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CMS-PAS-SMP-16-017
Measurement of the $ \mathrm{ pp \rightarrow ZZ } $ production cross section, $\mathrm{ Z } \to {4\ell} $ branching fraction and constraints on anomalous triple gauge couplings at $ \sqrt{s} = $ 13 TeV
Abstract: Four-lepton production in proton-proton collisions, ${\mathrm{ p }\mathrm{ p }} \to(\mathrm{ Z } / \gamma^*)(\mathrm{ Z }/ \gamma^*) \to {4\ell} $, where $\ell, \ell' = \mathrm{ e }$ or $\mu$, is studied at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The ZZ production cross section, $\sigma(\mathrm{ p }\mathrm{ p } \to \mathrm{ Z }\mathrm{ Z }) =$ 17.8 $\pm$ 0.6 (stat)$ _{-0.6}^{+0.7}$ (syst) $\pm$ 0.4 (theory) $\pm$ 0.5 (lumi) pb, measured for events with two opposite-sign, same-flavor lepton pairs produced in the mass region 60 $ < m_{\ell^+\ell^-}, m_{\ell^{\prime +}\ell^{\prime -}} < $ 120 GeV is consistent with standard model predictions. Differential cross sections are measured and well described by the theoretical predictions. The Z boson branching fraction to four leptons is measured to be $\mathcal{B}(\mathrm{ Z } \to {4\ell}) =$ 4.74 $\pm$ 0.16 (stat)$ _{-0.17}^{+0.18}$ (syst) $\pm$ 0.08 (theory) $\pm$ 0.12 (lumi) $\times 10^{-6}$ the four-lepton invariant mass in the range 80 $ < m_{{4\ell} } < $ 100 GeV and dilepton mass $m_{{\ell^+\ell^-} } > $ 4 GeV for all opposite-sign, same-flavor lepton pairs. The invariant mass distribution of the four-lepton system is used to set limits on anomalous $\mathrm{ Z }\mathrm{ Z }\mathrm{ Z }$ and $\mathrm{ Z }\mathrm{ Z }\gamma$ couplings at the 95% confidence level: $ - 0.00117 < f_4^\mathrm{ Z } < 0.00110 $, $ - 0.00100 < f_5^\mathrm{ Z } < 0.00125 $, $ -0.00133 < f_4^{\gamma} < 0.00132 $, and $ -0.00123 < f_5^{\gamma} < 0.00130 $. The results agree with standard model predictions.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Distributions of (left) the four-lepton invariant mass $m_{ {4\ell } }$ and (right) the invariant mass of the dilepton candidates in all selected four-lepton events, including both $\mathrm{ Z } _1$ and $\mathrm{ Z } _2$ in each event. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 1-a:
Distribution of the four-lepton invariant mass $m_{ {4\ell } }$. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 1-b:
Distribution of the invariant mass of the dilepton candidates in all selected four-lepton events, including both $\mathrm{ Z } _1$ and $\mathrm{ Z } _2$ in each event. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 2:
At (left) the distribution of the reconstructed mass of the $\mathrm{ Z } _1$ candidate. Points represent the data, while shaded histograms represent the SM prediction and background estimate. At (right) the reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events, with distinctive markers for each final state.

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Figure 2-a:
The distribution of the reconstructed mass of the $\mathrm{ Z } _1$ candidate. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 2-b:
The reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events, with distinctive markers for each final state.

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Figure 3:
At (left), the distribution of the reconstructed four-lepton mass $m_{ {4\ell } }$ for events selected with $m_{ {4\ell } } < $ 100 GeV. Points represent the data, while shaded histograms represent the SM prediction and background estimate. At (right) the reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events selected with $m_{ {4\ell } }$ between 80 and 100 GeV, with distinctive markers for each final state.

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Figure 3-a:
The distribution of the reconstructed four-lepton mass $m_{ {4\ell } }$ for events selected with $m_{ {4\ell } } < $ 100 GeV. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 3-b:
The reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events selected with $m_{ {4\ell } }$ between 80 and 100 GeV, with distinctive markers for each final state.

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Figure 4:
Distributions of (left) the four-lepton invariant mass $m_{ {4\ell } }$ and (right) dilepton candidate mass for four-lepton events selected with both Z bosons on-shell. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 4-a:
Distribution of the four-lepton invariant mass $m_{ {4\ell } }$. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 4-b:
Distribution of the dilepton candidate mass for four-lepton events selected with both Z bosons on-shell. Points represent the data, while shaded histograms represent the SM prediction and background estimate.

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Figure 5:
The total $ {\mathrm{ Z } \mathrm{ Z } } $ cross section as a function of the proton-proton center-of-mass energy. Results from the CMS experiment are compared to predictions from matrix and mcfm with NNPDF3.0 PDF sets and fixed scales $\mu _F = \mu _R = m_\mathrm{ Z } $. Details of the calculations and uncertainties are given in the text. Measurements at the same center-of-mass energy are shifted slightly along the horizontal axis for clarity.

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Figure 6:
Differential cross sections normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of $ {p_{\mathrm {T}}} $ for (upper left) the highest ${p_{\mathrm {T}}}$ lepton in the event, (upper right) all Z bosons in $ {\mathrm{ Z } \mathrm{ Z } } $ events, and (lower left) the ZZ system. Figure (lower right) shows the normalized $ {\mathrm {d}}\sigma / {\mathrm {d}}{m_{\mathrm{ Z } \mathrm{ Z } }}$ distribution. Points represent the unfolded data, and the shaded histogram represent the POWHEG+MCFM predictions for ZZ signal, and the solid curves correspond to the results of the MadGraph5-aMC@NLO+MCFM calculations. The two lower plots in each subfigure represent the ratio of the measured cross section to the expected distributions from POWHEG+MCFM (middle plot) and MadGraph5-aMC@NLO+MCFM (bottom plot). The hatched areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only.

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Figure 6-a:
Differential cross section normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of the $ {p_{\mathrm {T}}} $ of the highest-${p_{\mathrm {T}}}$ lepton in the event. Points represent the unfolded data, and the shaded histogram represent the POWHEG+MCFM predictions for ZZ signal, and the solid curves correspond to the results of the MadGraph5-aMC@NLO+MCFM calculations. The two lower plots represent the ratio of the measured cross section to the expected distributions from POWHEG+MCFM (middle plot) and MadGraph5-aMC@NLO+MCFM (bottom plot). The hatched areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only.

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Figure 6-b:
Differential cross section normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of

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Figure 6-c:
Differential cross section normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of

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Figure 6-d:
Normalized $ {\mathrm {d}}\sigma / {\mathrm {d}}{m_{\mathrm{ Z } \mathrm{ Z } }}$ distribution. Points represent the unfolded data, and the shaded histogram represent the POWHEG+MCFM predictions for ZZ signal, and the solid curves correspond to the results of the MadGraph5-aMC@NLO+MCFM calculations. The two lower plots represent the ratio of the measured cross section to the expected distributions from POWHEG+MCFM (middle plot) and MadGraph5-aMC@NLO+MCFM (bottom plot). The hatched areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only.

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Figure 7:
Differential cross sections normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of (left) azimuthal separation of the two Z bosons and (right) $\Delta R$ between the Z-bosons. Points represent the unfolded data, and the shaded histogram represent the POWHEG+MCFM predictions for ZZ signal, and the solid curves correspond to the results of the MadGraph5-aMC@NLO+MCFM calculations. The two lower plots in each subfigure represent the ratio of the measured cross section to the expected distributions from POWHEG+MCFM (middle plot) and MadGraph5-aMC@NLO+MCFM (bottom plot). The hatched areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only.

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Figure 7-a:
Differential cross sections normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of the azimuthal separation of the two Z bosons. Points represent the unfolded data, and the shaded histogram represent the POWHEG+MCFM predictions for ZZ signal, and the solid curves correspond to the results of the MadGraph5-aMC@NLO+MCFM calculations. The two lower plots represent the ratio of the measured cross section to the expected distributions from POWHEG+MCFM (middle plot) and MadGraph5-aMC@NLO+MCFM (bottom plot). The hatched areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only.

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Figure 7-b:
Differential cross sections normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of the $\Delta R$ between the Z-bosons. Points represent the unfolded data, and the shaded histogram represent the POWHEG+MCFM predictions for ZZ signal, and the solid curves correspond to the results of the MadGraph5-aMC@NLO+MCFM calculations. The two lower plots represent the ratio of the measured cross section to the expected distributions from POWHEG+MCFM (middle plot) and MadGraph5-aMC@NLO+MCFM (bottom plot). The hatched areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only.

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Figure 8:
Differential cross sections normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of the four-lepton mass for events subject only to the common requirements of Table 4. SM $\gamma \gamma \to \mathrm{ H } \to \mathrm{ Z } \mathrm{ Z } ^* $ production is included, simulated with POWHEG. Points represent the unfolded data, and the shaded histogram represent the POWHEG+MCFM predictions for ZZ signal, and the solid curves correspond to the results of the MadGraph5-aMC@NLO+MCFM+POWHEG calculations. The two lower plots represent the ratio of the measured cross section to the expected distributions from POWHEG+MCFM. The hatched areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only.

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Figure 9:
Distribution of the four-lepton reconstructed mass for the combined 4e, 4$\mu $, and 2e2$\mu$ channels. Points represent the data, the shaded histograms represent the SM prediction including signal and irreducible background Monte Carlo and the data-driven background estimate. Dashed histograms represent example aTGC signals (colors), and the SHERPA SM prediction (black), included to illustrate the expected shape differences between the SHERPA and POWHEG samples. The SHERPA distributions are normalized such that the SM sample has the same total yield as the POWHEG sample predicts. The last bin includes the "overflow'' contribution from events at masses above 1.2 TeV.

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Figure 10:
Two-dimensional exclusion limits at 68% (dashed contour) and 95% (solid contour) CL on the $\mathrm{ Z } \mathrm{ Z } \mathrm{ Z } $ and $\mathrm{ Z } \mathrm{ Z } \gamma $ aTGCs. The left (right) plot shows the exclusion contour in the $(f_{4(5)}^\mathrm{ Z }, f_{4(5)}^\gamma)$ parameter planes. The values of couplings outside of contours are excluded at the corresponding confidence level. The solid dot is the point at which the likelihood is at its maximum. No form factor is used.

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Figure 10-a:
Two-dimensional exclusion limits at 68% (dashed contour) and 95% (solid contour) CL on the $\mathrm{ Z } \mathrm{ Z } \mathrm{ Z } $ and $\mathrm{ Z } \mathrm{ Z } \gamma $ aTGCs. The plot shows the exclusion contour in the $(f_{4}^\mathrm{ Z }, f_{4}^\gamma)$ parameter planes. The values of couplings outside of contours are excluded at the corresponding confidence level. The solid dot is the point at which the likelihood is at its maximum. No form factor is used.

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Figure 10-b:
Two-dimensional exclusion limits at 68% (dashed contour) and 95% (solid contour) CL on the $\mathrm{ Z } \mathrm{ Z } \mathrm{ Z } $ and $\mathrm{ Z } \mathrm{ Z } \gamma $ aTGCs. The plot shows the exclusion contour in the $(f_{5}^\mathrm{ Z }, f_{5}^\gamma)$ parameter planes. The values of couplings outside of contours are excluded at the corresponding confidence level. The solid dot is the point at which the likelihood is at its maximum. No form factor is used.
Tables

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Table 1:
The contributions of each source of signal systematic uncertainty in the cross section measurements. The integrated luminosity uncertainty and the PDF and scale uncertainties are considered separately. All other uncertainties are added in quadrature into a single systematic uncertainty. Uncertainties that vary by decay channel are listed as a range.

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Table 2:
The observed and expected yields of four-lepton events in the mass region 80 $ < m_{ {4\ell } } < $ 100 GeV and estimated yields of background events evaluated from data, shown for each final state and summed in the total expected yield. The first uncertainty is statistical, the second one is systematic. The systematic uncertainties do not include the uncertainty on the integrated luminosity.

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Table 3:
The observed and expected yields of ZZ events, and estimated yields of background events evaluated from data, shown for each final state and summed in the total expected yield. The first uncertainty is statistical, the second one is systematic. The systematic uncertainties do not include the uncertainty on the integrated luminosity.

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Table 4:
Fiducial definitions for the reported cross sections. The common requirements are applied for both measurements.
Summary
Results have been presented for a study of four-lepton final states in proton-proton collisions at $ \sqrt{s} = $ 13 TeV with the CMS detector at the LHC. The $\mathrm{ pp \to ZZ }$ cross section has been measured to be $\sigma(\mathrm{ p }\mathrm{ p } \to \mathrm{ Z }\mathrm{ Z }) = $ 17.8 $\pm$ 0.6 (stat)$ _{-0.6}^{+0.7}$ (syst) $\pm$ 0.4 (theory) $\pm$ 0.5 (lumi) pb measured for Z boson masses in the range 60 $ < m_{\mathrm{ Z }} < $ 120 GeV. The branching fraction for Z boson decays to four leptons has been measured to be $\mathcal{B}(\mathrm{ Z } \to {4\ell}) = $ 4.74 $\pm$ 0.16 (stat)$ _{-0.17}^{+0.18}$ (syst) $\pm$ 0.08 (theory) $\pm$ 0.12 (lumi) $\times 10^{-6}$ for four-lepton mass in the range 80 $ < m_{{4\ell} } < $ 100 GeV and dilepton mass $m_{{\ell^+\ell^-} } > $ 4 GeV for all oppositely charged same-flavor lepton pairs. The results are consistent with SM predictions. The normalized differential cross sections have also been measured and agree well with the SM predictions. Improved limits on anomalous $\mathrm{ Z } \mathrm{ Z } \mathrm{ Z }$ and $\mathrm{ Z } \mathrm{ Z } \gamma$ triple gauge couplings are established, significantly restricting their allowed ranges.
Additional Figures

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Additional Figure 1:
The total ZZ cross section as a function of the proton-proton center-of-mass energy. Results from the CMS and ATLAS experiments are compared to predictions from matrix at NNLO in QCD, and MCFM at NLO in QCD. The MCFM prediction further includes gluon-gluon initiated production at LO in QCD. Both predictions use NNPDF3.0 PDF sets and fixed scales $\mu _F = \mu _R = m_{\mathrm{Z}} $. Details of the calculations and uncertainties are given in the text of SMP-16-017. ATLAS measurements were performed with a Z mass window of 66-116 GeV, and are corrected for the resulting 1.6%difference. Measurements at the same center-of-mass energy are shifted slightly along the horizontal axis for clarity.
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