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 ${}^{6,8}$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. 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.

Figure 1

Two-neutron GSM density (in fm${}^{-2}$) of the 0${}^{+}$ ground state of ${}^6$He: (a) total; (b) $S=0$ component; (c) $S=1$ component; (d) density in $p_{3/2}$ model space.

Figure 2

Two-neutron GSM density (in fm${}^{-2}$) of the 2${}_1^{+}$ resonance in ${}^6$He: (a) total; (b) $S=0$ component; (c) $S=1$ component; (d) density with the two-body interaction turned off.

Figure 3

Angular $2n$ GSM densities ($S=0$, 1, and total) for g.s. configurations of ${}^6$He (solid lines) and ${}^8$He (dashed lines).

Figure 4

Different contributions to the charge radius of ${}^6$He (solid line, squares) and ${}^8$He (dashed line, dots) calculated in the GSM. The core swelling contribution is taken from GFMC calculations of Ref. [22]. Recently revised experimental charge radii come from [7] (triangles). The NCSM [10] (stars) and GFMC [9] (pentagons) results are marked for comparison. The inset shows GSM rms neutron radii compared to experimental results [34].

Figure 5

(a) Evolution of the GSM rms neutron radius of ${}^6$He as a function of the $2n$ separation energy $S_{2n}$. The position of the $2n$ threshold is varied by (i) changing the strength of the MN interaction (squares) or by (ii) changing the strength of the WS potential (circles). The complex energy $E-i\Gamma /2$ (in MeV) of the $0p_{3/2}$ Gamow resonant state of the WS potential is marked for three values of $S_{2n}$. (b) Correlation energy, i.e., the expectation value of the MN interaction, in the cases (i) and (ii). The $2n$ threshold $S_{2n}=0$ is marked by a vertical line.