  We present a new method aiming at a non-perturbative, model-independent
determination of the momentum dependence of the form factors entering
semileptonic decays using unitarity and analyticity constraints. We extend the
original proposal and, using suitable two-point functions computed
non-perturbatively, we determine the form factors at low-momentum transfer
$q^2$ from those computed explicitly on the lattice at large $q^2$, without
making any assumption about their $q^2$ dependence. As a training ground we
apply the new method to the analysis of the lattice data of the semileptonic $D
\rightarrow K \ell \nu_{\ell}$ decays obtained both at finite values of the
lattice spacing and at the physical pion point in the continuum limit. We show
that, starting from a limited set of data at large $q^2$, it is possible to
determine quite precisely the form factors in a model independent way in the
full kinematical range, obtaining a remarkable agreement with the direct
calculation of the form factors. This finding opens the possibility to obtain
non-perturbatively the form factors entering the semileptonic B decays in the
full kinematical range.
