Nuclear Axial-Charge Transitions in Chiral Perturbation Theory

We develop a systematic chiral perturbation expansion for the calculation of meson-exchange currents in nuclei and apply the formalism to nuclear axial currents. We summarize the principal results of such a calculation to one loop order on nuclear axial-charge transitions which provides a strong support to the conjecture of ``chiral filter phenomenon" in nuclear medium. The use of heavy baryon chiral perturbation theory enables us to obtain a remarkably simple result valid next to the leading order in chiral counting. The dominant role of a soft-pion exchange in axial-charge transitions in heavy nuclei is confirmed. An important, {\it albeit} indirect, consequence of our result on the empirically observed enhancement in axial-charge transitions in heavy nuclei is pointed out.