Search for Higgs boson decays into a pair of pseudoscalar particles in the $\gamma \gamma {\tau }_{\text{had}}{\tau }_{\text{had}}$ final state using $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

Abstract

A search for exotic decays of the 125 GeV Higgs boson into a pair of new spin-0 particles, $H\to aa$, where one decays into a photon pair and the other into a $\tau$-lepton pair, is presented. Hadronic decays of the $\tau$-leptons are considered and reconstructed using a dedicated tagger for collimated $\tau$-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 $\gamma \gamma \tau \tau$ final state, $\mathcal{B}(H\to aa\to \gamma \gamma \tau \tau )$, ranging from 0.2$\%$ to 2$\%$, depending on the $a$-boson mass hypothesis.

38 pages in total, author list starting page 21, 5 figures, 1 tables, published in JHEP. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HMBS-2024-39/

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