Radiative semileptonic $ \overline{B} $ decays

We consider the form factors for the radiative semileptonic decays $$ \overline{B} $$B ¯ (v) → D^(*)(v′)ℓ $$ \overline{\nu} $$ν ¯ _ℓγ 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^(′) ∙ 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^(′) ∙ k < Λ_QCD.