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 τ-lepton pair, in the mass range 12 $\leq m_{a} \leq$ 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.