Two-photon Exchange Corrections Research Articles

Global analysis of proton elastic form factor data with two-photon exchange corrections

We use the world's data on elastic electron-proton scattering and calculations of two-photon exchange effects to extract corrected values of the proton's electric and magnetic form factors over the full ${Q}^{2}$ range of the existing data. Our analysis combines the corrected Rosenbluth cross section and polarization transfer data, and is the first extraction of ${G}_{E}$ and ${G}_{M}$ including explicit two-photon exchange corrections and their associated uncertainties. In addition, we examine the angular dependence of the corrected cross sections to examine the possible nonlinearities of the cross section as a function of \ensuremath{\varepsilon}.

Precise determination of low-Qnucleon electromagnetic form factors and their impact on parity-violatinge-pelastic scattering

The extraction of the strangeness form factors from parity-violating elastic electron-proton scattering is sensitive to the electromagnetic form factors at low ${Q}^{2}$. We provide parametrizations for the form factors and uncertainties, including the effects of two-photon exchange corrections to the extracted electromagnetic form factors. We study effect of the correlations between different form factors, in particular as they impact the parity-violating asymmetry and the extraction of the strangeness form factors. We provide a prescription to extract the strangeness form factors from the asymmetry that provides an excellent approximation of the full two-photon correction. The corrected form factors are also appropriate as input for other low-$Q$ analyses, although the effects of correlations and two-photon exchange corrections may be different.

Two-photon exchange at lowQ2

We studied two-photon exchange for elastic electron-proton scattering at low ${Q}^{2}$. Compact approximate formulas for the amplitudes were obtained. Numerical calculations were done for ${Q}^{2}\ensuremath{\leqslant}0.1\phantom{\rule{0.3em}{0ex}}{\mathrm{GeV}}^{2}$ with several realistic form factor parametrizations, yielding similar results. They indicate that the corrections to the magnetic form factor can visibly affect the cross-section and proton radii. For low-${Q}^{2}$ electron-neutron scattering two-photon exchange corrections were shown to be negligibly small.

Two-Photon Physics in Hadronic Processes

Two-photon exchange contributions to elastic electron scattering are reviewed. The apparent discrepancy between unpolarized Rosenbluth and polarization transfer experiments in the extraction of elastic nucleon form factors is discussed, as well as the understanding of this puzzle in terms of two-photon exchange corrections. Calculations of such corrections within both partonic and hadronic frameworks are reviewed. In view of recent spin-dependent electron scattering data, the relation of the two-photon exchange process to the hyperfine splitting in hydrogen is critically examined. The imaginary part of the two-photon exchange amplitude as can be accessed from the beam-normal spin asymmetry in elastic electron-nucleon scattering is reviewed. Further extensions and open issues in this field are outlined.

Calculation of two-photon exchange effects for Δ production in electron–proton collisions

We present an evaluation of a two-photon exchange correction to the cross section for unpolarized Δ isobar production in electron–proton collisions, using a relativistic, crossing symmetric and gauge invariant approach. The calculated box and crossed-box diagrams include nucleon and Δ intermediate states. We find a relation between the angular nonlinearity of the two-photon exchange contribution and the value of the γΔΔ coupling constant.

Experimental constraints on nonlinearities induced by two-photon effects in elastic and inelastic Rosenbluth separations

The effects of two-photon exchange corrections, suggested to explain the difference between measurements of the proton elastic electromagnetic form factors using the polarization transfer and Rosenbluth techniques, have been studied in elastic and inelastic scattering data. Such corrections could introduce epsilon-dependent non-linearities in inelastic Rosenbluth separations, where epsilon is the virtual photon polarization parameter. It is concluded that such non-linear effects are consistent with zero for elastic, resonance, and deep-inelastic scattering for all Q^2 and W^2 values measured.

Two-Photon-Exchange Effects in the Electroexcitation of theΔResonance

We examine the two-photon exchange contribution to the eN --> edelta(1232) --> epiN process with the aim of a precision study of the ratios of electric quadrupole (E2) and Coulomb quadrupole (C2) to the magnetic dipole (M1) gamma*Ndelta transitions. We relate the two-photon exchange amplitude to the N --> delta generalized parton distributions and obtain a quantitative estimate of the two-photon effects. The two-photon exchange corrections to the C2/M1 ratio depend strongly on whether this quantity is obtained from an interference cross section or from the Rosenbluth-type cross sections, in similarity with the elastic, eN --> eN, process.

ΔResonance Contribution to Two-Photon Exchange in Electron-Proton Scattering

We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate Delta resonance. The Delta two-photon exchange contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and Delta two-photon exchange corrections has an angular dependence compatible with both the polarization-transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.

Two photon effects in electron scattering

The apparent discrepancy between the Rosenbluth and the polarization transfer method for the ratio of the electric to magnetic proton form factors can be explained by a two-photon exchange correction which does not destroy the linearity of the Rosenbluth plot. Though intrinsically small, of the order of a few percent of the cross section, this correction is kinematically enhanced in the Rosenbluth method while it is small for the polarization transfer method, at least in the range of Q 2 where it has been used until now.

How to Reconcile the Rosenbluth and the Polarization Transfer Methods in the Measurement of the Proton Form Factors

The apparent discrepancy between the Rosenbluth and the polarization transfer methods for the ratio of the electric to magnetic proton form factors can be explained by a two-photon exchange correction which does not destroy the linearity of the Rosenbluth plot. Though intrinsically small, of the order of a few percent of the cross section, this correction is accidentally amplified in the case of the Rosenbluth method.

Rigorous quantum electrodynamics treatment of electron correlations in high-Z ions: Beyond the Breit and effective-potential approximations

Abstract We discuss the present status of auantum electrodynamics (QED) investigations of electron correlations in second order in α for heavy fewelectron atoms. A complete evaluation of all two-electron QED contributions of order α2 is presented for the 2p½-2s transition in high-Z Li-like ions. In particular, the two-photon exchange correction beyond the Breit approximation is calculated for highly charged ions with Z < 50. The total theoretical value of 280.44(9)(20) eV is found for the 2p½-2s transition energy in Li-like U.

Two-photon exchange corrections to the 2p(1/2)-2s transition energy in Li-like high- Z ions

A rigorous QED calculation of the two-photon exchange corrections to the 2p(1/2)- 2s transition energy in Li-like high- Z ions is presented. The contribution due to an exchange by more than two photons is evaluated within the Breit approximation. The resulting theoretical value of the 2p(1/2)- 2s transition energy in Li-like uranium is found to be 280.44(20) eV.

Reference state contributions to the two-photon interaction corrections for the energy shifts in multicharged few-electron ions

The second-order alpha corrections in QED, in particular the two-photon exchange corrections, are investigated by the line profile approach. The expressions for equal-energy 'reference state' corrections and for the unequal-energy Coulomb-transverse corrections are given. The numerical calculations for the latter corrections of 'box' and 'cross' type in the case of (1s1/2)22s1/2 and (1s1/2)22p1/2 states of Li-like heavy ions are presented.