Shell evolution in neutron-rich carbon isotopes: Unexpected enhanced role of neutron-neutron correlation

TL;DR

This study uses shell-model calculations to reveal that neutron-neutron correlations significantly influence shell evolution in neutron-rich carbon isotopes, especially in N=14 nuclei, challenging prior assumptions about proton-neutron dominance.

Contribution

It demonstrates the unexpectedly large role of neutron-neutron monopole interactions in shell evolution, highlighting enhanced many-body correlations in neutron-rich carbon isotopes.

Findings

Neutron-neutron monopole terms significantly reduce the N=14 shell gap in C and N isotopes.

Enhanced configuration mixing due to many-body correlations explains the large B(E2) value in $^{20}$C.

Neutron-neutron correlations can be as influential as proton-neutron interactions in shell evolution.

Abstract

Full shell-model diagonalization has been performed to study the structure of neutron-rich nuclei around $^{20}$C. We investigate in detail the roles played by the different monopole components of the effective interaction in the evolution of the N=14 shell in C, N and O isotopes. It is found that the relevant neutron-neutron monopole terms, $V^{nn}_{d_{5/2}d_{5/2}}$ and $V^{nn}_{s_{1/2}s_{1/2}}$, contribute significantly to the reduction of the N=14 shell gap in C and N isotopes in comparison with that in O isotopes. The origin of this unexpectedly large effect, which is comparable with (sometimes even larger than) that caused by the proton-neutron interaction, is related to the enhanced configuration mixing in those nuclei due to many-body correlations. Such a scheme is also supported by the large B(E2) value in the nucleus $^{20}$C which has been measured recently.