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 the neutron halo nuclei ${}^{6,8}$He. We find that the experimentally observed decrease of the charge radius between ${}^6$He and ${}^8$He 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 $2_1^{+}$ resonance of ${}^6$He differ markedly from the ground-state correlations in ${}^{6,8}$He.