Limits on a CP-violating scalar axion-nucleon interaction

TL;DR

This paper reviews experimental constraints on CP-violating axion-nucleon interactions, highlighting that laboratory limits currently provide the most restrictive bounds on these hypothetical forces.

Contribution

It synthesizes existing experimental and astrophysical constraints to establish the most stringent limits on CP-violating axion-nucleon couplings.

Findings

Laboratory limits on g_s(N) are the most restrictive.

Stellar energy-loss arguments strongly constrain g_p(e) and g_p(N).

Torsion-balance experiments limit g_p(e) g_s(N).

Abstract

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.