PT Journal AU ATLAS Collaboration (Aad, Gea Aparisi Pozo, JA Bailey, AJ Cabrera Urban, S Castillo, FL Castillo Gimenez, V Costa, MJ Escobar, C Estrada Pastor, O Ferrer, A Fiorini, L Fullana Torregrosa, E Fuster, J Garcia, C Garcia Navarro, JE Gonzalez de la Hoz, S Gonzalvo Rodriguez, GR Guerrero Rojas, JGR Higon-Rodriguez, E Lacasta, C Lozano Bahilo, JJ Madaffari, D Mamuzic, J Marti-Garcia, S Martinez Agullo, P Mitsou, VA Moreno Llacer, M Poveda, J Rodriguez Bosca, S Ruiz-Martinez, A Salt, J Santra, A Sayago Galvan, I Soldevila, U Sanchez, J Valero, A Valls Ferrer, JA Vos, M TI Reconstruction and identification of boosted di-tau systems in a search for Higgs boson pairs using 13 TeV proton-proton collision data in ATLAS SO Journal of High Energy Physics JI J. High Energy Phys. PY 2020 BP 163 EP 47pp VL 11 IS 11 DI 10.1007/JHEP11(2020)163 LA English DE Beyond Standard Model; Hadron-Hadron scattering (experiments); Higgs physics; Tau Physics AB In this paper, a new technique for reconstructing and identifying hadronically decaying tau (+)tau (-) pairs with a large Lorentz boost, referred to as the di-tau tagger, is developed and used for the first time 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 bbbar pair and the other into a boosted tau (+)tau (-) pair, with two hadronically decaying tau -leptons in the final state. Using 139 fb(-1) of proton-proton collision data recorded at a centre-of-mass energy of 13 TeV, the efficiency of the di-tau tagger is determined and the background with quark- or gluon-initiated jets misidentified as di-tau objects is estimated. The search for a heavy, narrow, 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. ER