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 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)=({4.2-0.8+0.9 (stat.)±0.2(syst.))×times 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 ›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.[Figure not available: see fulltext.] © 2012 CERN for the benefit of the CMS collaboration.