FIZIKA B 20 (2011) 3, 253-260

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for the A4 Collaboration
Institut für Kernphysik, Johannes Gutenberg-Universität, Mainz, Germany
E-mail address:

Received 31 January 2011;   Accepted 14 December 2011
Online 9 February 2012

The structure of the nucleon can be described by the electromagnetic form factors GE and GM. These form factors can be determined by elastic electron-proton scattering with the well-known Rosenbluth technique. Alternatively, the ratio of the electric and the magnetic form factors GE/GM can be determined by the so-called polarization-transfer technique. The measurements reveal a large discrepancy between the two methods at large momentum transfers. There is evidence that two-photon exchange contributions to the elastic scattering have been underestimated. While the two-photon exchange amplitude is difficult to compute from theory due to the excited intermediate hadronic states, its imaginary part can be accessed experimentally by measuring the asymmetry A in the cross section of elastic ep scattering where the electrons are transversely polarized parallel or antiparallel to the normal of the scattering plane, respectively. The PVA4 experiment at MAMI is currently performing such measurements covering a large range of momentum transfers.

PACS numbers: 14.20.Dh, 13.40.Gp
UDC 539.125, 539.171

Keywords: nucleon structure, electromagnetic form factors, elastic electron scattering, polarization-transfer technique, two-photon exchange contribution

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