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

Abstract

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.

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