https://doi.org/10.1140/epjc/s10052-021-08929-9
Regular Article – Experimental Physics
Search for type-III seesaw heavy leptons in dilepton final states in pp collisions at 
with the ATLAS detector
1
Department of Physics, University of Adelaide, Adelaide, Australia
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Physics Department, SUNY Albany, Albany, NY, USA
3
Department of Physics, University of Alberta, Edmonton, AB, Canada
4
Department of Physics, Ankara University, Ankara, Turkey
5
Istanbul Aydin University, Application and Research Center for Advanced Studies, Istanbul, Turkey
6
Division of Physics, TOBB University of Economics and Technology, Ankara, Turkey
7
LAPP, Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS/IN2P3, Annecy, France
8
High Energy Physics Division, Argonne National Laboratory, Argonne, IL, USA
9
Department of Physics, University of Arizona, Tucson, AZ, USA
10
Department of Physics, University of Texas at Arlington, Arlington, TX, USA
11
Physics Department, National and Kapodistrian University of Athens, Athens, Greece
12
Physics Department, National Technical University of Athens, Zografou, Greece
13
Department of Physics, University of Texas at Austin, Austin, TX, USA
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Bahcesehir University, Faculty of Engineering and Natural Sciences, Istanbul, Turkey
15
Istanbul Bilgi University, Faculty of Engineering and Natural Sciences, Istanbul, Turkey
16
Department of Physics, Bogazici University, Istanbul, Turkey
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Department of Physics Engineering, Gaziantep University, Gaziantep, Turkey
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Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan
19
Institut de Física d’Altes Energies (IFAE), Barcelona Institute of Science and Technology, Barcelona, Spain
20
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China
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Physics Department, Tsinghua University, Beijing, China
22
Department of Physics, Nanjing University, Nanjing, China
23
University of Chinese Academy of Science (UCAS), Beijing, China
24
Institute of Physics, University of Belgrade, Belgrade, Serbia
25
Department for Physics and Technology, University of Bergen, Bergen, Norway
26
Physics Division, Lawrence Berkeley National Laboratory and University of California, Berkeley, CA, USA
27
Institut für Physik, Humboldt Universität zu Berlin, Berlin, Germany
28
Albert Einstein Center for Fundamental Physics and Laboratory for High Energy Physics, University of Bern, Bern, Switzerland
29
School of Physics and Astronomy, University of Birmingham, Birmingham, UK
30
Facultad de Ciencias y Centro de Investigaciónes, Universidad Antonio Nariño, Bogotá, Colombia
31
Departamento de Física, Universidad Nacional de Colombia, Bogotá, Colombia
32
Dipartimento di Fisica, INFN Bologna and Universita’ di Bologna, Bologna, Italy
33
INFN Sezione di Bologna, Bologna, Italy
34
Physikalisches Institut, Universität Bonn, Bonn, Germany
35
Department of Physics, Boston University, Boston, MA, USA
36
Department of Physics, Brandeis University, Waltham, MA, USA
37
Transilvania University of Brasov, Brasov, Romania
38
Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania
39
Department of Physics, Alexandru Ioan Cuza University of Iasi, Iasi, Romania
40
National Institute for Research and Development of Isotopic and Molecular Technologies, Physics Department, Cluj-Napoca, Romania
41
University Politehnica Bucharest, Bucharest, Romania
42
West University in Timisoara, Timisoara, Romania
43
Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia
44
Department of Subnuclear Physics, Institute of Experimental Physics of the Slovak Academy of Sciences, Kosice, Slovak Republic
45
Physics Department, Brookhaven National Laboratory, Upton, NY, USA
46
Departamento de Física, Universidad de Buenos Aires, Buenos Aires, Argentina
47
California State University, Long Beach, CA, USA
48
Cavendish Laboratory, University of Cambridge, Cambridge, UK
49
Department of Physics, University of Cape Town, Cape Town, South Africa
50
iThemba Labs, Western Cape, South Africa
51
Department of Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa
52
Department of Physics, University of South Africa, Pretoria, 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, Réseau Universitaire de Physique des Hautes Energies, Université Hassan II, Casablanca, Morocco
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Faculté des Sciences, Université Ibn-Tofail, Kénitra, Morocco
57
Faculté des Sciences Semlalia, Université Cadi Ayyad, LPHEA-Marrakech, Marrakesh, Morocco
58
Faculté des Sciences, Université Mohamed Premier and LPTPM, Oujda, Morocco
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Faculté des sciences, Université Mohammed V, Rabat, Morocco
60
CERN, Geneva, Switzerland
61
Enrico Fermi Institute, University of Chicago, Chicago, IL, USA
62
LPC, Université Clermont Auvergne, CNRS/IN2P3, Clermont-Ferrand, France
63
Nevis Laboratory, Columbia University, Irvington, NY, USA
64
Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
65
Dipartimento di Fisica, Università della Calabria, Rende, Italy
66
INFN Gruppo Collegato di Cosenza, Laboratori Nazionali di Frascati, Frascati, Italy
67
Physics Department, Southern Methodist University, Dallas, TX, USA
68
Physics Department, University of Texas at Dallas, Richardson, TX, USA
69
National Centre for Scientific Research “Demokritos”, Agia Paraskevi, Greece
70
Department of Physics, Stockholm University, Stockholm, Sweden
71
Oskar Klein Centre, Stockholm, Sweden
72
Deutsches Elektronen-Synchrotron DESY, Hamburg and Zeuthen, Germany
73
Lehrstuhl für Experimentelle Physik IV, Technische Universität Dortmund, Dortmund, Germany
74
Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany
75
Department of Physics, Duke University, Durham, NC, USA
76
SUPA-School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
77
INFN e Laboratori Nazionali di Frascati, Frascati, Italy
78
Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
79
II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany
80
Département de Physique Nucléaire et Corpusculaire, Université de Genève, Geneva, Switzerland
81
Dipartimento di Fisica, Università di Genova, Genoa, Italy
82
INFN Sezione di Genova, Genoa, Italy
83
II. Physikalisches Institut, Justus-Liebig-Universität Giessen, Giessen, Germany
84
SUPA-School of Physics and Astronomy, University of Glasgow, Glasgow, UK
85
LPSC, Université Grenoble Alpes, CNRS/IN2P3, Grenoble INP, Grenoble, France
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Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge, MA, USA
87
Department of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei, China
88
Institute of Frontier and Interdisciplinary Science and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Qingdao, China
89
School of Physics and Astronomy, Shanghai Jiao Tong University, KLPPAC-MoE, SKLPPC, Shanghai, China
90
Tsung-Dao Lee Institute, Shanghai, China
91
Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
92
Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
93
Faculty of Applied Information Science, Hiroshima Institute of Technology, Hiroshima, Japan
94
Department of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong
95
Department of Physics, University of Hong Kong, Pok Fu Lam, Hong Kong
96
Department of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
97
Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
98
IJCLab, Université Paris-Saclay, CNRS/IN2P3, 91405, Orsay, France
99
Department of Physics, Indiana University, Bloomington, IN, USA
100
INFN Gruppo Collegato di Udine, Sezione di Trieste, Udine, Italy
101
ICTP, Trieste, Italy
102
Dipartimento Politecnico di Ingegneria e Architettura, Università di Udine, Udine, Italy
103
INFN Sezione di Lecce, Lecce, Italy
104
Dipartimento di Matematica e Fisica, Università del Salento, Lecce, Italy
105
INFN Sezione di Milano, Milan, Italy
106
Dipartimento di Fisica, Università di Milano, Milan, Italy
107
INFN Sezione di Napoli, Naples, Italy
108
Dipartimento di Fisica, Università di Napoli, Naples, Italy
109
INFN Sezione di Pavia, Pavia, Italy
110
Dipartimento di Fisica, Università di Pavia, Pavia, Italy
111
INFN Sezione di Pisa, Pisa, Italy
112
Dipartimento di Fisica E. Fermi, Università di Pisa, Pisa, Italy
113
INFN Sezione di Roma, Rome, Italy
114
Dipartimento di Fisica, Sapienza Università di Roma, Rome, Italy
115
INFN Sezione di Roma Tor Vergata, Rome, Italy
116
Dipartimento di Fisica, Università di Roma Tor Vergata, Rome, Italy
117
INFN Sezione di Roma Tre, Rome, Italy
118
Dipartimento di Matematica e Fisica, Università Roma Tre, Rome, Italy
119
INFN-TIFPA, Trento, Italy
120
Università degli Studi di Trento, Trento, Italy
121
Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität, Innsbruck, Austria
122
University of Iowa, Iowa City, IA, USA
123
Department of Physics and Astronomy, Iowa State University, Ames, IA, USA
124
Joint Institute for Nuclear Research, Dubna, Russia
125
Departamento de Engenharia Elétrica, Universidade Federal de Juiz de Fora (UFJF), Juiz de Fora, Brazil
126
Universidade Federal do Rio De Janeiro COPPE/EE/IF, Rio de Janeiro, Brazil
127
Instituto de Física, Universidade de São Paulo, São Paulo, Brazil
128
KEK, High Energy Accelerator Research Organization, Tsukuba, Japan
129
Graduate School of Science, Kobe University, Kobe, Japan
130
AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland
131
Marian Smoluchowski Institute of Physics, Jagiellonian University, Krakow, Poland
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Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland
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Faculty of Science, Kyoto University, Kyoto, Japan
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Kyoto University of Education, Kyoto, Japan
135
Research Center for Advanced Particle Physics and Department of Physics, Kyushu University, Fukuoka, Japan
136
Instituto de Física La Plata, Universidad Nacional de La Plata and CONICET, La Plata, Argentina
137
Physics Department, Lancaster University, Lancaster, UK
138
Oliver Lodge Laboratory, University of Liverpool, Liverpool, UK
139
Department of Experimental Particle Physics, Jožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, Slovenia
140
School of Physics and Astronomy, Queen Mary University of London, London, UK
141
Department of Physics, Royal Holloway University of London, Egham, UK
142
Department of Physics and Astronomy, University College London, London, UK
143
Louisiana Tech University, Ruston, LA, USA
144
Fysiska institutionen, Lunds universitet, Lund, Sweden
145
Centre de Calcul de l’Institut National de Physique Nucléaire et de Physique des Particules (IN2P3), Villeurbanne, France
146
Departamento de Física Teorica C-15 and CIAFF, Universidad Autónoma de Madrid, Madrid, Spain
147
Institut für Physik, Universität Mainz, Mainz, Germany
148
School of Physics and Astronomy, University of Manchester, Manchester, UK
149
CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France
150
Department of Physics, University of Massachusetts, Amherst, MA, USA
151
Department of Physics, McGill University, Montreal, QC, Canada
152
School of Physics, University of Melbourne, Melbourne, VIC, Australia
153
Department of Physics, University of Michigan, Ann Arbor, MI, USA
154
Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA
155
B.I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, Minsk, Belarus
156
Research Institute for Nuclear Problems of Byelorussian State University, Minsk, Belarus
157
Group of Particle Physics, University of Montreal, Montreal, QC, Canada
158
P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia
159
National Research Nuclear University MEPhI, Moscow, Russia
160
D.V. Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Moscow, Russia
161
Fakultät für Physik, Ludwig-Maximilians-Universität München, Munich, Germany
162
Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), Munich, Germany
163
Nagasaki Institute of Applied Science, Nagasaki, Japan
164
Graduate School of Science and Kobayashi-Maskawa Institute, Nagoya University, Nagoya, Japan
165
Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM, USA
166
Institute for Mathematics, Astrophysics and Particle Physics, Radboud University/Nikhef, Nijmegen, The Netherlands
167
Nikhef National Institute for Subatomic Physics and University of Amsterdam, Amsterdam, The Netherlands
168
Department of Physics, Northern Illinois University, DeKalb, IL, USA
169
Budker Institute of Nuclear Physics and NSU, SB RAS, Novosibirsk, Russia
170
Novosibirsk State University Novosibirsk, Novosibirsk, Russia
171
Institute for High Energy Physics of the National Research Centre Kurchatov Institute, Protvino, Russia
172
Institute for Theoretical and Experimental Physics named by A.I. Alikhanov of National Research Centre “Kurchatov Institute”, Moscow, Russia
173
Department of Physics, New York University, New York, NY, USA
174
Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo, Japan
175
Ohio State University, Columbus, OH, USA
176
Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK, USA
177
Department of Physics, Oklahoma State University, Stillwater, OK, USA
178
Palacký University, RCPTM, Joint Laboratory of Optics, Olomouc, Czech Republic
179
Institute for Fundamental Science, University of Oregon, Eugene, OR, USA
180
Graduate School of Science, Osaka University, Osaka, Japan
181
Department of Physics, University of Oslo, Oslo, Norway
182
Department of Physics, Oxford University, Oxford, UK
183
LPNHE, Sorbonne Université, Université de Paris, CNRS/IN2P3, Paris, France
184
Department of Physics, University of Pennsylvania, Philadelphia, PA, USA
185
Konstantinov Nuclear Physics Institute of National Research Centre “Kurchatov Institute”, PNPI, St. Petersburg, Russia
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, Universidade do Minho, Braga, Portugal
192
Departamento de Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain
193
Dep Física and CEFITEC of Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal
194
Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
195
Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic
196
Czech Technical University in Prague, Prague, Czech Republic
197
Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic
198
Particle Physics Department, Rutherford Appleton Laboratory, Didcot, UK
199
IRFU, CEA, Université Paris-Saclay, Gif-sur-Yvette, France
200
Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz, CA, USA
201
Departamento de Física, Pontificia Universidad Católica de Chile, Santiago, Chile
202
Department of Physics, Universidad Andres Bello, Santiago, Chile
203
Instituto de Alta Investigación, Universidad de Tarapacá, Arica, Chile
204
Departamento de Física, Universidad Técnica Federico Santa María, Valparaíso, Chile
205
Universidade Federal de São João del Rei (UFSJ), São João del Rei, Brazil
206
Department of Physics, University of Washington, Seattle, WA, USA
207
Department of Physics and Astronomy, University of Sheffield, Sheffield, UK
208
Department of Physics, Shinshu University, Nagano, Japan
209
Department Physik, Universität Siegen, Siegen, Germany
210
Department of Physics, Simon Fraser University, Burnaby, BC, Canada
211
SLAC National Accelerator Laboratory, Stanford, CA, USA
212
Physics Department, Royal Institute of Technology, Stockholm, Sweden
213
Departments of Physics and Astronomy, Stony Brook University, Stony Brook, NY, USA
214
Department of Physics and Astronomy, University of Sussex, Brighton, UK
215
School of Physics, University of Sydney, Sydney, Australia
216
Institute of Physics, Academia Sinica, Taipei, Taiwan
217
E. Andronikashvili Institute of Physics, Iv. Javakhishvili Tbilisi State University, Tbilisi, Georgia
218
High Energy Physics Institute, Tbilisi State University, Tbilisi, Georgia
219
Department of Physics, Technion, Israel Institute of Technology, Haifa, Israel
220
Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
221
Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece
222
International Center for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo, Japan
223
Graduate School of Science and Technology, Tokyo Metropolitan University, Tokyo, Japan
224
Department of Physics, Tokyo Institute of Technology, Tokyo, Japan
225
Tomsk State University, Tomsk, Russia
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
Department of Physics and Astronomy, University of Uppsala, Uppsala, Sweden
233
Department of Physics, University of Illinois, Urbana, IL, USA
234
Instituto de Física Corpuscular (IFIC), Centro Mixto Universidad de Valencia-CSIC, Valencia, Spain
235
Department of Physics, University of British Columbia, Vancouver, BC, Canada
236
Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada
237
Fakultät für Physik und Astronomie, Julius-Maximilians-Universität Würzburg, Würzburg, Germany
238
Department of Physics, University of Warwick, Coventry, UK
239
Waseda University, Tokyo, Japan
240
Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, Israel
241
Department of Physics, University of Wisconsin, Madison, WI, USA
242
Fakultät für Mathematik und Naturwissenschaften, Fachgruppe Physik, Bergische Universität Wuppertal, Wuppertal, Germany
243
Department of Physics, Yale University, New Haven, CT, USA
244
Borough of Manhattan Community College, City University of New York, New York, NY, USA
245
Centro Studi e Ricerche Enrico, Fermi, Italy
246
CERN, Geneva, Switzerland
247
CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France
248
Département de Physique Nucléaire et Corpusculaire, Université de Genève, Geneva, Switzerland
249
Departament de Fisica de la Universitat Autonoma de Barcelona, Barcelona, Spain
250
Department of Financial and Management Engineering, University of the Aegean, Chios, Greece
251
Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA
252
Department of Physics and Astronomy, University of Louisville, Louisville, KY, USA
253
Department of Physics, Ben Gurion University of the Negev, Beer Sheva, Israel
254
Department of Physics, California State University, East Bay, USA
255
Department of Physics, California State University, Fresno, USA
256
Department of Physics, California State University, Sacramento, USA
257
Department of Physics, King’s College London, London, UK
258
Department of Physics, St. Petersburg State Polytechnical University, St. Petersburg, Russia
259
Department of Physics, University of Fribourg, Fribourg, Switzerland
260
Dipartimento di Matematica, Informatica e Fisica, Università di Udine, Udine, Italy
261
Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow, Russia
262
Giresun University, Faculty of Engineering, Giresun, Turkey
263
Graduate School of Science, Osaka University, Osaka, Japan
264
Hellenic Open University, Patras, Greece
265
IJCLab, Université Paris-Saclay, CNRS/IN2P3, 91405, Orsay, France
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 for Particle and Nuclear Physics, Wigner Research Centre for Physics, Budapest, Hungary
270
Institute of Particle Physics (IPP), Montreal, Canada
271
Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan
272
Instituto de Fisica Teorica, IFT-UAM/CSIC, Madrid, Spain
273
Istanbul University, Dept. of Physics, Istanbul, Turkey
274
Joint Institute for Nuclear Research, Dubna, Russia
275
Louisiana Tech University, Ruston, LA, USA
276
Moscow Institute of Physics and Technology State University, Dolgoprudny, Russia
277
National Research Nuclear University MEPhI, Moscow, Russia
278
Physics Department, An-Najah National University, Nablus, Palestine
279
Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
280
The City College of New York, New York, NY, USA
281
TRIUMF, Vancouver, BC, Canada
282
Universita di Napoli Parthenope, Naples, Italy
283
University of Chinese Academy of Sciences (UCAS), Beijing, China
284
CERN, 1211, Geneva 23, Switzerland
Received:
19
August
2020
Accepted:
31
January
2021
Published online:
5
March
2021
A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 
of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on the final state with two light leptons (electrons or muons) of different flavour and charge combinations, with at least two jets and large missing transverse momentum. No significant excess over the Standard Model expectation is observed. The results are translated into exclusion limits on heavy-lepton masses, and the observed lower limit on the mass of the type-III seesaw heavy leptons is 790 GeV at 95% confidence level.
© CERN for the benefit of the ATLAS collaboration 2021
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