The first observation of the $B^0_s\to\eta'\eta'$ decay is reported. The study is based on a sample of proton-proton collisions corresponding to $3.0$ ${ \rm fb^{ -1}}$ 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 $[3.31 \pm 0.64\,{ \rm (stat)} \pm 0.28\,{ \rm (syst)} \pm 0.12\,{ \rm (norm)}]\times10^{ -5}$, where the third uncertainty comes from the $B^{ \pm}\to\eta' K^{ \pm}$ branching fraction that is used as a normalisation. In addition, the charge asymmetries of $B^{ \pm}\to\eta' K^{ \pm}$ and $B^{ \pm}\to\phi K^{ \pm}$, 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.
Categories: hep-ex