Extensive Search for Axion Dark Matter over 1 GHz with CAPP’S Main Axion Experiment

We report an extensive high-sensitivity search for axion dark matter above 1 GHz at the Center for Axion and Precision Physics Research (CAPP). The cavity resonant search, exploiting the coupling between axions and photons, explored the frequency (mass) range of 1.025 GHz ($4.24\mathrm{\mu }\mathrm{eV}$) to 1.185 GHz ($4.91\mathrm{\mu }\mathrm{eV}$). We have introduced a number of innovations in this field, demonstrating the practical approach of optimizing all the relevant parameters of axion haloscopes, extending presently available technology. The CAPP 12 T magnet with an aperture of 320 mm made of ${\mathrm{Nb}}_3\mathrm{Sn}$ and NbTi superconductors surrounding a 37 l ultralight-weight copper cavity is expected to convert Dine-Fischler-Srednicki-Zhitnitsky axions into approximately ${10}^2$ microwave photons per second. A powerful dilution refrigerator, capable of keeping the core system below 40 mK, combined with quantum-noise-limited readout electronics, achieved a total system noise of about 200 mK or below, which corresponds to a background of roughly $4\times {10}^3$ photons per second within the axion bandwidth. The combination of all those improvements provides unprecedented search performance, imposing the most stringent exclusion limits on axion-photon coupling in this frequency range to date. These results also suggest an experimental capability suitable for highly sensitive searches for axion dark matter above 1 GHz.