Nucleon Electromagnetic Form Factors Research Articles

Odd-parity nucleon electromagnetic transitions in lattice QCD

The parity-expanded variational analysis (PEVA) technique enables the isolation of opposite-parity eigenstates at finite momentum. The approach has been used to perform the first lattice QCD calculations of excited-baryon form factors. In particular, these calculations show that the low-lying odd-parity nucleon excitations are described well by constituent quark models at moderate u and d quark masses approaching the strange quark mass. Herein, we extend the PEVA technique to establish a formalism for the determination of odd-parity nucleon electromagnetic transition form factors in lattice QCD. The formalism is implemented in the first calculation of the helicity amplitudes for transitions from the ground state nucleon to the first two odd-parity excitations. Through a comparison with constituent quark-model calculations of these amplitudes, these new results give important insight into the structure of these excitations. This work is a critical step towards confronting experimental electroproduction amplitudes for the N*(1535) and N*(1650) resonances with lattice QCD calculations. Published by the American Physical Society 2024

Light quark mass dependence of nucleon electromagnetic form factors in dispersively modified chiral perturbation theory

Light quark mass dependence of nucleon electromagnetic form factors in dispersively modified chiral perturbation theory

New insights into the oscillations of the nucleon electromagnetic form factors

The electromagnetic form factors of the proton and the neutron in the timelike region are investigated. Electron–positron annihilation into antinucleon-nucleon (N¯N) pairs is treated in distorted wave Born approximation, including the final-state interaction in the N¯N system. The latter is obtained by a Lippmann–Schwinger equation for N¯N potentials derived within SU(3) chiral effective field theory. By fitting to the phase shifts and (differential) cross section data, a high quality description is achieved. With these amplitudes, the oscillations of the electromagnetic form factors of the proton and the neutron are studied. It is found that each of them can be described by two fractional oscillators. One is characterized as “overdamped” and dominates near the threshold, while the other is “underdamped” and plays an important role in the high-energy region. These two oscillators are essential to understand the distributions of polarized electric charges induced by hard photons for the nucleons.

The proton radius and its relatives - much ado about nothing?

I summarize the dispersion-theoretical analysis of the nucleon electromagnetic form factors. Special emphasis is given on the extraction of the proton charge radius and its relatives, the proton magnetic radius as well as the neutron magnetic radius. Some recent work on the hyperfine splitting in leptonic hydrogen and on radiative corrections to muon-proton scattering is also discussed. Some views on future studies are given.

Nucleon Electromagnetic Form Factors at Large Momentum Transfer from Lattice QCD

Nucleon Electromagnetic Form Factors at Large Momentum Transfer from Lattice QCD

Generalized parton distributions at zero skewness

We present a new determination of the generalized parton distributions (GPDs) with their uncertainties at zero skewness, $ \xi =0 $, through a simultaneous analysis of all available experimental data of the nucleon electromagnetic form factors (FFs), nucleon charge and magnetic radii, proton axial FFs (AFFs) and wide-angle Compton scattering (WACS) cross sections for the first time, and we investigate whether there is any tension between these data. This can be considered the most comprehensive analysis of GPDs at $ \xi=0 $ performed so far. We show that such an analysis provides the simultaneous determination of three kinds of GPDs, namely $ H^q $, $ \widetilde{H}^q $ and $ E^q $, considering also the sea-quark contributions. As a result, we find that the inclusion of the WACS and AFF data at larger values of the momentum transfer squared $ Q^2=-t $ can put new constraints on GPDs and change them drastically in some cases. We show that there is a considerable tension between the WACS and the proton magnetic form factor ($ G_M^p $) data, especially at larger values of $ -t $. However, we indicate that the results for the gravitational FF $ M_2 $ and the proton total angular momentum $ J^p $ calculated using the extracted GPDs are in relatively good agreement with the light-cone QCD sum rules (LCSRs) and lattice QCD predictions when the sea-quark contributions are considered and both WACS and $ G_M^p $ data are included in the analyses simultaneously.

Toy model to understand the oscillatory behavior in timelike nucleon form factors

To understand the oscillatory behavior exhibited in the timelike electromagnetic form factors of nucleons, we propose a toy model based on the Jost function of the $N\bar N$ pair into the timelike form factors with the help of the distorted-wave Born approximation. By constructing a simple square-well potential reflecting the final-state interaction of $N\bar N$, we naturally represent the damped oscillatory phenomenon in the timelike electromagnetic form factors of nucleons. Especially, our study reveals that the ``period" of the oscillation is approximately determined by the Yukawa interaction range $1/m_\pi$. Other possible potentials are also discussed. The threshold enhancements of the cross sections for $e^+e^-\to n\bar n$, $\Lambda\bar\Lambda$, and $\Lambda_c\bar\Lambda_c$ can also be understand within this scenario.

Nucleon Resonance Electroexcitation Amplitudes and Emergent Hadron Mass

Understanding the strong interaction dynamics that govern the emergence of hadron mass (EHM) represents a challenging open problem in the Standard Model. In this paper we describe new opportunities for gaining insight into EHM from results on nucleon resonance (N*) electroexcitation amplitudes (i.e., γvpN* electrocouplings) in the mass range up to 1.8 GeV for virtual photon four-momentum squared (i.e., photon virtualities Q2) up to 7.5 GeV2 available from exclusive meson electroproduction data acquired during the 6-GeV era of experiments at Jefferson Laboratory (JLab). These results, combined with achievements in the use of continuum Schwinger function methods (CSMs), offer new opportunities for charting the momentum dependence of the dressed quark mass from results on the Q2-evolution of the γvpN* electrocouplings. This mass function is one of the three pillars of EHM and its behavior expresses influences of the other two, viz. the running gluon mass and momentum-dependent effective charge. A successful description of the Δ(1232)3/2+ and N(1440)1/2+ electrocouplings has been achieved using CSMs with, in both cases, common momentum-dependent mass functions for the dressed quarks, for the gluons, and the same momentum-dependent strong coupling. The properties of these functions have been inferred from nonperturbative studies of QCD and confirmed, e.g., in the description of nucleon and pion elastic electromagnetic form factors. Parameter-free CSM predictions for the electrocouplings of the Δ(1600)3/2+ became available in 2019. The experimental results obtained in the first half of 2022 have confirmed the CSM predictions. We also discuss prospects for these studies during the 12-GeV era at JLab using the CLAS12 detector, with experiments that are currently in progress, and canvass the physics motivation for continued studies in this area with a possible increase of the JLab electron beam energy up to 22 GeV. Such an upgrade would finally enable mapping of the dressed quark mass over the full range of distances (i.e., quark momenta) where the dominant part of hadron mass and N* structure emerge in the transition from the strongly coupled to perturbative QCD regimes.

Pion and nucleon relativistic electromagnetic four-current distributions

The quantum phase-space approach allows one to define relativistic spatial distributions inside a target with arbitrary spin and arbitrary average momentum. We apply this quasiprobabilistic formalism to the whole electromagnetic four-current operator in the case of spin-0 and spin-$\frac{1}{2}$ targets, study in detail the frame dependence of the corresponding spatial distributions, and compare our results with those from the light-front formalism. While former works focused on the charge distributions, we extend here the investigations to the current distributions. We clarify the role played by the Wigner rotation and argue that electromagnetic properties are most naturally understood in terms of Sachs form factors, contrary to what the light-front formalism previously suggested. Finally, we illustrate our results using the pion and nucleon electromagnetic form factors extracted from experimental data.

Polarization phenomena in the reaction e++e−→N+N¯+π0 in the framework of the nonresonant mechanism

The dependence of the nucleon polarization in the reaction ${e}^{+}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}N+\overline{N}+{\ensuremath{\pi}}^{0}$ over different invariant variables is derived for the nonresonant mechanism. The nucleon polarization is expressed in terms of six invariant complex amplitudes, assuming the conservation of the hadron electromagnetic currents and the P invariance of the hadron electromagnetic interaction. An inclusive experimental setup when the proton (or the antiproton) and the pion are detected in coincidence is considered. Numerical estimations are performed for the so-called normal polarization in the energy range from threshold up to $s=16\text{ }\text{ }{\mathrm{GeV}}^{2}$, using selected parametrizations of the nucleon electromagnetic form factors in frame of a formalism derived in a previous work for the calculation of the unpolarized differential cross section.

Heavy quark contribution to the electromagnetic properties of the nucleon

Quantum chromodynamics (QCD) predicts the existence of both nonperturbative intrinsic and perturbative extrinsic heavy quark contributions to the fundamental structure of hadrons. The existence of intrinsic charm at the 3-standard-deviation level in the proton has recently been established from structure function measurements by the NNPDF Collaboration. Here we revisit the physics of intrinsic heavy quarks using light-front holographic QCD (LFHQCD) - a novel comprehensive approach to hadron structure which provides detailed predictions for dynamical properties of the hadrons, such as form factors, distribution amplitudes, structure functions, etc. We will extend this nonperturbative light-front QCD approach to study the heavy quark-antiquark contribution to the electromagnetic properties of nucleon. Our framework is based on a study of the eigenfunctions of the QCD light-front Hamiltonian, the frame-independent light-front wave functions (LFWFs) underlying hadron dynamics. We analyze the heavy quark content in the proton, induced either directly by the nonperturbative $|uud+Q\bar Q\rangle$ Fock state or by the $|uud+g\rangle$ Fock state, where the gluon splits into a heavy quark-antiquark pair. The specific form of these LFWFs are derived from LFHQCD. Using these LFWFs, we construct light-front representations for the heavy quark-antiquark asymmetry, the electromagnetic form factors of nucleons induced by heavy quarks, including their magnetic moments and radii.

Contributions to the nucleon form factors from bubble and tadpole diagrams* *Supported by the National Natural Science Foundation of China (11975241)

The nonlocal chiral effective theory is applied to investigate the electromagnetic and strange form factors of nucleons. The bubble and tadpole diagrams are included in the calculation. With the contributions from bubble and tadpole diagrams, the obtained electromagnetic form factors are close to the results without these contributions as long as the low energy constants and are properly chosen, while the magnitudes of strange form factors become larger. The electromagnetic form factors are in good agreement with the experimental results, while the magnitudes of strange form factors are larger than the lattice data.

Precision calculation of the recoil–finite-size correction for the hyperfine splitting in muonic and electronic hydrogen

We present a high-precision calculation of the recoil–finite-size correction to the hyperfine splitting (HFS) in muonic and electronic hydrogen based on nucleon electromagnetic form factors obtained from dispersion theory. This will help guide the upcoming searches of the HFS transition in muonic hydrogen, and will allow a precise determination of the sum of the polarizability and Zemach radius contributions when this transition is found.

Recent results on nucleon electromagnetic form factors at BESIII


 
 
 We report recent results of Nucleon Electromagnetic Form Factors at the BESIII experiment. The BESIII detector is installed at the BEPCII electron-positron collider in Beijing (PRC) with a center-of-mass energy range between between 2.0 and 4.9 GeV. The Nucleon Electromagnetic Form Factors has been measured in BESIII both via direct e+e− annihilation and initial-state-radiation technique.
 
 

Timelike nucleon electromagnetic form factors: All about interference of isospin amplitudes

A striking feature of the timelike nucleon electromagnetic form factors, investigated in ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}N\overline{N}$ annihilation reactions, is the modulation by local structures of small magnitude and oscillatory form, showing up above the $N\overline{N}$ threshold. Starting from an isospin decomposition of the proton and neutron form factors it is shown that such structures are the natural consequence of the interference of a large and a small amplitude, resulting in a sinusoidal behavior as a function of the ``invariant energy'' if the relative phase shift varies with energy. Thus, periodic oscillations superimposed on a smooth background will be observed. In this scenario, the equal size of the modulation for neutrons and protons (discovered by recent BESIII data) implies the particular isoscalar or isovector nature of these local structures, or their orthogonal interference. Hence, it specifies their origin as excited vector mesons whose widths are tied to the modulation frequency. We clarify that the phase difference of the modulation between neutrons and protons as BESIII data found, but not the modulation itself, is the evidence of an imaginary part of the timelike nucleon electromagnetic form factors, which is associated with the rescattering processes.

Parametrization of quark and gluon generalized parton distributions in a dynamical framework

We present a parametrization of the chiral even generalized parton distributions, $H$, $E$, $\stackrel{\texttildelow{}}{H}$, $\stackrel{\texttildelow{}}{E}$, for the quark, antiquark, and gluon, in the perturbative QCD (pQCD)-parton framework. Parametric analytic forms are given as a function of two equivalent sets of variables $x$, $\ensuremath{\xi}$, $t$ (symmetric frame) and $X$, $\ensuremath{\zeta}$, $t$ (asymmetric frame), at an initial scale, ${Q}_{o}^{2}$. In the $X>\ensuremath{\zeta}$ region, a convenient and flexible form is obtained as the product of a Regge term $\ensuremath{\propto}{X}^{\ensuremath{-}\ensuremath{\alpha}+{\ensuremath{\alpha}}^{\ensuremath{'}}t}$, describing the low $X$ behavior, times a spectator model-based functional form depending on various mass parameters; the behavior at $X<\ensuremath{\zeta}$ is determined using the generalized parton distributions symmetry and polynomiality properties. The parameters are constrained using data on the flavor separated nucleon electromagnetic elastic form factors, the axial and pseudoscalar nucleon form factors, and the parton distribution functions from both the deep inelastic unpolarized and polarized nucleon structure functions. For the gluon distributions we use, in particular, constraints provided by recent lattice QCD moments calculations. The parametrization's kinematical range of validity is $0.0001\ensuremath{\le}X\ensuremath{\le}\phantom{\rule{0ex}{0ex}}0.85$, $0.01\ensuremath{\le}\ensuremath{\zeta}\ensuremath{\le}0.85$, $0\ensuremath{\le}\ensuremath{-}t\ensuremath{\le}1\text{ }\text{ }{\mathrm{GeV}}^{2}$, $2\ensuremath{\le}{Q}^{2}\ensuremath{\le}100\text{ }\text{ }{\mathrm{GeV}}^{2}$. With the simultaneous description of the quark, antiquark, and gluon sectors, this parametrization represents a first tool enabling a global QCD analysis of deeply virtual exclusive experiments.

Electromagnetic Form Factors of Σ Hyperons

Electromagnetic form factors (EMFFs) are fundamental observable of baryons that intimately related to their internal structure and dynamics, where the EMFFs of hyperons provide valuable insight into the behavior of the strangeness. The EMFFs of hyperons can also help to understand those of nucleons as they are connected with the flavor SU(3) symmetry. The EMFFs of nucleons can be measured in both spacelike and timelike regions. However, it is difficult to probe the EMFFs of hyperons in spacelike region due to the unstable nature of hyperons. By means of electron-positron annihilation, the EMFFs of hyperons in timelike region is accessible via the production of hyperon-antihyperon pair. The timelike EMFFs of the isospin triplet Σ hyperons measured at Babar, CLEO-c and BESIII experiments are reviewed in this paper. Besides, the relevant theoretical discussion based on the experimental results are also presented.

Effects of neutrino magnetic moment and charge radius constraints and medium modifications of the nucleon form factors on the neutrino mean free path in dense matter

Effects of neutrino charge radius and magnetic moment constraints obtained from the astrophysical observations and reactor experiments, as well as in-medium modifications of the weak and electromagnetic nucleon form factors of the matter on the neutrino electroweak interaction with dense matter, are estimated. We use a relativistic mean-field model for the in-medium effective nucleon mass and quark-meson coupling model for nucleon form factors. We analyze the neutrino scattering cross section and mean free path in cold nuclear matter. We find that the increase of the cross section relative to that without neutrino form factors results in the decrease of the neutrino mean free path when neutrino form factors and the in-medium modifications of the nucleon weak and electromagnetic form factors are simultaneously considered. The quenching of the neutrino mean free path is evaluated to be about 12-58% for the values of μν=3×10−12μB and Rν=3.5×10−5MeV−1 compared with that obtained for the μν=0 and Rν=0. The decrease of the neutrino mean free path is expected to decelerate the cooling of neutron stars. Each contribution of the neutrino form factors to the neutrino mean free path is discussed.

Prediction of neutron electromagnetic form factors behaviors just by the proton electromagnetic form factors data

First time a theoretical prediction of the neutron electromagnetic form factors behaviors in the whole region of their definition by the use of the proton electromagnetic form factors data only is demonstrated. This could be achieved in the framework of the unitary and analytic approach to a description of the nucleon electromagnetic form factors with canonical normalization conditions, the QCD (up to the logarithmic correction) asymptotic behaviors and specific transformation properties of the nucleon electromagnetic current in the isotopic space.

Nπ -state contamination in lattice calculations of the nucleon electromagnetic form factors

The nucleon-pion-state contribution to QCD two-point and three-point functions relevant for lattice calculations of the nucleon electromagnetic form factors are studied in chiral perturbation theory. To leading order the results depend on a few experimentally known low-energy constants only, and the nucleon-pion-state contribution to the form factors can be estimated. The nucleon-pion-state contribution to the electric form factor $G_{\rm E}(Q^2)$ is at the +5 percent level for a source-sink separation of 2 fm, and it increases with increasing momentum transfer $Q^2$. For the magnetic form factor the nucleon-pion-state contribution leads to an underestimation of $G_{\rm M}(Q^2)$ by about 5 percent that decreases with increasing $Q^2$. For smaller source-sink separations that are accessible in present-day lattice simulations the impact is larger, although the ChPT results may not be applicable for such small time separations. Still, a comparison with lattice data at $t\approx 1.6$ fm works reasonably well.