Indirect Signs of the Peccei-Quinn Mechanism

TL;DR

This paper investigates how certain higher-dimensional operators in the Standard Model effective field theory can induce observable CP-violation signals, providing indirect evidence for the Peccei-Quinn mechanism solving the strong CP problem.

Contribution

It analyzes the quadratic divergences in the QCD vacuum angle caused by dimension-6 SMEFT operators and explores their potential detection in current and future experiments.

Findings

Quadratic divergences in $ heta$ can be linked to dimension-6 SMEFT operators.

Electric dipole moment experiments can probe these divergences.

Future colliders may detect signals indicating the Peccei-Quinn mechanism.

Abstract

In the Standard Model, the renormalization of the QCD vacuum angle $\theta$ is extremely tiny, and small $\theta$ is technically natural. In the general Standard Model effective field theory (SMEFT), however, $\Delta\theta$ is quadratically divergent, reflecting the fact that new sources of hadronic CP-violation typically produce $\mathcal O(1)$ threshold corrections to $\theta$. The observation of such CP-violating interactions would therefore be in tension with solutions to the strong CP problem in which $\theta=0$ is an ultraviolet boundary condition, pointing to the Peccei-Quinn mechanism as the explanation for why $\theta$ is small in the infrared. We study the quadratic divergences in $\theta$ arising from dimension-6 SMEFT operators and discuss the discovery prospects for these operators at electric dipole moment experiments, the LHC, and future proton-proton colliders.