Baryoid Dark Matter from $\mathbb{Z}_N$ Domain Walls: The $(N-1):1$ origin of the dark matter-baryon coincidence

TL;DR

This paper explains the dark matter-baryon coincidence through collapsing $ ext{Z}_N$ domain walls forming baryoids, a novel dark matter candidate, with a natural 6:1 energy-density ratio for $N=7$.

Contribution

It introduces baryoids formed from $ ext{Z}_N$ domain walls as a new dark matter candidate and analyzes their properties and phenomenological implications.

Findings

Baryoids have asteroid-scale mass and nuclear-scale energy density.

The collapse of domain walls yields a baryon-number ratio of $(N-1):1$.

The resulting dark matter-to-baryon energy-density ratio is close to 6:1 for $N=7$.

Abstract

We propose an explanation for the dark matter-baryon coincidence based on collapsing $\mathbb{Z}_N$ domain walls, which form a novel compact baryonic state: the baryoid. A baryoid has an asteroid-scale mass and up-to-nuclear-scale energy density, and can serve as a dark matter candidate. Starting from equal baryon numbers in the domains formed in the early universe, the collapse of the domain walls after the QCD phase transition leads to a baryon-number ratio of $(N-1):1$ between the false- and true-vacuum domains. Since baryons are slightly lighter in the false-vacuum domains than in the true-vacuum domain, the resulting dark matter-to-baryon energy-density ratio is naturally close to, but slightly smaller than, $(N-1):1$, or $6:1$ for $N=7$. We calculate the domain-wall dynamics and the efficiency of baryon-number trapping, derive the resulting baryoid properties, and discuss a broad set of phenomenological probes.