Observation of the $B^0_s\toη'η'$ decay

The first observation of the B s 0 → η ′ η ′ decay is reported. The study is based on a sample of proton-proton collisions corresponding to 3.0 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 ± 0.64 (stat) ± 0.28 (syst) ± 0.12 (norm)] × 10 - 5, where the third uncertainty comes from the B ± → η ′ K ± branching fraction that is used as a normalization. In addition, the charge asymmetries of B ± → η ′ K ± and B ± → Φ K ±, which are control channels, are measured to be (- 0.2 ± 1.3) % and (+ 1.7 ± 1.3) %, respectively. All results are consistent with theoretical expectations.