Eur. Phys. J. C (2025) 85: 886
https://doi.org/10.1140/epjc/s10052-025-14507-0

Regular Article - Experimental Physics

Search for a new pseudoscalar decaying into a pair of bottom and antibottom quarks in top-associated production in $\sqrt{s}=13$ TeV proton–proton collisions 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, Türkiye
⁴ Division of Physics, TOBB University of Economics and Technology, Ankara, Türkiye
⁵ LAPP, CNRS/IN2P3, Université Savoie Mont Blanc, 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, Türkiye
²⁵ Department of Physics Engineering, Gaziantep University, Gaziantep, Türkiye
²⁶ Department of Physics, Istanbul University, Istanbul, Türkiye
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, Slovak Republic
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 Facultad de Ciencias Exactas y Naturales, Departamento de Física, y CONICET, Instituto de Física de Buenos Aires (IFIBA), Universidad de Buenos Aires, 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, 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, LPHEA-Marrakech, Université Cadi Ayyad, Marrakech, 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 Formerly Covered by a Cooperation Agreement with CERN, Geneva, Switzerland
⁶³ Affiliated with an International Laboratory Covered by a Cooperation Agreement with CERN, Geneva, Switzerland
⁶⁴ Enrico Fermi Institute, University of Chicago, Chicago, IL, USA
⁶⁵ LPC, CNRS/IN2P3, Université Clermont Auvergne, Clermont-Ferrand, France
⁶⁶ Nevis Laboratory, Columbia University, Irvington, NY, USA
⁶⁷ Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
⁶⁸ Dipartimento di Fisica, Università della Calabria, Rende, Italy
⁶⁹ Laboratori Nazionali di Frascati, INFN Gruppo Collegato di Cosenza, Cosenza, Italy
⁷⁰ Physics Department, Southern Methodist University, Dallas, TX, USA
⁷¹ National Centre for Scientific Research “Demokritos”, Agia Paraskevi, Greece
⁷² Department of Physics, Stockholm University, Stockholm, Sweden
⁷³ Oskar Klein Centre, Stockholm, Sweden
⁷⁴ Deutsches Elektronen-Synchrotron DESY, Hamburg and Zeuthen, Germany
⁷⁵ Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
⁷⁶ Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany
⁷⁷ Department of Physics, Duke University, Durham, NC, USA
⁷⁸ SUPA-School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
⁷⁹ INFN e Laboratori Nazionali di Frascati, Frascati, Italy
⁸⁰ Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
⁸¹ II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany
⁸² Département de Physique Nucléaire et Corpusculaire, Université de Genève, Geneva, Switzerland
⁸³ Dipartimento di Fisica, Università di Genova, Genoa, Italy
⁸⁴ INFN Sezione di Genova, Genoa, Italy
⁸⁵ II. Physikalisches Institut, Justus-Liebig-Universität Giessen, Giessen, Germany
⁸⁶ SUPA-School of Physics and Astronomy, University of Glasgow, Glasgow, UK
⁸⁷ LPSC, Université Grenoble Alpes, CNRS/IN2P3, Grenoble INP, Grenoble, France
⁸⁸ Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge, MA, USA
⁸⁹ Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei, China
⁹⁰ Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
⁹¹ Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
⁹² Department of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China
⁹³ Department of Physics, University of Hong Kong, Hong Kong, China
⁹⁴ Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
⁹⁵ Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁹⁶ IJCLab, CNRS/IN2P3, Université Paris-Saclay, 91405, Orsay, France
⁹⁷ Centro Nacional de Microelectrónica (IMB-CNM-CSIC), Barcelona, Spain
⁹⁸ Department of Physics, Indiana University, Bloomington, IN, USA
⁹⁹ INFN Gruppo Collegato di Udine, Sezione di Trieste, Udine, Italy
¹⁰⁰ ICTP, Trieste, Italy
¹⁰¹ Dipartimento Politecnico di Ingegneria e Architettura, Università di Udine, Udine, Italy
¹⁰² INFN Sezione di Lecce, Lecce, Italy
¹⁰³ Dipartimento di Matematica e Fisica, Università del Salento, Lecce, Italy
¹⁰⁴ INFN Sezione di Milano, Milan, Italy
¹⁰⁵ Dipartimento di Fisica, Università di Milano, Milan, Italy
¹⁰⁶ INFN Sezione di Napoli, Naples, Italy
¹⁰⁷ Dipartimento di Fisica, Università di Napoli, Naples, Italy
¹⁰⁸ INFN Sezione di Pavia, Pavia, Italy
¹⁰⁹ Dipartimento di Fisica, Università di Pavia, Pavia, Italy
¹¹⁰ INFN Sezione di Pisa, Pisa, Italy
¹¹¹ Dipartimento di Fisica E. Fermi, Università di Pisa, Pisa, Italy
¹¹² INFN Sezione di Roma, Rome, Italy
¹¹³ Dipartimento di Fisica, Sapienza Università di Roma, Rome, Italy
¹¹⁴ INFN Sezione di Roma Tor Vergata, Rome, Italy
¹¹⁵ Dipartimento di Fisica, Università di Roma Tor Vergata, Rome, Italy
¹¹⁶ INFN Sezione di Roma Tre, Rome, Italy
¹¹⁷ Dipartimento di Matematica e Fisica, Università Roma Tre, Rome, Italy
¹¹⁸ INFN-TIFPA, Povo, Italy
¹¹⁹ Università degli Studi di Trento, Trento, Italy
¹²⁰ Universität Innsbruck, Department of Astro and Particle Physics, Innsbruck, Austria
¹²¹ University of Iowa, Iowa City, IA, USA
¹²² Department of Physics and Astronomy, Iowa State University, Ames, IA, USA
¹²³ Istinye University, Sariyer, Istanbul, Türkiye
¹²⁴ Departamento de Engenharia Elétrica, Universidade Federal de Juiz de Fora (UFJF), Juiz de Fora, Brazil
¹²⁵ Universidade Federal do Rio De Janeiro COPPE/EE/IF, Rio de Janeiro, Brazil
¹²⁶ Instituto de Física, Universidade de São Paulo, São Paulo, Brazil
¹²⁷ Rio de Janeiro State University, Rio de Janeiro, Brazil
¹²⁸ Federal University of Bahia, Bahia, Brazil
¹²⁹ KEK, High Energy Accelerator Research Organization, Tsukuba, Japan
¹³⁰ Graduate School of Science, Kobe University, Kobe, Japan
¹³¹ AGH University of Krakow, Faculty of Physics and Applied Computer Science, Kraków, Poland
¹³² Marian Smoluchowski Institute of Physics, Jagiellonian University, Kraków, Poland
¹³³ Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland
¹³⁴ Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates
¹³⁵ University of Sharjah, Sharjah, United Arab Emirates
¹³⁶ Faculty of Science, Kyoto University, Kyoto, Japan
¹³⁷ Research Center for Advanced Particle Physics and Department of Physics, Kyushu University, Fukuoka, Japan
¹³⁸ L2IT, CNRS/IN2P3, UPS, Université de Toulouse, Toulouse, France
¹³⁹ Instituto de Física La Plata, Universidad Nacional de La Plata and CONICET, La Plata, Argentina
¹⁴⁰ Physics Department, Lancaster University, Lancaster, UK
¹⁴¹ Oliver Lodge Laboratory, University of Liverpool, Liverpool, UK
¹⁴² Department of Experimental Particle Physics, Jožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, Slovenia
¹⁴³ Department of Physics and Astronomy, Queen Mary University of London, London, UK
¹⁴⁴ Department of Physics, Royal Holloway University of London, Egham, UK
¹⁴⁵ Department of Physics and Astronomy, University College London, London, UK
¹⁴⁶ Louisiana Tech University, Ruston, LA, USA
¹⁴⁷ Fysiska institutionen, Lunds universitet, Lund, Sweden
¹⁴⁸ Departamento de Física Teorica C-15 and CIAFF, Universidad Autónoma de Madrid, Madrid, Spain
¹⁴⁹ Institut für Physik, Universität Mainz, Mainz, Germany
¹⁵⁰ School of Physics and Astronomy, University of Manchester, Manchester, UK
¹⁵¹ CPPM, CNRS/IN2P3, Aix-Marseille Université, Marseille, France
¹⁵² Department of Physics, University of Massachusetts, Amherst, MA, USA
¹⁵³ Department of Physics, McGill University, Montreal, QC, Canada
¹⁵⁴ School of Physics, University of Melbourne, Victoria, Australia
¹⁵⁵ Department of Physics, University of Michigan, Ann Arbor, MI, USA
¹⁵⁶ Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA
¹⁵⁷ Group of Particle Physics, University of Montreal, Montreal, QC, Canada
¹⁵⁸ Fakultät für Physik, Ludwig-Maximilians-Universität München, Munich, Germany
¹⁵⁹ Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Munich, Germany
¹⁶⁰ Graduate School of Science and Kobayashi-Maskawa Institute, Nagoya University, Nagoya, Japan
¹⁶¹ Department of Physics, Nanjing University, Nanjing, China
¹⁶² School of Science, Shenzhen Campus of Sun Yat-sen University, Guangzhou, China
¹⁶³ University of Chinese Academy of Science (UCAS), Beijing, China
¹⁶⁴ Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM, USA
¹⁶⁵ Institute for Mathematics, Astrophysics and Particle Physics, Radboud University/Nikhef, Nijmegen, Netherlands
¹⁶⁶ Nikhef National Institute for Subatomic Physics and University of Amsterdam, Amsterdam, Netherlands
¹⁶⁷ Department of Physics, Northern Illinois University, DeKalb, IL, USA
¹⁶⁸ New York University Abu Dhabi, Abu Dhabi, United Arab Emirates
¹⁶⁹ United Arab Emirates University, Al Ain, United Arab Emirates
¹⁷⁰ Department of Physics, New York University, New York, NY, USA
¹⁷¹ Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo, Japan
¹⁷² Ohio State University, Columbus, OH, USA
¹⁷³ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK, USA
¹⁷⁴ Department of Physics, Oklahoma State University, Stillwater, OK, USA
¹⁷⁵ Palacký University, Joint Laboratory of Optics, Olomouc, Czech Republic
¹⁷⁶ Institute for Fundamental Science, University of Oregon, Eugene, OR, USA
¹⁷⁷ Graduate School of Science, University of Osaka, Osaka, Japan
¹⁷⁸ Department of Physics, University of Oslo, Oslo, Norway
¹⁷⁹ Department of Physics, Oxford University, Oxford, UK
¹⁸⁰ LPNHE, CNRS/IN2P3, Sorbonne Université, Université Paris Cité, Paris, France
¹⁸¹ Department of Physics, University of Pennsylvania, Philadelphia, PA, USA
¹⁸² Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA, USA
¹⁸³ Laboratório de Instrumentação e Física Experimental de Partículas - LIP, Lisbon, Portugal
¹⁸⁴ Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisbon, Portugal
¹⁸⁵ Departamento de Física, Universidade de Coimbra, Coimbra, Portugal
¹⁸⁶ Centro de Física Nuclear da Universidade de Lisboa, Lisbon, Portugal
¹⁸⁷ Departamento de Física, Escola de Ciências, Universidade do Minho, Braga, Portugal
¹⁸⁸ Departamento de Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain
¹⁸⁹ Departamento de Física, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
¹⁹⁰ Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic
¹⁹¹ Czech Technical University in Prague, Prague, Czech Republic
¹⁹² Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic
¹⁹³ Particle Physics Department, Rutherford Appleton Laboratory, Didcot, UK
¹⁹⁴ IRFU, CEA, Université Paris-Saclay, Gif-sur-Yvette, France
¹⁹⁵ Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz, CA, USA
¹⁹⁶ Departamento de Física, Pontificia Universidad Católica de Chile, Santiago, Chile
¹⁹⁷ Millennium Institute for Subatomic Physics at High Energy Frontier (SAPHIR), Santiago, Chile
¹⁹⁸ Instituto de Investigación Multidisciplinario en Ciencia y Tecnología y Departamento de Física, Universidad de La Serena, La Serena, Chile
¹⁹⁹ Universidad Andres Bello, Department of Physics, Santiago, Chile
²⁰⁰ Instituto de Alta Investigación, Universidad de Tarapacá, Arica, Chile
²⁰¹ Departamento de Física, Universidad Técnica Federico Santa María, Valparaíso, Chile
²⁰² Department of Physics, Institute of Science, Tokyo, Japan
²⁰³ Department of Physics, University of Washington, Seattle, WA, USA
²⁰⁴ Institute of Frontier and Interdisciplinary Science and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Qingdao, China
²⁰⁵ School of Physics, Zhengzhou University, Zhengzhou, China
²⁰⁶ State Key Laboratory of Dark Matter Physics, School of Physics and Astronomy, Shanghai Jiao Tong University, Key Laboratory for Particle Astrophysics and Cosmology (MOE), SKLPPC, Shanghai, China
²⁰⁷ State Key Laboratory of Dark Matter Physics, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai, China
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, Thessaloníki, Greece
²²⁴ International Center for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo, Japan
²²⁵ Graduate School of Science and Technology, Tokyo Metropolitan University, Tokyo, Japan
²²⁶ Department of Physics, University of Toronto, Toronto, ON, Canada
²²⁷ TRIUMF, Vancouver, BC, Canada
²²⁸ Department of Physics and Astronomy, York University, Toronto, ON, Canada
²²⁹ Division of Physics and Tomonaga Center for the History of the Universe, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan
²³⁰ Department of Physics and Astronomy, Tufts University, Medford, MA, USA
²³¹ Department of Physics and Astronomy, University of California Irvine, Irvine, CA, USA
²³² University of West Attica, Athens, Greece
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 Formerly 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 Interdisciplinary Research and Innovation (CIRI-AUTH), Thessaloníki, Greece
²⁵⁰ Centre of Physics of the Universities of Minho and Porto (CF-UM-UP), Porto, Portugal
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 Mathematical Sciences, University of South Africa, Johannesburg, South Africa
²⁵⁷ Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei, China
²⁵⁸ Department of Physics, Bolu Abant Izzet Baysal University, Bolu, Türkiye
²⁵⁹ Department of Physics, King’s College London, London, UK
²⁶⁰ Department of Physics, Stanford University, Stanford, CA, USA
²⁶¹ Department of Physics, Stellenbosch University, South Africa
²⁶² Department of Physics, University of Fribourg, Fribourg, Switzerland
²⁶³ Department of Physics, University of Thessaly, Volos, Greece
²⁶⁴ Department of Physics, Westmont College, Santa Barbara, USA
²⁶⁵ Faculty of Physics, Sofia University ‘St. Kliment Ohridski’, Sofia, Bulgaria
²⁶⁶ Faculty of Physics, University of Bucharest, Bucharest, Romania
²⁶⁷ Hellenic Open University, Patras, Greece
²⁶⁸ Henan University, Henan, China
²⁶⁹ Imam Mohammad Ibn Saud Islamic University, Riyadh, Saudi Arabia
²⁷⁰ Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona, Spain
²⁷¹ Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany
²⁷² Institute for Nuclear Research and Nuclear Energy (INRNE) of the Bulgarian Academy of Sciences, Sofia, Bulgaria
²⁷³ Institute of Applied Physics, Mohammed VI Polytechnic University, Ben Guerir, Morocco
²⁷⁴ Institute of Particle Physics (IPP), Toronto, Canada
²⁷⁵ Institute of Physics and Technology, Mongolian Academy of Sciences, Ulaanbaatar, Mongolia
²⁷⁶ Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan
²⁷⁷ Institute of Theoretical Physics, Ilia State University, Tbilisi, Georgia
²⁷⁸ National Institute of Physics, University of the Philippines Diliman (Philippines), Quezon City, Philippines
²⁷⁹ The Collaborative Innovation Center of Quantum Matter (CICQM), Beijing, China
²⁸⁰ TRIUMF, Vancouver, BC, Canada
²⁸¹ Università di Napoli Parthenope, Naples, Italy
²⁸² Department of Physics, University of Colorado Boulder, Colorado, USA
²⁸³ University of Sienna, Siena, Italy
²⁸⁴ Washington College, Chestertown, MD, USA
²⁸⁵ Physics Department, Yeditepe University, Istanbul, Türkiye
²⁸⁶ CERN, 1211, Geneva 23, Switzerland

^a atlas.publications@cern.ch

Received: 24 March 2025
Accepted: 10 July 2025
Published online: 19 August 2025

Abstract

A search for a pseudoscalar a produced in association with a top-quark pair, or in association with a single top quark plus a W boson, with the pseudoscalar decaying into b-quarks ($a\to b\overline{b}$), is performed using the full Run 2 data sample using a dileptonic decay mode signature. The search covers pseudoscalar boson masses between 12 and 100 GeV and involves both the kinematic regime where the decay products of the pseudoscalar are reconstructed as two standard b-tagged small-radius jets, or merged into a large-radius jet due to its Lorentz boost. No significant excess relative to expectations is observed. Assuming a branching ratio $\text{BR}(a\to b\overline{b})=100\%$, the range of pseudoscalar masses between 50 and 80 GeV is excluded at 95% confidence level for a coupling of the pseudoscalar to the top quark of 0.5, while a coupling of 1.0 is excluded at 95% confidence level for the masses considered, with the coupling defined as the strength modifier of the Standard Model Yukawa coupling.