On the possible existence of a soft dipole mode in 8He

TL;DR

This study uses covariant density functional theory to investigate the dipole response of 8He, finding no evidence for a soft dipole mode despite its extreme neutron richness, due to the decoupling of spurious center-of-mass effects.

Contribution

The paper applies a covariant DFT approach to analyze 8He’s dipole response, highlighting the decoupling of spurious motion and challenging the existence of a soft dipole mode in this nucleus.

Findings

No soft dipole mode observed in 8He.

Spurious center-of-mass contamination shifts to zero energy.

Strong mixing of isoscalar and isovector modes expected.

Abstract

With an extreme neutron-to-proton ratio of N/Z=3, 8He provides an ideal laboratory for the study of a variety of exotic phenomena, such as the emergence of a soft dipole mode that is dominated by transitions into the continuum. In this contribution, a covariant density functional theory (DFT) framework is used to compute ground-state properties and the dipole response of 8He. Although 8He is admittedly too light for DFT to be applicable, the great merit of the approach is that the spurious contamination associated with the center-of-mass motion is guaranteed to decouple from the physical response. Given that a strong mixing between the isoscalar and isovector dipole modes is expected for a system with such a large neutron-proton asymmetry as 8He, the narrow structures that emerged at low energies in the isovector dipole response are attributed to the shift of the spurious strength to zero (or near zero) excitation energy. Thus, the theoretical framework implemented here disfavors the emergence of a soft dipole mode in 8He.