Axion Production and Detection Using a Dual NMR-type Experiment

TL;DR

This work proposes a laboratory approach to search for axions through the axial current coupling to nuclei by using a dual NMR-type setup: one device acts as an axion source via driven spin precession, while a nearby device detects the axion-induced field with a high-sensitivity magnetometer. The key idea is that the near-field axion gradient scales as $\sim1/R$ and that the axion field is produced coherently at the spin-precession frequency $\omega_0$, enabling long interrogation times and broad mass coverage independent of the axion mass. The authors derive the production and detection physics, include a detailed noise model and a likelihood framework, and project sensitivities showing that two centimeter-scale devices with 15-day integration can surpass astrophysical bounds on $g_{aN}$, with 10 cm devices achieving similar sensitivity in only 1 hour. The results indicate a feasible path to explore a wide axion-mass range up to values near the inverse source-detector distance, and suggest future enhancements via material, geometry, and integration-time optimizations. Overall, the work demonstrates a viable, mass-agnostic axion search modality leveraging geometry-enhanced near-field gradients and resonant NMR detection, potentially complementing other axion-coupling searches.

Abstract

Axions that couple to nuclear spins via the axial current interaction can be both produced and detected using nuclear magnetic resonance (NMR) techniques. In this scheme, nuclei driven by a real oscillating magnetic field in one device act as an axion source, which can drive NMR in a nearby spin-polarized sample interrogated with a sensitive magnetometer. We study the prospects for detecting axions through this method and identify two key characteristics that result in compelling detection sensitivity. First, the gradient of the generated axion field can be substantial, set by the inverse distance from the source. In the near zone, it reduces to the inverse of the source's geometric size. Second, because the generated axion field is produced at a known frequency, the detection medium can be tuned precisely to this frequency, enabling long interrogation times. We show that the experimental sensitivity of a pair of centimeter-scale NMR devices operating over a 15-day integration time can already surpass existing astrophysical bounds on the axion-nucleon coupling. A similar sensitivity can be achieved with 10 centimeter-scale NMR devices with only 1 hour of integration time. These dual NMR configurations are capable of probing a wide range of axion masses, up to values comparable to the inverse distance between the source and the sensor.

Figures (5)

• Figure 1: Summary of the experimental proposal. Left: The sketch of the experiment. Right: The projection of the 95% C.L. upper limit of axion-nucleon coupling $g_{aN}$ as a function of axion mass $m_a$. The material in both the spin source and spin sensor is $^{129}\mathrm{Xe}$, with a number density $n_N=1.3\times10^{22}\,{\rm cm}^{-3}$ and a Larmor frequency $\omega_0 =0.74\mathrm{GHz}$ which corresponds to a static magnetic field of $B_0=10\,\mathrm{T}$, and we take the transverse relaxation time to be $T_2=100$ s. The source and the detector cylinders radii are varied by $R=10\,$cm, 5 cm, 1 cm, and 1 mm, with their heights fixed at $H=3R$. The two cylinder axes are separated by $3R$ and the pickup loop is placed at the near end of the sensor medium. The observation time for the solid lines is $T=1\,\mathrm{yr}$, while the dashed lines use the same setups as the solid lines in the same colors, but run for different observation times. The astrophysical bounds on $g_{aN}$ from neutron star (NS) cooling Buschmann:2021juv and SN 1987A Lella:2023bfb are shown in gray-shaded regions, same the those taken in Ref. Dror:2022xpi. The QCD axion parameter space is shown in the orange-shaded region.
• Figure S-1: The "spin charge" distribution in a cylindrical, uniform, spin precessing medium and the gradient of axion field it produces in the near zone.
• Figure S-2: The amplitude of the driving force as a function of the height-to-radius ratio.
• Figure S-3: Amplitude of $B_x$ and $M_x$ along $x$-axis for the two-cylinder setups in the main text (height-to-radius ratio $H/R=3$, near end of the sensor at $\mathbf{x}_0=(2R,0,0)$). Left: $k_0R=0$, which corresponds to the near-field limit of the produced axion field. Right: $k_0R=1$ in the $m_a\geq\omega_0$ regime. The gray dashed lines represent the boundary of the sensor medium, outside which $M_x$ is zero. The curves are normalized with the amplitude of $M_x$ at the near end $\mathbf{x}_0$.
• Figure S-4: Left: $|\mathcal{G}(\mathbf{x}_0)|$ as a function of $k_0R=\sqrt{\omega_0^2-m_a^2}R$ for $m_a \leq \omega_0$. Right: $|\mathcal{G}(\mathbf{x}_0)|$ as a function of $\kappa_0R=\sqrt{m_a^2-\omega_0^2}R$ for $m_a > \omega_0$. In both plots, ${\mathbf{x}_0} = (2R,0,0)$ and the height-to radius ratio of the cylindrical spin source is $H/R=3$.