COMMENT ON VUB FROM EXCLUSIVE SEMILEPTONIC B AND D DECAYS

Abstract The prospects for determining |Vub| from exclusive B semileptonic decay are discussed. The double ratio of form factors (f (B→ρ) /f (B→K ∗ ) )/(f (D→ρ) /f (D→K ∗ ) ) is calculated using chiral perturbation theory. Its deviation from unity due to contributions that are non-analytic in the symmetry breaking parameters is very small. Combining experimental data obtainable from B→ρ l ν l , B→K ∗ l l and D→ρ l ν l can lead to a model independent determination of |Vub| with an uncertainty from theory of about 10%.