Limits on a CP-violating scalar axion-nucleon interaction

Axions or similar hypothetical pseudoscalar bosons may have a small CP-violating scalar Yukawa interaction g(s)(N) with nucleons, causing macroscopic monopole-dipole forces. Torsion-balance experiments constrain g(p)(e)g(s)(N), whereas g(p)(N)g(s)(N) is constrained by the depolarization rate of ultra-cold neutrons or spin-polarized nuclei. However, the pseudoscalar couplings g(p)(e) and g(p)(N) are strongly constrained by stellar energy-loss arguments and g(s)(N) by searches for anomalous monopole-monopole forces, together providing the most restrictive limits on g(p)(e)g(s)(N) and g(p)(N)g(s)(N). The laboratory limits on g(s)(N) are currently the most restrictive constraints on CP-violating axion interactions.