New Experiment Sets Limits on Axion-like Dark Matter
A physics experiment using solid-state nuclear magnetic resonance and a SQUID magnetometer has searched for ultralight axion-like dark matter, establishing
A new physics experiment has searched for ultralight axion-like dark matter and set new limits on how strongly it could interact with ordinary matter. The Cosmic Axion Spin Precession Experiment-electric (CASPEr-e) probed specific mass ranges using solid-state nuclear magnetic resonance (NMR) techniques.
The experiment, detailed in a paper on arXiv, targeted axion-like particles with masses between 19.5 and 20.5 neV and between 21.5 and 22 neV. Researchers performed NMR on ensembles of lead-207 ($^{207}$Pb) nuclei embedded within a polarized ferroelectric crystal. The core idea is that a background field of axion-like dark matter would exert an oscillating torque on these nuclear spins via a specific coupling known as the electric dipole moment coupling, denoted as $g_{d}$.
The Experimental Method
To detect this subtle signal, the team used a superconducting quantum interference device (SQUID) magnetometer inductively coupled to the crystal sample through a broadband circuit. They swept an external magnetic field applied to the sample, which allowed them to tune into and detect the nuclear magnetic resonance of the $^{207}$Pb spins at specific frequencies. These frequencies correspond to the Compton frequencies of the hypothetical axions.
The experiment was calibrated using pulsed magnetic resonance measurements. This calibration covered two distinct radio-frequency bands: 4.6 to 5.0 MHz and 5.2 to 5.3 MHz. These bands directly match the mass ranges being investigated.
Results and New Constraints
After analyzing their data, the researchers found no conclusive signal for axion-like dark matter. Consequently, they were able to establish upper bounds on the strength of the electric dipole moment coupling $g_{d}$. With 95% confidence, the experiment rules out couplings larger than $4 times 10^{-4}$ GeV$^{-2}$ in the searched frequency bands.
The paper states, "We establish the upper bounds $|g_{d}| < 4times 10^{-4}$ GeV$^{-2}$ with 95% confidence in these frequency bands." This result represents a new constraint in the ongoing hunt for these elusive particles.
Significance for Dark Matter Searches
This work demonstrates a viable method for detecting axion-like dark matter in the nanoelectron-volt mass range. A key technical achievement was the successful detection and calibration of low-field $^{207}$Pb nuclear magnetic resonance. The use of a broadband SQUID magnetometry setup is important for scanning across the targeted frequencies with high sensitivity.
The experiment specifically probes the axion-nuclear electric dipole moment coupling, a different interaction channel than those searched by many other axion experiments, which often look for couplings to photons. The researchers note that their approach allows for sensitivity to axion-like dark matter in a mass range that is challenging to access with other techniques.
Future iterations of the experiment could improve sensitivity by increasing measurement time or optimizing the sample and detection apparatus. The search for dark matter continues to push the boundaries of experimental physics.
