Background: The four-neutron correlation has been attracting much attention for decades. In addition to the study on the tetraneutron system, it is worthwhile to investigate the correlation in bound systems.

Purpose: The ${}^8\mathrm{He}$ nucleus is a system where four neutrons are weakly bound around the ${}^4\mathrm{He}$ core. The dineutron ($2n$) correlation has been long discussed in various weakly bound neutron-rich nuclei such as ${}^6\mathrm{He}$ and ${}^{11}\mathrm{Li}$, whereas the ${}^8\mathrm{He}$ nucleus gives us an opportunity to investigate the $2n\text{−}2n$ type four-neutron correlation.

Methods: We introduce a microscopic ${}^4\mathrm{He}+4n$ model and describe the ground-state structure of ${}^8\mathrm{He}$. The mixing of the two-$2n$ component in the ground state is examined. The ground-state wave function is verified by investigating various observables including high-energy scattering cross sections.

Results: Our model reasonably reproduces the available experimental data, the binding energy, charge radius, total reaction cross section, and proton-nucleus elastic scattering cross section data. We find that the significant mixing of the two-$2n$ cluster configurations around ${}^4\mathrm{He}$ in the ground state of ${}^8\mathrm{He}$: The ground state has a squared overlap of about 45% with a $2n\text{−}{}^4\mathrm{He}\text{−}2n$ configuration with the ${}^4\mathrm{He}\text{−}2n$ distance of 3 fm and opening angle of ${80}^{\circ }$.

Conclusions: The ground state of ${}^8\mathrm{He}$ contains a certain amount of the two-$2n$ cluster component, indicating the strong nuclear deformation, which was experimentally observed recently.

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• Binding energy & masses
• Cluster models
• Elastic scattering reactions
• Nuclear radii
• Nucleon distribution
• 6 ≤ A ≤ 19