Dineutron-dineutron correlation in $^8$He

Background: The four-neutron correlation has been attracting much attention for decades. In addition to the study on the tetra-neutron system, it is worthwhile to investigate the correlation in bound systems. Purpose: The $^8$He nucleus is a system where four neutrons are weakly bound around the $^4$He core. The dineutron ($2n$) correlation has been long discussed in various weakly-bound neutron-rich nuclei such as $^6$He and $^{11}$Li, whereas the $^8$He nucleus gives us an opportunity to investigate the $2n$-$2n$ type four-neutron correlation. Methods: We introduce a microscopic $^4{\rm He}+4n$ model and describe the ground-state structure of $^8$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 $^4$He in the ground state of $^8$He: The ground state has a squared overlap of about 45% with a $2n$-$^4$He-$2n$ configuration with the $^4$He-$2n$ distance of 3 fm and opening angle of 80$^\circ$. Conclusion: The ground state of $^8$He contains a certain amount of the two-$2n$ cluster component, indicating the strong nuclear deformation, which was experimentally observed recently.