Intruder states in Be-8 - 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 mode l spaces consisting of up to 10 (h) over bar Omega excitations above the un perturbed ground state with the basis state dimensions reaching 1.87x10(8). To gain predictive power we derive and use effective interactions from rea listic nucleon-nucleon (NN) potentials in a way that guarantees convergence to the exact solution with increasing model space. Our 0 (h) over bar Omeg a 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 t he 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 sta bilize around 12 MeV. We hypothesize that these states might be the broad r esonance intruder states needed in R-matrix analysis of alpha - alpha elast ic scattering. In addition, we present our predictions for the A = 8 bindin g energies with the CD-Bonn NN potential.