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 the letter (Phys. Rev. C106 (2022) L021302). The five-body unbound 1− states of 8He are obtained with the 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 the 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.

Possible interpretation of the complex expectation values associated with resonances

“…In Table I, we list the resonance energies E r and the decay widths for the resonance poles in five nuclei taken from [32,35,40]. These complex eigenenergies are obtained using the complex scaling.…”

“…Some of them are consistent with the experimental observations such as 6 He(2 + 1 ) and 6 Be(2 + 1 ). We recently predict the 1 − resonance of 8 He with a relatively higher resonance energy and a large decay width [31,32]. This state is a candidate of the soft dipole resonance (SDR) [44], in which the four valence neutrons (4n) are in a collective motion with a dipole oscillation against the α cluster core.…”

“…where 0 is the initial bound state. So far, we have discussed the cases of monopole, dipole, and quadrupole transitions for nuclei and investigated the contributions of resonances [27][28][29][30][31][32].…”

“…The Hamiltonian of 6 He, 6 Be, 8 He, and 8 C is the same as used in the previous studies [16,31,32,40]:…”

“…This resonance corresponds to the dipole excitation of the relative motion between the α cluster core and 4n from the ground state. The detailed structure and the dominant configurations of each resonance are given in the previous studies [31,32,35,40].…”