Kavli Affiliate: Jia Liu
| First 5 Authors: Shuailiang Ge, Jinhui Guo, Jia Liu, ,
| Summary:
We present a novel mechanism for the primordial black hole (PBH) production
within the QCD axion framework. We take the case where the Peccei-Quinn
symmetry breaks during inflation, resulting in a $N_{rm DW}=1$ string-wall
network that re-enters horizon sufficiently late. Therefore, closed axion
domain walls naturally arising in the network are sufficiently large to
collapse into PBHs. Our numerical simulation shows that $0.8%$ of the total
wall area is in the form of closed walls. Notably, this fraction is independent
of any axion parameters, as its determination is firmly grounded in the
principles of percolation theory. In addition, the relic abundance of dark
matter is accounted for by free axions from the collapse of open walls bounded
by strings. This framework yields a calculated PBH fraction of dark matter as
0.0256. The PBHs uniformly share the same mass, which spans from about
$10^{-10}$ to $10^{-1}$ solar masses, corresponding to the classical QCD axion
mass window $10^{-5}-10^{-2}$ eV and the re-entering horizon temperature
$300-1$ MeV. Intriguingly, PBHs in this mechanism can naturally account for the
gravitational-lensing events observed by the OGLE collaboration.
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