Diphotons, New Vacuum Angles, and Strong CP

TL;DR

This paper explores the implications of a nonzero topological angle in new strong dynamics models that could explain a potential 750 GeV diphoton resonance, linking it to the strong CP problem and vacuum structure.

Contribution

It analyzes the role of the topological angle ig in hypercolor models and its effects on phenomenology and the strong CP problem, providing new theoretical insights.

Findings

Nonzero ig can cause large threshold corrections to the QCD vacuum angle 

Small ig values may support UV solutions to the strong CP problem

ig influences the phenomenology of new strong sector resonances

Abstract

The Standard Model contains a well-understood, natural, spin-0 diphoton resonance: the $\pi^0$. Numerous studies have pointed out that the hint of a new diphoton resonance at 750 GeV could be a pion analog, identified with the pseudo-Nambu-Goldstone boson of a chiral symmetry spontaneously broken by new strong dynamics at the TeV scale. These "hypercolor" models are generically expected to violate parity through a topological angle $\tilde\theta$. We discuss the physics of $\tilde\theta$ and its impact on the phenomenology of the new sector. We also describe some of the theoretical implications of a nonzero $\tilde\theta$. In particular, $\tilde\theta$ can generate an ${\cal O}(1)$ threshold correction to the QCD vacuum angle $\theta$ near the TeV scale, sharply constraining ultraviolet solutions to the strong CP problem. Alternatively, finding that $\tilde\theta$ is small may be interpreted as evidence in favor of UV solutions to strong CP, particularly those based on spontaneously broken P or CP symmetries.