Small instantons and the strong CP problem in composite Higgs models

We show that QCD instantons can generate large effects at small length scales in the ultraviolet in standard composite Higgs models that utilize 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 realize partial compositeness.

Automated summary

In standard composite Higgs models, QCD instantons can have significant effects at small length scales in the ultraviolet, which may aid in solving the strong CP problem. Even without axions, the contribution of instantons alone can generate the up quark mass entirely, thus achieving a massless up quark solution.