Shell spectroscopy sensitivity via the ground state population of ²⁶O from halo nuclei in proton removal reactions

The ground state of ²⁶O is unbound to 2n decay by only 18 ± 3 (stat) ± 4 (sys) keV. The signature of this state enables a unique sensitivity to probe the halo structure of ²⁹Ne. The invariant mass technique was used to reconstruct the decay energies of ²⁵O and ²⁶O produced from ²⁷F (105.3 MeV/u) and ²⁹Ne (112.8 MeV/u) beams impinging on a Be target using the MoNA-Sweeper setup at the National Superconducting Cyclotron Laboratory. The measured distributions obtained from the two beams differ substantially, especially in the low energy region where the low lying 18 keV state is present for ²⁷F but absent for the ²⁹Ne for both ²⁵O and ²⁶O decay energies. This behavior is believed to originate from the halo nature of ²⁹Ne which has a ground state with a J^π = 3/2^-. A density matrix renormalization group (DMRG) calculation predicts energy levels of ²⁷O with respect to a ²⁴O core with a J^π = 3/2^- excited state located at 5.653 MeV above its J^π = 3/2⁺ (0.637 MeV) ground state. Coupled with the lower probability of ²⁹Ne(-2p1n) compared to ²⁹Ne(-2p) to produce the ²⁶O, the most likely scenario is a population of such predicted 3/2^- state of ²⁷O.