Title: High Quality Alternatives to the QCD Axion

Abstract:The QCD axion was proposed to solve the “strong CP problem”: the question of why we observe CP violation in the quark mixing matrix, but not in the strong interactions. An alternative solution, which requires no new light degrees of freedom, is to suppose that CP is a fundamental symmetry of the theory—so that the theta term vanishes in the UV—but one which is spontaneously broken at low energies, giving rise to the observed phase in quark mixing. In this talk, I will review this “Nelson–Barr” solution to the strong CP puzzle and explain some of the model-building challenges that come with it. I will then show how these challenges (including the “quality problem” analogous to that in axion models) can be ameliorated by invoking additional chiral gauge symmetries or by appealing to strong dynamics. Focusing on the latter, I will discuss in detail how a QCD-like hidden sector provides a natural source of spontaneous CP breaking, and how this naturally ameliorates the fine-tuning and quality issues, while also predicting new dark composite states that could serve as dark matter.

https://lbnl.zoom.us/j/94928022788?pwd=emVQWG1mTnhSbHVqekVuenk0VEVQZz09