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

TL;DR

This work targets high-frequency gravitational waves in the GHz range by reanalyzing CAPP-12T MC haloscope data near 5.311 GHz to search for monochromatic signals from axion-cloud superradiance around rotating black holes. Using an electromagnetic cavity in a 12 T field with a quantum-limited readout, the authors derive the GW-to-EM conversion power via the inverse Gertsenshtein effect, construct sky-dependent sensitivity maps, and set a 90% C.L. limit of $h_0 \approx 3.9\times 10^{-21}$ in the most sensitive directions. Interpreted within black-hole superradiance, these limits exclude BHs of mass $M_{\mathrm{BH}} \approx 1.22\times10^{-6}\,M_\odot$ within $\mathcal{O}(10^{-2})\ \mathrm{AU}$ for benchmark parameters, demonstrating that haloscope datasets can constrain well-motivated HFGW sources. The results motivate extended searches for both long-lived and transient GHz-band signals and point to substantial gains from improved GW-to-EM coupling and broader frequency coverage in future experiments.

Abstract

Monochromatic high-frequency gravitational waves (HFGW) 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 MC (multi-cell) axion haloscope experiment [Phys. Rev. Lett. 133,051802 (2024)]. 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 $h_0 \approx 3.9 \times 10^{-21}$ 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 $M_{\mathrm{BH}} \simeq 1.22 \times 10^{-6}\,M_\odot$ 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 HFGW and motivates future searches for both long-lived and transient signals.

Figures (10)

• Figure 1: Schematic of the CAPP-12T MC haloscope setup CAPP2024_12TMC. Red solid lines (between the 50 $\Omega$ noise source and the directional coupler, and between the isolator at mixing plate of physcial temperature 40 mK and the HEMT amplifiers) indicate superconducting RF lines that prevent thermal links.
• Figure 2: Schematic of the GW propagation plane with respect to the cylindrical cavity. The external magnetic field is applied along $\hat{z}$ axis. GW propagates in the $y’–z$ plane, making an angle $\beta$ with the cavity axis $\hat{z}$. The azimuthal angle $\phi$ defines the orientation of the GW propagation direction with respect to the $x–y$ plane.
• Figure 3: Gravitational coupling coefficients as a function of $\beta$ in polar coordinates for different polarization states and azimuthal angles $\phi$. Left: plus polarization, $C_{GW}^{+}$. Right: cross polarization, $C_{GW}^{\times}$.
• Figure 4: Relative location of the GW source and the cavity in the equatorial coordinates. The GW source position is specified by right ascension $\alpha$ and declination $\delta$, with the zenith direction along $\hat{z}$. The dashed curve represents the $\beta=\pi/2$ plane.
• Figure 5: Gravitational-wave with cross polarization coupling coefficients in equatorial coordinates for the CAPP–12T MC cavity mode, evaluated for arbitrary gravitational-wave sources.