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Document Type
Dissertation
Degree
Doctor of Philosophy
Department
Physics
Advisor's Name
Brian A. Raue
Advisor's Title
Committee Chair
Advisor's Name
Julian Edward
Advisor's Name
Misak Sargsian
Advisor's Name
William K. Brooks
Advisor's Name
Werner U. Boeglin
Date of Defense
3-24-2009
Abstract
The two-photon exchange phenomenon is believed to be responsible for the discrepancy observed between the ratio of proton electric and magnetic form factors, measured by the Rosenbluth and polarization transfer methods. This disagreement is about a factor of three at Q2 of 5.6 GeV2. The precise knowledge of the proton form factors is of critical importance in understanding the structure of this nucleon. The theoretical models that estimate the size of the two-photon exchange (TPE) radiative correction are poorly constrained. This factor was found to be directly measurable by taking the ratio of the electron-proton and positron-proton elastic scattering cross sections, as the TPE effect changes sign with respect to the charge of the incident particle. A test run of a modified beamline has been conducted with the CEBAF Large Acceptance Spectrometer (CLAS) at Thomas Jefferson National Accelerator Facility. This test run demonstrated the feasibility of producing a mixed electron/positron beam of good quality. Extensive simulations performed prior to the run were used to reduce the background rate that limits the production luminosity. A 3.3 GeV primary electron beam was used that resulted in an average secondary lepton beam of 1 GeV. As a result, the elastic scattering data of both lepton types were obtained at scattering angles up to 40 degrees for Q2 up to 1.5 GeV2. The cross section ratio displayed an dependence that was Q2 dependent at smaller Q2 limits. The magnitude of the average ratio as a function of was consistent with the previous measurements, and the elastic (Blunden) model to within the experimental uncertainties. Ultimately, higher luminosity is needed to extend the data range to lower where the TPE effect is predicted to be largest.
Recommended Citation
Moteabbed, Maryam, "A Precise Measurement of the Two-Photon Exchange Effect" (2009). FIU Electronic Theses and Dissertations. Paper 280.
http://digitalcommons.fiu.edu/etd/280