Electroweak form factors of non-strange baryons

We compute electroweak form factors of the nucleon and photon transition form factors of non-strange baryon resonances up to the third resonance region in a model with instanton-induced interaction. The calculation is based on the Bethe-Salpeter equation for three light constituent quarks and is fully relativistic (U. L?ring et al., Eur. Phys. J. A 10, 309 (2001)). Static nucleon properties and photon resonance couplings are in good agreement with experiment and the Q² behaviour of the experimentally known form factors up to large momentum transfer is accounted for. Received: 4 April 2002 / Accepted: 2 May 2002     

Effect of the interference between orbital and spin currents on form factors for nuclear electroexcitation

The momentum-transfer dependence of the transverse and longitudinal form factors is considered for all transitions forming electric multipole resonances in cross sections for nuclear electroexcitation. The contributions of the matrix elements of orbital and spin currents to transverse EJ form factors are analyzed. The special features of the form factors due to interference between nucleon current are revealed. A universal character of the destructive interference between currents is proven for transitions dominant in the wave function for the giant photonuclear dipole resonance.     

Empirical transverse charge densities in the nucleon and the nucleon-to-delta transition

Using only the current empirical information on the nucleon electromagnetic form factors we map out the transverse charge density in proton and neutron as viewed from a light front moving towards a transversely polarized nucleon. These charge densities are characterized by a dipole pattern, in addition to the monopole field corresponding with the unpolarized density. Furthermore, we use the latest empirical information on the N-->Delta transition form factors to map out the transition charge density which induces the N-->Delta excitation. This transition charge density in a transversely polarized N and Delta contains both monopole, dipole and quadrupole patterns, the latter corresponding with a deformation of the N and Delta charge distribution.     

Understanding nucleon structure using lattice simulations

This review focuses on the discussion of three key results of nucleon structure calculations on the lattice. These three results are the quark contribution to the nucleon spin, J_q, the nucleon-Δ transition form factors, and the nucleon axial coupling, g_A. The importance for phenomenology and experiment is discussed and the requirements for future simulations are pointed out.     

Calculations of electromagnetic nucleon form factors and electroexcitation amplitudes of isobars

In this paper, we present numerical results for electroproduction amplitudes of proton resonances and electromagnetic nucleon form factors calculated in a relativized quark model. Interactions with both transversely and longitudinally polarized virtual photons were considered. Contributions of the different effects included in our approach have been analysed through a sample comparison with the available data. We also discuss the validity of the usual single-quark transition ansatz and possible parametrizations of the potential acting between the constituent quarks of the baryon. Impressive agreement is obtained with the nucleon form factor data up to squared momentum transfers of 2.5 GeV², but still some problems remain with the Δ(1232) and higher resonances.     

Nucleon form factors in a simple three-body quark model

We construct a simple 3-body quark model for the non strange nucleon resonances and we give results for the spectrum, the helicity amplitudes and the transition form factors. All the observables, in particular the transition form factors, are evaluated analytically and the results are compared with those of other models. Received: 15 November 1997     

Tensor Form Factors and Transverse Spin Structures of the Nucleon

We briefly review recent investigations on the transverse spin structure of the nucleon, based on the chiral quark-soliton model. We discuss the results of the tensor and anomalous tensor magnetic form factors and the corresponding transverse quark spin densities inside a nucleon. The results are compared with those of the recent lattice calculation. While their qualitative features are similar to the lattice ones, there are some discrepancies in the momentum dependence of the form factors and accordingly in the strengths of the transverse spin densities.     

Baryon resonance analysis from MAID

The unitary isobar model MAID2007 has been used to analyze the recent data of pion electroproduction. The model contains all four-star resonances in the region below W = 2 GeV and both single-Q~2 and Q~2 dependent transition form factors could be obtained for the Delta, Roper, D_(13)(1520), S_(11) (1535), S_(31) (1620), S_(11)(1650), D_(15)(1675), F_(15)(1680) and P_(13)(1720). From the complete world data base, including also π~- data on the neutron, also Q~2 dependent neutron form factors are obtained. For all transition form factors we also give convenient numerical parameterizations that can be used in other reactions. Furthermore, we show how the transition form factors can be used to obtain empirical transverse charge densities and our first results are given for the Roper, the S_(11) and D_(13) resonances.     

Review of two-photon exchange in electron scattering

We review the role of two-photon exchange (TPE) in electron–hadron scattering, focusing in particular on hadronic frameworks suitable for describing the low and moderate Q² region relevant to most experimental studies. We discuss the effects of TPE on the extraction of nucleon form factors and their role in the resolution of the proton electric to magnetic form factor ratio puzzle. The implications of TPE on various other observables, including neutron form factors, electroproduction of resonances and pions, and nuclear form factors, are summarized. Measurements seeking to directly identify TPE effects, such as through the angular dependence of polarization observables, nonlinear ε contributions to the cross sections, and via e⁺p to e⁻p cross section ratios, are also outlined. In the weak sector, we describe the role of TPE and γZ interference in parity-violating electron scattering, and assess their impact on the extraction of the strange form factors of the nucleon and the weak charge of the proton.     

On Gravitational Form Factors and Transverse Spin Sum Rule

Using the light front wave functions of the scalar quark–diquark model for nucleon predicted by the soft-wall AdS/QCD, we calculate the flavor dependent gravitational form factors. We evaluate the matrix element of Pauli–Lubanski operator in this model and show that the intrinsic spin sum rule involves the higher twist form factor \({\bar{C}}\). The longitudinal momentum densities in the transverse impact parameter space are also discussed for both unpolarized and transversely polarized nucleons.     

Transverse spin structure of hadrons from lattice QCD

This paper presents recent lattice QCD calculations of transverse spin densities of quarks in hadrons.² Based on our simulation results for the tensor generalized form factors, we find substantial correlations between spin and coordinate degrees of freedom in the nucleon and the pion. They lead to strongly distorted transverse spin densities of quarks in the nucleon and a surprisingly non-trivial transverse spin structure of the pion. Following recent arguments by Burkardt [M. Burkardt, Phys. Rev. D 72 (2005) 094020], our results imply that the Boer-Mulders function , describing correlations of the transverse spin and intrinsic transverse momentum of quarks, is large and negative for up-quarks in the proton and the π⁺. This supports the recent hypothesis that all Boer-Mulders functions are alike [arXiv:0705.1573], and also provides additional motivation for future studies of azimuthal asymmetries in πp Drell-Yan production at, e.g., COMPASS.     

Single π0electro-production in the resonance region with CLAS＊

We report the analysis status of single π0electroproduction in the resonance region to study the electromagnetic excitation of the nucleon resonances. The study is aimed at understanding of the internal structure and dynamics of the nucleon. The experiment was performed using an unpolarized cryogenic hydrogen target and 2.0 and 5.8 GeV polarized electron beam during the ele and e1-6 run periods with CLAS at Jefferson Lab. The new measurements will produce a data base with high statistics and large kinematic coverage for the hadronic invariant mass （W） up to 2.0 CeV in the momentum transfer （Q2） range of 0.3--6.0 GeV2. Preliminary results will be presented and compared with the various model calculations.     

Form factors in lattice QCD

Lattice simulations of QCD have produced precise estimates for the masses of the lowest-lying hadrons which show excellent agreement with experiment. By contrast, lattice results for the vector and axial vector form factors of the nucleon show significant deviations from their experimental determination. We present results from our ongoing project to compute a variety of form factors with control over all systematic uncertainties. In the case of the pion electromagnetic form factor we employ partially twisted boundary conditions to extract the pion charge radius directly from the linear slope of the form factor near vanishing momentum transfer. In the nucleon sector we focus specifically on the possible contamination from contributions of higher excited states. We argue that summed correlation functions offer the possibility of eliminating this source of systematic error. As an illustration of the method we discuss our results for the axial charge, g _A , of the nucleon.     

Photo- and electroproduction of eta mesons

Eta photo- and electroproduction off the nucleon is investigated in an effective lagrangian approach that contains Born terms and both vector meson and nucleon resonance contributions. In particular, we review and develop the formalism for coincidence experiments with polarization degrees of freedom. The different response functions appearing in single and double polarization experiments have been studied. We will present calculations for structure functions and kinematical conditions that are most sensitive to details of the lagrangian, in particular with regard to contributions of nucleon resonances beyond the dominantS ₁₁(1535) resonance.This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 201)     

Structure and Dynamical Evolution of Low Lying Nucleon Resonances

Within a multi-channels multi-resonances reaction model of pion nucleon reactions, we examine the properties of the nucleon resonances with masses less than 2?GeV. The Roper resonance and the second resonance in P ₁₁ are found to evolve dynamically from the same bare state. With the exception of the P ₁₃ and P ₃₁ resonances, our resonance pole positions agree well with the 3 and 4 star resonances listed by the Particle Data Group.     

Photoexcitation of baryon resonances on nucleons and nuclei

Photoproduction of η-mesons off the proton, the deuteron and other light nuclei, and off heavy nuclei has been studied in detail during the last decade at the electron accelerators MAMI in Mainz, ELSA in Bonn, ERSF in Grenoble, CEBAF in Newport News, at KEK, and at Tohoku. The physics topics range from the detailed investigation of the properties of known nucleon resonances (in particular S ₁₁(1535)), over the search of new states, to the in-medium properties of baryon resonances, and the possible formation of η-nucleus (quasi)bound states (η-mesic nuclei). It is thus an excellent example for the different aspects of the photoexcitation of nucleon resonances. Here we report new preliminary results from the CBELSA/TAPS experiment for this reaction at higher incident photon energies for the deuteron and heavy nuclei.     

Baryon structure from Lattice QCD

We present recent lattice results on the baryon spectrum, nucleon electromagnetic and axial form factors, nucleon to △ transition form factors as well as the △ electromagnetic form factors. The masses of the low lying baryons and the nucleon form factors are calculated using two degenerate flavors of twisted mass fermions down to pion mass of about 270 MeV. We compare to the results of other collaborations. The nucleon to △ transition and △ form factors are calculated in a hybrid scheme, which uses staggered sea quarks and domain wall valence quarks. The dominant magnetic dipole nucleon to △ transition form factor is also evaluated using dynamical domain wall fermions. The transverse density distributions of the △ in the infinite momentum frame are extracted using the form factors determined from lattice QCD.     

Analytical investigation of γN→N*,Δ* transition helicity amplitudes

We study the structure of nonstrange baryons by analytically calculating the electromagnetic transition helicity amplitudes of the nucleon and Δ resonances. We employ an improved hypercentral constituent quark model and obtain the corresponding eigenenergies and eigenfunctions in closed forms. Then, we calculate the transverse and longitudinal helicity amplitudes for nucleon and Δ resonances. The comparison of evaluated observables and experimental data indicates good agreement between the proposed model and available data.