Limits on a $CP$-violating scalar axion-nucleon interaction

Limits on a $C P$ -violating scalar axion-nucleon interaction

Georg Raffelt

Phys. Rev. D 86, 015001 – Published 5 July 2012

Abstract

Axions or similar hypothetical pseudoscalar bosons may have a small $C P$ -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 $C P$ -violating axion interactions.