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 ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, and low-energy proton weak capture on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. 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 ${\mathit{v}}_{28}$ and ${\mathit{v}}_{28\mathit{Q}}$ 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 ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is ≊86 μb, compared to an experimental value of 55±3 μb; the astrophysical S factor for proton weak capture on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is predicted to be in the range (1.4–3.2)×${10}^{\mathrm{-}23}$ MeV b.