# 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 ($\varepsilon$) and momentum transfer ($Q^2$) simultaneously, as well as to cancel luminosity-related systematic errors. The cross section ratio increases with decreasing $\varepsilon$ 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$^2$.
Document type :
Journal articles

http://hal.in2p3.fr/in2p3-01088523
Contributor : Emmanuelle Vernay <>
Submitted on : Friday, November 28, 2014 - 10:30:49 AM
Last modification on : Friday, April 30, 2021 - 10:24:08 AM

### Citation

D. Adikaram, D. Rimal, L. B. Weinstein, B. Raue, P. Khetarpal, et al.. Towards a resolution of the proton form factor problem: new electron and positron scattering data. Physical Review Letters, American Physical Society, 2015, 114, pp.062003. ⟨10.1103/PhysRevLett.114.062003⟩. ⟨in2p3-01088523⟩

Record views