[en] It is shown that the helicity amplitudes A_1/2 and S_1/2 in the γN → N(1535) reaction, can be well related by S_1/2 = √1+τ/√2 M_S²-M²/2M_SQ A_1/2 in the region Q² > 2 GeV², where M and M_S are the nucleon and N(1535) masses, q² = -Q² the four-momentum transfer squared, and τ = Q²/(M_S + M)². This follows from the fact that the Pauli-type transition form factor F*₂ extracted from the experimental data, turns up to show F*₂ ≅ 0 for Q² > 1.5 GeV². The observed relation is tested by the experimental data and the MAID parametrization. A direct consequence of the relation is that the assumption,|A_1/2| >> |S_1/2|, is not valid for high Q². Instead, both amplitudes A_1/2 and S_1/2 have the same Q² dependence in the high Q² region, aside from that S_1/2 has an extra factor, - 1/√ M_S-M/2M_s. The origin of this relation is interpreted in a perspective of a quark model.

[en] We study the octet baryon electromagnetic properties by applying the covariant spectator quark model, and provide covariant parametrization that can be used to study baryon electromagnetic reactions. While we use the lattice QCD data in the large pion mass regime (small pion cloud effects) to determine the parameters of the model in the valence quark sector, we use the nucleon physical and octet baryon magnetic moment data to parameterize the pion cloud contributions. The valence quark contributions for the octet baryon electromagnetic form factors are estimated by extrapolating the lattice parametrization in the large pion mass regime to the physical regime. As for the pion cloud contributions, we parameterize them in a covariant, phenomenological manner, combined with SU(3) symmetry. We also discuss the impact of the pion cloud effects on the octet baryon electromagnetic form factors and their radii.

[en] Using SU(3) symmetry to constrain the π BB couplings, assuming SU(3) breaking comes only from one-loop pion cloud contributions, and using the the covariant spectator theory to describe the photon coupling to the quark core, we show how the experimental masses and magnetic moments of the baryon octet can be used to set a model independent constraint on the strength of the pion cloud contributions to the octet, and hence the nucleon, form factors at Q² = 0.

[en] Expressions for the nucleon wave functions in the covariant spectator theory (CST) are derived. The nucleon is described as a system with a off-mass-shell constituent quark, free to interact with an external probe, and two spectator constituent quarks on their mass shell. Integrating over the internal momentum of the on-mass-shell quark pair allows us to derive an effective nucleon wave function that can be written only in terms of the quark and diquark (quark-pair) variables. The derived nucleon wave function includes contributions from S, P and D-waves.

[en] The covariant spectator formalism is used to model the nucleon and the Δ (1232) as a system of three constituent quarks with their own electromagnetic structure. The definition of the 'fixed-axis' polarization states for the diquark emitted from the initial state vertex and absorbed into the final state vertex is discussed. The helicity sum over those states is evaluated and seen to be covariant. Using this approach, all four electromagnetic form factors of the nucleon, together with the magnetic form factor, G_M^*, for the γ N → Δ transition, can be described using manifestly covariant nucleon and Δ wave functions with zero orbital angular momentum L, but a successful description of G_M^* near Q²=0 requires the addition of a pion cloud term not included in the class of valence quark models considered here. We also show that the pure S$-wave model

[en] Using the covariant spectator theory (CST), we present the results of a valence quark-diquark model calculation of the nucleon structure function f(x) measured in unpolarized deep inelastic scattering (DIS), and the structure functions g1(x) and g2(x) measured in DIS using polarized beams and targets. Parameters of the wave functions are adjusted to fit all the data. The fit fixes both the shape of the wave functions and the relative strength of each component. Two solutions are found that fit f(x) and g1(x), but only one of these gives a good description of g2(x). This fit requires the nucleon CST wave functions contain a large D-wave component (about 35%) and a small P-wave component (about 0.6%). The significance of these results is discussed.

[en] A covariant quark model, based both on the spectator formalism and on Vector Meson Dominance, and previously calibrated by the physical data, is here extended to the unphysical region of the lattice data by means of one single extra adjustable parameter - the constituent quark mass in the chiral limit. We calculated the Nucleon (N) and the Gamma N -> Delta form factors in the universe of values for that parameter described by quenched lattice QCD. A qualitative description of the Nucleon and Gamma N -> Delta form factors lattice data is achieved for light pion masses.

[en] All four nucleon electromagnetic form factors can be very well described by a manifestly covariant nucleon wave function with zero orbital angular momentum. The same model gives a qualitative description of deep inelastic scattering. The results for the G^*_M form factor of the N ? ? transition are consistent with other quark models.

[en] Using a covariant spectator constituent quark model we predict an electric quadrupole moment -0.042 efm2 and a magnetic octupole moment -0.0035 efm3 for the Delta+ excited state of the nucleon.

[en] A constituent quark model based on the spectator formalism is applied to the gamma N -> N* transition for the three cases, where N* is the nucleon, the Delta and the Roper resonance. The model is covariant, and therefore can be used for the predictions at higher four-momentum transfer squared, Q2. The baryons are described as an off-mass-shell quark and a spectator on-mass-shell diquark systems. The quark electromagnetic current is described by quark form factors, which have a form inspired by the vector meson dominance. The valence quark contributions of the model are calibrated by lattice QCD simulations and experimental data. Contributions of the meson cloud to the inelastic processes are explicitly included.