First observation of the decay Bs⁰→K^*0K̄^*0

The first observation of the decay B _s ⁰→K ^*0K̄ ^*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 Bs0→(K ⁺π ^-)(K ^-π ⁺) events are observed within ±50 MeV/c ² of the B _s ⁰ mass and 746 MeV/c ²Kπ<1046 MeV/c ², mostly coming from a resonant B _s ⁰→K ^*0K̄ ^*0 signal. The branching fraction and the CP-averaged K ^*0 longitudinal polarization fraction are measured to be B(B _s ⁰→K ^*0K̄ ^*0)=(2.81±0.46(stat.)±0.45(syst.)±0.34(f _s/f _d))×10 ^-5 and f _L=0.31±0.12(stat.)±0.04(syst.).