Sensitivity of a Gigahertz Fabry-Pérot Resonator for Axion Dark Matter Detection

TL;DR

The paper tackles the challenge of searching for axion dark matter at frequencies above 30 GHz, where conventional closed cavities suffer from reduced signal power. It introduces FAxE, an open Fabry-Pérot resonator in a magnetic field, which enables larger transverse volumes at microwave frequencies and leverages a calibrated coupling scheme to extract a potential axion signal. Through finite-element method simulations and careful optimization of mirror geometry, it demonstrates that a cryogenic baseline configuration can reach sensitivities near $g_{a\gamma}$ values predicted by certain axion models, and shows that graded-phase mirrors can significantly improve the form factor and scan rate. The work outlines a practical pathway to probe QCD axion benchmarks in a challenging frequency band, highlighting the need for higher fields, larger mirrors, and superconducting coatings to realize a full FAxE strong program, and positions FAxE as a complementary approach to existing high-frequency haloscopes. All key equations and parameters are presented with explicit dependencies on $g_{a\gamma}$, $\rho_a$, $m_a$, $V$, $B_e$, $Q_L$, $C$, and $\beta$, enabling straightforward interpretation and comparison.

Abstract

Axions are hypothetical pseudo-Nambu Goldstone bosons that could explain the observed cold dark matter density and solve the strong CP problem of quantum chromodynamics (QCD). Haloscope experiments commonly employ resonant cavities to search for a conversion of axion dark matter into photons in external magnetic fields. As the expected signal power degrades with increasing frequency, this approach becomes challenging at frequencies beyond tens of Gigahertz. Here, we propose a novel haloscope design based on an open Fabry-Pérot resonator. Operating a small-scale resonator at cryogenic temperatures and at modest magnetic fields should already lead to an unparalleled sensitivity for photon-axion couplings $g_{aγ} \gtrsim 3\times10^{-12}\,\mathrm{GeV}^{-1}$ at 35GHz. We demonstrate how this sensitivity could be further improved using graded-phase mirrors and sketch possibilities to probe benchmark models of the QCD axion.

Figures (8)

• Figure 1: Diagram of the proposed Fabry-Pérot haloscope. The different components are described in the text. Colored lines show the different calibration chains.
• Figure 2: The projected FAxE sensitivity for different considered configurations (see Table \ref{['tab:configs']}) in terms of axion mass and photon-axion coupling for an assumed local axion dark matter density of $\rho_a = 0.45G\eV\per\cubic cm$ (in accordance with, e.g., Ref. 2021RPPh...84j4901D). We also show the KSVZ and DFSZ QCD axion benchmark models 1979PhRvL..43..103Ksvz1980dfs1981z1980 for different ratios $E/N$ of the electromagnetic and QCD anomaly coefficients, the ALP cogenesis model prediction 2021JHEP...01..172C, as well as the predictions for trapped misalignment 2021arXiv210201082D. Also shown are existing exclusions from MADMAX 2024arXiv240911777A, ORGAN 2017PDU....18...67M2022SciA....8.3765Q2024PhRvL.132c1601Q2024arXiv240718586Q, RADES 2021JHEP...10..075A2024arXiv240307790A, QUAX 2024PhRvD.110b2008R, as well as from the CAST experiment CAST:2024eil. Future sensitivities of the haloscope experiments ALPHA 2023PhRvD.107e5013M, DALI 2024PhRvD.109f2002D, MADMAX 2020arXiv200310894B, and ORGAN 2017PDU....18...67M are shown as blue dashed-dotted, solid, dashed, and dotted lines, respectively.
• Figure 3: FEM simulation of an FPR with aperture. The contours show the normalized electric field of the fundamental resonating eigenmode.
• Figure 4: The figure of merit as a function of waist radius for a Gaussian beam FPR for different values of $Q_{\mathrm{other}}$, stemming from the different conductivities of aluminum at different temperatures.
• Figure 5: Signal power of a simulated FPR with fixed mirror geometry and aperture parameters as a function of resonance frequency which is tuned by changing the distance between the spherical and graded-phase mirrors (see also Sec. \ref{['sec:graded-phase-mirrors']}). Mirror and aperture geometry were optimized to $f = 36GHz$.