Soft dipole resonance in neutron-rich 8He

Abstract In neutron-rich 8He, we study the soft dipole resonance, which is regarded as a dipole oscillation of four valence neutrons against the 4He core, and its effect on the low-energy electric dipole strength with a 4He+n+n+n+n five-body cluster model. This work is an extended study of an earlier letter [T. Myo and K. Katō, Phys. Rev. C 106, L021302 (2022)]. The five-body unbound 1− states of 8He are obtained with complex-energy eigenvalues by using the complex scaling method and the dipole strength is calculated in terms of the complex-scaled Green’s function. Two kinds of dominant excitation modes are confirmed in the dipole strength below 20 MeV of the excitation energy. The strengths below 10 MeV are exhausted by the 7He+n channel, which sequentially decays to 6He+n+n. Above 10 MeV, the strengths arise from the soft dipole mode of four neutrons (4n) oscillating against the 4He core. We further explore the possibility of the soft dipole resonance for this state by carefully searching for the resonance pole and finally predict the corresponding resonance with the excitation energy of 14 MeV and the decay width of 21 MeV. The soft dipole resonance exhausts about half of the dipole strength in the relative motion between the 4He core and 4n.