Search for Higgs boson decays into a pair of pseudoscalar particles in the $γγτ_{\text{had}}τ_{\text{had}}$ 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 $τ$-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 $\sqrt{s}=13$ 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, $\mathcal{B}(H\to aa\to γγττ)$, ranging from 0.2$\% $ to 2$\% $, depending on the $a$-boson mass hypothesis.