APPLICABILITY OF PERTURBATIVE QCD TO B-MESON DECAYS

We discuss the applicability of the perturbative QCD formalism, which includes the resummation of large radiative corrections, to B meson de cays, concentrating on the semileptonic decay B --> pi/upsilon. It is found that the perturbative QCD analysis of this process is reliable f or the energy fraction of the pion above 0.4. Combining predictions fr om soft pion theorems, we estimate that the upper limit of the matrix element \V-ub\ is 2.8 - 4.8 x 10(-3) from different models of B meson wave functions.