Soft contribution to $B\to \gamma \ell \nu_\ell$ and the $B$-meson distribution amplitude

Abstract

The $B\to \gamma \ell \nu_\ell$ decay at large energies of the photon receives a numerically important soft-overlap contribution which is formally of the next-to-leading order in the expansion in the inverse photon energy. We point out that this contribution can be calculated within the framework of heavy-quark expansion and soft-collinear effective theory, making use of dispersion relations and quark-hadron duality. The soft-overlap contribution is obtained in a full analogy with the similar contribution to the $\gamma^* \gamma \to \pi$ transition form factor. This result strengthens the case for using the $B\to \gamma \ell \nu_\ell$ decay to constrain the $B$-meson distribution amplitude and determine its most important parameter, the inverse moment $\lambda_B$.