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

Abstract The mass and width of the lightest scalar open-charm state listed in the Review of Particle Physics, the $$D_0^*(2300)$$ 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 $$B\rightarrow D \pi \ell \nu $$ B → D π ℓ ν decays that allow for a direct extraction of the $$D\pi $$ D π 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 $$D_0^*(2100)$$ D 0 ∗ ( 2100 ) using the currently available data set. We also investigate the possibility and necessary statistics of measuring the $$D\pi $$ D π 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.