  Presented is the search for anomalous Higgs boson decays into two axion-like
particles (ALPs) using the full Run 2 data set of 140 fb$^{-1}$ of
proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the
ATLAS experiment. The ALPs are assumed to decay into two photons, providing
sensitivity to recently proposed models that could explain the $(g-2)_\mu$
discrepancy. This analysis covers an ALP mass range from 100 MeV to 62 GeV and
ALP-photon couplings in the range $10^{-5}\,
\text{TeV}^{-1}<C_{a\gamma\gamma}/\Lambda<1\, \text{TeV}^{-1}$, and therefore
includes signatures with significantly displaced vertices and highly collinear
photons. No significant excess of events above the Standard Model background is
observed. Upper limits at 95% confidence level are placed on the branching
ratio of the Higgs boson to two ALPs in the four-photon final state, and are in
the range of $ 10^{-5}$ to $3\times 10^{-2}$, depending on the hypothesized ALP
mass and ALP-photon coupling strength.
