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Measurements of the ratio of the proton elastic form factors ($\mu_pG_E/G_M$)using Rosenbluth separation and those using polarization-based techniques showa strong discrepancy, which increases as a function of $Q^2$. The contributionof hard two-photon exchange (TPE) to $e^-p$ scattering, which is neglected inthe standard treatments of elastic $e^-p$ scattering, is the mostwidely-accepted hypothesis for the explanation of this discrepancy. Whilecalculations of the hard TPE contribution are highly model dependent, theeffect may be quantified experimentally by precisely measuring the ratio of thepositron-proton and electron-proton elastic scattering cross sections. TheOLYMPUS experiment collected approximately 4 fb$^{-1}$ of $e^+p$ and $e^-p$scattering data at the DORIS storage ring at DESY in 2012, with the goal ofmeasuring the elastic $\sigma_{e^+p}/\sigma_{e^-p}$ ratio over the kinematicrange $(0.4 \leq \epsilon \leq 0.9)$, $(0.6 \leq Q^2 \leq 2.2)$ GeV$^2/c^2$ ata fixed lepton beam energy of 2.01 GeV. Initial results from OLYMPUS werepresented, and subsequently results on $R_{2\gamma} =\sigma_{e^+p}/\sigma_{e^-p}$ from the experiment have been publicly releasedand are in preparation for publication.

Measurement of the Two-Photon Exchange Contribution to Elastic Lepton-Proton Scattering at the OLYMPUS Experiment