M. T. Peña

A Pure S-wave covariant model for the nucleon

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.

D-state effects in the electromagnetic N Delta transition

We consider here a manifestly covariant quark model of the nucleon and the Delta, where one quark is off-shell and the other two quarks form an on-shell diquark pair. Using this model, we have shown previously that the nucleon form factors and the dominant form factor for the gamma N -> Delta transition (the magnetic dipole (M1) form factor) can be well described by nucleon and Delta wave functions with S-state components only. In this paper we show that non-vanishing results for the small electric (E2) and Coulomb (C2) quadrupole form factors can be obtained if D-state components are added to the

Electric quadrupole and magnetic octupole moments of the Δ

\Delta

Valence quark contribution for the gamma N ---> Delta quadrupole transition extracted from lattice QCD

valence quark wave function. We present a covariant definition of these components and compute their contributions to the form factors. We find that these components cannot, by themselves, describe the data. Explicit pion cloud contributions must also be added and these contributions dominate both the E2 and the C2 form factors. By parametrizing the p

Repulsive short range three-nucleon interaction

The pi-pi scattering amplitude calculated with a model for the quark-antiquark interaction in the framework of the Covariant Spectator Theory (CST) is shown to satisfy the Adler zero constraint imposed by chiral symmetry. The CST formalism is established in Minkowski space and our calculations are performed in momentum space. We prove that the axial-vector Ward-Takahashi identity is satisfied by our model. Then we show that, similar to what happens within the Bethe-Salpeter formalism, application of the axial-vector Ward-Takahashi identity to the CST pi-pi scattering amplitude allows us to sum the intermediate quark-quark interactions to all orders. The Adler self-consistency zero for pi-pi scattering in the chiral limit emerges as the result for this sum.

Electromagnetic Structure of Few-Nucleon Ground States

Using a covariant spectator constituent quark model we predict an electric quadrupole moment QΔ+=−0.043efm2 and a magnetic octupole moment OΔ+=−0.0035efm3 for the Δ+ excited state of the nucleon.

Linear confinement in momentum space: singularity-free bound-state equations

Abstract.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, GM * , for the γN → Δ transition, can be described using manifestly covariant nucleon and Δ wave functions with zero orbital angular momentum L , but a successful description of GM * 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 gives electric, GE * , 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 .

Nonexistence of a η NN quasibound state

Starting with a spectator quark model developed for the nucleon (N) and the Delta in the physical pion mass region, we extend the predictions of the reaction gamma N -> Delta 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 Delta 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 da