Observation of Z to 4l production in pp collisions at 7 TeV

We present the first observation of the Z boson decaying to 4 leptons in proton-proton collisions. The analyzed dataset corresponds to an integrated luminosity of 4.7~fb$^{-1}$ at $\sqrt{s}=7$~TeV collected by the CMS detector at the Large Hadron Collider. A pronounced resonance peak, with the statistical significance of 8.9$\sigma$, is observed in the distribution of invariant mass of four leptons with its mean and width consistent with the Z boson. With the kinematic requirements imposed in the analysis, we observe 26~events in the mass window of 80--100 GeV, in agreement with the expected rate of 25.0~events, comprised of $24.6 \pm 2.2$ $\mathrm{Z} \to 4\ell$ events and $0.4 \pm 0.1$~events from backgrounds. The measured branching fraction of $\mathrm{Z} \to 4\ell$ decays with a cut on the minimum di-lepton mass $m_{2\ell} > 4$~GeV is $BR(Z \to 4\ell) = 4.4^{+1.0}_{-0.8} \mathrm{(stat)} \pm 0.2 \mathrm{(syst)} \times 10^{-6}$~and agrees with the standard model prediction of $4.45 \times 10^{-6}$. The measured cross section times branching fraction is $\sigma \times BR(Z \to 4\ell) = 125^{+26}_{-23} \mathrm{(stat)} ^{+9}_{-6} \mathrm{(syst)} ^{+7}_{-5} \mathrm{(lumi)}~\mathrm{fb}$, also consistent with the standard model prediction of 120~fb. The four-lepton mass peak arising from $\mathrm{Z} \to 4\ell$ decays provides a natural standard candle for the Higgs boson search in the $\mathrm{H} \to \mathrm{ZZ} \to 4\ell$ decay mode.