Search for short- and long-lived axion-like particles in H→ a a → 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⁻¹ 140 fb - 1 of proton-proton collisions at a centre-of-mass energy of 13\,TeV 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 ( g - 2 ) μ discrepancy. This analysis covers an ALP mass range from 100 to 62\,GeV 62 GeV and ALP-photon couplings in the range 10⁻⁷\,TeV⁻¹<Caγ γ/Λ <1 \,TeV⁻¹ 10 - 7 TeV - 1 < C a γ γ / 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 10 - 5 to 3× 10⁻² 3 × 10 - 2 , depending on the hypothesized ALP mass and ALP-photon coupling strength.