Cosmic Axion Spin Precession Experiment (CASPEr)

TL;DR

Cosmic Axion Spin Precession Experiment (CASPEr) addresses the problem of detecting QCD axion and ALP dark matter by exploiting an oscillating nucleon electric dipole moment induced by the axion field. The main approach uses a resonant, solid-state NMR–like setup where nuclear spins precess in an applied field, yielding a measurable transverse magnetization when the ALP frequency matches the nuclear Larmor frequency, i.e., on resonance at $2 mu B_ext approx m_a c^2$, with coherence time $tau_a approx 2*pi/(m_a*v^2)$. The paper provides a concrete experimental concept with sensitivity forecasts for two phases, showing that Phase 1 can probe unexplored ALP space and Phase 2 could reach the QCD axion region for $f_a > 10^{16}$ GeV, potentially covering much of the standard QCD axion range with further improvements. The method offers robustness against systematics due to the oscillatory nature of the signal, leverages the large number of spins in a solid, and provides a scalable, complementary path to traditional cavity searches like ADMX.

Abstract

We propose an experiment to search for QCD axion and axion-like-particle (ALP) dark matter. Nuclei that are interacting with the background axion dark matter acquire time-varying CP-odd nuclear moments such as an electric dipole moment. In analogy with nuclear magnetic resonance, these moments cause precession of nuclear spins in a material sample in the presence of an electric field. Precision magnetometry can be used to search for such precession. An initial phase of this experiment could cover many orders of magnitude in ALP parameter space beyond the current astrophysical and laboratory limits. And with established techniques, the proposed experimental scheme has sensitivity to QCD axion masses m_a < 10^-9 eV, corresponding to theoretically well-motivated axion decay constants f_a > 10^16 GeV. With further improvements, this experiment could ultimately cover the entire range of masses m_a < 10^-6 eV, complementary to cavity searches.

Figures (2)

• Figure 1: Geometry of the experiment. The applied magnetic field $\vec{B}_\text{ext}$ is colinear with the sample magnetization, $\vec{M}$. The effective electric field in the crystal $\vec{E}^*$ is perpendicular to $\vec{B}_\text{ext}$. The SQUID pickup loop is arranged to measure the transverse magnetization of the sample.
• Figure 2: Estimated constraints in the ALP parameter space in the EDM coupling $g_d$ (where the nucleon EDM is $d_n = g_d a$ and $a$ is the local value of the ALP field) vs. the ALP mass ALP space. The green region is excluded by the constraints on excess cooling of supernova 1987A ALP space. The blue region is excluded by existing, static nuclear EDM searches ALP space. The QCD axion is in the purple region, whose width shows the theoretical uncertainty ALP space. The solid red and orange regions show sensitivity estimates for our phase 1 and 2 proposals, set by magnetometer noise. The red dashed line shows the limit from magnetization noise of the sample for phase 2. The ADMX region shows what region of the QCD axion has been covered (darker blue) Asztalos:2009yp or will be covered (lighter blue) ADMXwebpagesnowdarktalk. Phase 1 is a modification of current solid state static EDM techniques that is optimized to search for a time varying signal and can immediately begin probing the allowed region of ALP dark matter. To calculate limits from previous (static) EDM searches as well as our sensitivity curves, we assume the ALP is all of the dark matter.