The double charm decays of $B_c$ Meson in the Perturbative QCD Approach
  We make a systematic investigation on the double charm decays of $B_c$ meson, by employing the perturbative QCD approach based on $k_T$ factorization. It is found that the non-factorizable emission diagrams are not negligible in these channels. We predict the branching ratios of these $B_c$ decays and also the transverse polarization fractions of $B_c\rightarrow D_{(s)}^{*+}\bar D^{*0}, D_{(s)}^{*+}D^{*0}$ decays, % where V denote the vector $D^*_{(s)}$ meson.   We find that the magnitudes of the branching ratios of the decays $B_c\rightarrow D_s\bar{D}^0$ and $B_c\rightarrow D_sD^0$ are very close to each other, which are well suited to extract the Cabibbo-Kobayashi-Maskawa angle $\gamma$ through the amplitude relations. In addition, a large transverse polarization contribution that can reach $50%\sim 60%$ is predicted in some of the $B_c$ meson decay to two vector charmed mesons. 
Zhou Rui, Zou Zhitian and Cai-Dian Lu
hep-ph