Revisiting B → ππℓν at large dipion masses

We revisit QCD factorization of B → ππ form factors at large dipion masses, by deriving new constraints based on the analyticity properties of these objects. We then propose a parametrization of the form factors, inspired by the leading-twist QCD factorization formula, that incorporates all known analytic properties. This parameterization is used to interpolate between the QCDF results and the constraints from the B^* pole. Based on this interpolation, we predict the B → ππℓν decay rate in a larger phase space region than previous studies could. We obtain a partially-integrated branching ratio up to $B\simeq O\left({10}^{-6}\right)$, which implies that a measurement of the non-resonant semileptonic decay is potentially within reach of the Belle II experiment.