Form Factors and Two-Photon Exchange in High-Energy Elastic Electron-Proton Scattering

TL;DR

This paper reports new high-precision measurements of elastic electron-proton scattering at high momentum transfer, improving form factor extraction and providing evidence for two-photon exchange effects extending to higher Q^2 values.

Contribution

It offers the most precise measurements to date at high Q^2 and extends the understanding of two-photon exchange contributions in electron-proton scattering.

Findings

Agreement with lower Q^2 polarization data suggests TPE effects are significant at high Q^2.

Extended the Q^2 range for observed discrepancies to over 8 (GeV/c)^2.

Quantified the size of TPE contributions needed to reconcile cross section differences.

Abstract

We present new precision measurements of the elastic electron-proton scattering cross section for momentum transfer (Q$^2$) up to 15.75~\gevsq. Combined with existing data, these provide an improved extraction of the proton magnetic form factor at high Q$^2$ and double the range over which a longitudinal/transverse separation of the cross section can be performed. The difference between our results and polarization data agrees with that observed at lower Q$^2$ and attributed to hard two-photon exchange (TPE) effects, extending to 8~(GeV/c)$^2$ the range of Q$^2$ for which a discrepancy is established at $>$95\% confidence. We use the discrepancy to quantify the size of TPE contributions needed to explain the cross section at high Q$^2$.