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Evidence for the Rare Decay B+→Λ¯pμ+μ−

R. Aaij38, A. S. W. Abdelmotteleb58, C. Abellan Beteta52, F. Abudinén58, T. Ackernley62, A. A. Adefisoye70, B. Adeva48, M. Adinolfi56, P. Adlarson86 et al. (LHCb Collaboration)

P. Adlarson86, C. Agapopoulou14, C. A. Aidala88, Z. Ajaltouni11, S. Akar11, K. Akiba38, M. Akthar40, P. Albicocco28, J. Albrecht19,b, R. Aleksiejunas82, F. Alessio50, P. Alvarez Cartelle57,48, R. Amalric16, S. Amato3, J. L. Amey56, Y. Amhis14, L. An6, L. Anderlini27, M. Andersson52, P. Andreola52, M. Andreotti26, S. Andres Estrada45, A. Anelli31,c, D. Ao7, C. Arata12, F. Archilli37, Z. Areg70, M. Argenton26, S. Arguedas Cuendis9,50, L. Arnone31,c, A. Artamonov44, M. Artuso70, E. Aslanides13, R. Ataíde Da Silva51, M. Atzeni66, B. Audurier12, J. A. Authier15, D. Bacher65, I. Bachiller Perea51, S. Bachmann22, M. Bachmayer51, J. J. Back58, P. Baladron Rodriguez48, V. Balagura15, A. Balboni26, W. Baldini26, Z. Baldwin80, L. Balzani19, H. Bao7, J. Baptista de Souza Leite2, C. Barbero Pretel48,12, M. Barbetti27, I. R. Barbosa71, R. J. Barlow64,a, M. Barnyakov25, S. Barsuk14, W. Barter60, J. Bartz70, S. Bashir40, B. Batsukh5, P. B. Battista14, A. Bavarchee81, A. Bay51, A. Beck66, M. Becker19, F. Bedeschi35, I. B. Bediaga2, N. A. Behling19, S. Belin48, A. Bellavista25, K. Belous44, I. Belov29, I. Belyaev36, G. Benane13, G. Bencivenni28, E. Ben-Haim16, A. Berezhnoy44, R. Bernet52, S. Bernet Andres47, A. Bertolin33, F. Betti60, J. Bex57, O. Bezshyyko87, S. Bhattacharya81, M. S. Bieker18, N. V. Biesuz26, A. Biolchini38, M. Birch63, F. C. R. Bishop10, A. Bitadze64, A. Bizzeti27,d, T. Blake58,e, F. Blanc51, J. E. Blank19, S. Blusk70, V. Bocharnikov44, J. A. Boelhauve19, O. Boente Garcia50, T. Boettcher69, A. Bohare60, A. Boldyrev44, C. Bolognani84, R. Bolzonella26,f, R. B. Bonacci1, N. Bondar44,50, A. Bordelius50, F. Borgato33,50, S. Borghi64, M. Borsato31,c, J. T. Borsuk85, E. Bottalico62, S. A. Bouchiba51, M. Bovill65, T. J. V. Bowcock62, A. Boyer50, C. Bozzi26, J. D. Brandenburg89, A. Brea Rodriguez51, N. Breer19, J. Brodzicka41, J. Brown62, D. Brundu32, E. Buchanan60, M. Burgos Marcos84, A. T. Burke64, C. Burr50, C. Buti27, J. S. Butter57, J. Buytaert50, W. 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  • 1School of Physics and Astronomy, Monash University, Melbourne, Australia
  • 2Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil
  • 3Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil
  • 4Department of Engineering Physics, Tsinghua University, Beijing, China
  • 5Institute Of High Energy Physics (IHEP), Beijing, China
  • 6School of Physics State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing, China
  • 7University of Chinese Academy of Sciences, Beijing, China
  • 8Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China
  • 9Consejo Nacional de Rectores (CONARE), San Jose, Costa Rica
  • 10Université Savoie Mont Blanc, CNRS, IN2P3-LAPP, Annecy, France
  • 11Université Clermont Auvergne, CNRS/IN2P3, LPC, Clermont-Ferrand, France
  • 12Université Paris-Saclay, Centre d’Etudes de Saclay (CEA), IRFU, Gif-Sur-Yvette, France
  • 13Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
  • 14Université Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France
  • 15Laboratoire Leprince-Ringuet, CNRS/IN2P3, Ecole Polytechnique, Institut Polytechnique de Paris, Palaiseau, France
  • 16Laboratoire de Physique Nucléaire et de Hautes Énergies (LPNHE), Sorbonne Université, CNRS/IN2P3, Paris, France
  • 17I. Physikalisches Institut, RWTH Aachen University, Aachen, Germany
  • 18Universität Bonn - Helmholtz-Institut für Strahlen und Kernphysik, Bonn, Germany
  • 19Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany
  • 20Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
  • 21Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany
  • 22Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany
  • 23School of Physics, University College Dublin, Dublin, Ireland
  • 24INFN Sezione di Bari, Bari, Italy
  • 25INFN Sezione di Bologna, Bologna, Italy
  • 26INFN Sezione di Ferrara, Ferrara, Italy
  • 27INFN Sezione di Firenze, Firenze, Italy
  • 28INFN Laboratori Nazionali di Frascati, Frascati, Italy
  • 29INFN Sezione di Genova, Genova, Italy
  • 30INFN Sezione di Milano, Milano, Italy
  • 31INFN Sezione di Milano-Bicocca, Milano, Italy
  • 32INFN Sezione di Cagliari, Monserrato, Italy
  • 33INFN Sezione di Padova, Padova, Italy
  • 34INFN Sezione di Perugia, Perugia, Italy
  • 35INFN Sezione di Pisa, Pisa, Italy
  • 36INFN Sezione di Roma La Sapienza, Roma, Italy
  • 37INFN Sezione di Roma Tor Vergata, Roma, Italy
  • 38Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands
  • 39Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, Netherlands
  • 40AGH - University of Krakow, Faculty of Physics and Applied Computer Science, Kraków, Poland
  • 41Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland
  • 42National Center for Nuclear Research (NCBJ), Warsaw, Poland
  • 43Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania
  • 44Authors affiliated with an institute formerly covered by a cooperation agreement with CERN
  • 45Universidade da Coruña, A Coruña, Spain
  • 46ICCUB, Universitat de Barcelona, Barcelona, Spain
  • 47La Salle, Universitat Ramon Llull, Barcelona, Spain
  • 48Instituto Galego de Física de Altas Enerxías (IGFAE), Universidade de Santiago de Compostela, Santiago de Compostela, Spain
  • 49Instituto de Fisica Corpuscular, Centro Mixto Universidad de Valencia - CSIC, Valencia, Spain
  • 50European Organization for Nuclear Research (CERN), Geneva, Switzerland
  • 51Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
  • 52Physik-Institut, Universität Zürich, Zürich, Switzerland
  • 53NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine
  • 54Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine
  • 55School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom
  • 56H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom
  • 57Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom
  • 58Department of Physics, University of Warwick, Coventry, United Kingdom
  • 59STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
  • 60School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom
  • 61School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom
  • 62Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom
  • 63Imperial College London, London, United Kingdom
  • 64Department of Physics and Astronomy, University of Manchester, Manchester, United Kingdom
  • 65Department of Physics, University of Oxford, Oxford, United Kingdom
  • 66Massachusetts Institute of Technology, Cambridge, MA, United States
  • 67University of Cincinnati, Cincinnati, OH, United States
  • 68University of Maryland, College Park, MD, United States
  • 69Los Alamos National Laboratory (LANL), Los Alamos, NM, United States
  • 70Syracuse University, Syracuse, NY, United States
  • 71Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil (associated with Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil)
  • 72Universidad Andres Bello, Santiago, Chile (associated with Physik-Institut, Universität Zürich, Zürich, Switzerland)
  • 73School of Physics and Electronics, Hunan University, Changsha City, China (associated with Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China)
  • 74State Key Laboratory of Nuclear Physics and Technology, South China Normal University, Guangzhou, China (associated with Department of Engineering Physics, Tsinghua University, Beijing, China)
  • 75Lanzhou University, Lanzhou, China (associated with Institute Of High Energy Physics (IHEP), Beijing, China)
  • 76School of Physics and Technology, Wuhan University, Wuhan, China (associated with Department of Engineering Physics, Tsinghua University, Beijing, China)
  • 77Henan Normal University, Xinxiang, China (associated with Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China)
  • 78Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia (associated with Laboratoire de Physique Nucléaire et de Hautes Énergies (LPNHE), Sorbonne Université, CNRS/IN2P3, Paris, France)
  • 79Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic (associated with Department of Physics and Astronomy, University of Manchester, Manchester, United Kingdom)
  • 80Ruhr Universitaet Bochum, Fakultaet f. Physik und Astronomie, Bochum, Germany (associated with Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany)
  • 81Eotvos Lorand University, Budapest, Hungary (associated with European Organization for Nuclear Research (CERN), Geneva, Switzerland)
  • 82Faculty of Physics, Vilnius University, Vilnius, Lithuania (associated with Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany)
  • 83Van Swinderen Institute, University of Groningen, Groningen, Netherlands (associated with Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands)
  • 84Universiteit Maastricht, Maastricht, Netherlands (associated with Nikhef National Institute for Subatomic Physics, Amsterdam, Netherlands)
  • 85Tadeusz Kosciuszko Cracow University of Technology, Cracow, Poland (associated with Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland)
  • 86Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden (associated with School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom)
  • 87Taras Schevchenko University of Kyiv, Faculty of Physics, Kyiv, Ukraine (associated with Université Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France)
  • 88University of Michigan, Ann Arbor, MI, United States (associated with Syracuse University, Syracuse, NY, United States)
  • 89Ohio State University, Columbus, United States (associated with Los Alamos National Laboratory (LANL), Los Alamos, NM, United States)

  • *Full author list given at the end of the Letter.
  • aDeceased.
  • bAlso at Lamarr Institute for Machine Learning and Artificial Intelligence, Dortmund, Germany.
  • cAlso at Università degli Studi di Milano-Bicocca, Milano, Italy.
  • dAlso at Università di Modena e Reggio Emilia, Modena, Italy.
  • eAlso at Department of Physics and Astronomy, University of Victoria, Victoria, Canada.
  • fAlso at Università di Ferrara, Ferrara, Italy.
  • gAlso at Universidade Estadual de Campinas (UNICAMP), Campinas, Brazil.
  • hAlso at Università di Bologna, Bologna, Italy.
  • iAlso at Università di Genova, Genova, Italy.
  • jAlso at Scuola Normale Superiore, Pisa, Italy.
  • kAlso at Università degli Studi di Milano, Milano, Italy.
  • lAlso at Center for High Energy Physics, Tsinghua University, Beijing, China.
  • mAlso at Universidad Nacional Autónoma de Honduras, Tegucigalpa, Honduras.
  • nAlso at Università di Cagliari, Cagliari, Italy.
  • oAlso at Centro Federal de Educacão Tecnológica Celso Suckow da Fonseca, Rio De Janeiro, Brazil.
  • pAlso at Università di Bari, Bari, Italy.
  • qAlso at Università di Perugia, Perugia, Italy.
  • rAlso at LIP6, Sorbonne Université, Paris, France.
  • sAlso at Università di Pisa, Pisa, Italy.
  • tAlso at Hangzhou Institute for Advanced Study, UCAS, Hangzhou, China.
  • uAlso at Università di Padova, Padova, Italy.
  • vAlso at Università di Bergamo, Bergamo, Italy.
  • wAlso at Universidad de Ingeniería y Tecnología (UTEC), Lima, Peru.
  • xAlso at Università di Siena, Siena, Italy.
  • yAlso at Universidad de Alcalá, Alcalá de Henares, Spain.
  • zAlso at Università di Urbino, Urbino, Italy.

Phys. Rev. Lett. 137, 051801 – Published 28 July, 2026

DOI: https://doi.org/10.1103/pl88-4435

Abstract

A search for the rare decay B+→Λ¯pμ+μ− is performed using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of s=13  TeV, corresponding to an integrated luminosity of 5.4  fb−1. An excess of events is found with respect to the background-only expectation, with a signal significance of 3.5 standard deviations, in the low invariant-mass region of m(Λ¯p)<2.8  GeV/c2. The branching fraction is measured to be Blow(B+→Λ¯pμ+μ−)=(1.70−0.56+0.65(stat)±0.17(syst)±0.14(ext))×10−8, where the last uncertainty is due to external inputs on B(B+→J/ψΛ¯p)×B(J/ψ→μ+μ−). With no significant signal observed in the high m(Λ¯p) region above 2.8  GeV/c2, an upper limit is set to be Bhigh(B+→Λ¯pμ+μ−)<2.8(3.7)×10−9 at the 90% (95%) confidence level.

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