Charge-symmetry breaking forces and isospin mixing in 8Be

We report Green's function Monte Carlo calculations of isospin-mixing (IM) matrix elements for the 2(+), 1(+), and 3(+) T = 0 and 1 pairs of states at 16-19 MeV excitation in Be-8. The realistic Argonne nu(18) (AV18) two-nucleon and Illinois-7 three-nucleon potentials are used to generate the nuclear wave functions. Contributions from the full electromagnetic interaction and strong class III charge-symmetry breaking (CSB) components of the AV18 potential are evaluated. We also examine two theoretically more complete CSB potentials based on rho-omega mixing, tuned to give the same neutron-neutron scattering length as AV18. The contribution of these different CSB potentials to the H-3-He-3, Li-7-Be-7, and Li-8-B-8 isovector energy differences is evaluated and reasonable agreement with experiment is obtained. Finally, for the Be-8 IM calculation we add the small class IV CSB terms coming from one-photon, one-pion, and one-rho exchange, as well as rho-omega mixing. The expectation values of the three CSB models vary by up to 20% in the isovector energy differences, but only by 10% or less in the IM matrix element. The total matrix element gives 85-90% of the experimental IM value of -145 keV for the 2(+) doublet, with about two thirds coming from the Coulomb interaction. We also report the IM matrix element to the first 2(+) state at 3 MeV excitation, which is the final state for various tests of the standard model for beta decay.