Measurement of two-photon exchange effect by comparing elastic $e^\pm p$ cross sections

TL;DR

This study measures the ratio of positron-proton to electron-proton elastic scattering cross sections to directly observe two-photon exchange effects, aiming to resolve discrepancies in proton form factor measurements.

Contribution

It provides the first direct experimental evidence of two-photon exchange effects in elastic lepton-proton scattering by comparing positron and electron scattering data.

Findings

Observed non-zero TPE contributions consistent with theoretical predictions.

Quantified the TPE effect's impact on proton form factor measurements.

Enhanced understanding of radiative corrections in elastic scattering.

Abstract

[Background] The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four momentum transfer ($Q^{2}$). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. [Purpose] We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. [Methods] We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron/positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm long liquid hydrogen (LH$_2$) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented.