Dark QCD perspective inspired by strong CP problem at QCD scale

TL;DR

This paper proposes a dark QCD-inspired composite axion model that addresses the strong CP problem, predicts new particles testable at colliders, and explores cosmological implications including dark matter candidates.

Contribution

It introduces a novel dark QCD-based composite axion framework compatible with baryogenesis and predicts testable flavor-changing processes and collider signatures.

Findings

Composite axion mimics QCD pion and can be flavorful.

Ultralight ALP acts as dark matter candidate.

Dark meson spectra constrained, accessible at colliders.

Abstract

We discuss a QCD-scale composite axion model arising from dark QCD coupled to QCD. The presently proposed scenario not only solves the strong CP problem, but also is compatible with the preheating setup for the QCD baryogenesis. The composite axion is phenomenologically required to mimic the QCD pion, but can generically be flavorful, which could be testable via the induced flavor changing processes at experiments. Another axionlike particle (ALP) is predicted to achieve the axion relaxation mechanism, which can phenomenologically act as the conventional QCD axion. This ALP can be ultralight, having the mass less than 1 eV, to be a dark matter candidate. The QCD $\times$ dark QCD symmetry structure constrains dark QCD meson spectra, so that the dark $\eta'$-like meson would only be accessible at the collider experiments. Still, the Belle II and Electron ion collider experiments can have a high enough sensitivity to probe the dark $\eta'$-like meson in the diphoton channel, which dominantly arises from the mixing with the QCD $\eta'$ and the pionic composite axion. We also briefly address nontrivial cosmological aspects, such as those related to the dark-chiral phase transition, the dark matter production, and an ultraviolet completion related to the ultralight ALP.