Strong CP from a Hidden Chiral Condensate

TL;DR

This paper proposes a novel solution to the strong CP problem using a hidden sector with a chiral condensate, reducing fine-tuning issues and linking dark matter candidates to confining gauge theories.

Contribution

It introduces a model with a hidden confining sector that naturally breaks CP spontaneously, alleviating fine-tuning and connecting dark matter with strong CP problem solutions.

Findings

Dark pions as natural dark matter candidates with correct relic abundance

Reduced fine-tuning in models employing hidden sector chiral condensates

Connection between strong CP problem and confining gauge theories

Abstract

Models which solve the strong CP problem by employing discrete spacetime symmetries generically suffer fine-tuning and quality problems. We demonstrate that these issues are greatly ameliorated when the only source of spontaneous CP breaking is from the chiral condensate of a strongly coupled hidden sector. This is shown explicitly in a model with the SM extended by a vector-like quark family and a complex scalar portal to QCD-like dark sector with $N_f$ families of dark fermions that confines at a high scale. The dark pions of the hidden sector are natural dark matter candidates, with the correct relic abundance obtained via freeze-in. These "confining" Nelson-Barr solutions connect phenomenological questions regarding the strong CP problem to recent developments in the understanding of confining gauge theories, and present ample room for further model building.