RADES axion search results with a High-Temperature Superconducting cavity in an 11.7 T magnet

TL;DR

This work reports a RADES haloscope axion search using a high-temperature-superconducting (HTS) cavity housed in an 11.7 T magnet to probe higher-mass axions around $m_a\approx 36.57\,\mu\mathrm{eV}$. Employing a ReBCO-taped cavity, PCA-based background subtraction, and a modified Lorentzian line-shape fit, the authors achieved a 554 kHz scan window and set a 95% CL limit on the axion-photon coupling $g_{a\gamma}$ in the range $6.3\times10^{-13}\!\gtrsim\! GeV^{-1}$ to $1.59\times10^{-13}\!\gtrsim\! GeV^{-1}$, with no detected signal ($>2\sigma$). The analysis demonstrates robust background suppression in a high-frequency haloscope and documents the benefits and current limitations of HTS coatings in large magnets, including a plan to overcome curvature-based constraints and to pursue higher-Q designs. Looking forward, the work outlines pathways toward stronger sensitivity and future RADES developments, potentially enabling contributions to babyIAXO-scale axion searches.

Abstract

We describe the results of a haloscope axion search performed with an 11.7 T dipole magnet at CERN. The search used a custom-made radio-frequency cavity coated with high-temperature superconducting tape. A set of 27 h of data at a resonant frequency of around 8.84 GHz was analysed. In the range of axion mass 36.5676 $μ$eV to 36.5699 $μ$eV, corresponding to a width of 554 kHz, no signal excess hinting at an axion-like particle was found. Correspondingly, in this mass range, a limit on the axion to photon coupling-strength was set in the range between g$_{aγ}\gtrsim$ 6.2e-13 GeV$^{-1}$ and g$_{aγ}\gtrsim$ 1.59e-13 GeV$^{-1}$ with a 95% confidence level.

Figures (9)

• Figure 1: Left: Direction of the surface currents (red showing the region of maximum surface current flow) for the TE$_{111}$ mode (axion mode) from CST Studio Suite® simulations. Center: The cavity assembly prototype uncoated and a coin for size comparison. Right: The copper coated cavity covered with ReBCO tape. The cavity body has an inner length of 80mm, a width of 18.8mm, and a height of 24mm. The corners are rounded with a radius of 9mm.
• Figure 2:
• Figure 3: Left: Comparison of the measured S-parameters (transmission S$_{21}$ - green, reflection S$_{22}$ - yellow) and the simulated S-parameters (transmission S$_{21}$ - red, reflection S$_{22}$ - blue) of the HTS tape cavity. The measured S-parameters were used for quality factor and coupling calculation of the HTS cavity during the calibration measurements at 1.9K and 11.7T. For the plot, cable losses were calibrated after the measurement. Right: Unloaded quality factor versus magnetic field for the HTS tape cavity at 1.9K for ramp-down and subsequent ramp-up in comparison to a copper coated reference cavity of the same shape at zero magnetic fields.
• Figure 4:
• Figure 5: Left: Typical spectra for two different LO frequencies ($l_1$ = 8.700481644G Hz in blue and $l_2$ = 8.704143756G Hz in orange). Upon changing the LO frequency, the cavity resonance peak changes position in the IF frequency. Right: In blue an example of a $\delta^d_{ik}$ spectrum obtained by the division of two spectra taken at two different LO frequencies. In red the fit function produced by equation \ref{['eq:fit-function']}.