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 (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>4.24</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi mathvariant="normal">μ</a:mi><a:mi>eV</a:mi></a:mrow></a:math>) to 1.185 GHz (<d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mrow><d:mn>4.91</d:mn><d:mtext> </d:mtext><d:mtext> </d:mtext><d:mi mathvariant="normal">μ</d:mi><d:mi>eV</d:mi></d:mrow></d:math>). 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 <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:msub><g:mrow><g:mi>Nb</g:mi></g:mrow><g:mrow><g:mn>3</g:mn></g:mrow></g:msub><g:mi>Sn</g:mi></g:mrow></g:math> and NbTi superconductors surrounding a 37 l ultralight-weight copper cavity is expected to convert Dine-Fischler-Srednicki-Zhitnitsky axions into approximately <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msup><i:mn>10</i:mn><i:mn>2</i:mn></i:msup></i:math> 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 <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mn>4</k:mn><k:mo>×</k:mo><k:msup><k:mn>10</k:mn><k:mn>3</k:mn></k:msup></k:math> 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. Published by the American Physical Society 2024