# Two-Photon Exchange: Myth and History

Two-Photon Exchange: Myth and History After recalling the arguments for possible excess of two-photon contribution over $$\alpha$$ α -counting, model independent statements about the consequences on the observables will be given. The relevant experimental data are discussed: (polarized and unpolarized) electron and positron elastic scattering on the proton, as well as annihilation data. A reanalysis of unpolarized electron–proton elastic scattering data is presented in terms of the electric to magnetic form factor squared ratio. This observable is in principle more robust against experimental correlations and global normalizations. The present analysis shows indeed that it is a useful quantity that contains reliable and coherent information. The comparison with the ratio extracted from the measurement of the longitudinal to transverse polarization of the recoil proton in polarized electron–proton scattering shows that the results are compatible. These results bring a decisive piece of information in the controversy on the deviation of the proton form factors from the dipole dependence. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Few-Body Systems Springer Journals

# Two-Photon Exchange: Myth and History

, Volume 59 (5) – Jun 4, 2018
8 pages

/lp/springer_journal/two-photon-exchange-myth-and-history-gSTKgNEsW8
Publisher
Springer Vienna
Subject
Physics; Particle and Nuclear Physics; Nuclear Physics, Heavy Ions, Hadrons; Atomic, Molecular, Optical and Plasma Physics
ISSN
0177-7963
eISSN
1432-5411
D.O.I.
10.1007/s00601-018-1416-5
Publisher site
See Article on Publisher Site

### Abstract

After recalling the arguments for possible excess of two-photon contribution over $$\alpha$$ α -counting, model independent statements about the consequences on the observables will be given. The relevant experimental data are discussed: (polarized and unpolarized) electron and positron elastic scattering on the proton, as well as annihilation data. A reanalysis of unpolarized electron–proton elastic scattering data is presented in terms of the electric to magnetic form factor squared ratio. This observable is in principle more robust against experimental correlations and global normalizations. The present analysis shows indeed that it is a useful quantity that contains reliable and coherent information. The comparison with the ratio extracted from the measurement of the longitudinal to transverse polarization of the recoil proton in polarized electron–proton scattering shows that the results are compatible. These results bring a decisive piece of information in the controversy on the deviation of the proton form factors from the dipole dependence.

### Journal

Few-Body SystemsSpringer Journals

Published: Jun 4, 2018

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