NUCLEON ELECTROMAGNETIC FORM FACTORS AND CHIRAL SINGULARITIES

Chiral singularities in both nucleon isovector radii are calculated in closed form. Within a one-meson-loop approximation we implement in an explicitly gauge invariant manner quark substructure in the γ π π and πNN vertex functions. The coefficients of log mπ and [Formula: see text] terms are directly related to gA and to the threshold behaviour of the πN amplitude A(-)(ν,0) which are 1 and 0, respectively, if only the N pole is considered in the πN scattering amplitude. This result is remarkably independent of the cut-off procedure. If mesonic substructure at the γ π π vertex is included, however, gA becomes slightly smaller than 1 and [Formula: see text] becomes moderately negative. This hints at important contributions to both gA and [Formula: see text] from the Δ(1232) and possibly other N*, Δ* resonances in this approach.