Search for decays of the Higgs boson into a pair of pseudoscalar particles decaying into $b\overline{b}{\tau }^{+}{\tau }^{-}$ using $pp$ collisions at $\sqrt{s}=13\mathrm{TeV}$ 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 $12\le m_a\le 60\mathrm{GeV}$. The analysis uses $pp$ collision data at $\sqrt{s}=13\mathrm{TeV}$ collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$. 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 $b\overline{b}{\tau }^{+}{\tau }^{-}$, $B(H\to aa\to b\overline{b}{\tau }^{+}{\tau }^{-}$), between 2.2% and 3.9% depending on the pseudoscalar mass.