Near-Quantum-Noise Axion Dark Matter Search at CAPP around 9.5 μeV

We report the results of an axion dark matter search over an axion mass range of $9.39--9.51\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{eV}$. A flux-driven Josephson parametric amplifier (JPA) was added to the cryogenic receiver chain. A system noise temperature of as low as 200 mK was achieved, which is the lowest recorded noise among published axion cavity experiments with phase-insensitive JPA operation. In addition, we developed a two-stage scanning method which boosted the scan speed by 26%. As a result, a range of two-photon coupling in a plausible model for the QCD axion was excluded with an order of magnitude higher in sensitivity than existing limits.