Towards a resolution of the proton form factor problem: new electron and positron scattering data

Abstract

There is a significant discrepancy between the values of the proton electric form factor, $G_E^p$, extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of $G_E^p$ from the unpolarized electron-proton cross sections. We determined the TPE contribution by measuring the ratio of positron-proton to electron-proton elastic scattering cross sections using a simultaneous, tertiary electron-positron beam incident on a liquid hydrogen target and detecting the scattered particles in the Jefferson Lab CLAS detector. This novel technique allowed us to cover a wide range in virtual photon polarization ($\epsilon$) and momentum transfer ($Q^2$) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing $\epsilon$ at $Q^2=1.45{\text{ GeV}}^2$. This measurement is consistent with the size of the form factor discrepancy at $Q^2\approx 1.75$ GeV${}^2$ and with hadronic calculations including nucleon and $\Delta$ intermediate states, which have been shown to resolve the discrepancy up to $2-3$ GeV${}^2$.

6 pages, 4 figures, submitted to PRL

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• Nucleon Electromagnetic Form Factors
• Global analysis of proton elastic form factor data with two-photon exchange corrections
• Nucleon electromagnetic form factors
• Two-Photon Physics in Hadronic Processes
• Flavor decomposition of the elastic nucleon electromagnetic form factors
• The nucleon electromagnetic form factors from Lattice QCD
• Parity-Violating Electron Scattering and the Electric and Magnetic Strange Form Factors of the Nucleon
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• Precision Measurement of the Beam-Normal Single-Spin Asymmetry in Forward-Angle Elastic Electron-Proton Scattering
• The Structure of the Nucleon: Elastic Electromagnetic Form Factors
• Measuring the lepton sign asymmetry in elastic electron-proton scattering with OLYMPUS
• Generalized Sachs Form Factors and the Possibility of Their Measurement in Processes without and with Proton Spin Flip
• Reliability Analysis of the Results of the Known Experiments on Measuring of the Sachs Form Factor Ratio Using the Rosenbluth Technique. Polarization of the Final Proton in the $e\vec{p}\to e\vec{p}$ Elastic Process
• CFNS Ad-Hoc meeting on Radiative Corrections Whitepaper