Hard two-photon contribution to elastic lepton-proton scattering determined by the OLYMPUS experiment

The OLYMPUS collaboration has recently made a precise measurement of the positron-proton to electron-proton elastic scattering cross section ratio, R-2 gamma, over a wide range of the virtual photon polarization, 0.456 < epsilon < 0.978. This provides a direct measure of hard two photon exchange in elastic lepton-proton scattering widely thought to explain the discrepancy observed between unpolarized and polarized measurements of the proton form factor ratio, mu(p)G(E)(p)/G(M)(p). The OLYMPUS results are small, within 1% on unity, over the range of momentum transfers measured and significantly lower than theoretical calculations that can explain part of the observed discrepancy in terms of two-photon exchange at higher momentum transfers. However, the results are in reasonable agreement with predictions based on phenomeno logical fits to the available form factor data. The motivation for measuring R-2 gamma will be presented followed by a description of the OLYMPUS experiment. The importance of radiative corrections in the analysis will be shown also. Then we will present the OLYMPUS results and compare with results from two similar experiments and theoretical calculations.