A measurement of the two-photon exchange effect in elastic electron-proton scattering with OLYMPUS
Name
970697390-MIT.pdf
Description
Full printable version
Size
5.35 MB
Format
Adobe PDF
Checksum (MD5)
b684432488f694739ebeee1a29eb98d7
Author(s)
Russell, Rebecca Lynn
Advisor(s)
Richard G. Milner.
Date Issued
2016
Publisher
Massachusetts Institute of Technology
Abstract
Elastic electron-proton scattering has long been the tool of choice for the study of the proton form factors, GE(Q 2 ) and GM(Q2 ), which describe the electric and magnetic distributions of the proton as a function of momentum transfer. Recent experiments, measuring the form factors from polarization observables in polarized elastic electron-proton scattering, have found values of the ratio GE(Q2 )/GM(Q2) at high Q2 that contradict the results from unpolarized measurements. A proposed explanation for this discrepancy is the unaccounted two-photon exchange radiative correction, which could affect the unpolarized measurements. As this effect is currently not possible to calculate in a model-independent way, the OLYMPUS experiment was designed to make a direct measurement of it by measuring the elastic positron-proton to electron-proton scattering cross section ratio. The experiment was run in 2012 at DESY using the BLAST spectrometer and the DORIS positron and electron beams at 2 GeV incident on a gaseous hydrogen target. To analyze the data, a careful reconstruction of the scattering events, detailed simulation of the experimental setup, and full radiative corrections to the measured cross sections were performed. Preliminary results for the experiment show a statistically significant two-photon exchange effect, increasing over the measurement range of 0.6 GeV2 < Q2 < 2.35 GeV2 . These preliminary results suggest that two-photon exchange could explain the GE(Q2)/Gm(Q2 ) puzzle.
Description
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 185-190).
Subjects
Physics.
MIT Department
Massachusetts Institute of Technology. Department of Physics
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