The OLYMPUS Experiment

Abstract

The OLYMPUS experiment was designed to measure the ratio between the positron-proton and electron-proton elastic scattering cross sections, with the goal of determining the contribution of two-photon exchange to the elastic cross section. Two-photon exchange might resolve the discrepancy between measurements of the proton form factor ratio, $\hat{I}{¼}_p{G}_E^p/G_M^p$, made using polarization techniques and those made in unpolarized experiments. OLYMPUS operated on the DORIS storage ring at DESY, alternating between 2.01~GeV electron and positron beams incident on an internal hydrogen gas target. The experiment used a toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight detectors to measure rates for elastic scattering over the polar angular range of approximately $25^{\circ }$--$75^{\circ }$. Symmetric Møller/Bhabha calorimeters at $1.29^{\circ }$ and telescopes of GEM and MWPC detectors at $12^{\circ }$ served as luminosity monitors. A total luminosity of approximately 4.5~fb${}^{-1}$ was collected over two running periods in 2012. This paper provides details on the accelerator, target, detectors, and operation of the experiment.

40 pages, 26 figures