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     

Relativistic study of nucleon electroweak properties in a constituent-quark model

We discuss the predictions of the Goldstone-boson-exchange constituent-quark model for the proton and neutron electric and magnetic form factors as well as the nucleon axial and induced pseudoscalar form factors. The results are calculated in a covariant framework using the point-form approach to relativistic quantum mechanics. The only input into the calculations is the nucleon wave function from the constituent-quark model. A remarkably consistent picture, with all aspects of the electroweak nucleon structure close to existing experimental data, is obtained.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.-x Phenomenological quark models - 13.66.-a Lepton-lepton interactions - 14.20.Dh Protons and neutrons     

Charge-symmetry-breaking nucleon form factors

A quantitative understanding of charge-symmetry breaking is an increasingly important ingredient for the extraction of the nucleon??s strange vector form factors. We review the theoretical understanding of the charge-symmetry-breaking form factors, both for single nucleons and for ⁴He.     

Covariant nucleon electromagnetic form factors from the Goldstone-boson-exchange quark model

We present a study of proton and neutron electromagnetic form factors for the recently proposed Goldstone-boson-exchange constituent quark model. Results for charge radii, magnetic moments, and electric as well as magnetic form factors are reported. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. All the predictions by the Goldstone-boson-exchange constituent quark model are found to fall remarkably close to existing experimental data.     

Electromagnetic form factors of the nucleon

Dynamical and thermal characterizations of excited nuclear systems produced during the collisions between two heavy ions at intermediate incident energies are presented by means of a review of experimental and theoretical work performed in the last two decades. Intensity interferometry, applied to both charged particles (light particles and intermediate mass fragments) and to uncharged radiation (gamma rays and neutrons) has provided relevant information about the space-time properties of nuclear reactions. The volume, lifetime, density and relative chronology of particle emission from decaying nuclear sources have been extensively explored and have provided valuable information about the dynamics of heavy-ion collisions. Similar correlation techniques applied to coincidences between light particles and complex fragments are also presented as a tool to determine the internal excitation energy of excited primary fragments as it appears in secondary-decay phenomena.     

Analysis of electromagnetic nucleon form factors

Electromagnetic nucleon form factors have been determined from Rosenbluth plots and, independently, by fitting a dispersion ansatz to electron-nucleon scattering cross sections, allowing for a renormalization of the data in both cases. The ?-exchange contribution was taken from a Frazer-Fulco-type analysis based on new πN. Pole terms with adjustable parameters were used for the other parts of the spectral functions. Only the Dirac isoscalar and the Pauli isovector spectral function show a pronounced dipole- like bump-dip structure. The bumps belong to ω- and ?-exchange and the dips presumably to ø- and ω′(1250)-exchange in the first case and to ?′(1250)-exchange in the second case. The results for vector meson-nucleon coupling constants are compared with predictions from SU(3). Values for the nucleon radii are given which are only weakly model dependent.     

Relativistic oscillator model with spin for nucleon resonances

The relativistic three-body problem is approached via the extension of the SL(2, C) group to the Sp(4, C) one. In terms of Sp(4, C) spinors, a Dirac-like equation with three-body kinematics is composed. After introducing the linear in coordinates interaction, it describes the spin-1/2 oscillator. For this system, the exact energy spectrum is derived and then applied to fit the Regge trajectories of baryon N-resonances in the (E ², J) plane. The model predicts linear trajectories at high total energy E with some form of nonlinearity at low E.     

Strangeness contributions to nucleon form factors

We review a recent theoretical determination of the strange quark content of the electromagnetic form factors of the nucleon. These are compared with a global analysis of current experimental measurements in parity-violating electron scattering.     

Veneziano model for the ρ contribution to the nucleon form factors

The Igi-Veneziano model forπN scattering, combined with the experimental pion form factor is used to study theρ contribution to the isovector electromagnetic nucleon form factors.     

Sidewise electromagnetic form factors of the nucleon

Electromagnetic form factors are evaluated for the nucleons using the method of sidewise dispersion relations. The threshold region is assumed to dominate the results. A π-meson is used in the intermediate state expansions. The results are presented graphically.     

Gravitational form factors and nucleon spin structure

Nucleon scattering by the classical gravitational field is described by the gravitational (energy-momentum tensor) form factors (GFFs), which also control the partition of nucleon spin between the total angular momenta of quarks and gluons. The equivalence principle (EP) for spin dynamics results in the identically zero anomalous gravitomagnetic moment, which is the straightforward analog of its electromagnetic counterpart. The extended EP (ExEP) describes its (approximate) validity separately for quarks and gluons and, in turn, results in equal partition of the momentum and total angular momentum. It is violated in quantum electrodynamics and perturbative quantum chromodynamics (QCD), but may be restored in nonperturbative QCD because of confinement and spontaneous chiral symmetry breaking, which is supported by models and lattice QCD calculations. It may, in principle, be checked by extracting the generalized parton distributions from hard exclusive processes. The EP for spin-1 hadrons is also manifested in inclusive processes (deep inelastic scattering and the Drell–Yan process) in sum rules for tensor structure functions and parton distributions. The ExEP may originate in either gravity-proof confinement or in the closeness of the GFF to its asymptotic values in relation to the mediocrity principle. The GFFs in time-like regions reveal some similarity between inflation and annihilation.