.64,{\rm (stat)} \pm 0.28,{\rm (syst)} \pm 0.12,{\rm (norm)}]\times10^{-5}$,wherethethirduncertaintycomesfromthe$B^{\pm}\to\eta’ K^{\pm}$ branching fraction that is used as a normalisation. In addition, the charge asymmetries of $B^{\pm }\to {\eta }^{\mathrm{\prime }}{K}^{\pm }$ and $B^{\pm }\to \varphi K^{\pm }$, which are control channels, are measured to be $(-0.2 \pm1.3)%$and$(+1.7\pm1.3)%$, respectively. All results are consistent with theoretical expectations.