First observation of the decay B-s(0) -> K*(0)(K)over-bar*(0)

The first observation of the decay B0s→ K∗0K∗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∗0K∗0 signal. The branching fraction and the CP-averaged K∗0 longitudinal polarization fraction are measured to be B(B0s→K∗0K∗0) = (2.81±0.46(stat.)±0.45(syst.)±0.34( fs/ fd))×10−5 and f L = 0.31±0.12(stat.)±0.04(syst.).