The Main Electrode System of the Nab Experiment and the Analysis of the Performance in the Measurement of the Fierz Term b

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Li, Huangxing, Physics - Graduate School of Arts and Sciences, University of Virginia
Baessler, Stefan, AS-Physics, University of Virginia

The Nab collaboration will study free neutron beta decay at the Spallation Neutron Source at Oak Ridge National Lab. A neutron decays into a proton, an electron and an anti-neutrino in this process, where the energy of the outgoing protons and electrons are collected to determine (1) the electron-antineutrino correlation co-efficient a to the precision of Δa/a <= 0.001 and (2) the Fierz interference term b to the precision of Δb <= 0.003.
From the measurement of a, we can calculate the axial-vector to vector coupling ratio λ. Together with the neutron lifetime measurement, we could also calculate the upper right element of the Cabbibo-Kobayashi-Maskawa Matrix, and test the unitarity of that matrix. The measurement of b could shed light on physics beyond the Standard Model since b is predicted to be 0 by the V-A structure of weak interaction in the Standard Model.
This thesis presents the design of Nab electrode system, and the solution to a major systematic effect in the measurement of a: the requirement of having a low electrical field environment in the neutron decay region. The electrode system has been built and installed successfully, and as our characterization and its analysis shows, the electrode system meets the required specifications.
This thesis also provides a systematic uncertainty study for b measurement, and provides a table of the requirements for the b measurement. The analysis of the systematic uncertainties of the Nab-b measurement shows that the main challenge for the measurement goal of b is the calibration and characterization of the detector, which is achievable. The total systematic uncertainty, as discussed in this thesis, is Δb_sys ≈ 0.0022. The statistical uncertainty, as discussed in this thesis, is at the level of Δb_stat ≈ 0.001 given 1e8 decay events. This conforms to the overall goal of Δb < 0.003. In the Nab experiment, we expect a decay rate of 1600 Hz, and the run time for 1e8 decay events is less than two days.

PHD (Doctor of Philosophy)
Related Links:
  • neutron beta decay
  • Nab experiment
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