Speaker
Description
The OLYMPUS collaboration has recently published 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 polarised and unpolarised measurements of the
proton form factor ratio, $\mu_{p} G_{E}^{p}/G_{M}^{p}$. The
OLYMPUS results are significantly lower than theoretical
calculations that explain the observed discrepancy in terms of
two-photon exchange but are in good agreement with predictions based
on phenomenological fits to the available form factor data.
This presentation will briefly review the motivation for measuring
$R_{2\gamma}$ followed by a description of the OLYMPUS experiment
and analysis. In particular, the importance of soft two-photon
contributions from radiative corrections included in the analysis
will be shown. Then we will present the OLYMPUS results and compare
these with results from two similar, recent experiments. Finally,
various theoretical calculations will be compared with the
experimental results and conclusions drawn.
