Nucleon Electromagnetic Form Factors Research Articles

Nucleon form factors, generalized parton distributions and quark angular momentum

We extract the individual contributions from u and d quarks to the Dirac and Pauli form factors of the proton, after a critical examination of the available measurements of electromagnetic nucleon form factors. From this data we determine generalized parton distributions for valence quarks, assuming a particular form for their functional dependence. The result allows us to study various aspects of nucleon structure in the valence region. In particular, we evaluate Ji’s sum rule and estimate the total angular momentum carried by valence quarks at the scale μ=2 GeV to be $J_{v}^{u} = 0.230^{+ 0.009}_{- 0.024}$ and $J_{v}^{d} = -0.004^{+ 0.010}_{- 0.016}$ .

Possible Uncertainties of Parity Violation in Electron Quasi-Elastic Scattering

From the leading order of the interference of the electromagnetic current and the electroweak neutral current, parity violating electron scattering on a nucleus in the quasi-elastic region is investigated by using a relativistic single particle model. We use 208Pb as a target nucleus, and the incident electron energy range is exploited up to 1 GeV. We enumerate possible ambiguities from 208Pb by the electromagnetic form factors, the strangeness form factors, the final states interaction, and the Coulomb effects on the parity violation (PV) asymmetry in the inclusive electrons scattering on the quasi-elastic region. The effect of nucleon electromagnetic form factors is studied by comparing the standard Sachs form factors and the parameterized form recently adjusted to the world data. The effect of the strangeness in the PV asymmetry is also investigated through the recent results of the strangeness form factors. We separately calculate the asymmetries of longitudinal and transverse response functions. The ...

Ideas of four-fermion operators in hadron physics

Four-fermion operators have been utilized in the past to link the quarkexchange processes in the interaction of hadrons with the effective mesonexchange amplitudes. In this paper, we apply the similar idea of Fierz rearrangement to the electromagnetic processes and focus on the electromagnetic form factors of nucleon and electron. We explain the motivation of using four-fermion operators and discuss the advantage of this method in computing electromagnetic processes.

Flavor decomposition of the nucleon electromagnetic form factors

Background: The spatial distribution of charge and magnetization in the proton and neutron are encoded in the nucleon electromagnetic form factors. The form factors are all approximated by a simple dipole function, normalized to the charge or magnetic moment of the nucleon. The differences between the proton and neutron form factors and the deviation of GEn from zero are sensitive to the difference between up- and down-quark contributions to the form factors. Methods: We combine recent measurements of the neutron form factors with updated extractions of the proton form factors, accounting for two-photon exchange corrections and including an estimate of the uncertainties for all of the form factors to obtain a complete set of measurements up to Q^2 approximately 4 (GeV/c)^2. We use this to extract the up- and down-quark contributions which we compare to recent fits and calculations. Results: We find a large differences between the up- and down-quark contributions to G_E and G_M, implying significant flavor dependence in the charge and magnetization distributions. The rapid falloff of the ratio GEp/GMp does not appear in the individual quark form factors, but arises from a cancellation between the up- and down-quark contributions. We see indications that the down-quark contributions to the Dirac and Pauli form factors deviate from the suggested 1/Q^4 scaling behavior suggested by a previous analysis. Finally, while recent models provide a generally good qualitative description of the data, the down-quark contribution to G_E/G_M and F_2/F_1 are not reproduced by any of the models. Finally, we note that while the inclusion of recent $\gmn$ data from CLAS modifies the high-$Q^2$ slightly, the tension between these data and previous measurements at lower $Q^2$ has a more significant impact, suggesting the need for additional data in this region.

Nucleon structure including high Fock states in AdS/QCD

We present a detailed analysis of nucleon electromagnetic and axial form factors in a holographic soft-wall model. This approach is based on an action which describes hadrons with broken conformal invariance and incorporates confinement through the presence of a background dilaton field.For Nc=3 we describe the nucleon structure in a superposition of a three valence quark state with high Fock states including an adjustable number of partons (quarks, antiquarks and gluons) via studying the dynamics of 5D fermion fields of different scaling dimension in anti-de Sitter (ADS) space. According to the gauge/gravity duality the 5D fermion fields of different scaling dimension correspond to the Fock state components with a specific number of partons. In the present application we restrict to the contribution of 3, 4 and 5 parton components in the nucleon Fock state. With a minimal number of free parameters (dilaton scale parameter, mixing parameters of partial contributions of Fock states, coupling constants in the effective Lagrangian) we achieve a reasonable agreement with data for the nucleon form factors.

Erratum: Timelike and spacelike electromagnetic form factors of nucleons, a unified description [Phys. Rev. D85, 113004 (2012)

Erratum: Timelike and spacelike electromagnetic form factors of nucleons, a unified description [Phys. Rev. D<b>85</b>, 113004 (2012)

Timelike and spacelike electromagnetic form factors of nucleons, a unified description

The extended Lomon-Gari-Kr\"umpelmann model of nucleon electromagnetic form factors, which embodies $\ensuremath{\rho}$, ${\ensuremath{\rho}}^{\ensuremath{'}}$, $\ensuremath{\omega}$, ${\ensuremath{\omega}}^{\ensuremath{'}}$, and $\ensuremath{\phi}$ vector meson contributions and the perturbative QCD high momentum transfer behavior has been extended to the timelike region. Breit-Wigner formulas with momentum-dependent widths have been considered for broad resonances in order to have a parametrization for the electromagnetic form factors that fulfills, in the timelike region, constraints from causality, analyticity, and unitarity. This analytic extension of the Lomon-Gari-Kr\"umpelmann model has been used to perform a unified fit to all the nucleon electromagnetic form factor data, in the spacelike and timelike region (where form factor values are extracted from ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\leftrightarrow}N\overline{N}$ cross sections data). The knowledge of the complete analytic structure of form factors enables predictions at extended momentum transfer, and also of timelike observables such as the ratio between electric and magnetic form factors and their relative phase.

Roy-Steiner equations for pion-nucleon scattering

Starting from hyperbolic dispersion relations, we derive a closed system of Roy-Steiner equations for pion-nucleon scattering that respects analyticity, unitarity, and crossing symmetry. We work out analytically all kernel functions and unitarity relations required for the lowest partial waves. In order to suppress the dependence on the high-energy regime we also consider once- and twice-subtracted versions of the equations, where we identify the subtraction constants with subthreshold parameters. Assuming Mandelstam analyticity we determine the maximal range of validity of these equations. As a first step towards the solution of the full system we cast the equations for the $\pi\pi\to\bar NN$ partial waves into the form of a Muskhelishvili-Omn\`es problem with finite matching point, which we solve numerically in the single-channel approximation. We investigate in detail the role of individual contributions to our solutions and discuss some consequences for the spectral functions of the nucleon electromagnetic form factors.

Nucleon electromagnetic form factors and electroexcitation of low-lying nucleon resonances in a light-front relativistic quark model

We utilize a light-front relativistic quark model (LF RQM) to predict the 3q core contribution to the electroexcitation amplitudes for the Delta(1232)P33, N(1440)P11, N(1520)D13, and N(1535)S11 up to Q2= 12GeV2. The parameters of the model have been specified via description of the nucleon electromagnetic form factors in the approach that combines 3q and pion-cloud contributions in the LF dynamics.

Chiral extrapolations for nucleon magnetic moments

Lattice QCD simulations have made significant progress in the calculation of nucleon electromagnetic form factors in the chiral regime in recent years. With simulation results achieving pion masses of order ~180 MeV, there is an apparent challenge as to how the physical regime is approached. By using contemporary methods in chiral effective field theory, both the quark-mass and finite-volume dependence of the isovector nucleon magnetic moment are carefully examined. The extrapolation to the physical point yields a result that is compatible with experiment, albeit with a combined statistical and systematic uncertainty of 10%. The extrapolation shows a strong finite-volume dependence; lattice sizes of L > 5 fm must be used to simulate results within 2% of the infinite-volume result for the magnetic moment at the physical pion mass.

A model for space and time-like proton (neutron) form factors

A model is suggested to interpret nucleon electromagnetic form factors both in space and time-like regions and to describe the existing data. It implies quarks to be colorless in the region of high intensity chromo-electromagnetic gluon field inside the nucleon.

Evaluation of fermion loops applied to the calculation of the [formula omitted] mass and the nucleon scalar and electromagnetic form factors

The exact evaluation of the disconnected diagram contributions to the flavor-singlet pseudo-scalar meson mass, the nucleon σ-term and the nucleon electromagnetic form factors is carried out utilizing GPGPU technology with the NVIDIA CUDA platform. The disconnected loops are also computed using stochastic methods with several noise reduction techniques. Various dilution schemes as well as the truncated solver method are studied. We make a comparison of these stochastic techniques to the exact results and show that the number of noise vectors depends on the operator insertion in the fermion loop.

New value of the proton charge root mean square radius

The non-dipole behavior of GEp(Q2) in the space-like region with the zero around Q2=13GeV2 is found in a global analysis of all existing nucleon electromagnetic form factor data by means of a sophisticated analytic model of nucleon electromagnetic structure. From such behavior of GEp(Q2), the charge proton root mean square (rms) radius is compatible with the value obtained recently from spectroscopy of the muon hydrogen atom.

Time-like electromagnetic form factors at PANDA

This proceeding is a summary based on the talk given at the 33rd international school of nuclear physics, Erice, Italy. An introduction following the historical development of a theoretical treatment of nucleon electromagnetic form factors will be given. A feasibility study on the time-like electromagnetic form factor at PANDA is presented based on a Monte Carlo simulation. Some recent progress on electromagnetic processes at PANDA is also given.

Hadron properties in AdS/QCD

We discuss a holographic soft-wall model developed for the description of mesons and baryons with adjustable quantum numbers n,J,L,S. This approach is based on an action which describes hadrons with broken conformal invariance and which incorporates confinement through the presence of a background dilaton field. Results obtained for heavy-light meson masses and decay constants are consistent with predictions of HQET. In the baryon sector, applications to the baryon masses, nucleon electromagnetic form factors and generalized parton distributions are discussed.

Properties of the bound nucleons

We present recent investigations on the properties of nucleons in nuclear matter and, in particular, nucleon electromagnetic form factors in the framework of the in-medium modified Skyrme model. Medium modifications are achieved by introducing the pion-nucleus optical potential for the pion fields in nuclear environment. In order to reproduce the bulk matter properties, the parametrization of the Skyrme term in the nu- clear matter has been adjusted and the sensitivity of results to the pion-nucleus optical potential parameters are discussed. In studying nuclear matter properties one can also start from an alternative approach: we first modify nucleon properties in nuclear environment according to some general rules and considerations, and then try to reproduce the properties of the nuclear matter itself. The in-medium modified Skyrme model (6) and its recently improved version (7) are based on a such approach and allow one to perform pertinent studies in a single and many-body levels. Initially, the in-medium modified Skyrme model (6) has been proposed to study single nucleon properties in nuclear environment not only by guessing the corresponding changes in input parameters of the Lagrangian according to the density changes but also relate those changes to the existing phenomenological data, i.e. to the pion physics in nuclear matter. These studies do not take into account the direct and obvious changes of the quark core of the nucleon but take a small influence of the surrounding environment to the nucleon core via the pion profile changes. Furthermore, to be more realistic and to utilize more phenomenological information from the many-body sector, the core part of the nucleon also was modified and that medium modification has been adjusted to reproduce the binding energy of the whole system (7). While there are no explicit quark degrees of freedom and the core of the nucleon in the Skyrme model is e ectively represented by the Skyrme term in the e ective Lagrangian, core modifications are related to the change of the Skyrme parameter. In more detailed treatments Skyrme's quartic stabilizing term may be revised by explicit vector meson degrees of freedom. In the present work, we discuss the changes in the electromagnetic properties of the bound nucle- ons, based on the e ective Lagrangian discussed in Ref. (7). In this context, the electromagnetic form factors of the bound nucleons can be directly related to the existing experimental indications coming from the structure studies of the matter under extreme conditions.

Electromagnetic form factors in the relativistic interacting quark-diquark model of baryons

A relativistic interacting quark-diquark model of baryons with direct and exchange interaction has been used to calculate the nucleon electromagnetic form factors. The results have been compared with experimental data.

Implications of the new proton charge rms radius from spectroscopy of muon hydrogen atom

It is demonstrated a coincidence of the proton electric rms radius from the global analysis of the JLab proton polarization data together with all other existing nucleon electromagnetic form factor data in the framework of the Unitary & Analytic model of nucleon electromagnetic structure with its value recently determined in a spectroscopy of muon hydrogen atom.The result confirms a non-dipole behavior of the proton electric form factor in the space-like region and strengthen a belief in an existence of the proton electric form factor zero revealed in global analysis of nucleon form factors data.

Nucleon electromagnetic form factors from the covariant Faddeev equation

We compute the electromagnetic form factors of the nucleon in the Poincare-covariant Faddeev framework based on the Dyson-Schwinger equations of QCD. The general expression for a baryon's electromagnetic current in terms of three interacting dressed quarks is derived. Upon employing a rainbow-ladder gluon-exchange kernel for the quark-quark interaction, the nucleon's Faddeev amplitude and electromagnetic form factors are computed without any further truncations or model assumptions. The form factor results show clear evidence of missing pion-cloud effects below a photon momentum transfer of ~2 GeV^2 and in the chiral region whereas they agree well with experimental data at higher photon momenta. Thus, the approach reflects the properties of the nucleon's quark core.

Flavor Decomposition of the Elastic Nucleon Electromagnetic Form Factors

The u- and d-quark contributions to the elastic nucleon electromagnetic form factors have been determined by using experimental data on G(E)(n), G(M)(n), G(E)(p), and G(M)(p). Such a flavor separation of the form factors became possible up to negative four-momentum transfer squared Q(2) = 3.4 GeV(2) with recent data on G(E)(n) from Hall A at Jefferson Lab. For Q(2) above 1 GeV(2), for both the u and the d quark, the ratio of the Pauli and Dirac form factors, F(2)/F(1), was found to be almost constant in sharp contrast to the behavior of F(2)/F(1) for the proton as a whole. Also, again for Q(2)>1 GeV(2), both F(2)(d) and F(1)(d) are roughly proportional to 1/Q(4), whereas the dropoff of F(2)(u) and F(1)(u) is more gradual.