Structure of the exotic $^9$He nucleus from the no-core shell model with continuum

摘要

The exotic $^9$He nucleus, which presents one of the most extremeneutron-to-proton ratios, belongs to the $N=7$ isotonic chain famous for thephenomenon of ground-state parity inversion with decreasing number of protons.Consequently, it would be expected to have an unnatural (positive) parityground state similar to $^{11}$Be and $^{10}$Li. Despite many experimental andtheoretical investigations, its structure remains uncertain. Apart from thefact that it is unbound, other properties including the spin and parity of itsground state and the very existence of additional low-lying resonances arestill a matter of debate. In this work we study the properties of $^9$He byanalyzing the $n+^8$He continuum in the context of the ab initio no-core shellmodel with continuum (NCSMC) formalism with chiral interactions as the onlyinput. The NCSMC is a state-of-the-art approach for the ab initio descriptionof light nuclei. With its capability to predict properties of bound states,resonances, and scattering states in a unified framework, the method isparticularly well suited for the study of unbound nuclei such as $^9$He. Ouranalysis produces an unbound $^9$He nucleus. Two resonant states are found atthe energies of ${\sim}1$ and ${\sim}3.5$ MeV, respectively, above the $n+^8$Hebreakup threshold. The first state has a spin-parity assignment of $J^{\pi} ={1/2}^-$ and can be associated with the ground state of $^9$He, while thesecond, broader state has a spin-parity of ${3/2}^-$. No resonance is found inthe ${1/2}^+$ channel, only a very weak attraction. We find that the $^9$Heground-state resonance has a negative parity and thus breaks theparity-inversion mechanism found in the $^{11}$Be and $^{10}$Li nuclei of thesame $N=7$ isotonic chain.