Description of the proton-decaying 0+2 resonance of the α particle
Abstract
The recent precise experimental determination of the monopole transition form factor from the ground state of 4He to its 0+2 resonance via electron scattering has reinvigorated discussions about the nature of this first excited state of the α 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 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], [3He + n], and [2H+2H] 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.
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