We consider the form factors for the radiative semileptonic decays $\bar{ B}(v) \rightarrow D^{ (*)}(v') \ell { \bar \nu}_\ell \gamma$ in the kinematic region where the photon momentum, $k$, is small enough that heavy quark symmetry (HQS) can be applied without the radiated photon changing the heavy quark velocity (i.e., $v^{ (\prime)} \cdot k < m_{ (b,c)}$). We find that HQS is remarkably powerful, leaving only four new undetermined form factors at leading order in $1/m_{ (b,c)}$. In addition, one of them is fixed in terms of the leading order Isgur-Wise function in the kinematic region, $v^{ (\prime)}\cdot k < \Lambda_\text{ QCD}$.
Categories: hep-ph hep-ex