Polarization, CP Asymmetry and Branching ratios in B \to K^* K^* with   Perturbative QCD approach
  We study the charmless rare decays $B \to K^{\ast} K^{\ast}$ within the Perturbative QCD picture. We calculate not only factorizable and non-factorizable diagrams, but also annihilation ones. Our predictions are the following: The longitudinal polarization fraction vary from 75% to 99% depending on channels, the branching ratios are of order $10^{-7}$ for $B^0(\bar{B}^0) \to K^{\ast0}\bar{K}^{\ast0}$ and $B^{\pm} \to K^{\ast\pm} \bar{K}^{\ast0}(K^{\ast0})$, much bigger than that for $B^0(\bar{B}^0) \to K^{\ast+}K^{\ast-}$($10^{-8}$). The direct CP asymmetry in $B^{\pm} \to K^{\ast\pm} \bar{K}^{\ast0}(K^{\ast0})$ and $B^0(\bar{B}^0) \to K^{\ast+}K^{\ast-}$ is about -15% and -65% if we choose $\alpha (\phi_2)$ as $95^{\circ}$. There's no direct CPV in $B^0(\bar{B}^0) \to K^{\ast0}\bar{K}^{\ast0}$ decays because of the pure $b \to d$ penguin topology. Our predictions will be tested in the future B experiments. 
Jin Zhu, Yue-Long Shen, Cai-Dian Lu