Using the manifestly covariant spectator theory, and modeling the nucleon as a system of three constituent quarks with their own electromagnetic structure, we show that all four nucleon electromagnetic form factors can be very well described by a manifestly covariant nucleon wave function with zero orbital angular momentum. Since the concept of wave function depends on the formalism, the conclusions of light-cone theory requiring nonzero angular momentum components are not inconsistent with our results. We also show that our model gives a qualitatively correct description of deep inelastic scattering, unifying the phenomenology at high and low momentum transfer. Finally we review two different definitions of nuclear shape and show that the nucleon is spherical in this model, regardless of how shape is defined.

Studies of the structure of excited baryons are key factors to the N* program at Jefferson Lab (JLab). Within the first year of data taking with the Hall B CLAS12 detector following the 12 GeV upgrade, a dedicated experiment will aim to extract the N* electrocouplings at high photon virtualities Q2. This experiment will allow exploration of the structure of N* resonances at the highest photon virtualities ever achieved, with a kinematic reach up to Q2 = 12 GeV 2. This high-Q2 reach will make it possible to probe the excited nucleon structures at distance scales ranging from where effective degrees of freedom, such as constituent quarks, are dominant through the transition to where nearly massless bare-quark degrees of freedom are relevant. In this document, we present a detailed description of the physics that can be addressed through N* structure studies in exclusive meson electroproduction. The discussion includes recent advances in reaction theory for extracting N* electrocouplings from meson electroproduction off protons, along with Quantum Chromodynamics (QCD)-based approaches to the theoretical interpretation of these fundamental quantities. This program will afford access to the dynamics of the nonperturbative strong interaction responsible for resonance formation, and will be crucial in understanding the nature of confinement and dynamical chiral symmetry breaking in baryons, and how excited nucleons emerge from QCD.

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The covariant spectator formalism is used to model the nucleon and the ∆(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 γN → ∆ transition, can be described using manifestly covariant nucleon and ∆ wave functions with zero orbital angular momentum L, but a successful description of G * M near Q 2 = 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 gives electric, G * E , and coulomb, G * C , transition form factors that are identically zero, showing that these form factors are sensitive to wave function components with L > 0.

Starting with a covariant spectator quark model developed for the nucleon (N ) and the ∆ in the physical pion mass region, we extend the predictions of the reaction γN → ∆ to the lattice QCD regime. The quark model includes S and D waves in the quark-diquark wavefunctions. Within this framework it is the D-wave part in the ∆ wavefunction that generates nonzero valence contributions for the quadrupole form factors of the transition. Those contributions are however insufficient to explain the physical data, since the pion cloud contributions dominate. To separate the two effects we apply the model to the lattice regime in a region where the pion cloud effects are negligible, and adjust the D-state parameters directly to the lattice data. This process allows us to obtain a better determination of the D-state contributions. Finally, by adding a simple parametrization of the pion cloud we establish the connection between the experimental data and the lattice data.

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The ∆(1232) → γ * N magnetic dipole form factor (G * M ) is described here within a new covariant model that combines the valence quark core together with the pion cloud contributions. The pion cloud term is parameterized by two terms: one connected to the pion electromagnetic form factor, the other to the photon interaction with intermediate baryon states. The model can be used in studies of pp and heavy ion collisions. In the timelike region this new model improves the results obtained with a constant form factor model fixed at its value at zero momentum transfer. At the same time, and in contrast to the Iachello model, this new model predicts a peak for the transition form factor at the expected position, i.e. at the ρ mass pole. We calculate the decay of the ∆ → γN transition, the Dalitz decay (∆ → e + e − N ), and the ∆ mass distribution function. The impact of the model on dilepton spectra in pp collisions is also discussed.Within the covariant spectator quark model framework the nucleon and the ∆ are dominated by the S-wave components of the quark-diquark configuration [6,25,26]. In this case the only non-vanishing form factor of the ∆ → γ * N transition is the magnetic dipole form factor, which anyway dominates in all circumstances.One can then write [6][7][8]

The pion electromagnetic form factor at spacelike momentum transfer is calculated in relativistic impulse approximation using the Covariant Spectator Theory. The same dressed quark mass function and the equation for the pion bound-state vertex function as discussed in the companion paper are used for the calculation, together with a dressed quark current that satisfies the Ward-Takahashi identity. The results obtained for the pion form factor are in agreement with experimental data, they exhibit the typical monopole behavior at high-momentum transfer, and they satisfy some remarkable scaling relations.