Axion Comagnetometry

If dark matter is an ultralight axion-like particle, the gradient of its classical field can be detected in precision atomic experiments, including helium-potassium comagnetometers. In this talk, I will discuss a re-analysis of existing comagnetometer data with world-leading sensitivity to axions in the mass range from $10^{-17}$ to $10^{-12}$ eV that couple to neutrons. Proper interpretation of this data requires a careful treatment of the dark matter velocity distribution, which leads to stochastic fluctuations of the signal amplitude. I will introduce a complete statistical treatment for these stochastic effects, which has implications for any experiment that is sensitive to the gradient of the axion field.