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| CMS-SMP-16-017 ; CERN-EP-2017-177 | ||
| Measurements of the ${\mathrm{p}}{\mathrm{p}}\to \mathrm{Z}\mathrm{Z}$ production cross section and the $\mathrm{Z} \to 4\ell$ branching fraction, and constraints on anomalous triple gauge couplings at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 25 September 2017 | ||
| Eur. Phys. J. C 78 (2018) 165 [Erratum] | ||
| Abstract: Four-lepton production in proton-proton collisions, ${\mathrm{p}}{\mathrm{p}} \to (\mathrm{Z} / \gamma^*)(\mathrm{Z} /\gamma^*) \to 4\ell$, where $\ell = $ 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}) = $ 17.2 $\pm$ 0.5 (stat) $\pm$ 0.7 (syst) $\pm$ 0.4 (theo) $\pm$ 0.4 (lumi) pb, measured using events with two opposite-sign, same-flavor lepton pairs produced in the mass region 60 $ < m_{\ell^+\ell^-} < $ 120 GeV, is consistent with standard model predictions. Differential cross sections are measured and are 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) = $ 4.8 $\pm$ 0.2 (stat) $\pm$ 0.2 (syst) $\pm$ 0.1 (theo) $\pm$ 0.1 (lumi) $ \times 10^{-6}$ for events with a four-lepton invariant mass in the range 80 $ < m_{4\ell} < $ 100 GeV and a dilepton mass $m_{\ell\ell} > $ 4 GeV for all opposite-sign, same-flavor lepton pairs. The results agree with standard model predictions. The invariant mass distribution of the four-lepton system is used to set limits on anomalous ZZZ and ZZ$\gamma$ couplings at 95% confidence level: $-0.0012 < f_4^\mathrm{Z} < 0.0010$, $-0.0010 < f_5^\mathrm{Z} < 0.0013$, $-0.0012 < f_4^{\gamma} < 0.0013$, $-0.0012 < f_5^{\gamma} < 0.0013$. | ||
| Links: e-print arXiv:1709.08601 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| 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 $\mathrm{Z} /\gamma ^* $ bosons in selected four-lepton events. Both selected dilepton candidates are included in each event. In the $m_{{4\ell}}$ distribution, bin contents are normalized to a bin width of 25 GeV ; horizontal bars on the data points show the range of the corresponding bin. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 1-a:
Distribution of the four-lepton invariant mass $m_{{4\ell}}$ in selected four-lepton events. Both selected dilepton candidates are included in each event. Bin contents are normalized to a bin width of 25 GeV ; horizontal bars on the data points show the range of the corresponding bin. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 1-b:
Distribution of the invariant mass of the dilepton candidates in all $\mathrm{Z} /\gamma ^* $ bosons in selected four-lepton events. Both selected dilepton candidates are included in each event. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 2:
(Left): the distribution of the reconstructed mass of $\mathrm{Z} _1$, the dilepton candidate closer to the nominal Z boson mass. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (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. For readability, only every fourth event is plotted. |
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Figure 2-a:
The distribution of the reconstructed mass of $\mathrm{Z} _1$, the dilepton candidate closer to the nominal Z boson mass. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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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. For readability, only every fourth event is plotted. |
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Figure 3:
(Left): the distribution of the reconstructed four-lepton mass $m_{{4\ell}}$ for events selected with 80 $ < m_{{4\ell}} < $ 100 GeV. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (Right): the reconstructed $m_{\mathrm{Z} _2}$ plotted against the reconstructed $m_{\mathrm{Z} _1}$ for all 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 80 $ < m_{{4\ell}} < $ 100 GeV. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 3-b:
The reconstructed $m_{\mathrm{Z} _2}$ plotted against the reconstructed $m_{\mathrm{Z} _1}$ for all 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 $m_{{\mathrm{Z} \mathrm{Z}}}$ and (right) dilepton candidate mass for four-lepton events selected with both Z bosons on-shell. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. In the $m_{{\mathrm{Z} \mathrm{Z}}}$ distribution, bin contents are normalized to the bin widths, using a unit bin size of 50 GeV ; horizontal bars on the data points show the range of the corresponding bin. |
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Figure 4-a:
Distribution of the four-lepton invariant mass $m_{{\mathrm{Z} \mathrm{Z}}}$ for four-lepton events selected with both Z bosons on-shell. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. Bin contents are normalized to the bin widths, using a unit bin size of 50 GeV ; horizontal bars on the data points show the range of the corresponding bin. |
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Figure 4-b:
Distribution of dilepton candidate masses for four-lepton events selected with both Z bosons on-shell. Points represent the data, while filled histograms represent the SM prediction and background estimate. Vertical bars on the data points show their statistical uncertainty. Shaded grey regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 5:
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 also includes gluon-gluon initiated production at LO in QCD. Both predictions use NNPDF3.0 PDF sets and fixed scales $\mu _\mathrm {F} = \mu _\mathrm {R} = m_\mathrm{Z} $. Details of the calculations and uncertainties are given in the text. The ATLAS measurements were performed with a Z boson mass window of 66-116 GeV, and are corrected for the resulting 1.6% difference. Measurements at the same center-of-mass energy are shifted slightly along the horizontal axis for clarity. |
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Figure 6:
Differential cross sections normalized to the fiducial cross section for the combined 4e, 4$\mu $, and 2e2$\mu $ decay channels as a function of mass (left) and $ {p_{\mathrm {T}}} $ (right) of the ZZ system. Points represent the unfolded data; the solid, dashed, and dotted histograms represent the POWHEG+MCFM, MadGraph 5\_aMC@NLO+MCFM, and matrix predictions for ZZ signal, respectively, and the bands around the predictions reflect their combined statistical, scale, and PDF uncertainties. PYTHIA v8 was used for parton showering, hadronization, and underlying event simulation in the POWHEG, MadGraph 5\_aMC@NLO, and MCFM samples. The lower part of each plot represents the ratio of the measured cross section to the theoretical distributions. The shaded grey areas around the points represent the sum in quadrature 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 mass of the ZZ system. Points represent the unfolded data; the solid, dashed, and dotted histograms represent the POWHEG+MCFM, MadGraph 5_aMC@NLO+MCFM, and matrix predictions for ZZ signal, respectively, and the bands around the predictions reflect their combined statistical, scale, and PDF uncertainties. PYTHIA v8 was used for parton showering, hadronization, and underlying event simulation in the POWHEG, MadGraph 5_aMC@NLO, and MCFM samples. The lower part of the plot represents the ratio of the measured cross section to the theoretical distributions. The shaded grey areas around the points represent the sum in quadrature 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 the $ {p_{\mathrm {T}}} $ of the ZZ system. Points represent the unfolded data; the solid, dashed, and dotted histograms represent the POWHEG+MCFM, MadGraph 5_aMC@NLO+MCFM, and matrix predictions for ZZ signal, respectively, and the bands around the predictions reflect their combined statistical, scale, and PDF uncertainties. PYTHIA v8 was used for parton showering, hadronization, and underlying event simulation in the POWHEG, MadGraph 5_aMC@NLO, and MCFM samples. The lower part of the plot represents the ratio of the measured cross section to the theoretical distributions. The shaded grey areas around the points represent the sum in quadrature of the statistical and systematic uncertainties, while the crosses represent the statistical uncertainties only. |
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Figure 7:
Normalized ZZ differential cross sections as a function of the $ {p_{\mathrm {T}}} $ of (left) all Z bosons and (right) the leading lepton in ZZ events. Other details are as described in the caption of Fig. 6. |
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Figure 7-a:
Normalized ZZ differential cross section as a function of the $ {p_{\mathrm {T}}} $ of all Z bosons in ZZ events. Other details are as described in the caption of Fig. 6. |
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Figure 7-b:
Normalized ZZ differential cross section as a function of the $ {p_{\mathrm {T}}} $ of the leading lepton in ZZ events. Other details are as described in the caption of Fig. 6. |
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Figure 8:
Normalized ZZ differential cross sections as a function of (left) the azimuthal separation of the two Z bosons and (right) $\Delta R$ between the $\mathrm{Z} $-bosons. Other details are as described in the caption of Fig. 6. |
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Figure 8-a:
Normalized ZZ differential cross sections as a function of the azimuthal separation of the two Z bosons. Other details are as described in the caption of Fig. 6. |
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Figure 8-b:
Normalized ZZ differential cross sections as a function of the azimuthal separation of $\Delta R$ between the $\mathrm{Z} $-bosons. Other details are as described in the caption of Fig. 6. |
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Figure 9:
The normalized differential four-lepton cross section as a function of the four-lepton mass, subject only to the common requirements of Table 4. SM $\mathrm{g} \mathrm{g} \to \mathrm{H} \to \mathrm{Z} \mathrm{Z} ^* $ production is included, simulated with POWHEG. Other details are as described in the caption of Fig. 6. |
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Figure 10:
Distribution of the four-lepton reconstructed mass for the combined 4e, 4$\mu $, and 2e2$\mu $ channels. Points represent the data, the filled histograms represent the SM expected yield including signal and irreducible background predictions from simulation and the data-driven background estimate. Unfilled histograms represent examples of aTGC signal predictions (dashed), and the SHERPA SM prediction (solid), included to illustrate the expected shape differences between the SHERPA and POWHEG predictions. Vertical bars on the data points show their statistical uncertainty. The SHERPA distributions are normalized such that the SM sample has the same total yield as the POWHEG sample predicts. Bin contents are normalized to the bin widths, using a unit bin size of 50 GeV ; horizontal bars on the data points show the range of the corresponding bin. The last bin includes the "overflow'' contribution from events at masses above 1.2 TeV. |
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Figure 11:
Two-dimensional observed 95% CL limits (solid contour) and expected 68 and 95% CL limits (dashed contour) on the ZZZ and ZZ$ \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. The solid lines at the center show the observed one-dimensional 95% CL limits for $f_{4,5}^\gamma $ (horizontal) and $f_{4,5}^\mathrm{Z} $ (vertical). No form factor is used. |
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Figure 11-a:
Two-dimensional observed 95% CL limits (solid contour) and expected 68 and 95% CL limits (dashed contour) on the ZZZ and ZZ$ \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. The solid lines at the center show the observed one-dimensional 95% CL limits for $f_{4}^\gamma $ (horizontal) and $f_{4}^\mathrm{Z} $ (vertical). No form factor is used. |
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Figure 11-b:
Two-dimensional observed 95% CL limits (solid contour) and expected 68 and 95% CL limits (dashed contour) on the ZZZ and ZZ$ \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. The solid lines at the center show the observed one-dimensional 95% CL limits for $f_{5}^\gamma $ (horizontal) and $f_{5}^\mathrm{Z} $ (vertical). No form factor is used. |
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Figure 12:
Expected and observed one-dimensional limits on the four aTGC parameters, as a function of an upper cutoff on the invariant mass of the four-lepton system. No form factor is used. |
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Figure 12-a:
Expected and observed one-dimensional limits on the $f_{4}^\gamma $ aTGC parameters, as a function of an upper cutoff on the invariant mass of the four-lepton system. No form factor is used. |
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Figure 12-b:
Expected and observed one-dimensional limits on the $f_{4}^\mathrm{Z}$ aTGC parameters, as a function of an upper cutoff on the invariant mass of the four-lepton system. No form factor is used. |
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Figure 12-c:
Expected and observed one-dimensional limits on the $f_{5}^\gamma $ aTGC parameters, as a function of an upper cutoff on the invariant mass of the four-lepton system. No form factor is used. |
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Figure 12-d:
Expected and observed one-dimensional limits on the $f_{5}^\mathrm{Z}$ aTGC parameters, as a function of an upper cutoff on the invariant mass of the four-lepton system. No form factor is used. |
| Tables | |
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Table 1:
The contributions of each source of 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, 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 in the integrated luminosity. |
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Table 3:
The observed and expected yields of ZZ events, and estimated yields of background events, 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 in 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 |
| A series of measurements of four-lepton final states in proton-proton collisions at $\sqrt{s} = $ 13 TeV have been performed with the CMS detector at the CERN LHC. The measured $\mathrm{pp} \to \mathrm{ZZ} $ cross section is $\sigma({\mathrm{p}}{\mathrm{p}} \to \mathrm{Z}\mathrm{Z}) = $ 17.2 $\pm$ 0.5 (stat) $\pm$ 0.7 (syst) $\pm$ 0.4 (theo) $\pm$ 0.4 (lumi) pb for Z boson masses in the range 60 $ < m_{\mathrm{Z}} < $ 120 GeV. The measured branching fraction for Z boson decays to four leptons is $\mathcal{B}(\mathrm{Z} \to {4\ell} ) = $ 4.8 $\pm$ 0.2 (stat) $\pm$ 0.2 (syst) $\pm$ 0.1 (theo) $\pm$ 0.1 (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. Normalized differential cross sections were also measured. All results agree well with the SM predictions. Improved limits on anomalous ZZZ and ZZ$ \gamma$ triple gauge couplings were established, the most stringent to date by approximately a factor of two. |
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