$^6$He nucleus in halo effective field theory

TL;DR

This paper develops a halo effective field theory for the Borromean nucleus $^6$He, using valence neutrons and an inert alpha core, and demonstrates how to renormalize the system at leading order with a three-body operator.

Contribution

The work introduces a Halo EFT for $^6$He that accounts for p-wave interactions and demonstrates renormalization with a single three-body parameter at leading order.

Findings

The two-neutron separation energy $S_{2n}$ can be reproduced with a three-body operator.

Cutoff independence is achieved for cutoffs above the EFT breakdown scale.

The running of the three-body counterterm differs from s-wave-only systems due to p-wave interactions.

Abstract

Background: In recent years properties of light rare isotopes have been measured with high accuracy. At the same time, the theoretical description of light nuclei has made enormous progress, and properties of, e.g., the Helium isotopes can now be calculated {\it ab initio}. These advances make those rare isotopes an ideal testing ground for effective field theories (EFTs) built upon cluster degrees of freedom. Purpose: Systems with widely separated intrinsic scales are well suited to an EFT treatment. The Borromean halo nucleus $^6$He exhibits such a separation of scales. In this work an EFT in which the degrees of freedom are the valence neutrons ($n$) and an inert $^4$He-core ($\alpha$) is employed. The properties of ${}^6$He can then be calculated using the momentum-space Faddeev equations for the $\alpha nn$ bound state to obtain information on ${}^6$He at leading order (LO) within the EFT. Results: The $nn$ virtual state and the $^2$P$_{3/2}$ resonance in $^5$He give the two-body amplitudes which are input to our LO three-body Halo EFT calculation. We find that without a genuine three-body interaction the two-neutron separation energy $S_{2n}$ of ${}^6$He is strongly cutoff dependent. We introduce a $nn \alpha$ "three-body" operator which renormalizes the system, adjusting its coefficient to reproduce the $S_{2n}$ of $^6$He. The Faddeev components are then cutoff independent for cutoffs of the order of, and above, the breakdown scale of the Halo EFT. Conclusions: As in the case of a three-body system where only resonant s-wave interactions are present, one three-body input is required for the renormalization of the EFT equations that describe $^6$He at LO. However, in contrast to the s-wave-only case, the running of the LO $nn\alpha$ counterterm does not exhibit discrete scale invariance, due to the presence of the p-wave $n\alpha$ interaction.