Determination of two-photon exchange via $e^+p/e^-p$ Scattering with CLAS12

TL;DR

This paper aims to measure two-photon exchange effects in electron-proton scattering over a broad kinematic range using CLAS12, to clarify their role in the proton form factor discrepancy and test theoretical models.

Contribution

It presents a high-precision experimental approach to measure two-photon exchange effects across a wide Q^2 range, addressing gaps in previous tests and theoretical validations.

Findings

Supports existence of small two-photon exchange effects

Provides data to test theoretical models at high Q^2

Aims to resolve the proton form factor discrepancy

Abstract

The proton elastic form factor ratio shows a discrepancy between measurements using the Rosenbluth technique in unpolarized beam and target experiments and measurements using polarization degrees of freedom. The proposed explanation of this discrepancy is uncorrected hard two-photon exchange (TPE), a type of radiative correction that is conventionally neglected. The effect size and agreement with theoretical predictions has been tested recently by three experiments. While the results support the existence of a small two-photon exchange effect, they cannot establish that theoretical treatments are valid. At larger momentum transfers, theory remains untested. This proposal aims to measure two-photon exchange over an extended and so far largely untested $Q^2$ and $\varepsilon$ range with high precision using the {\tt CLAS12} experiment. Such data are crucial to clearly confirm or rule out TPE as the driver for the discrepancy as well as test several theoretical approaches, believed valid in different parts of the tested $Q^2$ range.