Dispersive Approach to Semileptonic Form-Factors in Heavy-to-Light Meson   Decays
  We study the semileptonic decays of heavy mesons into light pseudoscalars by making use of dispersion relations. Constraints from heavy quark symmetry, chiral symmetry and perturbative QCD are implemented into a dispersive model for the form-factors. Large deviations from $B^*$-pole dominance are observed in $B\to\pi\ell\nu$. We discuss the model prediction for this mode and its possible impact on the extraction of $|V_{ub}|$. 
Gustavo Burdman (Fermilab) and Joachim Kambor (Orsay)