EFFECTS OF DELTA-ISOBAR DEGREES OF FREEDOM ON LOW-ENERGY ELECTROWEAK TRANSITIONS IN FEW-BODY NUCLEI

Variational wave functions with DELTA-isobar components are used to study trinucleon magnetic moments, the Gamow-Teller matrix element of tritium beta-decay, thermal neutron radiative capture on He-3, and low-energy proton weak capture on He-3. The DELTA-isobar components are generated by transition correlation operators acting on realistic nuclear wave functions. These correlations are obtained from a fit to exact two-body ground-state and low-energy scattering solutions for the Argonne upsilon-28 and upsilon-28Q interaction models, which include DELTA-isobar degrees of freedom. Contributions of DELTA-isobars to electroweak current operators appear at the one-body level in this formalism. Their effect on low-energy electroweak transitions is significantly smaller than that obtained in perturbation theory analyses, where DELTA-isobar effects are commonly subsumed into effective two-body current operators. The resulting theoretical cross section for thermal neutron radiative capture on He-3 is almost-equal-to 86-mu-b, compared to an experimental value of 55+/-3-mu-b; the astrophysical S factor for proton weak capture on He-3 is predicted to be in the range (1.4-3.2) X 10(-23) MeV b.