EXCLUSIVE SEMILEPTONIC DECAYS OF B-MESONS INTO LIGHT MESONS IN THE RELATIVISTIC QUARK-MODEL

The exclusive semileptonic B --> pi(rho w)ev decays are studied in the framework of the relativistic quark model based on the quasipotential approach in quantum field theory. The large recoil momentum of the final pi(rho) meson allows for the expansion of the decay form factors at q(2) = 0 in the inverse powers of b-quark mass. This considerably simplifies the analysis of these decays. The 1/m(b) expansion is carried out up to the first order. It is found that the q(2)-dependence of the axial form factor A(1) is different from the q(2)-behaviour of other form factors. We find Gamma(B --> pi e nu)=(3.1 +/- 0.6) x \V-ub\(2) x 10(12) s(-1) and Gamma(B --> rho e nu)=(5.7 +/- 1.2) x \V-ub\(2) x 10(12) s(-1). The relation between semileptonic and rare radiative B-decays is discussed.