Axion field and the quark nugget's formation at the QCD phase transition

TL;DR

This paper proposes a dark matter model where axion-induced topological defects during the QCD phase transition form quark nuggets, explaining the observed dark and visible matter densities without relying on traditional baryogenesis.

Contribution

It introduces a novel scenario where dark matter consists of baryon-charged nuggets formed via topological defects during the QCD phase transition, independent of axion mass.

Findings

Dark matter density is naturally explained by QCD-originated nuggets.

The model predicts a baryon-to-entropy ratio of about 10^{-10}.

The scenario is insensitive to the axion mass within the allowed window.

Abstract

We study a testable dark matter (DM) model outside of the standard WIMP paradigm in which the observed ratio $\Omega_{\rm dark} \simeq \Omega_{\rm visible}$ for visible and dark matter densities finds its natural explanation as a result of their common QCD origin when both types of matter (DM and visible) are formed at the QCD phase transition and both are proportional to $\Lambda_{\rm QCD}$. Instead of conventional "baryogenesis" mechanism we advocate a paradigm when the "baryogenesis" is actually a charge separation process which always occur in the presence of the $\cal{CP}$ odd axion field $a(x)$. In this scenario the global baryon number of the Universe remains zero, while the unobserved anti-baryon charge is hidden in form of heavy nuggets, similar to Witten's strangelets and compromise the DM of the Universe. We argue that the nuggets will be inevitably produced during the QCD phase transition as a result of Kibble-Zurek mechanism on formation of the topological defects during a phase transition. Relevant topological defects in our scenario are the closed bubbles made of the $N_{\rm DW}=1$ axion domain walls. These bubbles, in general, accrete the baryon (or anti baryon) charge, which eventually result in formation of the nuggets and anti-nuggets carrying a huge baryon (anti-baryon) charge. The main consequence of the model, $\Omega_{\rm dark} \approx \Omega_{\rm visible}$ is insensitive to the axion mass which may assume any value within the observationally allowed window $10^{-6} {\rm eV} \lesssim m_a \lesssim 10^{-3}{\rm eV}$. We also estimate the baryon to entropy ratio $\eta\equiv {n_B}/{n_{\gamma}}\sim 10^{-10}$ within this scenario. Finally, we comment on implications of these results to the axion search experiments, including microwave cavity and the Orpheus experiments.