Reconstruction and identification of boosted di-$\tau$ systems in a search for Higgs boson pairs using 13 TeV proton-proton collision data in ATLAS

In this note, a novel technique, referred to as the di-$\tau$ tagger, is designed to reconstruct and identify hadronically decaying $\tau^+\tau^-$ pairs with a large Lorentz boost in the ATLAS experiment at the Large Hadron Collider. A benchmark di-$\tau$ tagging selection is employed in the search for resonant Higgs boson pair production, where one Higgs boson decays into a boosted $b\bar{b}$ pair and the other into a boosted $\tau^+\tau^-$ pair, with two hadronically decaying $\tau$-leptons in the final state. The efficiency of the di-$\tau$ tagger is measured for this benchmark using 139 fb$^{-1}$ of proton-proton collision data recorded at a centre-of-mass energy of 13 TeV, and the background with quark- or gluon-initiated jets misidentified as di-$\tau$ objects is estimated. The search for a heavy, narrow-width, scalar resonance produced via gluon-gluon fusion and decaying into two Higgs bosons is carried out in the mass range 1-3 TeV using the same dataset. No deviations from the Standard Model predictions are observed and 95% confidence-level exclusion limits are set on this model.