Intruder states in8Be

Abstract

Low-lying intruder $T=0$ states in ${}^{8}\mathrm{Be}$ have been posited and challenged. To address this issue, we performed ab initio shell model calculations in model spaces consisting of up to $10\ensuremath{\Elzxh}\ensuremath{\Omega}$ excitations above the unperturbed ground state with the basis state dimensions reaching $1.87\ifmmode\times\else\texttimes\fi{}{10}^{8}.$ To gain predictive power we derive and use effective interactions from realistic nucleon-nucleon $(\mathrm{NN})$ potentials in a way that guarantees convergence to the exact solution with increasing model space. Our $0\ensuremath{\Elzxh}\ensuremath{\Omega}$ dominated states show good stability when the model space size increases. At the same time, we observe a rapid drop in excitation energy of the $2\ensuremath{\Elzxh}\ensuremath{\Omega}$ dominated $T=0$ states. In the $10\ensuremath{\Elzxh}\ensuremath{\Omega}$ space the intruder ${0}^{+}0$ state falls below 18 MeV of excitation and, also, below the lowest ${0}^{+}1$ state. Our extrapolations suggest that this state may stabilize around 12 MeV. We hypothesize that these states might be the broad resonance intruder states needed in R-matrix analysis of $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\alpha}$ elastic scattering. In addition, we present our predictions for the $A=8$ binding energies with the CD-Bonn $\mathrm{NN}$ potential.