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Measurements of the ratio of the elastic form factors of the proton($\mu_pG_E/G_M$) exhibit a strong discrepancy. Experiments using unpolarizedbeams and Rosenbluth separation to determine the form factors have found valuesof the ratio approximately consistent with unity over a wide range of $Q^2$,while polarization transfer experiments suggest that the ratio decreases as afunction of $Q^2$. The most widely-accepted hypothesis to explain thisdiscrepancy is that hard two-photon exchange (TPE) significantly contributes tothe elastic $ep$ cross section. Hard TPE has been neglected in previousanalyses of electron-proton scattering scattering experiments, in part due tothe fact that there exists no model independent way to calculate thecontribution. The effect of hard TPE may be measured experimentally, however,via precise determination of the ratio of the electron-proton andpositron-proton elastic cross sections. The OLYMPUS experiment collected morethan 3 fb$^{-1}$ of exclusive $e^- p$ and $e^+ p$ elastic scattering data atDESY in 2012, and has determined the elastic $\sigma_{e^+p}/\sigma_{e^-p}$ratio to unprecedented precision up to $Q^2\approx2.2$ (GeV/$c$)$^2$,$\epsilon\approx0.4$. This presentation will discuss the OLYMPUS experiment andanalysis, and present the recently published results from OLYMPUS in thecontext of the results from the other two TPE experiments.

Results from the OLYMPUS Experiment on the Contribution of Hard Two-Photon Exchange to Elastic Electron-Proton Scattering