Muonium Spectroscopy as a Quantum Sensor for Ultralight Axion Dark Matter

Abstract

High-intensity muon beams could enable a muonium-based search for ultralight axions through resonant quantum transitions between hyperfine states. Combining theoretical calculations with simulation results, we demonstrate that such a muonium-based experimental approach -exemplified by one of the facilities under development in Huizhou- could improve constraints on the axion-muon coupling by up to two orders of magnitude compared with existing limits from the muon g-2 measurement, over the axion mass range of $10^{-15}$ eV to $10^{-12}$ eV. These results establish muonium spectroscopy as a powerful probe of physics beyond the Standard Model.

Figures (4)

• Figure 1: (a) Schematic diagram of the experimental setup. $S$ represents a carbon foil. $D_1$, $D_2$ and $D_3$ represent detectors. $E_1$ and $E_2$ represent deflection electrodes. The "scan" region denotes where the muoniums interact with ALPs. "MC" represents multipass cavity. Laser beams consist of photons with wavelengths of 244 nm and 365 nm. BS and HR denote beam splitter and high reflection, respectively. (b) Diagram of experimental process.
• Figure 2: Experimental model constructed in Geant4 with an illustrated signal profile at the MCP.
• Figure 3: Dependence of the detected signal rate on the incident muon kinetic energy and magnetic field length $L$, evaluated for $m_a = 10^{-13}\ \mathrm{eV}$ with the corresponding sensitivity of $f_a = 10^3\ \mathrm{GeV}$. The signal arises from muonium atoms undergoing ALP-induced transitions followed by laser ionization.
• Figure 4: Projected sensitivity of the proposed scheme with one-year data. Mu-I limit (blue dashed line) and Mu-II limit (red dashed line) are obtained with muonium intensities of $8.5\times 10^2$ /s and $8.5\times 10^4$ /s, respectively. The gray shaded regions show the constraints from muon $g\!-\!2$ bound Buen-Abad:2021fwqAthron:2025etsMuong-2:2025xyk.