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We revisit QCD factorization of $B\to \pi\pi$ form factors at large dipionmasses, by deriving new constraints based on the analyticity properties ofthese objects. We then propose a parametrization of the form factors, inspiredby the leading-twist QCD factorization formula, that incorporates all knownanalytic properties. This parameterization is used to interpolate between theQCDF results and the constraints from the $B^*$ pole. Based on thisinterpolation, we predict the $B\to \pi\pi\ell\nu$ decay rate in a larger phasespace region than previous studies could. We obtain a partially-integratedbranching ratio up to $\mathcal{B} \simeq \mathcal{O}({10^{-6}})$, whichimplies that a measurement of the non-resonant semileptonic decay ispotentially within reach of the Belle II experiment.

Revisiting $B \to \pi\pi \ell \nu$ at Large Dipion Masses