Structure of the exotic $^9$He nucleus from the no-core shell model with continuum
Résumé
Background: The exotic He9 nucleus, which presents one of the most extreme neutron-to-proton ratios, belongs to the N=7 isotonic chain famous for the phenomenon of ground-state parity inversion with decreasing number of protons. Consequently, it would be expected to have an unnatural (positive) parity ground state similar to Be11 and Li10. Despite many experimental and theoretical investigations, its structure remains uncertain. Apart from the fact that it is unbound, other properties including the spin and parity of its ground state, and the very existence of additional low-lying resonances are still a matter of debate. Purpose: In this work, we study the properties of He9 by analyzing the n+He8 continuum in the context of the ab initio no-core shell model with continuum (NCSMC) formalism with chiral nucleon-nucleon interactions as the only input. Methods: The NCSMC is a state-of-the-art approach for the ab initio description of light nuclei. With its capability to predict properties of bound states, resonances, and scattering states in a unified framework, the method is particularly well suited for the study of unbound nuclei such as He9. Results: Our analysis produces an unbound He9 nucleus. Two resonant states are found at the energies of ∼1 and ∼3.5 MeV, respectively, above the n+He8 breakup threshold. The first state has a spin-parity assignment of Jπ=1/2− and can be associated with the ground state of He9, while the second, broader state has a spin parity of 3/2−. No resonance is found in the 1/2+ channel, only a very weak attraction. Conclusions: We find that the He9 ground-state resonance has a negative parity and thus breaks the parity-inversion mechanism found in the Be11 and Li10 nuclei of the same N=7 isotonic chain.