COMMENT ON V-UB FROM EXCLUSIVE SEMILEPTONIC B-DECAY AND D-DECAY

The prospects for determining \V-ub\ from exclusive B semileptonic dec ay are discussed. The double ratio of form factors (f((B --> rho))/f(( B --> K)))/(f((D --> rho))/f((D --> K*))) is calculated using chiral perturbation theory. Its deviation from unity due to contributions tha t are non-analytic in the symmetry breaking parameters is very small. Combining experimental data obtainable from B --> rho l(-)nu(l), B --> K l (l) over bar and D --> rho l nu(l) can lead to a model independe nt determination of \V-ub\ with an uncertainty from theory of about 10 %. (C) 1998 Elsevier Science B.V.