LAUSR

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 drip-line 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 constraining the core potential to $\ensuremath{\alpha}\text{\ensuremath{-}}n$ scattering phase shifts and adjusting the strength of the spin-singlet central neutron-neutron interaction, we reproduce experimental energies and widths of ${}^{5--8}\mathrm{He}$ within tens of keV precision. We predict a parity inversion of narrow resonances in $^{9}\mathrm{He}$ and show that the ground state of $^{10}\mathrm{He}$ is an $s$-wave-dominated configuration that could decay through two-neutron emission. This threshold state can be viewed as a ``double-halo'' structure in an analogy to the atomic $^{3}\mathrm{He}\phantom{\rule{0.16em}{0ex}}{}^{4}{\mathrm{He}}_{2}$ trimer.