A recent report of events by the BESIII collaboration suggests the presence of a structure at 3900 MeV. We argue that this structure, called in the past, is not in fact due to a new resonance but rather naturally emerges due to a combination of interference between nearby resonances and the opening of the channel. We further find that the appearance of this structure does not require suppression because of a radial node in the wave function, although a node improves fit quality. The measured coupling of is found to be substantially smaller than previously estimated. In addition, we report new corrections to the measured cross section at energies near . Published by the American Physical Society2024
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Measurements of the electron-helicity asymmetry in the quasi-elastic A(e→,e′p) process
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We search for the rare decay in a sample of electron-positron collisions at the resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying meson in events to suppress background from other decays of the signal candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the branching fraction of and , respectively. Combining the results, we determine the branching fraction of the decay to be , providing the first evidence for this decay at 3.5 standard deviations. The combined result is 2.7 standard deviations above the standard model expectation. Published by the American Physical Society2024more » « less