Small instantons and the strong CP problem in composite Higgs models

TL;DR

This paper explores how QCD instantons at small scales affect composite Higgs models and proposes novel solutions to the strong CP problem, including a heavy axion and a massless up-quark scenario, without extra fields.

Contribution

It demonstrates that instantons can generate large effects in UV completions of composite Higgs models, offering new solutions to the strong CP problem without additional fields.

Findings

Instantons can produce large effects at small scales in composite Higgs models.

A heavy axion can arise in models with an axion present.

The massless up-quark solution can be revived without extra fields.

Abstract

We show that QCD instantons can generate large effects at small length scales in the ultraviolet in standard composite Higgs models that utilise partial compositeness. This has important implications for possible solutions of the strong CP problem in these models. First we show that in the simplest known UV completions of composite Higgs models, if an axion is also present, it can have a mass much larger than the usual QCD axion. Even more remarkable is the case where there are no axions, but the strong CP problem can be solved by generating the up quark mass entirely from the contribution of instantons thus reviving the massless up-quark solution for these models. In both cases no additional field content is required apart from what is required to realise partial compositeness.