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

A bstract 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 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 and agrees with the standard model prediction of 4.45 × 10 −6 . The measured cross section times branching fraction is fb, also consistent with the standard model prediction of 120 fb. The four-lepton mass peak arising from Z → 4 ℓ decays provides a calibration channel for the Higgs boson search in the H → ZZ → 4 ℓ decay mode.