Original scientific paper
Two-photon physics at MAMI
Sebastian Baunack (for the A4 Collaboration)
; Institut fur Kernphysik, Johannes Gutenberg-Universitat, Mainz, Germany
Abstract
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 socalled polarization-transfer technique. The measurements reveal a large discrepancy between the two methods at large momentum transfers. There is evidence that twophoton 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.
Keywords
nucleon structure; electromagnetic form factors; elastic electron scattering; polarization-transfer technique; two-photon exchange contribution
Hrčak ID:
304721
URI
Publication date:
1.9.2011.
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