W-exchange and W-annihilation processes of B mesons

Using the PQCD method we calculate the W-exchange and the W-annihilation processes of B mesons, which in general involve a charm quark or anti-quark in the final state. The nonvanishing amplitudes of these processes are found to be suppressed by a factor of $m_c\mathrm{/}{m}_b$ compared to the tree or the time-like penguin processes, but some of them are within the reach of observation at the future B-factories, and ${\bar{B}}_d^0\to D_s^{+}{K}^{-}$ whose branching ratio is found to be $6.6\times {10}^{-6}$ can be found even before the B-factory era. Comparisons with the results based on the BSW model are also given.