Intruder states in ⁸Be

Low-lying intruder T=0 states in ⁸Be have been posited and challenged. To address this issue, we performed ab initio shell model calculations in model spaces consisting of up to 10ħΩ excitations above the unperturbed ground state with the basis state dimensions reaching 1.87×10⁸. To gain predictive power we derive and use effective interactions from realistic nucleon-nucleon (NN) potentials in a way that guarantees convergence to the exact solution with increasing model space. Our 0ħΩ 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 2ħΩ dominated T=0 states. In the 10ħΩ 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 α-α elastic scattering. In addition, we present our predictions for the A=8 binding energies with the CD-Bonn NN potential.