Observation of Z decays to four leptons with the CMS detector at the LHC
Baarmand, Marc M.
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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 < m4` < 100 GeV, where m4` is the invariant mass of the four leptons, and m`` > 4 GeV for all pairs of leptons, where m`` is the two-lepton invariant mass. The measured branching fraction is B(Z → 4`) = 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 → 4`) = 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 → 4` decays provides a calibration channel for the Higgs boson search in the H → ZZ → 4` decay mode.