Discovering the D₀⁽2100) in B semileptonic decays

The mass and width of the lightest scalar open-charm state listed in the Review of Particle Physics, the D₀⁽2300), are in puzzling tension with predictions from unitarized chiral perturbation theory (UChPT) and lattice QCD, which favor a lighter state at around $2100$ MeV. However, to date, no direct experimental evidence for this lighter state exists. In an effort to facilitate a direct observation, we introduce angular asymmetries of B→ D π ν decays that allow for a direct extraction of the $Dπ$ S-wave phase shift and discuss a novel measurement strategy for the Belle II experiment. We conduct a sensitivity study, finding that the Belle II experiment can determine the pole location with sufficient precision to firmly establish the D₀⁽2100) using the currently available data set. We also investigate the possibility and necessary statistics of measuring the $Dπ$ isospin 1/2 scattering length with an accuracy sufficient to distinguish between the predictions from both UChPT and lattice QCD and the measurement by ALICE using femtoscopy.