Historically, electron scattering from nucleons and nuclei was used to measure the size and the electric and magnetic properties of the nucleons and nuclei.  It was always assumed that the scattering was mediated by the exchange of a single photon.  Then, in 2000, measurements at JLab showed a significant discrepancy in the ratio of the electric to magnetic form factor of the proton as measured using the traditional, unpolarized technique and new measurements using polarized techniques.  The most likely explanation for the discrepancy was that two-photon exchange had a significant role in elastic electron scattering.

To quantify the contribution of two-photon exchange the OLYMPUS experiment was conceived.  The OLYMPUS experiment used the magnetic toroid, wire chambers, and time of flight scintillator bars from the former BLAST experiment at Bates.  These components were packed and shipped to the DESY laboratory in Hamburg, Germany and assembled on the DORIS electron/positron storage ring.  The internal hydrogen gas target used at OLYMPUS was also designed and built at Bates.  In addition, several new detector systems were made by the other institutes collaborating on the experiment.

The OLYMPUS experiment measured the ratio in the rate for elastically scattering electrons versus positrons from protons.  In single-photon exchange the rates would be identical and the ratio would be one.  However, if two-photon exchange plays a significant role in elastic scattering then the rates would differ and the ratio would deviate from unity.  Two other experiments have seen a deviation from unity but we are waiting on the high precision results from the ongoing OLYMPUS analysis to determine if this is sufficient to explain the discrepancy observed in the proton form factor ratio.  If it is confirmed then the old elastic scattering data will have to be re-evaluated to include two-photon exchange.