Enhanced EDMs from Small Instantons

TL;DR

This paper demonstrates that models with small instanton dynamics solving the strong CP problem can significantly enhance sensitivity to CP violation, with potential observable effects in future electric dipole moment experiments.

Contribution

It introduces a mechanism where small instanton effects amplify CP violation sensitivity, providing new constraints on high-scale new physics.

Findings

Enhanced axion mass from small instantons at ^{10} GeV scale.

CP violation effects can be probed up to the Planck scale.

Potential detection of CP violation in future EDM experiments.

Abstract

We show that models in which the strong $CP$ problem is solved by introducing an axion field with a mass enhanced by non-QCD UV dynamics at a scale $\Lambda_{\rm SI}$ exhibit enhanced sensitivity to external sources of $CP$ violation. In the presence of higher-dimensional $CP$-odd sources at a scale $\Lambda_{\rm CP}$, the same mechanisms that enhance the axion mass also modify the axion potential, shifting the potential minimum by a factor $\propto\Lambda^2_{\rm SI}/\Lambda^2_{\rm CP}$. This phenomenon of $CP$-violation enhancement, which puts stringent constraints on the scale of new physics, is explicitly demonstrated within a broad class of "small instanton" models with $CP$-odd sources arising from the dimension-six Weinberg gluonic and four-fermion operators. We find that for heavy axion masses $\gtrsim 100$MeV, arising from new dynamics at $\Lambda_{\rm SI}\lesssim 10^{10}$GeV, $CP$ violation generated up to the Planck scale can be probed by future electric dipole moment experiments.