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A Novel Search for Dark Photons


Over the course of nearly a century, astrophysical and cosmological evidence has suggested that a large portion of the universe's mass is comprised of some form of non-luminous matter. Called dark matter, its source remains an open question to the present day. Inspired by theories such as Supersymmetry (SUSY), in recent years, the weakly interacting massive particle (WIMP) hypothesis, has dominated the dark matter search landscape.

A generic, weak-scale, thermal relic WIMP could account for all of the observed dark matter in the universe, and experimenters continue to probe new WIMP parameter space by developing larger and more sensitive detectors. However, these experiments have detection thresholds for deposited energy, and they tend to lose sensitivity for low mass (below 10 GeV) dark matter particles. This leaves a large range of parameter space open for exploration, and new experiments are being launched to search in this region. Further, while some effort has been directed towards detection of ultra-low mass (below 100 eV) dark matter candidates, all efforts to date have yielded null results, and the regime remains largely unexplored.

Extending the Standard Model (SM) to include an extra U(1) gauge symmetry, requires a new gauge boson called a dark photon that can couple very weakly to electrically charged particles. If the dark photon has mass, it will mix with the Standard Model (SM) photon. The mixing parameter effectively translates to a coupling strength ε, between the dark photon and SM fermions. Due to its mass, it is also a candidate for low-mass dark matter.This work details the development of the Davis Dark Radio Experiment, which is a search for the feeble coupling between the dark photon and standard electromagnetism.

This thesis describes the development of the Dark E-Field Radio Experiment: A wideband search for the weak electric field signature of a dark photon in the 200-1250 neV (50-300 MHz) mass range. The experiment utilizes a commercial, real-time spectrum analyzer coupled to a wide bandwidth antenna. Over the course of 133 days, 3.8 hours of real-time data were collected and analyzed to set an upper limit on ε of ~1E-11.5. The experiment is the electromagnetic dual of magnetic detector dark radio experiments being carried out elsewhere. A paper based on this work has been peer-reviewed and published (Godfrey, 2021). Work done in preparation for the next phase of the experiment, along with suggestions for improvement to the current experimental design, are also described.

This work was supervised by Professors J. Anthony Tyson, S. Mani Tripathi, and Brian H. Kolner, and was performed, in collaboration with, scientists at Stanford University and the University of California, Davis.

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