Abstract
Electromagnetic form factors of the nucleon in the space-like region are
investigated within the framework of a covariant and confining
Nambu-Jona-Lasinio model. The bound state amplitude of the nucleon is obtained
as the solution of a relativistic Faddeev equation, where diquark correlations
appear naturally as a consequence of the strong coupling in the colour
$3$ $qq$ channel. Pion degrees of freedom are included as a perturbation
to the "quark-core" contribution obtained using the Poincaré covariant
Faddeev amplitude. While no model parameters are fit to form factor data,
excellent agreement is obtained with the empirical nucleon form factors
(including the magnetic moments and radii) where pion loop corrections play a
critical role for $Q^2 1\,$GeV$^2$. Using charge symmetry, the nucleon
form factors can be expressed as proton quark sector form factors. The latter
are studied in detail, leading, for example, to the conclusion that the
$d$-quark sector of the Dirac form factor is much softer than the $u$-quark
sector, a consequence of the dominance of scalar diquark correlations in the
proton wave function. On the other hand, for the proton quark sector Pauli form
factors we find that the effect of the pion cloud and axialvector diquark
correlations overcomes the effect of scalar diquark dominance, leading to a
larger $d$-quark anomalous magnetic moment and a form factor in the $u$-quark
sector that is slightly softer than in the $d$-quark sector.
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