On the Sensitivity of Spin-Precession Axion Experiments

TL;DR

This paper analyzes spin-precession experiments for axion detection, revealing that their sensitivity can be significantly higher than previously thought, especially for certain axion masses, due to ongoing signal growth beyond the axion coherence time.

Contribution

It demonstrates that spin-precession experiments can achieve up to 100 times greater sensitivity for axion detection than earlier estimates, especially when the transverse spin-relaxation time dominates.

Findings

Signal continues to grow beyond axion coherence time.

Sensitivity can be improved by up to a factor of 100 at 100 neV.

Results apply to both electric and magnetic dipole operators.

Abstract

A leading direction in the hunt for axion dark matter is to search for its influence on nuclear spins. The detection scheme involves polarizing a sample of nuclei within a strong static magnetic field and then looking for a spin precession induced by the oscillating axion field. We study the axion signal and background contributions that arise in such experiments (a prominent example being CASPEr), finding key differences with the existing literature. Most importantly, in the limit where the transverse spin-relaxation time of the material is the largest timescale of the problem, we show that the induced signal continues to grow even beyond the coherence time of the axion field. As a result, we find that spin-precession instruments are much more sensitive than what has been previously estimated in a sizable range of axion masses, with sensitivity improvement of up to a factor of 100 at an axion mass of 100 neV using a Xenon-129 sample. This improves the detection prospects for the QCD axion, and we estimate the experimental requirements to reach this motivated target. Our results apply to both the axion electric and magnetic dipole moment operators.