| Abstrakt: |
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 = 7TeV 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 < m4l < 100 GeV, where m4l is the invariant mass of the four leptons, and mll > 4 GeV for all pairs of leptons, where mll is the two-lepton invariant mass. The measured branching fraction is B(Z → 4l) = (4:2-0:8+0:9(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) = 112-20+23(stat.)-5+7(syst.)-2+3(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. [ABSTRACT FROM AUTHOR] |
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