This paper introduces a new way to apply halo effective field theory (Halo EFT) to the neutron-rich nucleus 6He. The authors show that using finite-range, separable interactions avoids a technical problem that appears in the usual dimer-field implementation of Halo EFT. That standard method can generate energy-dependent interactions and a non-standard normalization of wave functions when a leading-order interaction requires two effective-range parameters. The finite-range approach keeps the interaction energy independent and so fits more cleanly into standard quantum mechanics.

Concretely, the authors implement separable finite-range potentials with Yamaguchi-like form factors. A Yamaguchi potential is a simple rank-one separable interaction specified by a coupling strength and a range parameter. They use these potentials to build a three-body model of 6He (an alpha core plus two neutrons) and solve the Faddeev equations for the bound state up to next-to-leading order (NLO) in the Halo EFT power counting. A three-body force is included to renormalize the three-body sector, as is standard in few-body EFT treatments.

From their bound-state wave function they compute two key observables. First, the root-mean-square (rms) charge radius of 6He comes out as 2.06 ± 0.35 fm at leading order (LO, the first approximation) and 2.00 ± 0.09 fm at next-to-leading order. Those numbers agree with experimental measurements. Second, they calculate the ground-state E1 strength distribution. The E1 distribution is the electric-dipole response that controls low-energy Coulomb breakup of 6He and is therefore directly comparable to experimental breakup spectra.

To get the full E1 breakup spectrum the authors include final-state interactions among the outgoing particles. They approximate the full three-body scattering operator first by single Møller operators and then by products of up to three Møller operators. (A Møller operator is a standard tool in scattering theory used to account for interactions in the final state.) With this approximation the NLO E1 strength distribution they obtain agrees with the experimental data within the quoted theory uncertainties. They also verify that the shape of the E1 distribution in the finite-range formulation matches that found in previous dimer-based EFT work, but without needing the non-standard wave-function normalization.