Abstract
Background: The exotic He-9 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 Be-11 and Li-10. 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 He-9 by analyzing the n + He-8 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 NCSMCis 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 He-9.
Results: Our analysis produces an unbound He-9 nucleus. Two resonant states are found at the energies of similar to 1 and similar to 3.5MeV, respectively, above the n + 8He breakup threshold. The first state has a spin-parity assignment of J(pi) = 1/2(-) and can be associated with the ground state of He-9, 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 He-9 ground-state resonance has a negative parity and thus breaks the parity-inversion mechanism found in the Be-11 and Li-10 nuclei of the same N = 7 isotonic chain.