Weak form factors for heavy meson decays
  We calculate the form factors for weak decays of $B_{(s)}$ and $D_{(s)}$ mesons to light pseudoscalar and vector mesons within a relativistic dispersion approach based on the constituent quark picture.   This approach gives the form factors as relativistic double spectral representations in terms of the wave functions of the initial and final mesons. The form factors have the correct analytic properties and satisfy general requirements of nonperturbative QCD in the heavy quark limit. The effective quark masses and meson wave functions are determined by fitting the quark model parameters to lattice QCD results for the $B\to \rho$ transition form factors at large momentum transfers and to the measured $D\to (K,K^*)l\nu$ decay rates. This allows us to predict numerous form factors for all kinematically accessible $q^2$ values. 
Dmitri Melikhov and Berthold Stech