CHIRAL-SYMMETRY RESTORATION AND PARITY MIXING

We derive the expressions of the vector and axial current from a chira l Lagrangian restricted to nucleons and pions. They display mixing ter ms between the axial and vector currents. We study the modifications i n the nuclear medium of the coupling constants of the axial current, n amely the pion decay constant and the nucleonic axial one due to the r equirements of chiral symmetry. We express the renormalizations in ter ms of the local scalar pion density. The latter also governs the quark condensate evolution and we discuss the link between this evolution a nd the renormalizations. In the case of the nucleon axial coupling con stant this renormalization corresponds to a new type of pion exchange currents, with two exchanged pions, We give an estimate for the result ing quenching. Although moderate it helps explaining the quenching exp erimentally observed. (C) 1998 Elsevier Science B.V.