PERTURBATIVE QCD STUDY OF B-]D(ASTERISK) DECAYS

We compute various form factors involved in B-->D() transitions based on the perturbative QCD formalism, which includes Sudakov effects fro m the resummation of large radiative corrections in a heavy-light syst em. A two-parameter model wave function for D() mesons is fixed using data of the nonleptonic decays B-->D()pi; from which the ratio of th e decay constants f(D)/f(D)=0.92 is obtained. We then derive the spec trum of the semileptonic decay B-->Dlv in the fast recoil region of t he D meson, and extract the CKM matrix element \V-cb\=0.043x(0.12 GeV /f(B))x(0.14 GeV/f(D)), f(B) and f(D) being the B and D meson decay co nstants, respectively. Here we adopt the convention with the pion deca y constant f(pi)=93 MeV. With these outcomes, we evaluate the decay ra te of B-->DDs, and estimate the ratio f(Ds)/f(D)=0.98 from data. Contr ibutions of internal W-emission and W-exchange diagrams are briefly di scussed.