Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering: Determined by the OLYMPUS Experiment

Abstract

The OLYMPUS collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, $R_{2\gamma }$, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01~GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of $\approx 20°$ to $80°$. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved GEM and MWPC detectors at $12°$, as well as symmetric Møller/Bhabha calorimeters at $1.29°$. A total integrated luminosity of 4.5~fb${}^{-1}$ was collected. In the extraction of $R_{2\gamma }$, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of $R_{2\gamma }$, presented here for a wide range of virtual photon polarization $0.456<\epsilon <0.978$, are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.

5 pages, 3 figures, 2 tables

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