Covariant electroweak nucleon form factors in a chiral constituent-quark model

Results for the proton and neutron electric and magnetic form factors as well as the nucleon axial and induced pseudoscalar form factors are presented for the chiral constituent-quark model based on Goldstone-boson exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The direct predictions of the model yield a remarkably consistent picture of the electroweak nucleon structure. Received: 28 February 2002 / Accepted: 14 March 2002     

Meson-Cloud Effects in the Electromagnetic Nucleon Structure

We study how the electromagnetic structure of the nucleon is influenced by a pion cloud. To this aim we make use of a constituent-quark model with instantaneous confinement and a pion that couples directly to the quarks. To derive the invariant 1-photon-exchange electron-nucleon scattering amplitude we employ a Poincaré-invariant coupled-channel formulation which is based on the point-form of relativistic quantum mechanics. We argue that the electromagnetic nucleon current extracted from this amplitude can be reexpressed in terms of pure hadronic degrees of freedom with the quark substructure of the pion and the nucleon being encoded in electromagnetic and strong vertex form factors. These are form factors of bare particles, i.e. eigenstates of the pure confinement problem. First numerical results for (bare) photon-nucleon and pion-nucleon form factors, which are the basic ingredients of the further calculation, are given for a simple 3-quark wave function of the nucleon.     

Electromagnetic properties of baryons in a relativistic quark model

Results on electromagnetic form factors of the nucleon and photon transition form factors of non-strange baryon resonances calculated in a relativistically covariant quark model based on the Bethe-Salpeter equation are presented. The relevance of the instanton-induced quark interaction on these properties is discussed.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.10.St Bound and unstable states; Bethe-Salpeter equations - 12.39.Ki Relativistic quark model - 12.40.Yx Hadron mass models and calculations - 13.40.Gp Electromagnetic form factors     

Relationship Between the Proton Charge Form Factor and the Quark-Quark Interaction

We demonstrate how the parameterized proton elastic charge form factor may be used to determine the quark-quark interaction assuming a constituent-quark model of the nucleon and non-relativistic kinematics. The relation between the hypercentral component of the three-quark bound-state wave function and the charge density is obtained by using an Abel-type integral equation. To a high degree of accuracy, this component fixes the hypercentral potential, from which the quark-quark interaction may be determined exactly via another Abel transform. We find that the presence of the meson cloud makes it impossible to determine the long-range behaviour of the quark-quark potential. However, our results indicate that the Gari-Krüpelmann and dipole parametrizations of the form factor both generate potentials that are singular at short distances, indicating a degree of sensitivity of the form factor to the quark-quark interaction at small distances. We also generalize our technique to systems of arbitrarily large number of particles. Received June 21, 1994; revised October 3, 1994; accepted for publication November 24, 1994     

Search for missing baryon resonances via associated strangeness photoproduction

Differential cross-section and single polarization observables in the process γp→K ⁺ Λ are investigated within a constituent-quark model and a dynamical coupled-channel formalism. The effects of two new nucleon resonances and of the K ^*(892)- and K1(1270)-exchanges are briefly presented.     

Electromagnetic form factors of the baryon octet in the perturbative chiral quark model

We apply the perturbative chiral quark model at one loop to analyze the electromagnetic form factors of the baryon octet. The analytic expressions for baryon form factors, which are given in terms of fundamental parameters of low-energy pion-nucleon physics (weak pion decay constant, axial nucleon coupling, strong pion-nucleon form factor), and the numerical results for baryon magnetic moments, charge and magnetic radii are presented. Our results are in good agreement with experimental data.Received: 7 January 2003, Revised: 4 November 2003, Published online: 15 April 2004PACS: 12.39.Ki Relativistic quark model - 13.40.Gp Electromagnetic form factors - 14.20.Dh Protons and neutrons - 14.20.Jn Hyperons     

Nucleon form factors in a projected chiral soliton model with dynamical confinement

We calculate nucleon form factors in the framework of a chiral chromodielectric model. The model state describing the nucleon is an angular momentum and isospin eigenstate with obtained by means of Peierls-Yoccoz projection from the hedgehog. We present results for the electromagnetic form factors and also for the axial form factors of the nucleon. There is a fairly good agreement with the data for small momentum transfers. For high momentum transfers (i.e. q² > 0.1 GeV² the agreement becomes poorer. As a general rule the calculated form factors fall too rapidly.     

Baryon structure from Lattice QCD

We present recent lattice results on the baryon spectrum, nucleon electromagnetic and axial form factors, nucleon to △ transition form factors as well as the △ electromagnetic form factors. The masses of the low lying baryons and the nucleon form factors are calculated using two degenerate flavors of twisted mass fermions down to pion mass of about 270 MeV. We compare to the results of other collaborations. The nucleon to △ transition and △ form factors are calculated in a hybrid scheme, which uses staggered sea quarks and domain wall valence quarks. The dominant magnetic dipole nucleon to △ transition form factor is also evaluated using dynamical domain wall fermions. The transverse density distributions of the △ in the infinite momentum frame are extracted using the form factors determined from lattice QCD.     

Nucleon Form Factors and N-△ Transitions in a Hypercentral Constituent Quark Model with Meson Cloud

We study the nucleon form factors and the nucleon-△（1232） transitions in a framework of hypercentral constituent quark model. The pion meson cloud effect is taken into account explicitly. Our results show that the pion cloud contributes substantially to the nucleon form factors as well as to the helicity amplitudes of △（1232）, and it gives an improved agreement compared to the experimental.     

On a possible estimation of the constituent-quark parameters from Jefferson Lab experiments on the pion form factor

The charge form factor of the pion is calculated for the momentum transfer range of the Jefferson Lab experiments. The approach is based on the instant form of the relativistic Hamiltonian dynamics. It is shown that the form-factor dependence on the choice of the model for the quark wave function in the pion is weak, while the dependence on the constituent-quark mass is rather significant. It is possible to estimate the mass of the constituent quark and the sum of the anomalous magnetic moments of the u- and -quarks from the JLab experiments. Received: 3 April 2000 / Revised version: 25 February 2001 / Published online: 25 April 2001     

Poincaré-Covariant Quark Models of Baryon Form Factors

Poincaré-covariant quark models of the nucleon, the resonance and their excitations are explored. The baryon states are represented by eigenfunctions of the four-velocity and a confining mass operator, which reproduces the empirical baryon masses up to 1700 MeV to an accuracy of . Models of constituent-quark currents provide the relations between ground-state properties and transition amplitudes. Received July 22, 1997; revised June 16, 1998; accepted for publication July 29, 1998     

Nucleon structure on the lattice

A few chosen nucleon properties are described from a lattice QCD perspective: the nucleon sigma term and the scalar strangeness in the nucleon; the vector form factors in the nucleon, including the vector strangeness contribution, as well as parity breaking effects like the anapole and electric dipole moment; and finally the axial and tensor charges of the nucleon. The status of the lattice calculations is presented and their potential impact on phenomenology is discussed.     

Electroweak form factors of non-strange baryons

We compute electroweak form factors of the nucleon and photon transition form factors of non-strange baryon resonances up to the third resonance region in a model with instanton-induced interaction. The calculation is based on the Bethe-Salpeter equation for three light constituent quarks and is fully relativistic (U. L?ring et al., Eur. Phys. J. A 10, 309 (2001)). Static nucleon properties and photon resonance couplings are in good agreement with experiment and the Q² behaviour of the experimentally known form factors up to large momentum transfer is accounted for. Received: 4 April 2002 / Accepted: 2 May 2002     

Determination of the strange form factors of the Nucleon from nup, nup, and parity-violating e-->p elastic scattering

A new method of obtaining the strange form factors of the nucleon is presented, in which forward-angle parity-violating e-->p elastic scattering data are combined with nup and nu;p elastic scattering data to extract all three strange form factors: electric, magnetic, and axial (G(s)(E), G(s)(M), and G(s)(A)). In this Letter, nup and nu;p data from the Brookhaven E734 experiment are combined with the Jefferson Laboratory HAPPEX e-->p data to obtain two distinct solutions for the strange form factors at Q(2)=0.5 GeV2. More generally, combining the neutrino elastic scattering data from E734 with the existing and upcoming e-->p data will yield the strange form factors of the nucleon for Q2 of 0.45-1.05 GeV2. Measurement of G(s)(A) is crucial to the determination of the strange quark contribution to the nucleon spin Deltas.     

Electric and magnetic form factors of strange baryons

Predictions for the electromagnetic form factors of the , and hyperons are presented. The numerical calculations are performed within the framework of the fully relativistic constituent-quark model developed by the Bonn group. The computed magnetic moments compare favorably with the experimentally known values. Most magnetic form factors G_M (Q²) can be parameterized in terms of a dipole with cutoff masses ranging from 0.79 to 1.14 GeV.Received: 2 October 2003, Published online: 20 April 2004PACS: 11.10.St Bound and unstable states; Bethe-Salpeter equations - 12.39.Ki Relativistic quark model - 13.40.Gp Electromagnetic form factors     

Nucleon electromagnetic form factors in a quark–gluon core model

We study the nucleon electromagnetic form factors in a quark–gluon core model framework, which can be viewed as an extension of the Isgur–Karl model of baryons. Using this picture we derive nucleon electromagnetic dipole form factors at low Q ² and the deviation from the dipole form at high Q ², that are consistent with the existing experimental data.     

Electromagnetic and Axial Form Factors of Octet and Decuplet Baryons

We report from a study of the elastic electromagnetic and axial form factors of all lowest baryon states with flavors up, down, and strange along relativistic constituent-quark models. We consider the baryons as relativistic bound states of three constituent quarks and solve the eigenvalue problem of the invariant mass operator. The corresponding eigenstates are employed to calculate manifestly covariant form factors within the point form of Poincaré-invariant quantum mechanics. The electromagnetic and axial current operators are constructed along the spectator model in point-form relativistic dynamics. We have thus obtained covariant predictions for the electroweak form factors, for momentum transfers up to Q ² ~ 4 GeV², as well as the electric radii, magnetic moments, and axial charges. The theoretical results in general agree very well with existing phenomenological data. In cases, where no experimental information is yet available, the results are well compatible with data from lattice quantum chromodynamics.     

Another look at meson-exchange interactions

We calculate the nucleon structure in a two-phase model, assuming forlow Q ²: meson dynamics, and forhigh Q ²: quark-gluon dynamics, and find a low meson scale in accordance with electron-nucleon scattering. Using these form factors we show that with the increased importance of nucleon structure in the nucleon-nucleon interaction, higher-mass meson exchange (m>1 GeV) becomes important. We argue that these contributions can be taken into account by a sort of closure relation, leading to contact interactions of the physical nucleons. A one-boson-exchange potential is constructed including these direct nucleon structure interactions. Implications concerning the corresponding three-nucleon interaction are also discussed. A partial-wave calculation of neutron-proton scattering is performed showing satisfactory agreement with experiment. The model has several advantages over conventional bosonexchange models.Work is supported by Deutsche Forschungsgemeinschaft (Ga 153/11-2 and Ga 153/11-3) and partially by NATO (0581/87) and COSY-KFA Jülich (41092744 and 41116381)