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