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

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 colour3 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.

The Dirac form factor predicts the Pauli form factor in the Endpoint Model

“…Work by Miller et al, Lin et al, and Cloet et al [16][17][18] has reproduced the experimentally observed momentum dependence of F 2 . These calculations emphasize the importance of the quark wave functions, i.e.…”

Baryons as relativistic three-quark bound states

“…3.12. Studies on the impact of pion cloud effects in this framework (also in the quark-diquark approximation) can be found in [345,346]. Results for the genuine three-body system have not been published so far.…”

“…For example, the rainbow-ladder truncation becomes a vector-vector contact interaction and in the quark-diquark equation an additional static approximation for the exchange quark propagator is made. The NJL model was recently employed to calculate elastic nucleon and hyperon form factors [346,507], building upon a body of work on other structure properties such as parton distribution functions and nuclear medium effects; see also [508] and references therein. Closely related is the contactinteraction model developed in Refs.…”

Flavor dependence of the pion and kaon form factors and parton distribution functions

“…Here the proper-time scheme is chosen, because it simulates aspects of quark confinement by eliminating on-shell quark propagation, while maintaining the symmetries of the theory, such as the Poincaré and chiral symmetries. As a result it has been widely used [38,39,51,[59][60][61][62][63][64]. Formally the proper-time regularization scheme is defined by…”

“…For example, the NJL model encapsulates dynamical chiral symmetry breaking (DCSB), which gives rise to dynamically generated dressed quark masses. The NJL model has been successfully used to investigate a broad range of phenomena, including hadron properties [12,[46][47][48][49][50][51][52], heavy ion collisions [53], neutron stars [45,54,55], quark fragmentation functions [56,57] and transverse momentum dependent phenomena [58].…”