Intruder states in 8Be -: art. no. 051301

Low-lying intruder T=0 states in Be-8 have been posited and challenged. To address this issue, we performed ab initio shell model calculations in model spaces consisting of up to 10 (h) over bar Omega excitations above the unperturbed ground state with the basis state dimensions reaching 1.87x10(8). 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 0 (h) over bar 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 (h) over bar Omega dominated T=0 states. In the 10 (h) over bar 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 alpha - alpha elastic scattering. In addition, we present our predictions for the A = 8 binding energies with the CD-Bonn NN potential.