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 ?, 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 4? < 100 GeV, where m 4? 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 ?(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.

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