Search for Higgs boson decays into a pair of pseudoscalar particles in the γγτ_hadτ_had final state using pp collisions at √<i>s</i>=13 TeV with the ATLAS detector

A bstract A search for exotic decays of the 125 GeV Higgs boson into a pair of new spin-0 particles, H → aa , where one decays into a photon pair and the other into a τ -lepton pair, is presented. Hadronic decays of the τ -leptons 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 $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> = 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, $$ \mathcal{B} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>B</mml:mi> </mml:math> ( H → aa → γγττ ), ranging from 0.2% to 2%, depending on the a -boson mass hypothesis.