Effects of Δ-isobar degrees of freedom on low-energy electroweak transitions in few-body nuclei

Variational wave functions with {Δ}-isobar components are used to study trinucleon magnetic moments, the Gamow-Teller matrix element of tritium {β} decay, thermal neutron radiative capture on ³He, and low-energy proton weak capture on ³He. The {Δ}-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 v₂₈ and v28Q interaction models, which include {Δ}-isobar degrees of freedom. Contributions of {Δ} 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 {Δ}-isobar effects are commonly subsumed into effective two-body current operators. The resulting theoretical cross section for thermal neutron radiative capture on ³He is {}86 {μ}b, compared to an experimental value of 55±{}3 {μ}b; the astrophysical S factor for proton weak capture on ³He is predicted to be in the range (1.4--3.2)×{}10^-23 MeV b.