On axion-mediated macroscopic forces again

In a supersymmetric unified theory or in a generic model where a large neutron electric dipole moment $d_N$ is expected, close to the present bound, we estimate the relation $g_{aNN}=10^{-21\pm1}(d_N/10^{-25}\ecm)(10^{10}\GeV/f_a)$ between $d_N$ itself, the scalar coupling $g_{aNN}$ to nucleons of the axion, assumed to exist, and the breaking scale, $f_a$, of the Peccei-Quinn symmetry. Newly developing techniques to search for sub-cm macroscopic forces might reveal a signal due to axion exchange at least in a favorable range of $f_a$.