Nucleon and gamma N -> Delta lattice form factors in a constituent quark model

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 pions.Comment: To appear in J.Phys.

A covariant model for the γ∗N→N∗(1520)\gamma^\ast N \to N^\ast(1520) reaction

We apply the covariant spectator quark model to the study of the electromagnetic structure of the $N^\ast(1520)$ state ($J^{P}= \frac{3}{2}^-$), an important resonance from the second resonance region in both spacelike and timelike regimes. The contributions from the valence quark effects are calculated for the $\gamma^\ast N \to N^\ast(1520)$ helicity amplitudes. The results are used to parametrize the meson cloud dominant at low $Q^2$.Comment: 3 figure, 3 pages. Contribution to the 13th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU 2013), Rome, September-October 201

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

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 data.Comment: To appear in Phys. Rev.

Fixed-axis polarization states: covariance and comparisons

Addressing the recent criticisms of Kvinikhidze and Miller, we prove that the spectator wave functions and currents based on ``fixed-axis'' polarization states (previously introduced by us) are Lorentz covariant, and find an explicit connection between them and conventional direction-dependent polarization states. The discussion shows explicitly how it is possible to construct pure $S$-wave models of the nucleon.Comment: Changed title and introductory material to match accepted pape

The shape of the Δ\Delta baryon in a covariant spectator quark model

Using a covariant spectator quark model that describes the recent lattice QCD data for the $\Delta$ electromagnetic form factors and all available experimental data on $\gamma N \to \Delta$ transitions, we analyze the charge and magnetic dipole distributions of the $\Delta$ baryon and discuss its shape. We conclude that the quadrupole moment of the $\Delta$ is a good indicator of the deformation and that the $\Delta^+$ charge distribution has an oblate shape. We also calculate transverse moments and find that they do not lead to unambiguous conclusions about the underlying shape.Comment: Extended introduction, references added, other small modifications. To appear in Phys. Rev. D. 14 pages, 8 figure

N*(1535) electroproduction at high Q2

A covariant spectator quark model is applied to study the gamma N -> N*(1535) reaction in the large Q2 region. Starting from the relation between the nucleon and N*(1535) systems, the N*(1535) valence quark wave function is determined without the addition of any parameters. The model is then used to calculate the gamma N -> N*(1535) transition form factors. A very interesting, useful relation between the A12 and S12 helicity amplitudes for Q2 > GeV^2, is also derived.Comment: Contribution to the proceedings of the 8th International Workshop on the Physics of Excited Nucleons (NSTAR2011), Jefferson Lab, Newport News, VA, USA, May 17-20, 201

Extracting the Omega- electric quadrupole moment from lattice QCD data

The Omega- has an extremely long lifetime, and is the most stable of the baryons with spin 3/2. Therefore the Omega- magnetic moment is very accurately known. Nevertheless, its electric quadrupole moment was never measured, although estimates exist in different formalisms. In principle, lattice QCD simulations provide at present the most appropriate way to estimate the Omega- form factors, as function of the square of the transferred four-momentum, Q2, since it describes baryon systems at the physical mass for the strange quark. However, lattice QCD form factors, and in particular GE2, are determined at finite Q2 only, and the extraction of the electric quadrupole moment, Q_Omega= GE2(0) e/(2 M_Omega), involves an extrapolation of the numerical lattice results. In this work we reproduce the lattice QCD data with a covariant spectator quark model for Omega- which includes a mixture of S and two D states for the relative quark-diquark motion. Once the model is calibrated, it is used to determine Q_Omega. Our prediction is Q_Omega= (0.96 +/- 0.02)*10^(-2) efm2 [GE2(0)=0.680 +/- 0.012].Comment: To appear in Phys. Rev. D. Version with small modifications. 8 pages, 1 figur

Covariant nucleon wave function with S, D, and P-state components

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.Comment: 13 pages and 1 figur