Search for resonant and non-resonant Higgs boson pair production in the $b\bar b\tau^+\tau^-$ decay channel using 13 TeV $pp$ collision data from the ATLAS detector

A search for Higgs boson pair production in events with two $b$-jets and two $\tau$-leptons is presented, using a $pp$ collision data set with an integrated luminosity of $139~\text{fb}^{-1}$ collected at $\sqrt{s}=13~\text{TeV}$ by the ATLAS experiment at the LHC. Higgs boson pairs produced non-resonantly or in the decay of a narrow-width scalar resonance in the mass range 251 to 1600 GeV are targeted. Events in which at least one $\tau$-lepton decays hadronically are considered, and multivariate discriminants are used to extract the signals. No significant excess of events above the expected background is observed in the non-resonant search. The largest excess in the resonant search is observed at a resonance mass of 1 TeV, with a local (global) significance of $3.0~\sigma$ ($2.0^{+0.4}_{-0.2}~\sigma$). Observed (expected) upper limits are set at the 95% confidence-level on the non-resonant Higgs boson pair production cross-section of 4.7 (3.9) times the Standard Model prediction, assuming Standard Model kinematics, and on the resonant Higgs boson pair production cross-section between 23 and 920 fb (12 and 840 fb), depending on the heavy resonance mass.