Search for decays of the Higgs boson into a pair of pseudoscalar particles decaying into $b\bar{b}{\tau }^{+}{\tau }^{-}$ using $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

This paper presents a search for exotic decays of the Higgs boson into a pair of new pseudoscalar particles, $H\to aa$, where one pseudoscalar decays into a $b$-quark pair and the other decays into a $\tau$-lepton pair, in the mass range $12\le m_a\le 60$ GeV. The analysis uses $pp$ collision data at $\sqrt{s}=13$ TeV collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 ${fb}^{-1}$. No significant excess above the Standard Model (SM) prediction is observed. Assuming the SM Higgs boson production cross-section, the search sets upper limits at 95% confidence level on the branching ratio of Higgs bosons decaying into $b\bar{b}{\tau }^{+}{\tau }^{-}$, $\mathcal{B}(H\to aa\to b\bar{b}{\tau }^{+}{\tau }^{-})$, between 2.2% and 3.9% depending on the pseudoscalar mass.