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The pure annihilation type B_c to M_2 M_3 decays in the perturbative QCD approach

In the standard model the two-body charmless hadronic $B_c$ meson decays can occur via annihilation diagrams only. In this work, we studied the $B_c \to PP, PV/VP,VV$ decays by employing the perturbative QCD (pQCD) factorization approach. From our calculations, we find that (a) the pQCD predictions for the branching ratios of the considered $B_c$ decays are in the range of $10^{-6}$ to $10^{-8}$; (b) for $B_c \to PV/VP, VV$ decays, the branching ratios of $ΔS= 0$ decays are much larger than those of $ΔS =1$ ones because the different Cabibbo-Kobayashi-Maskawa(CKM) factors are involved; (c) analogous to $B \to K η^{(\prime)}$ decays, we find $Br(B_c \to K^+ η^\prime) \sim 10 \times Br(B_c \to K^+ η)$, which can be understood by the destructive and constructive interference between the $η_q$ and $η_s$ contribution to the $B_c \to K^+ η$ and $B_c \to K^+ η^\prime$ decay; (d) the longitudinal polarization fractions of $B_c \to VV$ decays are in the range of $86%-95%$ and play the dominant role; and (e) there is no CP-violating asymmetries for the considered $B_c$ decays because only one type tree operators involved.