Publication Date:
2012-12-01
Description:
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 $ 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 σ, 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 $ mathcal{B}left( {Zo 4ell }
ight)=left( {4.2_{-0.8}^{+0.9}left( {mathrm{stat}.}
ight)pm 0.2left( {mathrm{syst}.}
ight)}
ight)imes {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}o mathrm{Z}}
ight)mathcal{B}left( {mathrm{Z}o 4ell }
ight)=112_{-20}^{+23}left( {mathrm{stat}.}
ight)_{-5}^{+7}left( {mathrm{syst}.}
ight)_{-2}^{+3}left( {mathrm{lumi}.}
ight) $ 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.
Print ISSN:
1126-6708
Electronic ISSN:
1029-8479
Topics:
Physics
Permalink