Axion dark matter search beyond KSVZ limit using the CAPP18T haloscope

Abstract

The axion is a promising dark matter candidate, initially introduced as a solution to the strong CP problem in quantum chromodynamics. This thesis presents the extended axion search using the CAPP18T haloscope. The CAPP18T experiment employs innovative technologies, including a High-Temperature Superconducting (HTS) magnet and a Josephson Parametric Converter. The CAPP18T detector was reconstructed following an unexpected HTS magnet quench incident. The system reconstruction involved rebuilding the magnet, enhancing the thermal coupling and impedance matching of the receiver system, and upgrading the data acquisition method. The total system noise temperature is approximately 0.6 K. The coupling between the cavity and the strong antenna was maintained at $\beta \simeq 2$ to maximize the axion search scanning speed. The scan frequency range spans from 4.8077 to 4.8181 GHz. No significant evidence of axion dark matter signatures was observed. The results establish the best upper bound on the axion-photon-photon coupling ($g_{a\gamma\gamma}$) within mass ranges of 19.883 to 19.926 $\mu$eV at approximately 0.7$\times|g_{a\gamma\gamma}^{\text{KSVZ}}|$ or 1.9$\times|g_{a\gamma\gamma}^{\text{DFSZ}}|$ with a 90\% confidence level. The findings demonstrate that a reliable search for high-mass dark matter axions beyond benchmark models can be achieved using the technology adopted in CAPP18T.