CP-Violating Axions: A Theory Review

Abstract: We review the physics case for CP-violating axions. In the first part, we focus on the Quantum Chromodynamics (QCD) axion and argue that new sources of CP violation beyond QCD misalign the axion solution to the strong CP problem and can manifest themselves via a tiny scalar axion-nucleon component. We hence highlight recent advancements in calculating this scalar axion-nucleon coupling, a parameter that could be probed via axion-mediated force experiments. In the second part, we focus on axion-like particle (ALP) interactions entailing the most general sources of CP violation. After classifying the full set of CP-violating Jarlskog invariants, we report on recent calculations of ALP contributions to permanent electric dipole moments. We finally speculate on possible ultraviolet completions of the CP-violating ALP.