Prediction of Pure Annihilation Type B Decays

The rare decays $B^{+}\to D_s^{+}\varphi$, $B^0\to D_s^{(\ast )-}{K}^{+}$ and $B^+ \to D_s^{()+} \bar{K}^0$ can occur only via annihilation type diagrams in the standard model. We calculate these decays in perturbative QCD approach. We found that the calculated branching ratio of $B^0\to D_s^{-}{K}^{+}$ agree with the data which had been observed in the KEK and SLAC B factories. The decay $B^+ \to D_s^{()+} \bar K^0$hasaverysmallbranchingratioatO($10^{-8}$), due to the suppression from CKM matrix elements. The branching ratio of $B^+ \to D_s^+ \phi$isoforder$10^{-7}$ which may be measured in the near future by KEK and SLAC B factories. The small branching ratios predicted in the standard model make these channel sensitive to new physics contributions.