Electroweak interactions and exchange currents in nuclei

In the first part of the dissertation, a consistent model for the long-range part of the weak meson exchange currents (MEC) preserving basic symmetries of the strong interactions is developed within the framework of an hadronic field theory of nuclear structure (QHD). A model which builds the nucleon-nucleon interaction out of {dollar}\sigma,\ \omega{dollar} and {dollar}\pi{dollar} meson exchange is used to describe strong interactions in a nucleus. The scalar-pseudoscalar part of the problem coincides with the {dollar}\sigma{dollar}-model. In the linear realization of the sigma-model one obtains spatial axial exchange currents of order (1/M) in a non-relativistic decomposition in nucleon mass due to {dollar}\omega{dollar}-exchange. Consistency with the nuclear physics phenomenology requires the use of a very large {dollar}\rm m\sb{lcub}scalar{rcub},{dollar} and the low-mass {dollar}\sigma{dollar} cannot be introduced simply without breaking chiral invariance in this approach. A chiral transformation to the non-linear realization of the sigma-model is shown to be the natural way of treating the problem. PCAC is then satisfied identically for a one-body axial current even for a nucleon inside the nucleus. In this approach, the phenomenological low-mass {dollar}\sigma{dollar} can be incorporated in the model as a chiral singlet, still necessitating no additional exchange currents of order (1/M) to be present. Here the first appearance of the axial MEC is in the familiar {dollar}\pi{dollar}-exchange term of order {dollar}\rm (1/M\sp2){dollar} in the axial charge density. at the same time, there is now an additional relativistic one-body correction of order (1/M) in the spatial part of the weak axial current that is required to satisfy PCAC. These correction terms are included in a unified analysis of weak and electromagnetic processes with some selected light nuclei where transition densities have been previously determined from available electromagnetic data.;In the second part of the dissertation, a potential use of electroweak experiments with excited {dollar}(J\sp{lcub}\pi{rcub}T) = (0\sp+0){dollar} nuclear states in addition to the ground state with the same quantum numbers is discussed. Existing low momentum transfer {dollar}q\sp2{dollar} data on the inelastic charge form factor for the {dollar}\rm (0\sp+0)\sb{lcub}gnd{rcub}\to (0\sp+0)\sp{lcub}\*{rcub}{dollar} transition in {dollar}\rm\sp4He{dollar} are fit within simple nuclear models, and predictions are made for higher {dollar}q\sp2.{dollar}