Two-photon exchange: myth and history

Abstract

After recalling the arguments for possible excess of two-photon contribution over $\alpha$-counting, model independent statements about the consequences on the observables will be given. The relevant experimental data are discussed: (polarized and unpolarized) electron and positron elastic scattering on the proton, as well as annihilation data. A reanalysis of unpolarized electron-proton elastic scattering data is presented in terms of the electric to magnetic form factor squared ratio. This observable is in principle more robust against experimental correlations and global normalizations. The present analysis shows indeed that it is a useful quantity that contains reliable and coherent information. The comparison with the ratio extracted from the measurement of the longitudinal to transverse polarization of the recoil proton in polarized electron-proton scattering shows that the results are compatible. These results bring a decisive piece of information in the controversy on the deviation of the proton form factors from the dipole dependence.

12 pages, 5 figures, contribution to NSTAR 2017, to appear in Few Body Systems

References in corpus (11)

• Global analysis of proton elastic form factor data with two-photon exchange corrections
• Measurement of the two-photon exchange contribution to the elastic $e^{\pm }p$ scattering cross sections at the VEPP-3 storage ring
• Search for effects beyond the Born approximation in polarization transfer observables in $\vec{e}p$ elastic scattering
• Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment
• Polarization Transfer Observables in Elastic Electron Proton Scattering at $Q^2=$2.5, 5.2, 6.8, and 8.5 GeV${}^2$
• Two-photon exchange in dispersion approach
• Two-photon exchange in elastic electron-proton scattering : QCD factorization approach
• The structure function method applied to polarized and unpolarized electron-proton scattering: a solution of the G_E(p)/G_M(p) discrepancy
• Two-Photon Exchange Contribution to Proton Form Factors in Time-Like region
• On radiative corrections for unpolarized electron proton elastic scattering
• Measuring the deviation from the Rutherford formula

Cited by in corpus (3)

• On the Transfer of Polarization from the Initial to the Final Proton in the Elastic Process $e\vec{p}\to e\vec{p}$
• Reliability Analysis of the Results of the Known Experiments on Measuring of the Sachs Form Factor Ratio Using the Rosenbluth Technique. Polarization of the Final Proton in the $e\vec{p}\to e\vec{p}$ Elastic Process
• Two-Photon Exchange: Future experimental prospects