Axion Dark Matter Search with Photonic Bandgap Cavity Haloscope and Dielectric Tuning Rod over 10.25-10.45 GHz

The specific nature of dark matter has eluded scientists to date and remains a missing piece of our understanding of the fundamental particles and forces that make up our universe. One of the leading dark matter candidates is the axion, notable not only for its potential as dark matter but also for its resolution of the strong-CP problem in particle physics. These axions are considered "wave-like" particles due to their large number density and coherent behavior, allowing scientists to treat the axion field as a classical wave. From there, current axion experiments look for an axion's proposed coupling to electromagnetic fields, where an axion is predicted to occasionally convert into a microwave photon. This axion signature is resonantly accumulated in a microwave cavity cooled to cryogenic temperatures before being amplified by a linear amplifier operating near the standard quantum limit (SQL). However, because we do not know the axion mass a priori, any microwave cavity needs to be widely tunable and have a long lifetime to get the most out of the resonant accumulation of the dark matter signal.

In this thesis I report the development of a new tunable cavity and apply this cavity for a new exclusion on axion-to-photon coupling. We will discuss the modeling and simulation of this cavity that utilizes concentric sapphire shells and a sapphire tuning rod to achieve unloaded quality factors above 10⁵, roughly 25× larger than a bare copper cavity at the same frequency. The tuning rod allows this cavity mode to be tuned over the range of 10.1–11.7 GHz, approximately 16% of its resonant frequency, making it the largest tuning range for a resonant experiment near this frequency band. With this same cavity we demonstrate its sensitivity to new axion parameter space by tuning the cavity over the 200 MHz range between 10.25–10.45 GHz (42.4–43.2 μeV) to constrain axion-to-photon coupling to |g_aγγ| ≤ 1 × 10⁻¹² GeV⁻¹. This cavity can scan its tuning range about 9 times faster compared to a bare copper cavity when paired with a photon counting device, laying the groundwork for a definitive search for the QCD axion over 10.1–11.7 GHz.