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155143 https://hdl.handle.net/2286/R.I.40823 http://rightsstatements.org/vocab/InC/1.0/ All Rights Reserved 2016 xi, 147 pages : illustrations (some color) Doctoral Dissertation Academic theses Text eng Ice, Lauren Alarcon, Ricardo O Dugger, Michael Lebed, Richard Ritchie, Barry Arizona State University Partial requirement for: Ph.D., Arizona State University, 2016 Includes bibliographical references (pages 138-145) Field of study: Physics The OLYMPUS experiment measured the two-photon exchange contribution to elastic electron-proton scattering, over a range of four-momentum transfer from \(0.6 < Q^2 < 2.2\) \((\mathrm{GeV/c})^2\). The motivation for the experiment stemmed from measurements of the electric-to-magnetic form factor ratio of the proton \(\mu G_E/G_M\) extracted from polarization observables in polarized electron-proton scattering. Polarized electron-proton scattering experiments have revealed a significant decrease in \(\mu G_E/G_M\) at large \(Q^2\), in contrast to previous measurements from unpolarized electron-proton scattering. The commonly accepted hypothesis is that the discrepancy in the form factor ratio is due to neglected higher-order terms in the elastic electron-proton scattering cross section, in particular the two-photon exchange amplitude. <br/><br/>The goal of OLYMPUS was to measure the two-photon exchange contribution by measuring the positron-proton to electron-proton elastic scattering cross section ratio, \(\sigma_{e^+p}/\sigma_{e^-p}\). The two-photon exchange contribution is correlated to the deviation of the cross section ratio from unity.<br/><br/>In 2012, the OLYMPUS experiment collected over 4 fb\(^{-1}\) of \(e^+p\) and \(e^-p\) scattering data using electron and positron beams incident on a hydrogen gas target. The scattered leptons and protons were measured exclusively with a large acceptance spectrometer. OLYMPUS observed a slight rise in \(\sigma_{e^+p}/\sigma_{e^-p}\) of at most 1-2\% over a \(Q^2\) range of \(0.6 < Q^2 < 2.2\) \((\mathrm{GeV/c})^2\). This work discusses the motivations, experiment, analysis method, and the preliminary results for the cross section ratio as measured by OLYMPUS. Physics elastic electron scattering elastic positron scattering proton form factor ratio two-photon exchange Elastic scattering Electrons--Scattering. Protons--Scattering. Measurement of the two-photon exchange contribution to lepton-proton scattering with OLYMPUS