Search for short- and long-lived axion-like particles in H -> a a -> 4 gamma 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)_mu discrepancy. This analysis covers an ALP mass range from 100 to 62 GeV and ALP-photon couplings in the range 10^-7 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 fourphoton final state, and are in the range of 10−5 to 3 × 10−2, depending on the hypothesized ALP mass and ALP-photon coupling strength.