Modeling the electric dipole strength of neutron-rich $^8$He: Prediction of multineutron collective excitations

TL;DR

This paper models the electric dipole strength of neutron-rich $^8$He using a five-body approach, revealing coexistence of different excitation modes and suggesting a new collective neutron motion.

Contribution

It introduces a five-body model with complex scaling to analyze multineutron excitations and identifies new collective motion in $^8$He.

Findings

Strength below 10 MeV from $^7$He+$n$ channel indicates sequential breakup.

Strength above 10 MeV suggests a collective four-neutron oscillation.

Multiple excitation modes coexist in the dipole strength spectrum.

Abstract

We investigate the multineutron excitations of neutron-rich $^8$He in the electric dipole strength using a $^4$He+$n$+$n$+$n$+$n$ five-body model. Many-body unbound states of $^8$He are obtained in the complex scaling method. We found that the different excitation modes coexist in the dipole strength below the excitation energy of 20 MeV. The strength below 10 MeV comes from the $^7$He+$n$ channel, indicating the sequential breakup of $^8$He via the $^7$He resonance to $^6$He+$n$+$n$. Above 10 MeV, the strength is obtained from many-body continuum states with strong configuration mixings, suggesting a new collective motion of four neutrons with the dipole oscillation against $^4$He.