This is the home page for the OLYMPUS Collaboration who are proposing an experiment to measure the two photon contribution to elastic electron scattering using the DORIS storage ring at DESY, Hamburg, Germany and the BLAST detector from the MIT-Bates Linear Accelerator Center.
The motivation for this proposal grew from the startling
discrepancy discovered at JLAB in the ratio of the electric to
magnetic elastic form factors for electron scattering from the
proton. Previous unpolarised measurements utilising the Rosenbluth
technique to measue this ratio found values consistent with unity
for a wide range of momentum transfer. 
This was also consistent with the
idea that the electric and magnetic elastic form factors for the
proton were both well described by the same dipole distribution
(therefore their ratio would be unity). However, the JLAB
measurements utilising a polarisation transfer technique found the
ratio decreasing sharply with increasing momentum transfer.
A possible explanation for the discrepancy is that multi-photon processes are important at higher momentum transfers. The unpolarised data of course sums over all processes and thus does not distinguish between single and multi-photon processes. But techniques using polarisation are sensitive primarily to single photon exchange as the polarisation information is washed out in multi-photon processes.
If multi-photon processes do play a significant role in elastic
eletron scattering from nucleons then the unpolarised data must be
re-evaluated and better electric and magnetic form factors
extracted so that a correct model of the nucleon can be found for
comparison with theoretical calculations. 