Two-Photon Exchange and the Role of OLYMPUS
Prof. Michael Kohl (Hampton University, USA)

The dramatic discrepancy in the observed ratio of elastic proton form factors between the Rosenbluth separation and polarization transfer methods has invoked numerous theoretical and experimental investigations. The previously neglected effect from two-photon exchange has become the favored explanation for the discrepancy.
While the effect can not be calculated from first principles, it can be determined experimentally by comparing the positron-proton and electron-proton elastic cross sections.
The OLYMPUS experiment at DESY has been carried out to quantify the effect of two-photon exchange using intense stored positron and electron beams along with an internal unpolarized hydrogen target and a large acceptance detector to measure the ratio of the positron-proton and electron-proton elastic scattering cross sections. Particular emphasis has been put on optimal control of systematics, by redundantly monitoring luminosity, beam properties and detector efficiencies.
Data taking has been completed in January 2013.
I will give an overview of the experimental efforts to verify the effect of two-photon exchange and will discuss in more detail the OLYMPUS experiment along with the status of the analysis.