CMS-SMP-16-001 ; CERN-EP-2016-174 | ||
Measurement of the ZZ production cross section and $\mathrm{ Z } \to {\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}} $ branching fraction in pp collisions at $ \sqrt{s} = $ 13 TeV | ||
CMS Collaboration | ||
29 July 2016 | ||
Phys. Lett. B 763 (2016) 280 [Corrigendum] | ||
Abstract: Four-lepton production in proton-proton collisions, ${\mathrm{ p }\mathrm{ p }} \to \left(\mathrm{Z} / \gamma^*\right)\left(\mathrm{Z}/ \gamma^*\right) \to {\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}} $, 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 2.6 fb$^{-1}$. The ZZ production cross section, $\sigma(\mathrm{ p }\mathrm{ p } \to \mathrm{Z}\mathrm{Z}) =$ 14.6 $^{+1.9}_{-1.8}$ (stat) $^{+0.5}_{-0.3}$ (syst) $\pm$ 0.2 (theory) $\pm$ 0.4 (lumi) pb, is 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. The Z boson branching fraction to four leptons is measured to be $\mathcal{B}(\mathrm{Z} \to {\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}} ) =$ 4.9 $_{-0.7}^{+0.8}$ (stat) $_{-0.2}^{+0.3}$ (syst) $_{-0.1}^{+0.2}$ (theory) $\pm$ 0.1 (lumi) $\times 10^{-6}$ for the four-lepton invariant mass in the range 80 $< m_{{\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}} } <$ 100 GeV and dilepton mass $m_{\ell^+\ell^{\prime -} } > $ 4 GeV for all opposite-sign lepton pairs. The results are in agreement with standard model predictions. | ||
Links: e-print arXiv:1607.08834 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1-a:
Distributions of (a) the four-lepton invariant mass $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ and (b) 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. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 1-b:
Distributions of (a) the four-lepton invariant mass $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ and (b) 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. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 2-a:
(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. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (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. Clusters of events corresponding to $\mathrm{ Z } \mathrm{ Z } \to {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} $, $\mathrm{ Z } \gamma ^\ast \to {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} $, and $\mathrm{ Z } \to {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} $ production can be seen. |
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Figure 2-b:
(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. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (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. Clusters of events corresponding to $\mathrm{ Z } \mathrm{ Z } \to {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} $, $\mathrm{ Z } \gamma ^\ast \to {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} $, and $\mathrm{ Z } \to {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} $ production can be seen. |
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Figure 3-a:
(a) The distribution of the reconstructed four-lepton mass $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ for events selected with $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} } < $ 110 GeV. Points represent the data, while shaded histograms represent the SM prediction and background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (b) The reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events selected with $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ between 80 and 100 GeV, with distinctive markers for each final state. |
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Figure 3-b:
(a) The distribution of the reconstructed four-lepton mass $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ for events selected with $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} } < $ 110 GeV. Points represent the data, while shaded histograms represent the SM prediction and background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. (b) The reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events selected with $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ between 80 and 100 GeV, with distinctive markers for each final state. |
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Figure 4-a:
Distributions of (a) the four-lepton invariant mass $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ and (b) 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. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and integrated luminosity uncertainties. |
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Figure 4-b:
Distributions of (a) the four-lepton invariant mass $m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} }$ and (b) 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. Hatched 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 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 x-axis for clarity. |
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 $< m_{ {\ell ^+\ell ^-\ell ^{\prime +}\ell ^{\prime -}} } <$ 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. |
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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. |
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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 $pp \to \mathrm{ZZ}$ cross section has been measured to be $\sigma(pp \to \mathrm{ZZ}) =$ 14.6 $^{+1.9}_{-1.8}$ (stat) $^{+0.5}_{-0.3}$ (syst) $\pm$ 0.2 (theory) $\pm$ 0.4 (lumi) pb 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 $\mathcal{B}(\mathrm{Z} \to {\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}} ) =$ 4.9 $_{-0.7}^{+0.8}$ (stat) $_{-0.2}^{+0.3}$ (syst) $_{-0.1}^{+0.2}$ (theory) $\pm$ 0.1 (lumi) $\times 10^{-6}$ for four-lepton mass in the range 80 $< m_{{\ell^+\ell^-\ell^{\prime+}\ell^{\prime-}} } <$ 100 GeV and dilepton mass $m_{\ell^+\ell^{\prime -} } >$ 4 GeV for all oppositely charged lepton pairs. The results are consistent with SM predictions. |
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Compact Muon Solenoid LHC, CERN |