Subleading power corrections in radiative leptonic $B$ decay

I discuss the two-particle subleading power corrections in radiative leptonic $B \to \gamma \ell \nu$ decay at next-to-leading order in $\alpha_s$ with the dispersion approach. Employing the method of regions, factorization of the $B \to \gamma^{\ast}$ form factors is demonstrated explicitly, at one loop, for a space-like hard-collinear photon. The two-particle soft (end-point) contribution is shown to be suppressed by one power of $\Lambda/m_b$, in the heavy quark limit, compared with the leading power contribution computed from QCD factorization. I further report the recent calculation on the three-particle subleading power contribution to the on-shell $B \to \gamma$ form factors at tree level and demonstrate that the "soft" and the "hard" three-particle corrections are of the same power, in contrast to the two-particle counterparts, as already speculated from the rapidity divergence in the corresponding factorization formulae. Phenomenological implications of the subleading power contributions to the $B \to \gamma \ell \nu$ amplitude are also addressed in detail, focusing on the determination of the inverse moment of the leading-twist $B$-meson distribution amplitude.