Eur. Phys. J. C (2025) 85: 561
https://doi.org/10.1140/epjc/s10052-025-14075-3

Regular Article - Experimental Physics

Reconstruction and identification of pairs of collimated $\tau$-leptons decaying hadronically using $\sqrt{s}=13$ TeV pp collision data with the ATLAS detector

¹ Department of Physics, University of Adelaide, Adelaide, Australia
² Department of Physics, University of Alberta, Edmonton, AB, Canada
³ Department of Physics, Ankara University, Ankara, Turkey
⁴ Division of Physics, TOBB University of Economics and Technology, Ankara, Turkey
⁵ LAPP, Université Savoie Mont Blanc, CNRS/IN2P3, Annecy, France
⁶ APC, Université Paris Cité, CNRS/IN2P3, Paris, France
⁷ High Energy Physics Division, Argonne National Laboratory, Argonne, IL, USA
⁸ Department of Physics, University of Arizona, Tucson, AZ, USA
⁹ Department of Physics, University of Texas at Arlington, Arlington, TX, USA
¹⁰ Physics Department, National and Kapodistrian University of Athens, Athens, Greece
¹¹ Physics Department, National Technical University of Athens, Zografou, Greece
¹² Department of Physics, University of Texas at Austin, Austin, TX, USA
¹³ Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan
¹⁴ Institut de Física d’Altes Energies (IFAE), Barcelona Institute of Science and Technology, Barcelona, Spain
¹⁵ Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China
¹⁶ Physics Department, Tsinghua University, Beijing, China
¹⁷ Institute of Physics, University of Belgrade, Belgrade, Serbia
¹⁸ Department for Physics and Technology, University of Bergen, Bergen, Norway
¹⁹ Physics Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
²⁰ University of California, Berkeley, CA, USA
²¹ Institut für Physik, Humboldt Universität zu Berlin, Berlin, Germany
²² Albert Einstein Center for Fundamental Physics and Laboratory for High Energy Physics, University of Bern, Bern, Switzerland
²³ School of Physics and Astronomy, University of Birmingham, Birmingham, UK
²⁴ Department of Physics, Bogazici University, Istanbul, Turkey
²⁵ Department of Physics Engineering, Gaziantep University, Gaziantep, Turkey
²⁶ Department of Physics, Istanbul University, Istanbul, Turkey
²⁷ Facultad de Ciencias y Centro de Investigaciónes, Universidad Antonio Nariño, Bogotá, Colombia
²⁸ Departamento de Física, Universidad Nacional de Colombia, Bogotá, Colombia
²⁹ Dipartimento di Fisica e Astronomia A. Righi, Università di Bologna, Bologna, Italy
³⁰ INFN Sezione di Bologna, Bologna, Italy
³¹ Physikalisches Institut, Universität Bonn, Bonn, Germany
³² Department of Physics, Boston University, Boston, MA, USA
³³ Department of Physics, Brandeis University, Waltham, MA, USA
³⁴ Transilvania University of Brasov, Brasov, Romania
³⁵ Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania
³⁶ Department of Physics, Alexandru Ioan Cuza University of Iasi, Iasi, Romania
³⁷ National Institute for Research and Development of Isotopic and Molecular Technologies, Physics Department, Cluj-Napoca, Romania
³⁸ National University of Science and Technology Politechnica, Bucharest, Romania
³⁹ West University in Timisoara, Timisoara, Romania
⁴⁰ Faculty of Physics, University of Bucharest, Bucharest, Romania
⁴¹ Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia
⁴² Department of Subnuclear Physics, Institute of Experimental Physics of the Slovak Academy of Sciences, Kosice, Slovak Republic
⁴³ Physics Department, Brookhaven National Laboratory, Upton, NY, USA
⁴⁴ Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Departamento de Física, y CONICET, Instituto de Física de Buenos Aires (IFIBA), Buenos Aires, Argentina
⁴⁵ California State University, Long Beach, CA, USA
⁴⁶ Cavendish Laboratory, University of Cambridge, Cambridge, UK
⁴⁷ Department of Physics, University of Cape Town, Cape Town, South Africa
⁴⁸ iThemba Labs, Cape Town, Western Cape, South Africa
⁴⁹ Department of Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa
⁵⁰ National Institute of Physics, University of the Philippines Diliman (Philippines), Quezon City, Philippines
⁵¹ University of South Africa, Department of Physics, Pretoria, South Africa
⁵² University of Zululand, KwaDlangezwa, South Africa
⁵³ School of Physics, University of the Witwatersrand, Johannesburg, South Africa
⁵⁴ Department of Physics, Carleton University, Ottawa, ON, Canada
⁵⁵ Faculté des Sciences Ain Chock, Université Hassan II de Casablanca, Casablanca, Morocco
⁵⁶ Faculté des Sciences, Université Ibn-Tofail, Kénitra, Morocco
⁵⁷ Faculté des Sciences Semlalia, Université Cadi Ayyad, LPHEA-Marrakech, Marrakesh, Morocco
⁵⁸ LPMR, Faculté des Sciences, Université Mohamed Premier, Oujda, Morocco
⁵⁹ Faculté des sciences, Université Mohammed V, Rabat, Morocco
⁶⁰ Institute of Applied Physics, Mohammed VI Polytechnic University, Ben Guerir, Morocco
⁶¹ CERN, Geneva, Switzerland
⁶² Affiliated with an Institute Covered by a Cooperation Agreement with CERN, Geneva, Switzerland
⁶³ Affiliated with an Institute Formerly Covered by a Cooperation Agreement with CERN, Geneva, Switzerland
⁶⁴ Affiliated with an International Laboratory Covered by a Cooperation Agreement with CERN, Geneva, Switzerland
⁶⁵ Enrico Fermi Institute, University of Chicago, Chicago, IL, USA
⁶⁶ LPC, Université Clermont Auvergne, CNRS/IN2P3, Clermont-Ferrand, France
⁶⁷ Nevis Laboratory, Columbia University, Irvington, NY, USA
⁶⁸ Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
⁶⁹ Dipartimento di Fisica, Università della Calabria, Rende, Italy
⁷⁰ INFN Gruppo Collegato di Cosenza, Laboratori Nazionali di Frascati, Frascati, Italy
⁷¹ Physics Department, Southern Methodist University, Dallas, TX, USA
⁷² Physics Department, University of Texas at Dallas, Richardson, TX, USA
⁷³ National Centre for Scientific Research “Demokritos”, Agia Paraskevi, Greece
⁷⁴ Department of Physics, Stockholm University, Stockholm, Sweden
⁷⁵ Oskar Klein Centre, Stockholm, Sweden
⁷⁶ Deutsches Elektronen-Synchrotron DESY, Hamburg and Zeuthen, Germany
⁷⁷ Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
⁷⁸ Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany
⁷⁹ Department of Physics, Duke University, Durham, NC, USA
⁸⁰ SUPA-School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
⁸¹ INFN e Laboratori Nazionali di Frascati, Frascati, Italy
⁸² Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
⁸³ II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany
⁸⁴ Département de Physique Nucléaire et Corpusculaire, Université de Genève, Geneva, Switzerland
⁸⁵ Dipartimento di Fisica, Università di Genova, Genoa, Italy
⁸⁶ INFN Sezione di Genova, Genoa, Italy
⁸⁷ II. Physikalisches Institut, Justus-Liebig-Universität Giessen, Giessen, Germany
⁸⁸ SUPA-School of Physics and Astronomy, University of Glasgow, Glasgow, UK
⁸⁹ LPSC, Université Grenoble Alpes, CNRS/IN2P3, Grenoble INP, Grenoble, France
⁹⁰ Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge, MA, USA
⁹¹ Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei, China
⁹² Institute of Frontier and Interdisciplinary Science and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Qingdao, China
⁹³ School of Physics and Astronomy, Shanghai Jiao Tong University, Key Laboratory for Particle Astrophysics and Cosmology (MOE), SKLPPC, Shanghai, China
⁹⁴ Tsung-Dao Lee Institute, Shanghai, China
⁹⁵ School of Physics, Zhengzhou University, Zhengzhou, China
⁹⁶ Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
⁹⁷ Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
⁹⁸ Department of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong
⁹⁹ Department of Physics, University of Hong Kong, Pok Fu Lam, Hong Kong
¹⁰⁰ Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
¹⁰¹ Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
¹⁰² IJCLab, Université Paris-Saclay, CNRS/IN2P3, 91405, Orsay, France
¹⁰³ Centro Nacional de Microelectrónica (IMB-CNM-CSIC), Barcelona, Spain
¹⁰⁴ Department of Physics, Indiana University, Bloomington, IN, USA
¹⁰⁵ INFN Gruppo Collegato di Udine, Sezione di Trieste, Udine, Italy
¹⁰⁶ ICTP, Trieste, Italy
¹⁰⁷ Dipartimento Politecnico di Ingegneria e Architettura, Università di Udine, Udine, Italy
¹⁰⁸ INFN Sezione di Lecce, Lecce, Italy
¹⁰⁹ Dipartimento di Matematica e Fisica, Università del Salento, Lecce, Italy
¹¹⁰ INFN Sezione di Milano, Milan, Italy
¹¹¹ Dipartimento di Fisica, Università di Milano, Milan, Italy
¹¹² INFN Sezione di Napoli, Naples, Italy
¹¹³ Dipartimento di Fisica, Università di Napoli, Naples, Italy
¹¹⁴ INFN Sezione di Pavia, Pavia, Italy
¹¹⁵ Dipartimento di Fisica, Università di Pavia, Pavia, Italy
¹¹⁶ INFN Sezione di Pisa, Pisa, Italy
¹¹⁷ Dipartimento di Fisica E. Fermi, Università di Pisa, Pisa, Italy
¹¹⁸ INFN Sezione di Roma, Rome, Italy
¹¹⁹ Dipartimento di Fisica, Sapienza Università di Roma, Rome, Italy
¹²⁰ INFN Sezione di Roma Tor Vergata, Rome, Italy
¹²¹ Dipartimento di Fisica, Università di Roma Tor Vergata, Rome, Italy
¹²² INFN Sezione di Roma Tre, Rome, Italy
¹²³ Dipartimento di Matematica e Fisica, Università Roma Tre, Rome, Italy
¹²⁴ INFN-TIFPA, Povo, Italy
¹²⁵ Università degli Studi di Trento, Trento, Italy
¹²⁶ Universität Innsbruck, Department of Astro and Particle Physics, Innsbruck, Austria
¹²⁷ University of Iowa, Iowa City, IA, USA
¹²⁸ Department of Physics and Astronomy, Iowa State University, Ames, IA, USA
¹²⁹ Istinye University, Sariyer, Istanbul, Turkey
¹³⁰ Departamento de Engenharia Elétrica, Universidade Federal de Juiz de Fora (UFJF), Juiz de Fora, Brazil
¹³¹ Universidade Federal do Rio de Janeiro COPPE/EE/IF, Rio de Janeiro, Brazil
¹³² Instituto de Física, Universidade de São Paulo, São Paulo, Brazil
¹³³ Rio de Janeiro State University, Rio de Janeiro, Brazil
¹³⁴ Federal University of Bahia, Bahia, Brazil
¹³⁵ KEK, High Energy Accelerator Research Organization, Tsukuba, Japan
¹³⁶ Graduate School of Science, Kobe University, Kobe, Japan
¹³⁷ Faculty of Physics and Applied Computer Science, AGH University of Krakow, Krakow, Poland
¹³⁸ Marian Smoluchowski Institute of Physics, Jagiellonian University, Krakow, Poland
¹³⁹ Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland
¹⁴⁰ Faculty of Science, Kyoto University, Kyoto, Japan
¹⁴¹ Research Center for Advanced Particle Physics and Department of Physics, Kyushu University, Fukuoka, Japan
¹⁴² L2IT, Université de Toulouse, CNRS/IN2P3, UPS, Toulouse, France
¹⁴³ Instituto de Física La Plata, Universidad Nacional de La Plata and CONICET, La Plata, Argentina
¹⁴⁴ Physics Department, Lancaster University, Lancaster, UK
¹⁴⁵ Oliver Lodge Laboratory, University of Liverpool, Liverpool, UK
¹⁴⁶ Department of Experimental Particle Physics, Jožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, Slovenia
¹⁴⁷ Department of Physics and Astronomy, Queen Mary University of London, London, UK
¹⁴⁸ Department of Physics, Royal Holloway University of London, Egham, UK
¹⁴⁹ Department of Physics and Astronomy, University College London, London, UK
¹⁵⁰ Louisiana Tech University, Ruston, LA, USA
¹⁵¹ Fysiska institutionen, Lunds universitet, Lund, Sweden
¹⁵² Departamento de Física Teorica C-15 and CIAFF, Universidad Autónoma de Madrid, Madrid, Spain
¹⁵³ Institut für Physik, Universität Mainz, Mainz, Germany
¹⁵⁴ School of Physics and Astronomy, University of Manchester, Manchester, UK
¹⁵⁵ CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France
¹⁵⁶ Department of Physics, University of Massachusetts, Amherst, MA, USA
¹⁵⁷ Department of Physics, McGill University, Montreal, QC, Canada
¹⁵⁸ School of Physics, University of Melbourne, Melbourne, VIC, Australia
¹⁵⁹ Department of Physics, University of Michigan, Ann Arbor, MI, USA
¹⁶⁰ Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA
¹⁶¹ Group of Particle Physics, University of Montreal, Montreal, QC, Canada
¹⁶² Fakultät für Physik, Ludwig-Maximilians-Universität München, Munich, Germany
¹⁶³ Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Munich, Germany
¹⁶⁴ Graduate School of Science and Kobayashi-Maskawa Institute, Nagoya University, Nagoya, Japan
¹⁶⁵ Department of Physics, Nanjing University, Nanjing, China
¹⁶⁶ School of Science, Shenzhen Campus of Sun Yat-sen University, Guangzhou, China
¹⁶⁷ University of Chinese Academy of Science (UCAS), Beijing, China
¹⁶⁸ Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM, USA
¹⁶⁹ Institute for Mathematics, Astrophysics and Particle Physics, Radboud University/Nikhef, Nijmegen, The Netherlands
¹⁷⁰ Nikhef National Institute for Subatomic Physics and University of Amsterdam, Amsterdam, The Netherlands
¹⁷¹ Department of Physics, Northern Illinois University, DeKalb, IL, USA
¹⁷² New York University Abu Dhabi, Abu Dhabi, United Arab Emirates
¹⁷³ United Arab Emirates University, Al Ain, United Arab Emirates
¹⁷⁴ Department of Physics, New York University, New York, NY, USA
¹⁷⁵ Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo, Japan
¹⁷⁶ Ohio State University, Columbus, OH, USA
¹⁷⁷ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK, USA
¹⁷⁸ Department of Physics, Oklahoma State University, Stillwater, OK, USA
¹⁷⁹ Palacký University, Joint Laboratory of Optics, Olomouc, Czech Republic
¹⁸⁰ Institute for Fundamental Science, University of Oregon, Eugene, OR, USA
¹⁸¹ Graduate School of Science, Osaka University, Osaka, Japan
¹⁸² Department of Physics, University of Oslo, Oslo, Norway
¹⁸³ Department of Physics, Oxford University, Oxford, UK
¹⁸⁴ LPNHE, Sorbonne Université, Université Paris Cité, CNRS/IN2P3, Paris, France
¹⁸⁵ Department of Physics, University of Pennsylvania, Philadelphia, PA, USA
¹⁸⁶ Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA, USA
¹⁸⁷ Laboratório de Instrumentação e Física Experimental de Partículas-LIP, Lisbon, Portugal
¹⁸⁸ Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisbon, Portugal
¹⁸⁹ Departamento de Física, Universidade de Coimbra, Coimbra, Portugal
¹⁹⁰ Centro de Física Nuclear da Universidade de Lisboa, Lisbon, Portugal
¹⁹¹ Departamento de Física, Escola de Ciências, Universidade do Minho, Braga, Portugal
¹⁹² Departamento de Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain
¹⁹³ Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
¹⁹⁴ Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic
¹⁹⁵ Czech Technical University in Prague, Prague, Czech Republic
¹⁹⁶ Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
¹⁹⁷ Particle Physics Department, Rutherford Appleton Laboratory, Didcot, UK
¹⁹⁸ IRFU, CEA, Université Paris-Saclay, Gif-sur-Yvette, France
¹⁹⁹ Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz, CA, USA
²⁰⁰ Departamento de Física, Pontificia Universidad Católica de Chile, Santiago, Chile
²⁰¹ Millennium Institute for Subatomic Physics at High Energy Frontier (SAPHIR), Santiago, Chile
²⁰² Instituto de Investigación Multidisciplinario en Ciencia y Tecnología y Departamento de Física, Universidad de La Serena, La Serena, Chile
²⁰³ Department of Physics, Universidad Andres Bello, Santiago, Chile
²⁰⁴ Instituto de Alta Investigación, Universidad de Tarapacá, Arica, Chile
²⁰⁵ Departamento de Física, Universidad Técnica Federico Santa María, Valparaíso, Chile
²⁰⁶ Department of Physics, Institute of Science, Tokyo, Japan
²⁰⁷ Department of Physics, University of Washington, Seattle, WA, USA
²⁰⁸ Department of Physics and Astronomy, University of Sheffield, Sheffield, UK
²⁰⁹ Department of Physics, Shinshu University, Nagano, Japan
²¹⁰ Department Physik, Universität Siegen, Siegen, Germany
²¹¹ Department of Physics, Simon Fraser University, Burnaby, BC, Canada
²¹² SLAC National Accelerator Laboratory, Stanford, CA, USA
²¹³ Department of Physics, Royal Institute of Technology, Stockholm, Sweden
²¹⁴ Departments of Physics and Astronomy, Stony Brook University, Stony Brook, NY, USA
²¹⁵ Department of Physics and Astronomy, University of Sussex, Brighton, UK
²¹⁶ School of Physics, University of Sydney, Sydney, Australia
²¹⁷ Institute of Physics, Academia Sinica, Taipei, Taiwan
²¹⁸ E. Andronikashvili Institute of Physics, Iv. Javakhishvili Tbilisi State University, Tbilisi, Georgia
²¹⁹ High Energy Physics Institute, Tbilisi State University, Tbilisi, Georgia
²²⁰ University of Georgia, Tbilisi, Georgia
²²¹ Department of Physics, Technion, Israel Institute of Technology, Haifa, Israel
²²² Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
²²³ Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece
²²⁴ International Center for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo, Japan
²²⁵ Department of Physics, University of Toronto, Toronto, ON, Canada
²²⁶ TRIUMF, Vancouver, BC, Canada
²²⁷ Department of Physics and Astronomy, York University, Toronto, ON, Canada
²²⁸ Division of Physics and Tomonaga Center for the History of the Universe, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan
²²⁹ Department of Physics and Astronomy, Tufts University, Medford, MA, USA
²³⁰ Department of Physics and Astronomy, University of California Irvine, Irvine, CA, USA
²³¹ University of West Attica, Athens, Greece
²³² University of Sharjah, Sharjah, United Arab Emirates
²³³ Department of Physics and Astronomy, University of Uppsala, Uppsala, Sweden
²³⁴ Department of Physics, University of Illinois, Urbana, IL, USA
²³⁵ Instituto de Física Corpuscular (IFIC), Centro Mixto Universidad de Valencia-CSIC, Valencia, Spain
²³⁶ Department of Physics, University of British Columbia, Vancouver, BC, Canada
²³⁷ Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada
²³⁸ Fakultät für Physik und Astronomie, Julius-Maximilians-Universität Würzburg, Würzburg, Germany
²³⁹ Department of Physics, University of Warwick, Coventry, UK
²⁴⁰ Waseda University, Tokyo, Japan
²⁴¹ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, Israel
²⁴² Department of Physics, University of Wisconsin, Madison, WI, USA
²⁴³ Fakultät für Mathematik und Naturwissenschaften, Fachgruppe Physik, Bergische Universität Wuppertal, Wuppertal, Germany
²⁴⁴ Department of Physics, Yale University, New Haven, CT, USA
²⁴⁵ Yerevan Physics Institute, Yerevan, Armenia
²⁴⁶ Affiliated with an Institute Covered by a Cooperation Agreement with CERN, Geneva, Switzerland
²⁴⁷ An-Najah National University, Nablus, Palestine
²⁴⁸ Borough of Manhattan Community College, City University of New York, New York, NY, USA
²⁴⁹ Center for High Energy Physics, Peking University, Beijing, China
²⁵⁰ Center for Interdisciplinary Research and Innovation (CIRI-AUTH), Thessaloniki, Greece
²⁵¹ CERN, Geneva, Switzerland
²⁵² CMD-AC UNEC Research Center, Azerbaijan State University of Economics (UNEC), Baku, Azerbaijan
²⁵³ Département de Physique Nucléaire et Corpusculaire, Université de Genève, Geneva, Switzerland
²⁵⁴ Departament de Fisica de la Universitat Autonoma de Barcelona, Barcelona, Spain
²⁵⁵ Department of Financial and Management Engineering, University of the Aegean, Chios, Greece
²⁵⁶ Department of Physics, California State University, Sacramento, USA
²⁵⁷ Department of Physics, King’s College London, London, UK
²⁵⁸ Department of Physics, Stanford University, Stanford, CA, USA
²⁵⁹ Department of Physics, Stellenbosch University, Stellenbosch, South Africa
²⁶⁰ Department of Physics, University of Fribourg, Fribourg, Switzerland
²⁶¹ Department of Physics, University of Thessaly, Volos, Greece
²⁶² Department of Physics, Westmont College, Santa Barbara, USA
²⁶³ Faculty of Physics, Sofia University, ‘St. Kliment Ohridski’, Sofia, Bulgaria
²⁶⁴ Hellenic Open University, Patras, Greece
²⁶⁵ Imam Mohammad Ibn Saud Islamic University, Riyadh, Saudi Arabia
²⁶⁶ Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona, Spain
²⁶⁷ Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany
²⁶⁸ Institute for Nuclear Research and Nuclear Energy (INRNE) of the Bulgarian Academy of Sciences, Sofia, Bulgaria
²⁶⁹ Institute of Applied Physics, Mohammed VI Polytechnic University, Ben Guerir, Morocco
²⁷⁰ Institute of Particle Physics (IPP), Toronto, Canada
²⁷¹ Institute of Physics and Technology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia
²⁷² Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan
²⁷³ National Institute of Physics, University of the Philippines Diliman (Philippines), Philippines
²⁷⁴ Technical University of Munich, Munich, Germany
²⁷⁵ The Collaborative Innovation Center of Quantum Matter (CICQM), Beijing, China
²⁷⁶ TRIUMF, Vancouver, BC, Canada
²⁷⁷ Università di Napoli Parthenope, Naples, Italy
²⁷⁸ University of Colorado Boulder, Department of Physics, Colorado, USA
²⁷⁹ Washington College, Chestertown, MD, USA
²⁸⁰ Yeditepe University, Physics Department, Istanbul, Turkey
²⁸¹ CERN, 1211, Geneva 23, Switzerland

^a atlas.publications@cern.ch

Received: 15 November 2024
Accepted: 14 March 2025
Published online: 22 May 2025

Abstract

This paper describes an algorithm for reconstructing and identifying a highly collimated hadronically decaying $\tau$-lepton pair with low transverse momentum. When two $\tau$-leptons are highly collimated, their visible decay products might overlap, degrading the reconstruction performance for each of the $\tau$-leptons. A dedicated treatment attempting to tag the $\tau$-lepton pair as a single object is required. The reconstruction algorithm is based on a large radius jet and its associated two leading subjets, and the identification uses a boosted decision tree to discriminate between signatures from ${\tau }^{+}{\tau }^{-}$ systems and those arising from QCD jets. The efficiency of the identification algorithm is measured in $Z\gamma$ events using proton–proton collision data at $\sqrt{s}=13$ TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of $139{\text{fb}}^{-1}$. The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26 to 37%.