Measuring the Axion's CP-violating Couplings

Axions, which were first proposed in 1977 to explain the strong CP problem in QCD, have also became well motivated candidates for dark matter whose discovery would have far-reaching consequences. We describe an experiment to measure the CP-violating axion coupling constant $g_p g_s$ with both the neutron and electron using a $^3$He-K atomic co-magnetometer and a 200 kg source mass. It will enable us to surpass, for the first time in a laboratory experiment, the current tightest constraints on $g^N_p g^N_s$ derived by Raffelt\footnote{G. Raffelt, Phys. Rev. D \textbf{86}, 015001 (2012).} from astrophysical observations by an order of magnitude. With an expected sensitivity of $g^N_p g^N_s \sim 6 \times 10^{-33}$, we would also exceed the current tightest laboratory constraints on $g^N_p g^N_s$ at large distances from Youdin \emph{et. al}\footnote{A. N. Youdin, D. Krause, Jr., K. Jagannathan, and L. R. Hunter, Phys. Rev. Lett. \textbf{77}, 11 (1996).} by 3 orders of magnitude.