Calculation of pure annihilation type decay $B^{+}\to D_s^{+}\varphi$

The rare decay $B^{+}\to D_s^{+}\varphi$ can occur only via annihilation type diagrams in the standard model. We calculate this decay in perturbative QCD approach with Sudakov resummation. We found that 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 ratio predicted in standard model makes this channel sensitive to new physics contributions.