Making use of the measurement of the $\overrightarrow{B}{K}^{*}\gamma$ branching ratio together with the relations following from the limit of high recoil energy, we obtain stringent constraints on the values of the form factors entering in heavy-to-light $\overrightarrow{B}{\mathrm{Vll}}^{\prime }$ processes such as $\overrightarrow{B}{K}^{*}{l}^{+}{l}^{-},$ $\overrightarrow{B}{K}^{*}\nu \overline{\nu }$ and $\overrightarrow{B}\rho l\nu$ decays. We show that the symmetry predictions, when combined with the experimental information on radiative decays, specify a severely restricted set of values for the vector and axial-vector form factors evaluated at zero momentum transfer, $q^2=0.\mathrm{}$ These constraints can be used to test model calculations and to improve our understanding of the $q^2$ dependence of semileptonic form factors. We stress that the constraints remain stringent even when corrections are taken into account.

• Received 21 December 2000

DOI:https://doi.org/10.1103/PhysRevD.63.113008