Description of the Proton-Decaying 02+ Resonance of the α Particle

The recent precise experimental determination of the monopole transition form factor from the ground state of He-4 to its 0(2)(+) resonance via electron scattering has reinvigorated discussions about the nature of this first excited state of the a particle. The 0(2)(+) state has been traditionally interpreted in the literature as the isoscalar monopole resonance (breathing mode) or, alternatively, as a particle-hole shell-model excitation. To better understand the nature of this state, which lies only similar to 410 keV above the proton emission threshold, we employ the coupled-channel representation of the no-core Gamow shell model. By considering the [3H + p], [He-3 + n], and [H-2 + H-2] reaction channels, we explain the excitation energy and monopole form factor of the 0(2)(+) state. We argue that the continuum coupling strongly impacts the nature of this state, which carries characteristics of the proton decay threshold.