Soft contribution to B→γℓνℓ from dispersion relations and duality.

The decay B→γℓνℓ receives a numerically large soft-overlap contribution which is formally next-to-leading order in the expansion in the inverse photon energy, 1/Eγ. We point out that this contribution can be calculated with little model-dependence making use of dispersion relations and duality, in full analogy with the similar contribution to the γγ*→π transition form factor. This result strengthens the case for the utility of the B→γℓνℓ decay for constraining the B-meson distribution amplitude and determining its most important parameter, the inverse moment λB. [ABSTRACT FROM AUTHOR]