Abstract
The OLYMPUS collaboration has recently made a precise measurement of the positron-proton to electron-proton elastic scattering cross section ratio, R2γ, over a wide range of the virtual photon polarization, 0.456 < ε < 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, . 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 phenomenological fits to the available form factor data.The motivation for measuring R2γ 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.
Highlights
The OLYMPUS Collaboration reports on a precision measurement of the positron-proton to electronproton elastic cross section ratio, R2γ, a direct measure of the contribution of hard two-photon exchange to the elastic cross section
One hypothesis for the cause of this discrepancy is a contribution to the cross section from hard two-photon exchange (TPE), which is not included in standard radiative corrections and would affect the two measurement techniques differently [15,16,17,18,19,20]
Hard TPE can be quantified from a measurement of R2γ, the ratio of positron-proton to electron-proton elastic cross sections that have been corrected for the standard set of radiative effects, including soft TPE
Summary
The OLYMPUS Collaboration reports on a precision measurement of the positron-proton to electronproton elastic cross section ratio, R2γ, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering Determined by the OLYMPUS Experiment
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