Document Type


Publication Date




Publication Title

Physical Review Letters






062003 (6 pages)


There is a significant discrepancy between the values of the proton electric form factor, GpE, extracted using unpolarized and polarized electron scattering. Calculations predict that small two-photon exchange (TPE) contributions can significantly affect the extraction of GpEfrom 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 (ε) and momentum transfer (Q2) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing ε at Q2=1.45 GeV2. This measurement is consistent with the size of the form factor discrepancy at Q2 ≈ 1.75 GeV2and with hadronic calculations including nucleon and Δ intermediate states, which have been shown to resolve the discrepancy up to 2-3 GeV2.


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Original Publication Citation

CLAS Collaboration, Adikaram, D., Rimal, D., Weinstein, L. B., Raue, B., Khetarpal, P., . . . Zonta, I. (2015). Towards a resolution of the proton form factor problem: New electron and positron scattering data. Physical Review Letters, 114(6), 062003. doi:10.1103/PhysRevLett.114.062003


0000-0001-5416-2900 (Weinstein), 0000-0001-7282-8120 (Hyde), 0000-0003-2243-6836 (Kuhn)