Energy Spectrum of Neutron-Rich Helium Isotopes: Complex Made Simple

Abstract

We demonstrate that the intricate energy spectrum of neutron-rich helium isotopes can be straightforwardly described by taking advantage of the low-energy properties of neutron-neutron interaction and the scale separation that is present in diluted dripline systems. By using arguments based on the halo effective field theory, we carry out a parameter reduction of the complex-energy configuration interaction framework in the $spd$ space, including resonant and scattering states. By adjusting only one parameter, the strength of the spin-singlet central neutron-neutron interaction, we reproduce experimental energies and widths of ${}^{5-8}$He within tens of keV precision. We predict a parity inversion of narrow resonances in ${}^9$He and show that the ground state of ${}^{10}$He is an $s$-wave-dominated threshold configuration that could decay through two-neutron emission.