Two-Photon Exchange and Electromagnetic Proton Form Factors

The presence of $2\gamma$-exchange in electron proton elastic scattering is discussed. From C-invariance and crossing symmetry, $2\gamma$ contribution induces a specific dependence of the reduced cross section on the variable $\epsilon$. No evidence of such dependence exists in the available experimental data.Comment: 4 pages, Contributed papers to Baryon 0

Is there model-independent evidence of the two-photon-exchange effect in the electron-proton elastic scattering cross section?

We re-analyze the data of the elastic electron proton scattering to look for model-independent evidence of the two-photon-exchange (TPE) effect. In contrast to previous analyses, TPE effect is parametrized in forms which are free of kinematical-singularity, in addition to being consistent with the constraint derived from crossing symmetry and the charge conjugation. Moreover, we fix the value of $R=G_E/G_M$ as determined from the data of the polarization transfer experiment. We find that, at high $Q^2 \geq 2 GeV^2$ values, the contribution of the TPE effect to the slope of $\sigma_R$ vs. $\epsilon$ is large and comparable with that arising from $G_{E}$. It also behaves quasi-linearly in the region of current data, namely, in the range of $0.2 < \epsilon < 0.95$. Hence the fact that the current elastic $ep$ cross section data shows little nonlinearity with respect to $\epsilon$ can not be used to exclude the presence of the TPE effect. More precise data at extreme angles will be crucial for a model-independent extraction of the TPE effect.Comment: 11 pages, 4 figure

Dispersive contributions to e+p/e−pe^+p/e^-p cross section ratio in forward regime

Two-photon exchange (TPE) contributions to elastic electron-proton scattering in the forward regime are considered. The imaginary part of TPE amplitude in these kinematics is related to the DIS nucleon structure functions. The real part of the TPE amplitude is obtained from the imaginary part by means of dispersion relations. We demonstrate that the dispersion integrals for the relevant elastic $ep$-scattering amplitude converge and do not need subtraction. This allows us to make clean prediction for the real part of the TPE amplitude at forward angles. We furthermore compare $e^+p$ and $e^-p$ cross sections which depends on the real part of TPE amplitude, and predict the positron cross section to exceed the electron one by a few per cent, with the difference ranging from 1.4% to 2.8% for electron $lab$ energies in the range from 3 to 45 GeV. We furthermore predict that the absolute value of this asymmetry grows with energy, which makes it promising for experimental tests.Comment: 10 pages, 6 figure

Beam normal spin asymmetry in elastic lepton-nucleon scattering

We discuss the two-photon exchange contribution to observables which involve lepton helicity flip in elastic lepton-nucleon scattering. This contribution is accessed through the spin asymmetry for a lepton beam polarized normal to the scattering plane. We estimate this beam normal spin asymmetry at large momentum transfer using a parton model and we express the corresponding amplitude in terms of generalized parton distributions.Comment: 17 pages, 5 figure

Nucleon Form Factors in a Light-Cone Quark Model with Two-Photon Exchange

We estimate the two-photon exchange corrections to both proton and neutron electromagnetic physical observables in a relativistic light cone quark model. At a fixed momentum transfer the corrections are found to be small in magnitudes, but with a strong angular dependence. Our results are comparable to those from simple hadronic model in a moderate momentum transfer region.Comment: 10 pages, 5 figure

Overview of nucleon structure studies

A brief overview of the recent activity in the measurement of the elastic electromagnetic proton and neutron form factors is presented. It is discussed how the quality of the data has been greatly improved by performing double polarization experiments, and the role of two-photon exchange processes will be highlighted. The spatial information on the quark charge distribibutions in the nucleon resulting from the form factors measurements will be discussed, as well as the steady rate of improvements made in the lattice QCD calculations. It is discussed how generalized parton distributions have emerged as a unifying theme in hadron physics linking the spatial densities extracted from form factors with the quark momentum distribution information residing in quark structure functions. The recent progress in the electromagnetic excitation of the $\Delta(1232)$ resonance will also briefly be discussed.Comment: prepared for Proceedings of International Nuclear Physics Conference (INPC07), typos corrected + references adde

Comment on "Nucleon elastic form factors and local duality"

We comment on the papers "Nucleon elastic form factors and local duality" [Phys. Rev. {\bf D62}, 073008 (2000)] and "Experimental verification of quark-hadron duality" [Phys. Rev. Lett. {\bf 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.Comment: to appear in Phys. Rev.

A Coherent Interpretation of the Form Factors of the Nucleon in Terms of a Pion Cloud and Constituent Quarks

The recent unbiased measurements of the electric form factor of the neutron suggest that its shape may be interpreted as a smooth broad distribution with a bump at Q^2 \approx 0.3(GeV/c)^2 superimposed. As a consequence the corresponding charge distribution in the Breit frame shows a negative charge extending as far out as 2fm. It is natural to identify this charge with the pion cloud. This realisation is then used to reanalyse all old and new data of the electric and magnetic from factors of the proton and the neutron by a phenomenological fit and by a fit based on the constituent quark model. It is shown that it is possible to fit all form factors coherently with both ansaetzen and that they all show the signal of the pion cloud.Comment: 17 pages, 17 figure

Electromagnetic proton form factors in large NcN_{c} QCD

The electromagnetic form factors of the proton are obtained using a particular realization of QCD in the large $N_c$ limit (${QCD}_{\infty}$), which sums up the infinite number of zero-width resonances to yield an Euler's Beta function (Dual-${QCD}_{\infty}$). The form factors $F_1(q^2)$ and $F_2(q^2)$, as well as $G_M(q^2)$ agree very well with reanalyzed space-like data in the whole range of momentum transfer. In addition, the predicted ratio $\mu_p G_E/G_M$ is in good agreement with recent polarization transfer measurements at Jefferson Lab.Comment: 10 page

Model independent analysis of polarization effects in elastic electron deuteron scattering in presence of two--photon exchange

The general spin structure of the matrix element, taking into account the two--photon exchange contribution, for the elastic electron (positron) --deuteron scattering has been derived using general symmetry properties of the hadron electromagnetic interaction, such as P--, C-- and T--invariances as well as lepton helicity conservation in QED at high energy. Taking into account also crossing symmetry, the amplitudes of $e^{\mp}d-$scattering can be parametrized in terms of fifteen real functions. The expressions for the differential cross section and for all polarization observables are given in terms of these functions. We consider the case of an arbitrary polarized deuteron target and polarized electron beam (both longitudinal and transverse). The transverse polarization of the electron beam induces a single--spin asymmetry which is non--zero in presence of two--photon exchange. It is shown that elastic deuteron electromagnetic form factors can still be extracted in presence of two photon exchange, from the measurements of the differential cross section and of one polarization observable (for example, the tensor asymmetry) for electron and positron deuteron elastic scattering, in the same kinematical conditions.Comment: 28 page