Effective Description of $${ {}}^{5-10}\text {He} $$ and the Search for a Narrow $${{}^{4}\text {n}}$$ Resonance

Open quantum systems that are at or beyond the limit of particle-emission stability exhibit generic features stemming from their coupling to an environment of positive energy states and decay channels. In nuclear physics, the exotic helium isotopes \({ {}^{5-10}\text {He} }\) and the four-neutron system \({ {}^{4}\text {n} }\) represent two prototypical cases of open quantum systems whose structures are highly impacted by the environment. In the first part, a practical approach inspired by halo effective field theory for the description of \({ {}^{5-10}\text {He} }\) within tens of keV uncertainties is presented and the parity inversion in \({ {}^{9}\text {He} }\), as well as the possible two-neutron decay of \({ {}^{10}\text {He} }\) are discussed. The second part discusses the last ab initio results on the four-neutron system obtained using chiral forces and including continuum couplings.