Abstract
The low-lying structure of ¹⁵C has been investigated via the neutron-removal ¹⁶C(d,t) reaction. Along with bound neutron sd-shell hole states, unbound p-shell hole states have been firmly confirmed. The excitation energies and the deduced spectroscopic factors of the cross-shell states are an important measure of the [(p)−1(sd)2] neutron configurations in ¹⁵C. Our results show a very good agreement with shell-model calculations using the SFO-tls interaction for ¹⁵C. However, a modification of the p-sd and sd-sd monopole terms was applied in order to reproduce the N=9 isotone ¹⁷O. In addition, the excitation energies and spectroscopic factors have been compared to the first calculations of ¹⁵C with the ab initio self-consistent Green's function method employing the NNLOsat interaction. The results show the sensitivity to the size of the N=8 shell gap and highlight the need of going beyond the current truncation scheme in the theory.