Constraints on form factors for exclusive semileptonic heavy to light meson decays

We use rigorous QCD dispersion relations to derive model-independent bounds on the B̄→πlν̄, D→πl̄ν, and D→K̄l̄ν form factors. These bounds are particularly restrictive when the value of the observable form factor at one or more kinematic points is assumed. With reasonable assumptions we find fB≤195 MeV and that the shape of the form factor becomes severely constrained. These constraints are useful both for model discrimination and for model-insensitive extraction of Cabibbo-Kobayashi-Maskawa mixing parameters.