CP-violating axion interactions in effective field theory

TL;DR

This paper uses effective field theory to analyze CP-violating axion interactions with Standard Model particles, exploring phenomenology and experimental constraints, and highlighting potential improvements from upcoming experiments.

Contribution

It systematically computes CP-odd axion couplings to leptons and hadrons within an effective field theory framework, incorporating beyond-Standard-Model CP violation sources.

Findings

Electric dipole moment limits are very restrictive.

ARIADNE experiment could improve constraints on axion CP violation.

Phenomenology of CP-odd axion couplings is thoroughly analyzed.

Abstract

Axions are introduced to explain the observed smallness of the $\bar \theta $ term of QCD. Standard Model extensions typically contain new sources of CP violation, for instance to account for the baryon asymmetry of the universe. In the presence of additional CP-violating sources a Peccei-Quinn mechanism does not remove all CP violation, leading to CP-odd interactions among axions and Standard Model fields. In this work, we use effective field theory to parametrize generic sources of beyond-the-Standard-Model CP violation. We systematically compute the resulting CP-odd couplings of axions to leptons and hadrons by using chiral perturbation theory. We discuss in detail the phenomenology of the CP-odd axion couplings and compare limits from axion searches, such as fifth force and monopole-dipole searches and astrophysics, to direct limits on the CP-violating operators from electric dipole moment experiments. While limits from electric dipole moment searches are tight, the proposed ARIADNE experiment can potentially improve the existing constraints in a window of axion masses.