  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 inverse femtobarns at sqrt(s) = 7 TeV collected
by the CMS detector at the Large Hadron Collider. A pronounced resonance peak,
with a statistical significance of 9.7 sigma, 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 to 4l) = (4.2
/+0.9/-0.8 (stat.) +/- 0.2 (syst.)) 10E-6 and agrees with the standard model
prediction of 4.45 10E-6. The measured cross section times branching fraction
is sigma(pp to Z) B(Z to 4 l) = 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 to 4 l decays provides a calibration
channel for the Higgs boson search in the H to ZZ to 4 l decay mode.
