APEX (A’ Experiment): The search for a dark photon at Jefferson Lab

Williamson, John Thomas Austin (2022) APEX (A’ Experiment): The search for a dark photon at Jefferson Lab. PhD thesis, University of Glasgow.

Abstract

The A’ Experiment (APEX) in Hall A of Jefferson Lab is a search for a new vector gauge boson, A’. The A’, or dark photon, serves as the mediator of a dark sector, a model for dark matter. Kinetic mixing, characterised by the strength parameter e², between the A’ and the Standard Model photon would allow for its experimental observation. For APEX the A’ production mechanism is dark bremsstrahlung in interactions of an electron beam with a high-Z target, which would be followed by decay to a lepton pair (the case of a visible dark photon). The High Resolution Spectrometers are set-up to record e⁺e⁻ pairs, from which an invariant mass distribution can be obtained. A resonance search is then performed on this invariant mass spectrum, looking for the A’. A test run for APEX was performed in 2010 with a beam energy of 2.260 GeV, which established limits down to e² ≃ 1 x 10⁻⁶ in the mass range 175–250 MeV [1]. This generated wide interest and proved the viability of the experiment.

An APEX production run was carried out in 2019, with a beam energy of 2.138 GeV. This thesis presents the analysis and preliminary results from this 2019 run period. Multiple stages of analysis were conducted in order to obtain a final invariant mass spectrum and to optimise the invariant mass resolution. A blinded peak search was performed on 10% of the final invariant mass distribution, to select background parameters without biasing a full search. The blinded peak search did not find any evidence for the A’ in the mass range 130-220 MeV, and established limits down to e² ≃ 6 x 10⁻⁷.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > TA Engineering (General). Civil engineering (General)
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Hamilton, Dr. David and Montgomery, Dr. Rachel
• Date of Award: 2022
• Depositing User: Theses Team
• Unique ID: glathesis:2022-83215
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 21 Oct 2022 14:22
• Last Modified: 21 Oct 2022 14:24
• Thesis DOI: 10.5525/gla.thesis.83215
• URI: https://theses.gla.ac.uk/id/eprint/83215