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Axion Dark Matter Detection with Cold Molecules

Peter W. Graham, Surjeet Rajendran

TL;DR

The paper proposes detecting axion dark matter in the high-$f_a$ region (GUT–Planck scales) by measuring oscillating nuclear electric dipole moments induced by the axion-gluon coupling in heavy, octupole-deformed nuclei within polar molecules. These oscillations generate time-varying shifts in molecular energy levels, amplified by Schiff moments and internal molecular fields, and measurable with molecular interferometry or precision clocks. The approach leverages non-derivative axion couplings to enable frequency scanning via external field control, with sensitivity enhanced by comagnetometry and long integration across coherence times, targeting $f_a$ in the range $10^{16}$–$10^{19}$ GeV. If realized, it would illuminate DM properties, confirm the axion as the solution to the strong CP problem, and provide insight into physics at energy scales far beyond current laboratory reach. The proposal outlines concrete setups with actinide-containing molecules (e.g., Radium, Plutonium, Protactonium, Francium) and discusses technological pathways in molecular cooling, trapping, and actinide manipulation to achieve the required sensitivity.

Abstract

Current techniques cannot detect axion dark matter over much of its parameter space, particularly in the theoretically well-motivated region where the axion decay constant f_a lies near the GUT and Planck scales. We suggest a novel experimental method to search for QCD axion dark matter in this region. The axion field oscillates at a frequency equal to its mass when it is a component of dark matter. These oscillations induce time varying CP-odd nuclear moments, such as electric dipole and Schiff moments. The coupling between internal atomic fields and these nuclear moments gives rise to time varying shifts to atomic energy levels. These effects can be enhanced by using elements with large Schiff moments such as the light Actinides, and states with large spontaneous parity violation, such as molecules in a background electric field. The energy level shift in such a molecule can be ~ 10^-24 eV or larger. While challenging, this energy shift may be observable in a molecular clock configuration with technology presently under development. The detectability of this energy shift is enhanced by the fact that it is a time varying shift whose oscillation frequency is set by fundamental physics and is therefore independent of the details of the experiment. This signal is most easily observed in the sub-MHz range, allowing detection when f_a is > 10^16 GeV, and possibly as low as 10^15 GeV. A discovery in such an experiment would not only reveal the nature of dark matter and confirm the axion as the solution to the strong CP problem, it would also provide a glimpse of physics at the highest energy scales, far beyond what can be directly probed in the laboratory.

Axion Dark Matter Detection with Cold Molecules

TL;DR

The paper proposes detecting axion dark matter in the high- region (GUT–Planck scales) by measuring oscillating nuclear electric dipole moments induced by the axion-gluon coupling in heavy, octupole-deformed nuclei within polar molecules. These oscillations generate time-varying shifts in molecular energy levels, amplified by Schiff moments and internal molecular fields, and measurable with molecular interferometry or precision clocks. The approach leverages non-derivative axion couplings to enable frequency scanning via external field control, with sensitivity enhanced by comagnetometry and long integration across coherence times, targeting in the range – GeV. If realized, it would illuminate DM properties, confirm the axion as the solution to the strong CP problem, and provide insight into physics at energy scales far beyond current laboratory reach. The proposal outlines concrete setups with actinide-containing molecules (e.g., Radium, Plutonium, Protactonium, Francium) and discusses technological pathways in molecular cooling, trapping, and actinide manipulation to achieve the required sensitivity.

Abstract

Current techniques cannot detect axion dark matter over much of its parameter space, particularly in the theoretically well-motivated region where the axion decay constant f_a lies near the GUT and Planck scales. We suggest a novel experimental method to search for QCD axion dark matter in this region. The axion field oscillates at a frequency equal to its mass when it is a component of dark matter. These oscillations induce time varying CP-odd nuclear moments, such as electric dipole and Schiff moments. The coupling between internal atomic fields and these nuclear moments gives rise to time varying shifts to atomic energy levels. These effects can be enhanced by using elements with large Schiff moments such as the light Actinides, and states with large spontaneous parity violation, such as molecules in a background electric field. The energy level shift in such a molecule can be ~ 10^-24 eV or larger. While challenging, this energy shift may be observable in a molecular clock configuration with technology presently under development. The detectability of this energy shift is enhanced by the fact that it is a time varying shift whose oscillation frequency is set by fundamental physics and is therefore independent of the details of the experiment. This signal is most easily observed in the sub-MHz range, allowing detection when f_a is > 10^16 GeV, and possibly as low as 10^15 GeV. A discovery in such an experiment would not only reveal the nature of dark matter and confirm the axion as the solution to the strong CP problem, it would also provide a glimpse of physics at the highest energy scales, far beyond what can be directly probed in the laboratory.

Paper Structure

This paper contains 10 sections, 15 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Axion Dark Matter
  3. Overview of the Detection Method
  4. Experimental Strategy
  5. The Setup
  6. Sensitivity
  7. Backgrounds
  8. Technology
  9. The Reach
  10. Conclusions and Future Vision

Figures (2)

  • Figure 1: (Color Online) The parameter space of the axion in $f_a$ (GeV). Values of $f_a < 10^9$ GeV are ruled out by astrophysical constraints (green). Values of $f_a \gtrsim 10^{11}$ GeV allow the axion to be the dark matter (purple). The (blue) region labelled "microwave cavity" shows the region of parameter space that is potentially observable with microwave cavity experiments, e.g. ADMX. The (red) region labelled "molecular interferometry" shows the range of $f_a$ which is potentially observable with the experiments proposed here. The lower limit on this region may in fact be lower than shown, depending on technological advances.
  • Figure 2: The molecules are polarized by an external electric field $\vec{E}_{\text{ext}} \sim 100 \, \frac{\text{kV}}{\text{cm}}$. They are then placed in a linear superposition of the two states $| \Psi_L \rangle_a$ and $| \Psi_L \rangle_o$, where the nuclear spin is either aligned or anti-aligned with the molecular axis respectively, leading to a phase difference between them in the presence of the axion induced nuclear dipole moment $d_n$. The external magnetic field $\vec{B}_{\text{ext}} \sim 0.1 \text{ T} \, \left( \frac{f_a}{M_{\text{GUT}}}\right)$ causes the spins to precess, so that the phase difference can be coherently accrued over several axion oscillations. The frequency can be scanned by dialing this magnetic field $\vec{B}_{\text{ext}}$ until it is resonant with the axion frequency.