Role of diquark correlations and the pion cloud in nucleon elastic form factors

TL;DR

This study uses a covariant NJL model to analyze nucleon electromagnetic form factors, highlighting the roles of diquark correlations and pion cloud effects, achieving excellent agreement with experimental data without parameter fitting.

Contribution

It introduces a covariant, confining NJL model with relativistic Faddeev equations to study nucleon form factors, emphasizing the impact of diquark correlations and pion loops, with no parameter fitting.

Findings

D-quark sector of Dirac form factor is much softer than u-quark sector.

Pion cloud significantly influences form factors for Q^2 less than 1 GeV^2.

Scalar diquark dominance affects the u-quark sector more than the d-quark sector.

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 $\bar{3}$ $qq$ channel. Pion degrees of freedom are included as a perturbation to the "quark-core" contribution obtained using the Poincar\'e 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 $Q^2 \lesssim 1\,$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.