Search for short- and long-lived axion-like particles in $H\rightarrow a a \rightarrow 4\gamma$ decays with the ATLAS experiment at the LHC

A search for anomalous Higgs boson decays into two axion-like particles (ALPs) is presented, using the full Run-2 data set recorded by the ATLAS experiment. The ALPs are assumed to decay into two photons, providing sensitivity to various recently proposed models that could explain the famous $(g-2)_\mu$ discrepancy. This analysis covers a mass range of ALPs between 100 MeV and 62 GeV, as well as ALP-photon couplings of $10^{-5}\,\text{TeV}^{-1}<C_{a\gamma\gamma}/\Lambda<1 \,\text{TeV}^{-1}$, 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 the four photon final state, and are in the range of $ 10^{-5}$ to $3\times 10^{-2}$, depending on the signal mass and ALP-photon coupling strength hypothesis.