Search for short- and long-lived axion-like particles in H → aa → 4γ decays with the ATLAS experiment at the LHC

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) _μ discrepancy. This analysis covers an ALP mass range from 100 to 62 GeV and ALP-photon couplings in the range 10^-7 TeV^-1 < C _aγγ/ Λ < 1 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 x 10^-2 , depending on the hypothesized ALP mass and ALP-photon coupling strength.