TL;DR
This paper reviews how CP-violating axion couplings can serve as a low-energy probe for high-energy sources of CP violation, emphasizing the unique role of the QCD axion in this context.
Contribution
It clarifies the physical origin of CP-violating axion couplings and highlights the QCD axion's role as a portal to high-energy CP violation sources.
Findings
CP-violating axion couplings originate from physics beyond QCD.
QCD axion acts as a low-energy portal to high-energy CP violation.
Experimental probes via axion-mediated force experiments are feasible.
Abstract
While the axion was originally introduced to wash out CP violation from strong interactions, new sources of CP violation beyond QCD might manifest themselves via a tiny scalar axion-nucleon component. The latter can be experimentally probed in axion-mediated force experiments, as suggested long ago by J.E. Moody and F. Wilczek. In the present note, I review the physical origin of CP-violating axion couplings and point out the special role of the QCD axion as a low-energy portal to high-energy sources of CP violation.
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