Constraining axion-like dark matter with a radio-frequency atomic magnetometer

TL;DR

This work presents a broadband laboratory search for axion-like particle (ALP) dark matter via gradient couplings to atomic spins, using a radio-frequency operated $^{87}\mathrm{Rb}$ magnetometer to detect a oscillating pseudomagnetic field $B_{\alpha}$ in the mass range $2.40\times10^{-10}$ to $2.11\times10^{-9}$ eV/$c^2$. By mapping ALP-fermion gradients to observables in the alkali-atom spin system, the authors derive limits on ALP couplings to electrons, protons, and neutrons, with the strongest improvement on $g_{\alpha pp}$ over prior laboratory searches. The analysis accounts for the DM halo coherence, employs a robust statistical framework with Monte Carlo thresholds, and demonstrates the viability of broadband atomic-spin probes for ALP-DM searches. While astrophysical bounds remain stronger, these results provide direct, complementary laboratory constraints on ALP couplings in a DM-halo context and outline pathways for substantial sensitivity gains in future work.

Abstract

We report on a broadband search for axion-like-particle (ALP) interactions using a radio-frequency-operated $^{87}\mathrm{Rb}$ atomic magnetometer. The instrument provides wide spectral coverage and sensitivity to an oscillating pseudomagnetic field that may be generated by the gradient coupling of the ALP field to the constituent fermions of atoms. We search for an ALP-gradient signature in the mass range $2.40\times10^{-10}\,\mathrm{eV}/c^{2}$--$2.11\times10^{-9}\,\mathrm{eV}/c^{2}$. No statistically significant signatures of an oscillating magnetic field are observed, and we derive upper limits on the corresponding ALP-proton, -neutron and -electron couplings, $g_{αpp}$, $g_{αnn}$ and $g_{αee}$, respectively. The result on $g_{αpp}$ improves over previous laboratory searches, while the limits on $g_{αnn}$ and $g_{αee}$ complement earlier laboratory searches and astrophysical bounds. The work extends searches for ALP-fermion interactions into a mass region largely unexplored in a dark-matter context, demonstrating the potential of our method for broadband axion-like particle searches targeting the Galactic dark-matter halo.

Figures (10)

• Figure 1: Simplified apparatus schematic. Rb atoms in a heated vapor cell are spin-polarized and probed using crossed laser beams. The cell is housed in an aluminum enclosure, surrounded by sets of coils to null the ambient field along $\hat{x}$ and $\hat{y}$, and set the leading field along $\hat{z}$. A set of rf coils is used to apply a known field to check the magnetic resonance response. The lab coordinate system $\hat{x}\hat{y}\hat{z}$ is shown in relation to the local coordinate system with axes $\hat{N}$ (north), $\hat{E}$ (east) and $\hat{U}$ (local zenith). Abbreviations: $\lambda/2$: half-wave plate; $\lambda/4$: quarter-wave plate; PBS: polarizing beam splitter; BPD: balanced photodetector.
• Figure 2: Magnetic field power spectra (blue) acquired in the three main experimental runs. Polynomial fits (red) to respective SG filter results (black) are used to determine the local mean noise level. The simulated lineshapes in the plots (to scale in frequency) show the squared-Lorentzian lineshape of a magnetometer resonance.
• Figure 3: Matched filter output $P_n^f(\nu)$ of the main data set $P_n(\nu)$ and detection threshold $X_{\rm thr}=1.183$ at the 95% confidence level.
• Figure 4: Exclusion plots for ALP--fermion couplings at the 95% confidence level, with the region excluded by this work shown in color. (a) Limits on $g_{\alpha pp}$. For comparison, constraints from NASDUCK-SERF (NS) BlochNatCom2023; Casimir and torsion-balance (TB) experiments MostepanenkoUniv2020AdelbergerPRL2007; solar-axion searches (SNO) BhusalPRL2021; and neutron-star cooling BuschmannPRL2022 are shown. (b) Limits on $g_{\alpha nn}$. Previous constraints include K--$^{3}$He VasilakisPRL2009; $^{129}$Xe SuPRL2024; torsion-balance (TB) experiments AdelbergerPRL2007; NS BlochNatCom2023; JEDI JEDI2022hxa; and SNO BhusalPRL2021. (c) Limits on $g_{\alpha ee}$. Additional constraints shown are from the electron $g-2$ measurement YanEPJC2019; torsion-pendulum (dipole--dipole force) experiments TerranoPRL2015; a fermionic axion interferometer (FAI) CresciniArxiv2023; and solar-neutrino observations GondoloPRD2009. Data from previous works are taken from Ohare2020axionlimits.
• Figure 5: Experimental setup, showing a cross section of the aluminum enclosure housing the Rb vapor cell. The apparatus coordinate system (with the $xz$ plane lying on the local horizontal plane) is shown in relation to the local ENU system, whose $\hat{E}$, $\hat{N}$ axes are on the same plane. Abbreviations: PBS: polarizing beam splitter;$\lambda/2$: half-wave plate; $\lambda/4$: quarter-wave plate; BPD: balanced photodetector; CPSS: computer-controlled power supply.