| Summary: | Multiple astrophysical and cosmological observations have shown that the visible matter described by the Standard Model of particle physics is only a small fraction of the energy density of the universe. We believe that there is about five times as much matter that is ‘dark’. The dark matter is likely comprised of massive particles that interact very little or not at all with other matter. Despite this lack of interaction, the ubiquity of dark matter has allowed it to have profound effects on the history of our universe—including seeding the formation of structures such as the galaxy in which we live.
One of the most well-motivated dark matter candidates is the axion, a hypothetical particle that is predicted by the solution to another long-standing mystery in physics, the strong CP problem. The Standard Model predicts that CP symmetry should be violated by the strong force. However, precision measurements have shown that strong interactions conserve CP symmetry to better than one part in 10¹⁰. At present, the most viable solution to this strong CP problem introduces a new particle, the axion. Within a wide range of parameters, the axion also satisfies all of the requirements to be dark matter.
This dissertation presents the first direct search for low-mass axion dark matter, using an innovative lumped-element detection method. In a lumped-element detector, a strong magnetic field interacts with the field of dark matter axions around us, inducing an effective current. This effective current is read out via a superconducting LC circuit and measured with high-sensitivity quantum sensors. The entire device must be kept only barely above absolute zero in order to reduce backgrounds that could mask a signal.
The prototype experiment that is the primary focus of this thesis, ABRACADABRA-10 cm, set world-leading limits on axion dark matter. Over the course of two month-long physics runs from 2018 to 2020, it excluded axions with masses 0.31—8.8 neV and couplings [formula]. This thesis will cover the lifetime of the experiment from design to construction to analysis.
The success of ABRACADABRA-10 cm has now set the stage for the DMRadio program, a series of larger detectors that will be capable of finding or definitively excluding axion-like particles and QCD axions over a wide range of masses below 1 μeV. I present initial sensitivity and design studies for the upcoming two generations of DMRadio, DMRadio-50 L and DMRadio-m³. I also discuss the path towards a future, large-scale experiment, DMRadio- GUT, which would probe of QCD axions at GUT-motivated masses.
The dark matter community is coalescing around the goal of probing the entire axion dark matter parameter space over the next couple of decades. The effort is driven by new ideas along with advances in cryogenic, magnet, and quantum sensing technology. The 1 L, 1 T ABRACADABRA-10 cm prototype experiment has formed the basis of a major component of the worldwide effort to find or exclude axions. The work in this dissertation represents the opening of new parameter space in the search for dark matter.
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