Eur. Phys. J. A 24, s1.59-s1.63 (2005)
DOI: 10.1140/epjad/s2005-05-010-2

Two-photon exchange in elastic electron-proton scattering

P.G. Blunden¹, W. Melnitchouk² and J.A. Tjon<sup>2, 3

1</sup>  Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, Canada R3T 2N2,  
²  Jefferson Lab, 12000 Jefferson Ave., Newport News, VA 23606, USA
³  Department of Physics, University of Maryland, College Park, MD 20742-4111, USA

(Received 27 September 2004 / Published online 08 February 2005)

Abstract

Two-photon exchange contributions to unpolarized and polarized elastic electron-proton scattering cross sections are evaluated taking into account nucleon finite size effects using realistic form factors. Contributions from nucleon elastic intermediate states are found to have a strong angular dependence, which partially resolves the discrepancy between Rosenbluth and polarization transfer measurements of the proton electric to magnetic form factor ratio. Two-photon contributions to the longitudinal and transverse polarizations are generally found to be small. A comparison is made of two-photon exchange effects with existing data on the ratio of e⁺ p to e^- p cross sections, which is predicted to be enhanced at backward angles.

PACS: 25.30.Bf Elastic electron scattering - 13.40.Gp Electromagnetic form factors

© Società Italiana di Fisica, Springer-Verlag 2005