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

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.

arXiv comment21 pages, 22 figures

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• Polarization Transfer Observables in Elastic Electron Proton Scattering at $Q^2=$2.5, 5.2, 6.8, and 8.5 GeV${}^2$
• The Present and Future of QCD
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• Form Factors and Two-Photon Exchange in High-Energy Elastic Electron-Proton Scattering
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• Up- and down-quark contributions to the nucleon electromagnetic form factors at low $Q^2$
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• Proton Form Factor Ratio, ${\mu }_p{G}_E^p/G_M^p$ from Double Spin Asymmetry
• Beam-normal single-spin asymmetry in elastic scattering of electrons from a spin-0 nucleus
• Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range
• Updated analysis of recent results on electron and positron elastic scattering on proton
• Model independent radiative corrections to elastic deuteron-electron scattering
• Lepton mass effects for beam-normal single-spin asymmetry in elastic muon-proton scattering
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• Parity-Violating Inelastic Electron-Proton Scattering at Low $Q^2$ Above the Resonance Region
• Polarization transfer in ${\vec{e}}^{+}p\to e^{+}\vec{p}$ scattering using the Super BigBite Spectrometer
• Polarization Observables using Positron Beams
• Impact of new particles on the ratio of Electromagnetic form factors
• Resonance Contributions to Radiative Corrections in Charged-Current Elastic (Anti)Neutrino-Nucleon Scattering at GeV Energies
• Charge and spin asymmetries in elastic lepton-nucleon scattering
• A measurement of two-photon exchange in Super-Rosenbluth separations with positron beams
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• Measurement of the Two-Photon Exchange Contribution to Elastic Lepton-Proton Scattering at the OLYMPUS Experiment
• Positrons at JLab - Advancing Nuclear Science in Hall B