The OLYMPUS Experiment at DESY.

Recent determinations of the proton electric to magnetic elastic form factor ratio from polarization transfer measurements at Jefferson Lab indicate an unexpected and dramatic discrepancy with the elastic form factor ratio obtained using the Rosenbluth separation technique in unpolarized cross section measurements. This discrepancy has been explained as the effect of two-photon exchange beyond the usual one-photon exchange approximation in the calculation of the elastic electron-proton scattering cross section. The OLYMPUS experiment at DESY, Hamburg, Germany has been proposed to definitively determine the effect of two-photon exchange in elastic lepton-proton scattering by precisely measuring the ratio of positron-proton to electron-proton elastic unpolarized cross sections. [ABSTRACT FROM AUTHOR]