The dipole strength distribution of $^8$He and decay characteristics

TL;DR

This study measures the dipole response of the neutron-rich nucleus $^8$He, including the four-neutron decay channel, and compares results with advanced theoretical models to understand its decay characteristics.

Contribution

First measurement of $^8$He dipole response including four-neutron decay, providing new insights into its decay modes and structure.

Findings

Total dipole strength is 0.95(16) e^2fm^2 for E*<15 MeV.

Dipole polarizability is 0.61(1) fm^3.

Dipole continuum dominated by two-neutron emission, with no evidence for four-neutron final-state correlation.

Abstract

The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus $^8$He is the most neutron-rich bound nucleus with a mass-to-charge ratio of $A/Z=4$. We measure the dipole response of $^8$He, including for the first time the four-neutron decay channel. A total dipole strength of $\sum B(E1)(E^*<15$~MeV$)=0.95(16)~e^2$fm$^2$ and a dipole polarizability of $\alpha_D = 0.61(1)$~fm$^3$ are extracted from the differential Coulomb-excitation cross section and compared to state-of-the-art theoretical calculations employing coupled cluster and three-body approaches. We find that the dipole continuum is dominated, even at high excitation energies well above the $4n$ decay threshold, by two-neutron emission, pointing to a $^6$He$+2n$ structure of the excited dipole mode. No indication was found for a $4n$ final-state correlation, while pronounced $nn$ and $^6$He-$n$ final-state correlations are apparent.