https://doi.org/10.1140/epjc/s10052-025-15035-7
Regular Article - Theoretical Physics
Discovering the 
in B semileptonic decays
1
School of Physics, University of Electronic Science and Technology of China, 611731, Chengdu, China
2
Institute of Theoretical Physics, Chinese Academy of Sciences, 100190, Beijing, China
3
School of Physical Sciences, University of Chinese Academy of Sciences, 100049, Beijing, China
4
Southern Center for Nuclear-Science Theory (SCNT), Institute of Modern Physics, Chinese Academy of Sciences, 516000, Huizhou, China
5
Institute for Advanced Simulation, Forschungszentrum Jülich, 52425, Jülich, Germany
6
Physics Institute, Universität Zürich, Winterthurerstrasse 190, 8057, Zürich, Switzerland
7
Helmholtz-Institut für Strahlen- und Kernphysik (Theorie) and Bethe Center for Theoretical Physics, Universität Bonn, 53115, Bonn, Germany
8
Institut für Experimentelle Teilchenphysik, Karlsruhe Institute of Technology (KIT), 76131, Karlsruhe, Germany
a
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Received:
25
September
2025
Accepted:
1
November
2025
Published online:
12
November
2025
Abstract
The mass and width of the lightest scalar open-charm state listed in the Review of Particle Physics, the 
, 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 
decays that allow for a direct extraction of the 
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 using the currently available data set. We also investigate the possibility and necessary statistics of measuring the 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.
© The Author(s) 2025
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Funded by SCOAP3.
