We use rigorous QCD dispersion relations to derive model-independent bounds on the $\overline{B}\to \pi l\overline{\nu }$, $D\to \pi \overline{l}\nu$, and $D\to \overline{K}\overline{l}\nu$ 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 $f_B\le 195$ 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.

• Received 22 December 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.4603