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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 has persisted both in modern experimental resultsand in re-analyses of previous data. The most widely accepted hypothesis toexplain this discrepancy is the treatment of the contributions from hardtwo-photon exchange (TPE) to elastic electron-proton scattering in theradiative corrections applied to the Rosenbluth separation measurements.Calculations of the hard TPE contribution are highly model dependent, but theeffect may be measured experimentally with a precise determination of the ratioof the positron-proton and electron-proton elastic scattering cross sections.  The OLYMPUS experiment collected approximately 4 fb$^{-1}$ of \pp and \epscattering data at the DORIS storage ring at DESY in 2012, with the goal ofmeasuring the elastic \ratio ratio over the kinematic range $(0.4 \leq \epsilon\leq 0.9)$, $(0.6 \leq Q^2 \leq 2.2)$ GeV$^2/c^2$ at a fixed lepton beam energyof 2.01 GeV. The detector for the OLYMPUS experiment consisted of refurbishedelements of the Bates Large Acceptance Spectrometer Toroid (BLAST) surroundingan internal gaseous hydrogen target, with the addition of multiple systems forthe monitoring of the luminosity collected by the experiment. A detailedsimulation of the experiment was developed to account for both radiativecorrections and various systematic effects.  This work presents preliminary results from the OLYMPUS data, demonstratingthat the elastic \ratio ratio rises to several percent at $\epsilon\approx 0.4$and indicating a significant contribution from TPE to \pmp scattering.Additionally, the value of \ratio has been measured to unprecedented precisionat $\epsilon=0.98$, which provides a valuable normalization point for otherexperimental data.

A precision measurement of the e+p/e-p elastic scattering cross section ratio at the OLYMPUS experiment