Chiral model of axial meson exchange currents in nuclei

The long-range part of the weak axial exchange currents (AXC) is considered in the framework of the chiral sigma-omega model, which preserves basic symmetries of QCD and includes sigma, omega, and pi mesons. Linear and nonlinear realizations of the model are considered, and it is explicitly demonstrated that the way one splits relativistic many-body effects into AXC contributions and one-body relativistic corrections to the traditional nuclear axial current is model dependent. In the linear realization of the sigma-omega model spatial two-body AXC are shown to be necessary to preserve the partial conservation of axial-vector current (PCAC) relation to order O(1/M) in inverse nucleon mass. In the nonlinear realization of the model PCAC is satisfied for the one-body current identically. Indeed, it is shown that there are no AXC of order O(1/M) in this realization, but one now has to include a new one-body relativistic correction of order O(1/M) in the analysis to preserve PCAC for a single nucleon. A familiar result for axial exchange charge is reproduced in both models. The obtained correction terms are estimated in a simple unified analysis of electroweak processes with a few light nuclei. [S0556-2813(98)07505-0].