On axion-mediated macroscopic forces again

In a supersymmetric unified theory or in a generic model where a large neutron electric dipole moment dn is expected, close to the present bound, we estimate the relation gann = 10−21±1(dn10−25e·cm) (1 010 GeV/fa) between dn itself, the scalar coupling gann to nucleons of the axion, assumed to exist, and the breaking scale, fa, 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 fa.