Soft dipole resonance in neutron-rich $^8$He

TL;DR

This study investigates the soft dipole resonance in neutron-rich $^8$He using a five-body cluster model, revealing two excitation modes and predicting a resonance at 14 MeV with a 21 MeV decay width.

Contribution

It provides a detailed analysis of the soft dipole resonance in $^8$He, identifying excitation modes and predicting a specific resonance state using complex scaling and Green's function methods.

Findings

Two dominant excitation modes below 20 MeV identified.

Resonance predicted at 14 MeV with a decay width of 21 MeV.

Soft dipole resonance accounts for about half of the dipole strength.

Abstract

In neutron-rich $^8$He, we study the soft dipole resonance, which is regarded as a dipole oscillation of four valence neutrons against the $^4$He core, and its effect on the low-energy electric dipole strength with a $^4$He+$n$+$n$+$n$+$n$ five-body cluster model. This work is an extended study of an earlier letter [T. Myo and K. Kat\=o, Phys. Rev. C106, L021302 (2022)]. The five-body unbound $1^-$ states of $^8$He 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 $^7$He+$n$ channel, which sequentially decays to $^6$He+$n$+$n$. Above 10 MeV, the strengths arise from the soft dipole mode of four neutrons ($4n$) oscillating against the $^4$He 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 $^4$He core and $4n$.