First Observation of the Decay B0s → ϕ anti-K∗0

The first observation of the decay B0s → ϕ anti-K*0 is reported. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0 fb^−1 of pp collisions at √s = 7 TeV, collected with the LHCb detector. A yield of 30 ± 6 B0s → (K+ K−)(K− π+) decays is found in the mass windows 1012.5 < M(K+ K−) < 1026.5 MeV/c^2 and 746 < M(K− π+) < 1046 MeV/c^2. The signal yield is found to be dominated by B0s → ϕ anti-K∗0 decays, and the corresponding branching fraction is measured to be B(B0s → ϕ anti-K∗0) = (1.10 ± 0.24 (stat) ± 0.14 (syst) ± 0.08 (f_d/f_s )) x 10^−6, where the uncertainties are statistical, systematic and from the ratio of fragmentation fractions f_d /f_s which accounts for the different production rate of B0 and B0s mesons. The significance of B0s → ϕ anti-K∗0 signal is 6.1 standard deviations. The fraction of longitudinal polarization in B0s → ϕ anti-K∗0 decays is found to be f_0 = 0.51 ± 0.15 (stat) ± 0.07 (syst).