Measuring the lepton sign asymmetry in elastic electron-proton scattering with OLYMPUS

Abstract

OLYMPUS is a particle physics experiment that collected data in 2012 at DESY, in Hamburg, Germany, on the asymmetry between positron-proton and electron-proton elastic scattering cross sections. A non-zero asymmetry is evidence of hard two-photon exchange, which has been hypothesized to cause the discrepancy in measurements of the proton's electromagnetic form factors. Alternating electron and positron beams, accelerated to 2 GeV, were directed through a windowless, gaseous, hydrogen target, and the scattered lepton and recoiling proton were detected in coincidence using a large acceptance magnetic spectrometer. Determining the relative integrated luminosity between the electron and positron data sets was critical, and a new technique, involving multi-interaction events, was developed to achieve the desired sub-percent accuracy. A detailed Monte Carlo simulation was built in order to reproduce the convolution of systematic effects at every stage of the experiment. The first stage in the simulation was new radiative event generator, which permitted the full simulation of the non-trivial, radiative corrections to the measurement. The analysis of the data and simulation showed that the lepton sign asymmetry rises by several percent between a momentum transfer of 0.5 GeV${}^2/c^2$ and 2.25 GeV${}^2/c^2$. This rise as a function of increasing momentum transfer confirms that two photon exchange at least partially contributes to the proton form factor discrepancy.

MIT PhD thesis, 273 pages, 102 figures, http://hdl.handle.net/1721.1/107040

References in corpus (9)

• Global analysis of proton elastic form factor data with two-photon exchange corrections
• Towards a resolution of the proton form factor problem: new electron and positron scattering data
• Two-photon exchange in elastic electron-proton scattering : QCD factorization approach
• Box diagram in the elastic electron-proton scattering
• The structure function method applied to polarized and unpolarized electron-proton scattering: a solution of the G_E(p)/G_M(p) discrepancy
• Contribution of spin 1/2 and 3/2 resonances to two-photon exchange effects in elastic electron-proton scattering
• Perturbative QCD predictions for two-photon exchange
• On Delta resonance contribution to two-photon exchange amplitude
• Dispersive contributions to $e^{+}p/e^{-}p$ cross section ratio in forward regime

Cited by in corpus (2)

• CFNS Ad-Hoc meeting on Radiative Corrections Whitepaper
• Measurement of the Two-Photon Exchange Contribution to Elastic Lepton-Proton Scattering at the OLYMPUS Experiment