Weak and electromagnetic form factors of hadrons

The elastic electromagnetic and weak form factors are studied for nucleons, π and K mesons. Experimental data as well as theoretical approaches are reviewed.     

Remarks on the electromagnetic form factors of hadrons in the quark model

Relations between transverse and longitudinal parts of elastic and quasi-elastic form factors are studied within the quark model. It is shown that for an even number of constituent quarks the longitudinal part dominates while for an odd number of quarks the transverse part is the largest one. Consequences from this result are considered for the deuteron form factor as well as for the matrix elements of electromagnetic transitions between π-, ?-, A₁-mesons.     

Asymptotic behavior of the pion electromagnetic form factors in Bethe-Salpeter model

Asymptotic behavior of the pion form factors has been discussed treating pion as a bound state of the elementary nucleon and antinucleon system. The matrix-element of the electromagnetic current is written in terms of the Bethe-Salpeter amplitude for the nucleon-antinucleon bound system. Using suitable approximations the Bethe-Salpeter equation for nucleon-antinucleon bound state with nucleons as Dirac spinors has been solved and the corresponding off-shell pion form factors are determined. The form factors are strongly interaction dependent. For coupling $\text{g}{}^2\text{16π}{}^2\text{=}\text{15}\text{4}$ the form factors vanish asymptotically like t^?1 and for $\text{14}\text{4}\text{<}\text{g}{}^2\text{16π}{}^2\text{<}\text{15}\text{4}$ the form factors still vanish but less rapidly than t^?1.     

Two new constraints on the electromagnetic form factors of hadrons

Two new axiomatic constraints on the electromagnetic form factors of hadrons are established under only very general assumptions which are rigorously proved in the traditional axiomatic approaches to quantum field theory and also under the experimentally supported assumption of the finiteness of the hadronic electromagnetic form factors.     

Dependence of electromagnetic form factors of hadrons on light-cone frames

A constituent quark model is developed for an arbitrary light-cone direction so that the light-front time is x_LF⁺ = ω · x with a constant lightlike four-vector ω. Form factors are obtained from free one-body electromagnetic current matrix elements. They are found to be ω-independent for spin-0 mesons, nucleons and the Λ-hyperon, while there is an ω-dependence for spin-1 systems like the deuteron.     

General expressions for VM dominated form factors of hadrons fulfilling asymptotic conditions

General expressions for vector-meson dominated (VMD) form factors of hadrons fulfilling asymptotic conditions, derived previously for n vector-meson parameterization of the electromagnetic form factor of any strongly interacting particle with the asymptotics (m < n) and form factor normalization conditions, are presented. The special case of m = n and the expression fulfilling asymptotic conditions without any form factor normalization are discussed too.Received: 27 January 2003, Revised: 31 August 2003, Published online: 27 November 2003     

Asymptotic behavior of nucleon electromagnetic form factors in time-like region

We study the asymptotic behavior of the ratio of Pauli and Dirac electromagnetic nucleon form factors, F₂/F₁, in time-like region, for different parametrizations built for the space-like region. We investigate how fast the ratio F₂/F₁ approaches the asymptotic limits according to the Phragmèn-Lindel?f theorem. We show that the QCD-inspired logarithmic behavior of this ratio results in very far asymptotics, experimentally unachievable. This is also confirmed by the normal component of the nucleon polarization, P_y, in e⁺ + e^-↦N + ˉ (in collisions of unpolarized leptons), which is a very interesting observable, with respect to this theorem. Finally we observe that the 1/Q parametrization of F₂/F₁ contradicts this theorem.     

Short-distance dominance in the elastic form factors of hadrons

The renormalization group analysis is applied to the high-energy behavior of the pion form factor in QCD. The anomalous dimension of the form factor is directly related to the short-distance behavior of the Bethe-Salpeter wave function with suitable choice of normalization. Absence of infrared singularities and short-distance dominance are proven by the use of a power counting argument in the axial gauge. The relation between our method and the “factorization” method is clarified. The applicability of our method to other elastic form factors is discussed.     

Covariant nucleon electromagnetic form factors from the Goldstone-boson-exchange quark model

We present a study of proton and neutron electromagnetic form factors for the recently proposed Goldstone-boson-exchange constituent quark model. Results for charge radii, magnetic moments, and electric as well as magnetic form factors are reported. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. All the predictions by the Goldstone-boson-exchange constituent quark model are found to fall remarkably close to existing experimental data.     

Flavor content of nucleon form factors in a VMD approach

The strange form factors of the nucleon are studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. A comparison with the available experimental world data from the SAMPLE, PVA4, HAPPEX and G0 Collaborations shows a good overall agreement. It is shown that the strangeness contribution to the electric and magnetic form factors is of the order of a few percent. In particular, the strange quark contribution to the charge radius is small 〈r ² _s〉_E = 0.005 fm^2 and to the magnetic moment it is positive μ_s = 0.315 μ_N .     

Sidewise electromagnetic form factors of the nucleon

Electromagnetic form factors are evaluated for the nucleons using the method of sidewise dispersion relations. The threshold region is assumed to dominate the results. A π-meson is used in the intermediate state expansions. The results are presented graphically.     

Phenomenology of the deuteron electromagnetic form factors

A rigorous extraction of the deuteron charge form factors from tensor polarization data in elastic electron-deuteron scattering, at given values of the 4-momentum transfer, is presented. Then the world data for elastic electron-deuteron scattering is used to parameterize, in three different ways, the three electromagnetic form factors of the deuteron in the 4-momentum transfer range 0-7 fm⁻¹. This procedure is made possible with the advent of recent polarization measurements. The parameterizations allow a phenomenological characterization of the deuteron electromagnetic structure. They can be used to remove ambiguities in the form factors extraction from future polarization data. Received: 28 January 2000 / Revised version: 25 February 2000     

Asymptotic behavior of the composite nucleon form factors

A new method is proposed for evaluating the nucleon electromagnetic form factors in terms of the p₁₁ pion-nucleon phase shift. The asymptotic behavior of the form factors is discussed.     

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.     

Analytic representation for form factors and determination of radii of hadrons and light nuclei

A simple exponential form with correct analytic properties is proposed for form factors. Its implications for the determination of radii of particles are discussed.     

Deuteron electromagnetic form factors with the light-front approach

The electromagnetic form factors and low-energy observables of the deuteron are studied with the help of the light-front approach, where the deuteron is regarded as a weakly bound state of a proton and a neutron. Both the S and D wave interacting vertexes among the deuteron, proton, and neutron are taken into account. Moreover,the regularization functions are also introduced. In our calculations, the vertex and the regularization functions are employed to simulate the momentum distribution inside the deuteron. Our numerical results show that the lightfront approach can roughly reproduce the deuteron electromagnetic form factors, like charge G_0, magnetic G_1, and quadrupole G_2, in the low Q~2 region. The important effect of the D wave vertex on G_2 is also addressed.     

Deuteron electromagnetic form factors in the light-front dynamics

The deuteron form factors are calculated in the framework of the relativistic nucleon-meson dynamics, by means of the explicitly covariant light-front approach. The inflluence of the nucleon electromagnetic form factors is discussed. At Q² < 3 (GeV/c)² the prediction for the structure function A(Q²) and for the tensor polarization observable t₂₀ are in agreement with the recent data of CEBAF/TJNAF.     

Relations between electromagnetic form factors of baryons

The inclusion of two-body exchange currents in the constituent quark model leads to new relations between the electromagnetic properties of octet and decuplet baryons. In particular, the N → Δ quadrupole transition form factor can be expressed in terms of the neutron charge form factor.