GeV-scale QCD Axion

TL;DR

This paper explores a GeV-scale QCD axion arising from a Peccei--Quinn symmetry broken below the QCD scale, which can evade many experimental constraints and be detectable at colliders.

Contribution

It proposes a novel scenario where the QCD axion is above GeV, broken below the QCD scale, and discusses its unique phenomenological signatures and constraints.

Findings

QCD axion can be above GeV and align with observed $ ext{eta}$ resonances.

Flavor-changing neutral currents are surprisingly small.

Collider signatures include heavy quark pairs and effects on Higgs and Z boson decays.

Abstract

In order to solve the strong CP problem, we study the possibility that the Peccei--Quinn symmetry is broken {\it below}\/ the QCD scale. We find that a QCD axion can be above GeV, and may be among the observed $\eta$ resonances. It is immune to quantum gravity corrections. The only fermion that has a $U(1)$ Peccei--Quinn charge is the right-handed up quark. Flavor-changing neutral currents are surprisingly small. All accelerator and astrophysical limits can be evaded. The most significant constraint is the mass splitting between $\pi^\pm$ and $\pi^0$. In a UV completed model, LHC can look for a heavy quark pair ${\cal D} \overline{\cal D}$ followed by the decay ${\cal D} \rightarrow W+u$ or a single production $q u \rightarrow q\, {\cal U}$ followed by ${\cal U} \rightarrow u Z, uh$. There can be an $O(1)$ contribution $h \rightarrow u\bar{u}\varphi$ in the measurement of $h \rightarrow gg$ or permille effects on the hadronic $Z$ width at a Higgs factory.