Two-Photon Exchange: Future experimental prospects

TL;DR

This paper reviews the current understanding of two-photon exchange effects in proton form factor measurements, highlighting discrepancies, recent experimental tests, and proposing future experiments at higher momentum transfers to better understand the phenomenon.

Contribution

It discusses the potential for future experiments to measure two-photon exchange effects at larger momentum transfer, addressing current discrepancies and untested theoretical predictions.

Findings

Small two-photon exchange effect observed experimentally

Discrepancy between experimental results and theoretical calculations

Future measurements could clarify two-photon exchange contributions

Abstract

The proton elastic form factor ratio is accessible in unpolarized Rosenbluth-type experiments as well as experiments which make use of polarization degrees of freedom. The extracted values show a distinct discrepancy, growing with $Q^2$. Three recent experiments tested the proposed explanation, two-photon exchange, by measuring the positron-proton to electron-proton cross section ratio. In the results, a small two-photon exchange effect is visible, significantly different from theoretical calculation. Theory at larger momentum transfer remains untested. This paper discusses the possibilities for future measurements at larger momentum transfer.