Two-photon exchange: myth and history

TL;DR

This paper reviews the history and debates surrounding two-photon exchange effects in electron-proton scattering, analyzing experimental data to clarify their impact on proton form factor measurements and resolve existing controversies.

Contribution

It provides a model-independent analysis of electron-proton scattering data, demonstrating the consistency of different measurement methods and clarifying the role of two-photon exchange effects.

Findings

The electric to magnetic form factor ratio is a reliable observable.

Experimental results from different methods are compatible.

The analysis supports the deviation of proton form factors from the dipole model.

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.