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FIZIKA B 20 (2011) 3, 253-260
 
TWO-PHOTON PHYSICS AT MAMI
SEBASTIAN BAUNACK
for the A4 Collaboration
Institut für Kernphysik, Johannes Gutenberg-Universität,
Mainz, Germany
E-mail address: baunack@kph.uni-mainz.de
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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