Electron emission in the deuteron-deuteron reaction supporting existence of the single-particle threshold resonance in ${}^4\mathrm{He}$ has been observed for the first time. The measured electron energy spectrum and the electron-proton branching ratio agree very well with the assumed electron-positron pair creation decay of the $0^{+}$ resonance state to the ground state and the detailed Monte Carlo simulations of the experimental energy spectrum.

• Received 5 June 2022
• Revised 26 November 2023
• Accepted 22 January 2024

DOI:https://doi.org/10.1103/PhysRevC.109.L021601

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Published by the American Physical Society