# Sublunar-Mass Primordial Black Holes from Closed Axion Domain Walls

## TL;DR

This paper proposes a model where sublunar-mass primordial black holes originate from closed axion domain walls near the QCD scale, linking axion parameters to PBH abundance and dark matter contributions.

## Contribution

It introduces a novel formation mechanism for PBHs from closed axion domain walls and constrains axion parameters based on PBH observational limits.

## Key findings

- PBHs in the mass range $10^{20}$-$10^{22}$ g can account for up to 1% of dark matter.
- The model favors axion mass around the meV scale ($f_{a} extasciitilde 10^{9}$ GeV).
- PBH abundance is sensitive to the formation efficiency of closed axion domain walls.

## Abstract

We study the formation of primordial black holes (PBHs) from the collapse of closed domain walls (DWs) which naturally arise in QCD axion models near the QCD scale together with the main string-wall network. The size distribution of the closed DWs is determined by percolation theory, from which we further obtain PBH mass distribution and abundance. Various observational constraints on PBH abundance in turn also constrain axion parameters. Our model prefers axion mass around the meV scale ($f_{a}\sim 10^{9}$ GeV). The corresponding PBHs are in the sublunar-mass window $10^{20}$-$10^{22}$ g (i.e., $10^{-13}$-$10^{-11}M_{\odot}$), one of few mass windows still available for PBHs contributing significantly to dark matter (DM). In our model, PBH abundance could reach $\sim1\%$ or even more of DM, sensitive to the formation efficiency of closed axion DWs.