Continuum effects in neutron-drip-line oxygen isotopes

The binding-energy pattern along the neutron-rich oxygen chain, governed by an interplay between shell effects and many-body correlations impacted by strong couplings to one- and two-neutron continua, make these isotopes a unique testing ground for nuclear models. In this work, we investigate ground states and low-lying excited states of ${}^{23-28}\mathrm{O}$ using the complex-energy Gamow shell model and density matrix renormalization group method with a finite-range two-body interaction optimized to the bound states and resonances of ${}^{23-26}\mathrm{O}$, assuming a core of ${}^{22}\mathrm{O}$. Our results suggest that the ground state of ${}^{28}\mathrm{O}$ has a threshold character, i.e., is very weakly bound or slightly unbound. We also predict narrow excited resonances in ${}^{25}\mathrm{O}$ and ${}^{27}\mathrm{O}$. The inclusion of the large continuum space significantly impacts predicted binding energies of ${}^{26-28}\mathrm{O}$. This implies that the careful treatment of a neutron continuum is necessary prior to assessing the spectroscopic quality of effective interactions in this region.

Figure 1

Ground-state binding energies of ${}^{25-28}\mathrm{O}$ relative to ${}^{24}\mathrm{O}$ obtained in various theoretical approaches using various interactions: SM USDB [27], $\mathrm{SM}+3\mathrm{NF}$ [32], $\mathrm{SM}+3\mathrm{NF}$ in the extended $sd{f}_{7/2}{p}_{3/2}$ space, CSM [29, 30], and IMSRG [46, 47]. If a given theoretical approach was used with several interactions, those predictions are marked by identical symbols and/or lines. Open boxes represent the results obtained in this work (variant B), and the shaded area shows the impact of the enlarged continuum space; see discussion in the text.

Figure 2

Energies of ${}^{23-28}\mathrm{O}$ obtained in GSM variants A and B and compared to experiment. The “GSM-pole” results are obtained by using the interaction of variant B and removing the scattering space. The widths are marked by shaded bands.

Figure 3

Ground-state energies of ${}^{26}\mathrm{O}$ and ${}^{28}\mathrm{O}$ relative to ${}^{24}\mathrm{O}$ computed in the extended $spdf$ space as a function of the renormalization factor for the interaction B. The $sd$-space prediction for ${}^{26}\mathrm{O}$ is marked by a dotted line.