Dark Matter Nuclear Magnetic Resonance is Sensitive to Dark Photons and the Axion-Photon Coupling

TL;DR

This paper shows that nuclear magnetic resonance experiments can detect dark matter candidates like dark photons and axions by observing their effects on nuclear spins, offering a new way to explore dark matter properties.

Contribution

It demonstrates the sensitivity of NMR-based searches to dark photons and axion-photon coupling, highlighting their potential to detect these particles at predicted coupling strengths.

Findings

Detectable and distinguishable signals for dark photons and axions.

Sensitivity to axion-nucleon coupling at QCD axion levels.

Potential to probe kinetic mixing parameters around 3×10⁻¹⁶.

Abstract

We demonstrate that nuclear magnetic resonance based searches for dark matter (DM) have intrinsic and powerful sensitivity to dark photons and the axion-photon coupling. The reason is conceptually straightforward. An instrument such as CASPEr-Gradient begins with a large sample of nuclear spins polarised in a background magnetic field. In the presence of axion DM coupled to nucleons, the spin ensemble feels an effective magnetic field $\mathbf{B} \propto \nabla a$ that tilts the spins, generating a potentially observable precession. If the magnetic field is real rather than effective, the system responds identically. A real field can be generated by a kinetically mixed dark photon within the shielded region the sample is placed or an axion coupled to photons through its interaction with the background magnetic field. We show that all three signals are detectable and distinguishable. If CASPEr-Gradient were to reach the QCD axion prediction of the axion-nucleon coupling, it would simultaneously be sensitive to kinetic mixings of $\epsilon \simeq 3 \times 10^{-16}$ and axion-photon couplings of $g_{a\gamma\gamma} \simeq 2 \times 10^{-16}\,{\rm GeV}^{-1}$ for $m \simeq 1\,\mu{\rm eV}$.