Search for high-frequency gravitational waves via reanalysis of cavity axion data

Monochromatic high-frequency gravitational waves (HFGWs) provide a distinctive probe of new physics scenarios, most notably axion clouds around rotating black holes formed via superradiance. We reanalyzed data from the CAPP-12T multicell axion haloscope experiment [Kim , ]. The study covers a continuous 2 MHz frequency span centered at 5.311 GHz. No rescan candidates were found, and we set 90% confidence-level exclusion limits on the gravitational-wave strain, reaching h 0 ≈ 3.9 × 10 − 21 in the most sensitive regions of the sky. Interpreted in the context of black hole superradiance from axion clouds, the results exclude black holes with mass M BH ≃ 1.22 × 10 − 6 M ⊙ within distances of O ( 10 − 2 ) AU from Earth, under benchmark assumptions. This work demonstrates the potential of electromagnetic resonant cavities as novel detectors of monochromatic HFGWs and motivates future searches for both long-lived and transient signals.