Cornering the axion with CP-violating interactions

Understand

Besides CP-preserving interactions, axions and axion-like particles may also have small CP-violating scalar Yukawa interactions with nucleons and electrons.

• Any such interaction will generate macroscopic monopole-dipole forces which can be searched for experimentally.
• When the best experimental limits on scalar interactions are combined with stellar energy-loss arguments constraining pseudoscalar interactions, strong bounds can be set on CP-violating axion couplings which almost intersect the expectation for QCD models.
• Over the years, both astrophysical and laboratory tests have improved.