Nucleon Elastic Form Factors Research Articles

An overview of the hypercentral constituent quark model

We report on the recent results of the hypercentral Constituent Quark Model (hCQM). The model contains a spin independent three-quark interaction which is inspired by Lattice QCD calculations and reproduces the average energy values of the SU(6) multiplets. The splittings are obtained with a SU(6)-breaking interaction, which can include also an isospin dependent term. The model has been used for predictions concerning the electromagnetic transition form factors giving a good description of the medium Q 2-behaviour. In particular the calculated helicity amplitude A 1 2 for the S 11(1535) resonance agrees very well with the recent CLAS data. Furthermore, we have shown for the first time that the decreasing of the ratio of the elastic form factors of the proton is due to relativistic effects. Finally, the elastic nucleon form factors have been calculated using a relativistic version of the hCQM and a relativistic quark current.

Overview on all reactions linked to GPDs

A short overview is given on how generalized parton distributions (GPDs) enter in a variety of hard exclusive processes such as deeply virtual Compton scattering (DVCS) and hard meson electroproduction reactions on the nucleon. We firstly discuss the links between GPDs and elastic nucleon form factors which represent powerful constraints on parametrizations of GPDs. Subsequently, we show some key observables which are sensitive to the various hadron structure aspects of the GPDs, and which are at present under experimental investigation at different facilities (HERMES, H1/ZEUS, JLab and Compass), or will be addressed by experiments in the near future.

Baryon form factors at high momentum transfer and generalized parton distributions

Nucleon form factors at high momentum transfer |t| are treated in the framework of generalized parton distributions (GPD's). The possibility of obtaining information about parton high transverse momentum components by application of GPD's to form factors is discussed. This is illustrated by applying an ad-hoc 2-body parton wave function to elastic nucleon form factors F1 and F2, the N to Delta magnetic form factor G*M, and wide angle Compton scattering (WACS) form factor R1.

Comment on “Nucleon elastic form factors and local duality”

We comment on the papers ``Nucleon elastic form factors and local duality'' [Phys. Rev. D 62, 073008 (2000)] and ``Experimental verification of quark-hadron duality'' [Phys. Rev. Lett. 85, 1186 (2000)]. Our main comment is that the reconstruction of the proton magnetic form factor, claimed to be obtained from the inelastic scaling curve thanks to parton-hadron local duality, is affected by an artifact.

Reply to “Comment on ‘Nucleon elastic form factors and local duality’ ”

Recently, we extracted [Phys. Rev. D 62, 073008 (2000)] the proton magnetic form factor from duality-averaging purely inelastic nucleon resonance data. The point stressed by Simula is that this extraction at larger values of momentum transfer is dominated by an unconstrained kinematic region. We will show at which momentum transfers this point is relevant, and argue the need for additional precise nucleon resonance data.

SU(6) breaking effects in the nucleon elastic electromagnetic form factors

The effects of SU(6) breaking on the nucleon elastic form factors are investigated within the constituent quark model formulated on the light-front. It is shown that the kinematical SU(6) breaking caused by the Melosh rotations of the quark spins as well as the dynamical SU(6) breaking due to the mixed-symmetry component generated in the nucleon wave function by the spin-dependent terms of the quark-quark interaction, can affect both GEn(Q**2) and GMp(Q**2)/GMn(Q**2). The calculated GEn(Q**2) is found to be qualitatively consistent with existing data. while the calculations of GM(Q**2) based on the plus component of the current are found to be plagued by spurious effects related to the loss of the rotational covariance in the light-front formalism. These unwanted effects can be avoided by considering the transverse y-component of the current. In this way our light-front predictions are found to be consistent with the data on both GEn(Q**2) and GMp(Q**2)/GMn(Q**2). Finally, it is shown that a suppression of the ratio GEp(Q**2)/GMp(Q**2) with respect to the dipole-fit prediction can be expected in the constituent quark model provided the relativistic effects generated by the Melosh rotations of the constituent spins are taken into account.

Nucleon elastic form factors and local duality

Using a newly obtained sample of inclusive electron-proton scattering data in the nucleon resonance region we have extracted the proton magnetic form factor, utilizing the quark-hadron duality concept. The extraction is in good agreement with direct measurements. Similarly, utilizing quark-hadron duality, a relation can be derived between the ratio of longitudinal to transverse deep inelastic electron-proton scattering cross sections and the ratio of the electric to magnetic proton elastic form factors. Again, the extracted ratio agrees reasonably with the existing world data. This agreement is a first experimental verification of duality in the longitudinal channel, and suggests that effects of higher-twist operators are suppressed on average in both the longitudinal and transverse electron-nucleon scattering processes.

Strange quarks and parity violation

We report on the study of parity violation in the scattering of polarized electrons to probe the contribution of strange quarks to the elastic nucleon form factors. We review the theoretical assumptions that are involved in isolating the strange form factors as well as the predictions for the size of the strange form factors. The experiments require the measurement of small asymmetries, typically ≤ 10 −5. We describe in detail the general experimental techniques that are used to obtain this level of precision. Finally, we summarize specific experiments that have been published recently as well as new experiments in progress.

HADRONIC FORM FACTORS AND PERTURBATIVE QCD

▪ Abstract The electromagnetic form factors of hadrons at large momentum transfer have been the subject of intense theoretical and experimental scrutiny over the past two decades, yet there is still not a universally accepted framework for their description. This review is a synopsis of their current status. The basic theoretical approaches to form factors at large momentum transfer are developed, emphasizing the valence quark and Feynman (soft) pictures. The discussion includes the relation of these descriptions to the parton model, as well as the roles of factorization, evolution, Sudakov resummation and QCD sum rules. This is followed by a discussion of the experimental status of pion and nucleon elastic form factors and resonance production amplitudes in the light of recent data; the successes and shortcomings of various theoretical proposals are highlighted.

Transition from meson to quark dynamics in the nucleon form factors

Even in the few GeV 2 region, there is already a pronounced deviation of the nucleon form factors from dipole parametrization and scaling. With the pion cloud model of nucleon structure formulated in the light-cone Fock approach, the Q 2 dependence of all four elastic nucleon form factors G E p,n(Q 2) and G M p,n( Q 2) is examined. Two different mesonic are included: the coupling through intermediate vector mesons to bare nucleons and the direct photon-pion interaction with pion-nucleon and pion-delta components in the nucleon. Contributions to the physical nucleon form factors from various sectors of the model space are calculated and compared. The model predicts scaling violation of roughly 10% which is consistent with perturbative treatment of the π- N and π-Δ. The observed deviations from dipole parametrization are attributed to pion cloud at small Q 2 and perturbative quark-gluon dynamics at large Q 2.

Asymptotic behaviour of weak formfactors within QCD

The highq2-behaviour of the six weak nucleon (quasi-)elastic formfactors as calculated from lowest order (two-gluon exchange) in QCD is presented. The expressions are also parametrized in the usual way by dipole (triplepole) formulae.

Electromagnetic Elastic Nucleon Form Factors in a Relativistic Quark Model

A quark model based on the relativistic wave equation is considered. The normalizable­ wave functions are used in the calculation of the nucleon elastic electromagnetic form factors, with the current defined via minimal coupling. A dipole behaviour of the magnetic form factors is obtained, and the neutron electric form factor is different from zero. Due to the choice of the spinor part of the wave function, there are no redundant factors in the calculated form factors.

Electromagnetic Inelastic Form Factors of Processesep→eN*in a Relativistic Extended Particle Model Based on the Quark Model

The nucleon elastic form factors are also discussed in the same framework of the model. In particular it is shown that the neutron electric form factor appearing as a relativistic effect is consistent with existing experimental data.

Bounds on Elastic and Inelastic Form Factors of the Nucleon and the Pion

In this paper we derive bounds on the nucleon form factor, the anomalous magnetic moment of nucleons, and the pion form factor. By using sidewise dispersion relations and the Schwarz inequality, we are able to bound the elastic nucleon form factors ${F}_{1}({q}^{2})$ and ${F}_{2}({q}^{2})$ by integrals over the structure functions for inelastic electron-nucleon scattering, ${W}_{1,2}({q}^{2},\ensuremath{\nu})$. At ${q}^{2}=0$, we then use unitarity to bound the anomalous magnetic moment by an integral over the nucleon propagator spectral function. Finally, by dispersing in ${q}^{2}$, the photon virtual mass, we are able to bound the pion form factor ${F}_{\ensuremath{\pi}}({q}^{2})$ by an integral over the total electron-positron annihilation cross section.

Phenomenological Analysis of theγNN*Form Factors

The data on electroexcitation of ${N}^{*}$ in the momentum-transfer range $\ensuremath{-}{q}^{2}=0.1\ensuremath{-}2.33$ Ge${\mathrm{V}}^{2}$ have been analyzed phenomenologically, using an isobar model. Assuming only the $M1$ transition, we obtained a phenomenological form factor for the $\ensuremath{\gamma}N{N}^{*}$ vertex. This form factor is found to decrease much faster than the elastic nucleon form factors. This implies that the ${N}^{*}$ has a larger spatial extension than the $N$.