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

The first observation of the decay B0s → K∗0 anti-K∗0 is reported using 35 pb^−1 of data collected by LHCb in proton–proton collisions at a centre-of-mass energy of 7 TeV. A total of 49.8 ± 7.5 B0s → (K+ π−)(K− π+) events are observed within ±50 MeV/c2 of the B0s mass and 746 MeV/c2 < mKπ < 1046 MeV/c2, mostly coming from a resonant B0s → K∗0 K∗0 signal. The branching fraction and the CP-averaged K∗0 longitudinal polarization fraction are measured to be B(B0s → K∗0 K∗0) = (2.81 ± 0.46(stat.) ± 0.45(syst.) ± 0.34(fs/fd)) x 10^−5 and f_L = 0.31 ± 0.12(stat.) ± 0.04(syst.).