Speaker
Description
Ultralight axion dark matter may couple to electromagnetic fields through the interaction term $g_{a\gamma\gamma}a\,\mu_0\mathbf{E}\!\cdot\!\mathbf{H}$ [1-4]. Traditional cavity haloscopes exploit this coupling using a strong external DC magnetic field [5,6], whereas upconversion experiments use the spatial overlap between resonant electromagnetic modes to probe axion frequencies below the cavity resonance [7-13].
In conventional two-mode schemes, however, practical limits on the minimum mode separation restrict sensitivity to the lowest axion masses [8]. Twisted anyon cavity resonators offer a single-mode alternative by supporting helical eigenmodes with nonzero electromagnetic helicity in free space, allowing the axion field to couple directly to a single resonant mode without an applied DC magnetic field [14]. This approach enables sensitivity to ultralight axions over the mass range $10^{-18}$-$10^{-13.7}\,\mathrm{eV}$. However, because the helicity of these modes depends sensitively on the full three-dimensional cavity boundary, high-performance geometries are difficult to identify using conventional heuristic design methods [15].
We present an inverse-design framework for optimising three-dimensional microwave cavities for single-mode ultralight axion searches. Parameterised cavity geometries are evaluated using finite-element eigenmode simulations and explored using a genetic algorithm [15]. The optimisation employs a composite figure of merit containing the cavity-dependent contribution to the expected axion-induced amplitude-modulation index, which quantifies the strength of the axion-induced amplitude modulation of the resonant cavity field,
\begin{equation}
F(\mathbf{x})=
\frac{\hat{Q}0^{\mathrm{opt}}
\left|\mathscr{H}{\mathrm{opt}}\right|}
{f_{\mathrm{opt}}}.
\end{equation}
Here, $\mathscr{H}_{\mathrm{opt}}$, $f_{\mathrm{opt}}$, and $\hat{Q}_0^{\mathrm{opt}}$ are the helicity, resonant frequency, and estimated intrinsic quality factor of the optimal cavity mode. Maximising this quantity increases the expected signal-to-noise ratio within the resonator bandwidth and reduces the measurement time required to reach a fixed axion sensitivity.
The framework is applied to cavity families compatible with additive and subtractive manufacturing in superconducting niobium, incorporating dimensional and fabrication constraints relevant to operation in a dilution refrigerator. Although additively manufactured geometries provide greater freedom to maximise helicity, the optimised subtractively manufactured design achieves the highest predicted performance because of the substantially lower surface resistance achievable in high-purity bulk niobium [16,17]. Compared with the original heuristically designed twisted anyon cavity haloscope [14], the optimised design improves the figure of merit by more than three orders of magnitude. This leads to a projected axion--photon coupling sensitivity of $g_{a\gamma\gamma}<10^{-11}\,\mathrm{GeV}^{-1}$ for an acquisition time of three months. These results demonstrate how inverse design can systematically identify fabrication-compatible, high-$Q$, helical microwave cavities for future ultralight axion searches.
REFERENCES:
[1] R. D. Peccei and H. R. Quinn, Phys. Rev. Lett. 38, 1440 (1977).
[2] S. Weinberg, Phys. Rev. Lett. 40, 223 (1978).
[3] F. Wilczek, Phys. Rev. Lett. 40, 279 (1978).
[4] R. D. Peccei, Lect. Notes Phys. 741, 3 (2008).
[5] P. Sikivie, arXiv:1009.0762 (2013).
[6] B. T. McAllister, et al., Phys. Rev. Lett. 116, 161804 (2016).
[7] M. Goryachev, et al., Phys. Dark Universe 26, 100345 (2019).
[8] C. Thomson, et al., Phys. Dark Universe 32, 100787 (2021).
[9] R. Lasenby, Phys. Rev. D 102, 015008 (2020).
[10] R. Lasenby, Phys. Rev. D 103, 075007 (2021).
[11] A. Berlin et al., J. High Energy Phys. 07, 001 (2020).
[12] C. A. Thomson et al., Phys. Rev. Lett. 126, 081803 (2021); 127, 019901(E) (2021).
[13] C. A. Thomson et al., Phys. Rev. D 107, 112003 (2023).
[14] J. F. Bourhill, et al., Phys. Rev. D 108, 052014 (2023).
[15] E. Paterson, et al., arXiv:2602.09037 (2026).
[16] P. Frigola et al., in Proc. SRF2015, THPB042 (2015).
[17] S. Posen et al., Phys. Rev. Applied 13, 014024 (2020).
| Primary Abstract Topic | Experiment: Axions and Wave-Like-DM |
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