Quantum Monte Carlo calculations of A=8 nuclei

We report quantum Monte Carlo calculations of ground and low-lying excited states for A = 8 nuclei using a realistic Hamiltonian containing the Argonne v(18) two-nucleon and Urbana IX three-nucleon potentials. The calculations begin with correlated eight-body wave functions that have a filled alpha-like core and four p-shell nucleons LS coupled to the appropriate (J(pi); T) quantum numbers for the state of interest. After optimization, these variational wave functions are used as input to a Green's function Monte Carlo calculation made with a new constrained path algorithm. We find that the Hamiltonian produces a Be-8 ground state that is within 2 MeV of the experimental resonance, but the other eight-body energies are progressively worse as the neutron-proton asymmetry increases. The Li-8 ground state is stable against breakup into subclusters, but the He-8 ground state is not. The excited state spectra are in fair agreement with experiment, with both the single-particle behavior of He-8 and Li-8 and the collective rotational behavior of Be-8 being reproduced. We also examine energy differences in the T = 1,2 isomultiplets and isospin-mixing matrix elements in the excited states of Be-8. Finally, we present densities, momentum distributions, and studies of the intrinsic shapes of these nuclei, with Be-8 exhibiting a definite 2 alpha cluster structure.