Study of η--η' mixing in B-meson decay with the LHCBb detector

The first ever observation of the decay process [B.sup.0] [right arrow] J/ψη' is described. The branching ratios were measured for the decay processes [B.sub.(s)] → J/ψη(') (and the η-η' mixing angles were determined on this basis. The first indication of the occurrence of the decay process [B.sub.s] → ψ(2S)η' is presented. The respective analysis was performed on the basis of data accumulated with the LHCb detector in proton--proton collisions at the c.m. collision energies of 7 and 8 TeV and the integrated luminosity of 3 [fb.sup.-1]