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Published December 2012 | Submitted + Published
Journal Article Open

Observation of Z decays to four leptons with the CMS detector at the LHC


The first observation of the Z boson decaying to four leptons in proton-proton collisions is presented. The analyzed data set corresponds to an integrated luminosity of 5.02 fb^(−1) at s√=7 TeV collected by the CMS detector at the Large Hadron Collider. A pronounced resonance peak, with a statistical significance of 9.7 σ, is observed in the distribution of the invariant mass of four leptons (electrons and/or muons) with mass and width consistent with expectations for Z boson decays. The branching fraction and cross section reported here are defined by phase space restrictions on the leptons, namely, 80 < m_(4ℓ) < 100 GeV, where m_(4ℓ) is the invariant mass of the four leptons, and m_(ℓℓ) > 4 GeV for all pairs of leptons, where m_(ℓℓ) is the two-lepton invariant mass. The measured branching fraction is B(Z→4ℓ)= 4.2^(+0.9)_(−0.8)(stat.)±0.2(syst.))×10^(−6) and agrees with the standard model prediction of 4.45 × 10^(−6). The measured cross section times branching fraction is σ(pp→Z)B(Z→4ℓ)=112^(+23)_(−20)(stat.)^(+7)_(−5)(syst.)^(+3)_(−2)(lumi.) fb, also consistent with the standard model prediction of 120 fb. The four-lepton mass peak arising from Z → 4ℓ decays provides a calibration channel for the Higgs boson search in the H → ZZ → 4ℓ decay mode.

Additional Information

© 2012 CERN for the benefit of the CMS collaboration. Received: October 14, 2012. Accepted: November 19, 2012. Published: December 7, 2012. This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited. We congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC machine. We thank the technical and administrative staff at CERN and other CMS institutes, and acknowledge support from BMWF and FWF (Austria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES (Croatia); RPF (Cyprus); MoER, SF0690030s09 and ERDF (Estonia); Academy of Finland, MEC, and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NKTH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Korea); LAS (Lithuania); CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mexico); MSI (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portugal); JINR (Armenia, Belarus, Georgia, Ukraine, Uzbekistan); MON, RosAtom, RAS and RFBR (Russia); MSTD (Serbia); SEIDI and CPAN (Spain); Swiss Funding Agencies (Switzerland); NSC (Taipei); ThEP, IPST and NECTEC (Thailand); TUBITAK and TAEK (Turkey); NASU (Ukraine); STFC (United Kingdom); DOE and NSF (USA).

Attached Files

Published - CMS_2012p034.pdf

Submitted - 1210.3844v2.pdf


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August 19, 2023
October 23, 2023