  Measurements of the proton's form factor ratio made with polarization
transfer show a striking discrepancy relative to the ratio extracted from
unpolarized elastic electron-proton scattering cross sections. One hypothesis
is that the discrepancy is caused by hard two-photon exchange (TPE), a
typically neglected radiative correction that may bias the two approaches
differently. This hypothesis has been challenging to confirm. Theoretical
estimates of TPE are model-dependent, and recent experimental determinations of
TPE lacked the kinematic reach to be conclusive. The possible impact of TPE
remains a cloud over our knowledge of the proton's form factors. Recently, the
OLYMPUS experiment published new elastic scattering cross sections that are
insensitive to the effects of TPE: specifically the average of electron-proton
and positron-proton cross sections. The OLYMPUS experiment, conducted at DESY,
Hamburg, measured elastic $e^-p$ and $e^+p$ scattering by detecting the
scattered lepton and recoiling proton in coincidence in a large-acceptance,
toroidal magnetic spectrometer. OLYMPUS was designed to measure the $e^+p/e^-p$
cross section raio to isolate the effects of TPE. By exploiting the over
determined kinematics of the reaction, the absolute efficiency of spectrometer
could be verified, allowing cross sections to be extracted from the data. These
results can help refine our knowledge of the proton's form factors, especially
in the squared momentum-transfer region of 1--2 GeV$^2$, where some previous
measurements are in tension.
