Chiral symmetry and axial charge density in the iso vector 0⁺ ↔ 1⁺ transition in the A = 12 system

Abstract

We present a detailed analysis of the low transfer data for the (ΔT = 1, 0⁺ ↔ 1⁺) transition in the A = 12 system. From the experimental observables obtained in electron scattering, in β-decay and in μ⁻ capture, we deduce the experimental form factors of the weak and electromagnetic nuclear currents. The theoretical values of these form factors are estimated with several standard 1p-shell nuclear models. The mesonic exchange currents are introduced in the form of one-pion and one ρ-meson exchange currents. We discuss the problem of radial overlap corrections and the problem of core polarization. The main effect of exchange currents is a large enhancement of the axial charge density with respect to impulse approximation. This is a manifestation of the almost exact chiral invariance of strong interactions. Our analysis shows that this enhancement is required by the experimental value of the corresponding form factor, and this conclusion does not rely upon a particular nuclear model. Finally we point out that all the observables of the transition can be fairly reproduced if we only retain the soft-pion exchange currents, which are determined without ambiguity by chiral symmetry.