Discovering the $D_0^*(2100)$ in B semileptonic decays

The mass and width of the lightest scalar opencharm state listed in the Review of Particle Physics, the D$^∗$$_0$ (2300), are in puzzling tension with predictions from unitarized chiral perturbation theory (UChPT) and lattice QCD, which favor a lighter state at around 2100 MeV. However, to date, no direct experimental evidence for this lighter state exists. In an effort to facilitate a direct observation, we introduce angular asymmetries of B → Dπν decays that allow for a direct extraction of the Dπ S-wave phase shift and discuss a novel measurement strategy for the Belle II experiment. We conduct a sensitivity study, finding that the Belle II experiment can determine the pole location with sufficient precision to firmly establish the D$^∗$$_0$ (2100) using the currently available data set. We also investigate the possibility and necessary statistics of measuring the Dπ isospin 1/2 scatteringlength with an accuracy sufficient to distinguish between the
predictions from both UChPT and lattice QCD and the measurement by ALICE using femtoscopy.