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.