Cornering the axion with CP-violating interactions

TL;DR

This paper reviews and updates experimental and astrophysical constraints on CP-violating axion interactions, highlighting recent improvements and proposing new experimental avenues to explore unexplored parameter space regions.

Contribution

It provides a comprehensive, updated compilation of bounds on CP-violating axion couplings and advocates for novel experiments independent of astrophysical assumptions.

Findings

Constraints improved by factors of 40 to 130 in some regions.

Strong bounds nearly reach QCD model expectations.

Proposes new experimental searches in less-explored parameter space.

Abstract

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. We provide a much-needed up-to-date compilation of these constraints, showing improvements in some regions of parameter space by factors between 40 and 130. We advocate experimental opportunities, without astrophysical or dark-matter assumptions, to track down the axion in the lesser-explored corners of its parameter space.