Search for an Invisibly Decaying Z′ Boson at Belle II in e+e- →μ+μ- (e±μ) Plus Missing Energy Final States

(Belle II Collaboration)

Research output: Contribution to journalArticlepeer-review


Theories beyond the standard model often predict the existence of an additional neutral boson, the Z′. Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the first searches for the invisible decay of a Z′ in the process e+e-→μ+μ-Z′ and of a lepton-flavor-violating Z′ in e+e-→e±μZ′. We do not find any excess of events and set 90% credibility level upper limits on the cross sections of these processes. We translate the former, in the framework of an Lμ-Lτ theory, into upper limits on the Z′ coupling constant at the level of 5×10-2-1 for MZ′≤6 GeV/c2.

Original languageEnglish
Article number141801
JournalPhysical Review Letters
Issue number14
StatePublished - 10 Apr 2020


FundersFunder number
7th FrameworkPIEF-GA-2013-622527
Chinese Academy of Sciences and ResearchQYZDJ-SSW-SLH011
Department of Atomic Energy and Department of Science and Technology
Horizon 2020 ERC-Advanced267104, 638528
Horizon 2020 Marie Sklodowska-Curie
National Natural Science Foundation of China and Research11521505, 11761141009, 11975076, 11575017, 11675166, 11705209
Science Committee of the Republic of Armenia18T-1C180
Shanghai Pujiang Program18PJ1401000
U.S. Department of Energy and ResearchDE-AC06-76RLO1830, DE-SC0010118, DE-SC0010504, DE-SC0012704, DE-SC0010073, DE-SC0009824, DE-SC0007983, DE-SC0009973, DE-SC0011784
National Science FoundationPHY-1807007, PHY-1913789
Alexander von Humboldt-Stiftung
National Foundation for Science and Technology Development103.99-2018.45
Univerzita Karlova v PrazeSVV 260448, GAUK 404316
Horizon 2020 Framework Programme700525, 644294
Thailand Center of Excellence in Physics
Compute Canada
Natural Sciences and Engineering Research Council of Canada
European Research Council
Australian Research CouncilDP170102204, DP180102629, DP150103061, FT130100018, DP170102389, FT130100303
Deutsche Forschungsgemeinschaft
Japan Society for the Promotion of Science16H06492, 18H05226, 18K03621, 26400255, 19H00682, 26220706, 16H03968, 17H01133, 17H05405, 16K05323, 18H03710, 16H03993
Ministry of Education, Culture, Sports, Science and Technology
United States-Israel Binational Science Foundation2016113
Ministerstvo Školství, Mládeže a TělovýchovyLTT17020
Ministry of Education of the People's Republic of China
Bundesministerium für Bildung und Forschung
Chinese Academy of Sciences
Austrian Science FundP 31361-N36
Ministry of Higher Education, MalaysiaFOINS-296, CB-180023, CB-236394, CB-254409, CB-221329
Generalitat Valenciana
Science and Technology Commission of Shanghai Municipality19ZR1403000
Ministry of Education and Science of the Russian Federation
Korea Institute of Science and Technology Information
National Research Foundation of Korea2016R1D1A1B02012900, 2018R1A2B3003643, 2018R1D1A1B07047294, 2019R1I1A3A01058933, 2016R1D1A1B01010135, 2019K1A3A7A09033840, 2018R1A4A1025334, 2018R1A6A1A06024970
Israel Science Foundation2476/17
Instituto Nazionale di Fisica Nucleare
Narodowe Centrum Nauki
Javna Agencija za Raziskovalno Dejavnost RSJ1-9124, P1-0135
Universiti Malaya
Ministerstwo Edukacji i Nauki
Centre National de la Recherche Scientifique
Akademi Sains Malaysia
Ministry of Education and Science of Ukraine
Ministry of Science Research and TechnologyMOST107-2119-M-002-035-MY3, MOST106-2112-M-002-005-MY3
Agencia Estatal de InvestigaciónCIDEGENT/2018/020, FPA2017-84445-P, FPA2014-55613-P
National Institute of Informatics
Institut National de Physique Nucléaire et de Physique des Particules
Bundesministerium für Bildung, Wissenschaft und Forschung
CAS Center for Excellence in Particle Physics
Liaoning Revitalization Talents ProgramXLYC1807135
Korea Research Environment Open Network


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