Electro-magnetic nucleon form factors and their spectral functions in soliton models

It is demonstrated that in simple soliton models essential features of the electro-magnetic nucleon form factors observed over three orders of magnitude in momentum transfert are naturally reproduced. The analysis shows that three basic ingredients are required: an extended object, partial coupling to vector mesons, and relativistic recoil corrections. We use for the extended object the standard skyrmion, one vector meson propagator for both isospin channels, and the relativistic boost to the Breit frame. Continuation to time-liket leads to quite stable results for the spectral functions in the regime from the 2- or 3-pion threshold to about two rho masses. Especially the onset of the continuous part of the spectral functions at threshold can be reliably determined and there are strong analogies to the results imposed on dispersion theoretic approaches by the unitarity constraint.     

Nucleon electroweak form factors: Analysis of their spectral functions

We investigate the imaginary parts of the nucleon electromagnetic and axial form factors close to threshold in the framework of heavy baryon chiral perturbation theory. For the isovector electromagnetic form factors, we recover the well-known strong enhancement near threshold. For the isoscalar ones, we show that there is no visible enhancement due to the three-pion continuum. This justifies the use of vector meson poles only in dispersion-theoretical calculations. We also calculate the imaginary part of the nucleon isovector axial form factor and show that it is small in the threshold region.     

Baryon spectra and electroweak nucleon form factors in constituent quark models

We discuss the spectra of light and strange baryons as well as the electroweak structure of the nucleons as described by relativistic constituent quark models. Special attention is paid to the performance of different types of quark dynamics and to the role of relativistic effects. It is found that the concept of constituent quark models, set up in a covariant framework, represents a promising tool to deal with low-energy hadron phenomena.     

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.     

Electron excitation of nucleon resonances: Structure functions and transition form factors

The electron excitation of nucleon resonances is discussed both from the theoretical and from the experimental point of view. This discussion is based on a phenomenological approach that employs the conservation of the electromagnetic and vector-meson hadronic currents and the requirements of limiting chiral invariance. For the electron excitation of J^π=1/2^±,3/2^±,5/2^±,... nucleon resonances, the structure functions are defined in terms of Sachs transition form factors. The resulting resonance structure functions for l+N → l+R processes are used in parametrizing smooth (background) structure functions for l+N → l+X inelastic scattering.     

Microscopic models for hadronic form factors and vertex functions

We review the status of nucleon (N) and few-nucleon form factors (f.f.'s) from the view-point of a gradual unfolding of successively inner degrees of freedom (d.o.f.) with increase inq ². To this end we focus attention on the problem of a microscopic formulation of hadronic vertex functions (v.f.) from the point of view of their key role in understanding the physics of a large variety of few-hadron reactions on the one hand, and their practical usefulness in articulating the internal dynamics of hadron and few-hadron systems on the other hand. The criterion of an integrated view from low-energy spectroscopy to high-q ² amplitudes is employed to emphasize the desirability of formulations in terms of relativistic dynamical equations based on Lorentz and gauge invariance in preference to phenomenological models, which often require additional assumptions beyond their original premises to extend their applicability domains. In this respect, the practical possibilities of the Bethe-Salpeter equation (BSE) in articulating the necessary dynamical ingredients are emphasized on a two-tier basis, the basic constant (3) being pre-determined from the mass spectral data (1st stage) in preparation for the construction of the hadron-quark vertex functions (2nd stage). An explicit construction is outlined for meson-quark and baryon-quark vertex functions as well as of meson-nucleon vertex functions in a stepwise fashion. The role of the latter as basic parameter-free ingredients is discussed for possible use in the more serious treatments in the current literature of quark-meson level () and meson-isobar () d.o.f. in 2-N and 3-N form factor studies. Since most of these studies are characterized by the use of RGM techniques at the six-quark level, a comparative discussion is also given of several contemporary RGM based models. Finally, the concrete prospects for employing such hardon-quark vertex functions for evaluating pp annihilation amplitudes are briefly indicated, keeping in view the LEAR programme for their studies.     

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.     

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.     

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.     

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.     

Valence and nonvalence vertex functions for timelike and spacelike nucleon form factors within light-front dynamics

A unified model for spacelike and timelike hadron form factors within light-front dynamics successfully applied to the pion is extended to the nucleon to study the role of valence and nonvalence vertex functions. Preliminary results for ?10(GeV/c)² ≤ q ² ≤ 10(GeV/c)² are presented.     

Exact quantum Sine-Gordon soliton form factors

Soliton form factors are constructed using Zamolodchikov's proposed exact massive Thirring model S-matrix. The asymptotic behaviour of the electromagnetic form factor is $\text{～(?t)}{}^{\mathrm{<mtext>?</mtext><mtext>g</mtext><mtext>2</mtext>}}$. The quasiclassical limit is not the previously accepted result.     

Pions in the light-cone nucleon and electromagnetic form factors

The light-cone Fock-state decomposition of the nucleon involves pion-quark core (π′N′ andπ′Δ′) components. The size of the quark core is at present of special importance for the analysis of the onset of colour transparency in the A(e,e′p) reactions. With the light-coneNπ′N′ andNπ′Δ′ vertex functions deduced from experimental data on high-energy proton fragmentation into nucleons and isobars, we evaluate the effect of the pionic cloud on the electromagnetic form factors of nucleons. We find the quark core radius to be about 10% smaller than the nucleon's radius. Analysis of the renormalization of the total nucleon-nucleon cross section for the presence of the pionic cloud suggests a similar core radius.     

Radiative corrections in nucleon time-like form factors measurements

The completely general radiative corrections to lowest order, including the final- and initial-state radiations, are studied in proton-antiproton annihilation into an electron-positron pair. Numerical estimates have been made in a realistic configuraton of the PANDA detector at FAIR for the proton time-like form factors measurements.