Charge radii and neutron correlations in helium halo nuclei

Within the complex-energy configuration-interaction framework, we study correlations of valence neutrons to explain the behavior of charge radii in neutron halo nuclei 6,8He. We find that the experimentally observed decrease of the charge radius between ⁶He and ⁸He is caused by a subtle interplay between three effects: dineutron correlations, a spin-orbit contribution to the charge radius, and a core swelling effect. We demonstrate that two-neutron angular correlations in the 2₁⁺ resonance of ⁶He differ markedly from the ground-state correlations in 6,8He. Finally, we discuss the impact of the neutron threshold position and valence neutron correlation energy on the neutron radius, i.e., the pairing-antihalo effect.