Selected nucleon form factors and a composite scalar diquark

A covariant, composite scalar diquark, Fadde'ev amplitude model for the nucleon is used to calculate pseudoscalar, isoscalar- and isovector-vector, axial-vector and scalar nucleon form factors. The last yields the nucleon {sigma} term and on-shell {sigma}-nucleon coupling. The calculated form factors are soft, and the couplings are generally in good agreement with experiment and other determinations. Elements in the dressed-quark-axial-vector vertex that are not constrained by the Ward-Takahashi identity contribute {approx}20% to the magnitude of g{sub A}. The calculation of the nucleon {sigma} term elucidates the only unambiguous means of extrapolating meson-nucleon couplings off the meson mass shell. (c) 2000 The American Physical Society.