Preparation for a Measurement of Charge Asymmetry in the Bethe-Heitler Process

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Chen, Haoyu, Physics - Graduate School of Arts and Sciences, University of Virginia
Norum, Blaine, AS-Physics (PHYS), University of Virginia

We have prepared a measurement of the energy asymmetry in wide- and medium-angle
electron/positron pair production off protons and heavy targets. This asymmetry is caused by the
interference between the first- and second-order Born diagrams and the Compton scattering
diagram. It directly probes aspects of QED, as well as providing a direct measurement of the real
part of the Compton amplitude. It will be conducted at the HI𝛾⃗S facility at Duke University, using
a 60 MeV photon beam. This dissertation serves as documentation of the preparation stage of the
Bethe-Heitler experiment. The major was the recommissioning of the vertical drift chambers
previously used in the Q-weak experiment at the Jefferson Lab. Cosmic test runs were conducted,
drift time data were collected and efficiency plateaus were measured. We made modifications to
the JLAB Hall A analyzer to suit the geometry and drift characteristics of these wire chambers.
The analyzer was used for the reconstruction of the trajectories of cosmic ray test runs with the
results confirmed by direct measurement of trigger geometry. Spatial and angular resolution is
estimated to ~300𝜇𝑚 and 0.17° respectively. Geant 4 simulations with generated Bethe-Heitler
pairs satisfying theoretical differential cross sections. It was used to check detector acceptance,
optimize apparatus layout, and estimate measurable energy asymmetry. The measureable
asymmetries from electron/positron pairs with polar angles around between approximately 5° and
8°, azimuthal angles differing by 180°, and energy differing by approximately 9 MeV to 15
MeV are predicted to be above 10%. The kinematics of primary vertices are reconstructed using
the data from wire chambers in the simulation. The energy resolution is determined to be better
than 1MeV.

PHD (Doctor of Philosophy)
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