First observation of the decay $B^0_s \to K^{*0} \overline{K}^{*0}$

We describe the first observation of the decay $B^0_s \to K^{*0} \bar{K}^{*0}$ with the $\sim$ 35 pb$^{-1}$ of data collected in the 2010 run by LHCb in proton-proton collisions at a centre-of-mass energy of 7 TeV. We find 34.0 $\pm$ 7.4 signal events in the mass interval $\pm$ 50 MeV/$c^2$ around the $B^0_s$ mass, on top of 14.7 $\pm$ 1.4 background events. The observation has a 7$\sigma$ significance. A preliminary measurement of the branching ratio is performed, giving the result $\cal{B}(B^0_s \to K^{*0}\overline{K}^{*0})$ = (1.95$\pm$0.47($stat.$)$\pm$0.51($syst.$)$\pm$0.29($f_d/f_s$)) x 10$^{-5}$.