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

Abstract

AbstractThe 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−1at$ \sqrt{s}=7 $TeV collected by the CMS detector at the Large Hadron Collider. A pronounced resonance peak, with a statistical significance of 9.7σ, is observed in the distribution of the invariant mass of four leptons (electrons and/or muons) with mass and width consistent with expectations for Z boson decays. The branching fraction and cross section reported here are defined by phase space restrictions on the leptons, namely, 80 <m4ℓ< 100 GeV, wherem4ℓis the invariant mass of the four leptons, andmℓℓ> 4 GeV for all pairs of leptons, wheremℓℓis the two-lepton invariant mass. The measured branching fraction is$ \mathcal{B}\left( {Z\to 4\ell } \right)=\left( {4.2_{-0.8}^{+0.9}\left( {\mathrm{stat}.} \right)\pm 0.2\left( {\mathrm{syst}.} \right)} \right)\times {10^{-6 }} $and agrees with the standard model prediction of 4.45 × 10−6. The measured cross section times branching fraction is$ \sigma \left( {\mathrm{pp}\to \mathrm{Z}} \right)\mathcal{B}\left( {\mathrm{Z}\to 4\ell } \right)=112_{-20}^{+23}\left( {\mathrm{stat}.} \right)_{-5}^{+7}\left( {\mathrm{syst}.} \right)_{-2}^{+3}\left( {\mathrm{lumi}.} \right) $fb, also consistent with the standard model prediction of 120 fb. The four-lepton mass peak arising fromZ→ 4ℓdecays provides a calibration channel for the Higgs boson search in theH→ZZ→ 4ℓdecay mode.