Sensitivity of Spin-Precession Axion Experiments.

We study the signal and background that arise in nuclear magnetic resonance searches for axion dark matter, finding key differences with the existing literature. 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 using a ^{129}Xe 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.