MESON CLOUD CORRECTIONS TO THE PION ELECTROMAGNETIC FORM-FACTOR IN THE NAMBU-JONA-LASINIO MODEL

We have calculated lowest order meson cloud corrections to the pion el ectromagnetic form factor in the two-flavor NJL model. The complete se t of gauge and chirally invariant Feynman diagrams falls naturally int o two distinct classes: hadronic tadpole and pion-pole diagrams, respe ctively. Only their sum respects gauge invariance, Goldstone's theorem , the Goldberger-Treiman relation and conserves the axial current. We demonstrate that hadronic tadpole diagrams can be neglected in the sta tic approximation of infinite quark masses, while only a single pion-p ole diagram survives that is, however, too weak to reproduce the stron g, rho-like vector correlations that are inferred experimentally above the pi pi-threshold. The meson cloud contributes less than 10% to the size of the basic NJL electromagnetic form factor of the pion, but dr astically alters its gradient at zero momentum transfer. This general feature is in the model due to a singularity introduced by the pion-po le diagram in the chiral limit and contributes the well-known logarith mic term to the pion electromagnetic radius, here at scale mu = m(sigm a) given by the sigma mode mass.