Search for Higgs boson decays into a pair of pseudoscalar particles in the γγτhadτhad final state using pp collisions at sqrt(s) = 13 TeV with the ATLAS detector

A search for exotic decays of the 125 GeV Higgs boson (H) into a pair of new spin-0 particles, H→aa, where one decays into a photon pair (γγ) and the other into a τ-lepton pair (ττ), is presented. Hadronic decays of the τ-leptons (τhad) are considered and reconstructed using a dedicated tagger for collimated τ-lepton pairs. The search uses 140 fb-1 of proton-proton collision data at a centre-of-mass energy of sqrt(s) =13 TeV recorded between 2015 and 2018 by the ATLAS experiment at the Large Hadron Collider. The search is performed in the mass range of the a boson between 10 GeV and 60 GeV. No significant excess of events is observed above the Standard Model background expectation. Model-independent upper limits at 95% confidence level are set on the branching ratio of the Higgs boson to the γγττ final state, B(H →aa→γγττ), ranging from 0.2% to 2%, depending on the a-boson mass hypothesis.