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CMS-SMP-12-009 ; CERN-PH-EP-2012-283
Observation of Z decays to four leptons with the CMS detector at the LHC
J. High Energy Phys. 12 (2012) 034
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 fb1 at 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 <m4< 100 GeV, where m4 is the invariant mass of the four leptons, and m> 4 GeV for all pairs of leptons, where m is the two-lepton invariant mass. The measured branching fraction is B(Z4)= ( 4.2 +0.90.8 (stat.) ± 0.2 (syst.) ) ×106 and agrees with the standard model prediction of 4.45 ×106. The measured cross section times branching fraction is σ(Z)B(Z4)= 112 +2320 (stat.) +75 (syst.) +32 (lumi.) fb, also consistent with the standard model prediction of 120 fb. The four-lepton mass peak arising from Z4 decays provides a calibration channel for the Higgs boson search in the HZZ4 decay mode.
Figures & Tables Summary CMS Publications
Figures

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Figure 1-a:
(a) Diagram of the Z4 process. (b) Diagram of the Zγ4 process for the irreducible background of Z2 production with the initial-state radiation undergoing an internal conversion γ2. Both Z and γ are present in all propagators. The choice of propagators shown in the figures corresponds to the dominant contributions in the phase space 80 <m4< 100 GeV.

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Figure 1-b:
(a) Diagram of the Z4 process. (b) Diagram of the Zγ4 process for the irreducible background of Z2 production with the initial-state radiation undergoing an internal conversion γ2. Both Z and γ are present in all propagators. The choice of propagators shown in the figures corresponds to the dominant contributions in the phase space 80 <m4< 100 GeV.

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Figure 2:
Four-lepton invariant mass distribution for events passing all selection requirements except that on m4. The data are shown by points. The filled histograms represent standard model expectations for ppZ/Zγ4 and for reducible backgrounds. The three final states, 4e, 4μ, and 2e2μ, are combined.

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Figure 3-a:
(a) Four-lepton mass distribution in simulation for pp4, without the Higgs boson (light shaded histogram), Z+X background (dark shaded histogram), and ppHZZ4 for a Higgs boson mass mH= 125 GeV. The three contributions are stacked. The standard model cross section for the Higgs boson is scaled up by a factor of 5. (b) Four-lepton mass distribution with data represented by the points with error bars. The three final states, 4e, 4μ, and 2e2μ, are combined. The solid line represents a simultaneous fit to the background and Z boson peak.

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Figure 3-b:
(a) Four-lepton mass distribution in simulation for pp4, without the Higgs boson (light shaded histogram), Z+X background (dark shaded histogram), and ppHZZ4 for a Higgs boson mass mH= 125 GeV. The three contributions are stacked. The standard model cross section for the Higgs boson is scaled up by a factor of 5. (b) Four-lepton mass distribution with data represented by the points with error bars. The three final states, 4e, 4μ, and 2e2μ, are combined. The solid line represents a simultaneous fit to the background and Z boson peak.
Tables

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Table 1:
Partial widths and branching fractions for Z boson decays to 4e, 4μ, 2e2μ final states with m> 4 GeV for all lepton pairs. The branching fractions are calculated with CalcHEP 3.2 [7] at LO using the total Z boson width Γtot= 2.4952 GeV [9]. Theoretical uncertainties are smaller than experimental uncertainties and are not shown in the table.

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Table 2:
Numbers of expected and observed events with 80 <m4< 100 GeV. The yields are determined by means of fits described in the text; the fit results in the 4e channel are not meaningful because only two events are selected.
Summary
The first observation of the Z boson decaying to four leptons (electrons and/or muons) in proton-proton collisions has been presented. The data set analyzed corresponds to an integrated luminosity of 5.02 fb1 at s= 7 TeV. A pronounced resonance peak, with a statistical significance of 9.7σ, is observed in the distribution of the invariant mass of four leptons with mass and width consistent with expectations for Z boson decays. The event yields, branching fraction, and cross section reported here are defined by phase space restrictions on the leptons, namely, 80 <m4< 100 GeV, where m4 is the invariant mass of the four leptons, and m> 4 GeV for all pairs of leptons, where m is the two-lepton invariant mass. We observe 28 events, in agreement with the expectation of 30.0 ± 2.6 events, comprised of 29.4 ± 2.6 Z4 events and 0.6 ± 0.2 events from backgrounds. The measured branching fraction is B(Z4)= ( 4.2 +0.90.8 (stat.) ± 0.2 (syst.) ) ×106, in agreement with the standard model prediction of 4.45 ×106. The measured cross section times branching fraction is σ(Z)B(Z4)= 112 +2320 (stat.) +75 (syst.) +32 (lumi.) fb, also consistent with the standard model prediction of 120 fb. The four-lepton mass peak arising from Z4 decays provides a calibration channel for the Higgs boson search in the HZZ4 decay mode.
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