Charmless $B\rightarrow VV$ decays at LHCb

$B^0_s$ meson decays into non--charmonium vector pairs are of particular interest to search for indirect $CP$ violation. The analyses of two specific decays are presented here, $B_s^0 \rightarrow \phi \phi$ and $B_s^0 \rightarrow K^{*0} \bar{K}^{*0}$. These studies have been performed using, respectively, $1fb^{-1}$ and $35pb^{-1}$ of data collected by LHCb in $pp$ collisions at a centre--of--mass energy of 7 TeV. In $B_s^0 \rightarrow \phi \phi$ decay, the $CP$--violating phase arising in the interference between $B_s^0$--$\bar{B}_s^0$ mixing and the $b\rightarrow s\bar{s}s$ penguin decay has been determined to be in the interval $[-2.46,-0.76]$ rad at 68% confidence level. The first observation of $B_s^0 \rightarrow K^{*0} \bar{K}^{*0}$ decay is also reported. The branching fraction and the $CP$--averaged $K^{*0}$ longitudinal polarization fraction are measured to be $\mathcal{B}(B_s^0 \rightarrow K^{*0} \bar{K}^{*0})$ = (2.81 $\pm$ 0.46(stat.) $\pm$ 0.45(syst.) $\pm$ 0.34($f_s/f_d$))$\times 10^{-5}$ and $f_L$ = 0.31 $\pm$ 0.12(stat.) $\pm$ 0.04(syst.).