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| CMS-PAS-SMP-16-001 | ||
| Measurement of the ZZ production cross section and $ \mathrm{Z} \to \ell \ell \ell' \ell' $ branching fraction in pp collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| April 2016 | ||
| Abstract: Four-lepton production in proton-proton collisions, ${\mathrm{pp}} \rightarrow {\mathrm{Z}} / \gamma^* {\mathrm{Z}}/ \gamma^* \rightarrow \ell\ell\ell'\ell'$, where ($\ell, \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 2.6 fb$^{-1}$. The ZZ production cross section, $\sigma( \mathrm{ pp \to ZZ }) =$ 14.6 $^{+1.9}_{-1.8}$ (stat) $^{+0.5}_{-0.3}$ (syst) $\pm$ 0.2 (theo) $\pm$ 0.4 (lumi) pb, is measured for both Z bosons produced in the mass region 60 $ < m_{\mathrm{Z}} < $ 120 GeV. The Z branching fraction to four leptons is measured to be $\mathcal{B}( \mathrm{ Z \to \ell\ell\ell'\ell' }) =$ 4.9 $_{-0.7}^{+0.8}$ (stat) $_{-0.2}^{+0.3}$ (syst) $_{-0.1}^{+0.2}$ (theo) $\pm$ 0.1 (lumi) $\times 10^{-6}$ for the four-lepton mass in the range 80 $ < m_{\ell\ell\ell'\ell'} < $ 100 GeV and dilepton mass $m_{\ell\ell'} > $ 4 GeV for all opposite-charge lepton pairs. The results are compared to standard model predictions. | ||
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Links:
CDS record (PDF) ;
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These preliminary results are superseded in this paper, PLB 763 (2016) 280. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1-a:
Distributions of (a) the four-lepton invariant mass $m_{\ell \ell \ell '\ell '}$ 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, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and luminosity uncertainties. |
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Figure 1-b:
Distributions of (a) the four-lepton invariant mass $m_{\ell \ell \ell '\ell '}$ 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, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and luminosity uncertainties. |
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Figure 2-a:
(a) The distribution of the reconstructed mass of $\mathrm{ Z } _1$. Points represent the data, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and 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. |
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Figure 2-b:
(a) The distribution of the reconstructed mass of $\mathrm{ Z } _1$. Points represent the data, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and 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. |
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Figure 3-a:
(a) The distribution of the reconstructed four-lepton mass $m_{\ell \ell \ell '\ell '}$ for events selected with $m_{\ell \ell \ell '\ell '} < $ 110 GeV. Points represent the data, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and luminosity uncertainties. (b) The reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events selected with $m_{\ell \ell \ell '\ell '}$ 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 $m_{\ell \ell \ell '\ell '}$ for events selected with $m_{\ell \ell \ell '\ell '} < $ 110 GeV. Points represent the data, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and luminosity uncertainties. (b) The reconstructed $m_{\mathrm{ Z } _2}$ plotted against the reconstructed $m_{\mathrm{ Z } _1}$ in data events selected with $m_{\ell \ell \ell '\ell '}$ 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 $m_{\ell \ell \ell '\ell '}$ and (b) dilepton candidate mass for four-lepton events selected with both $\mathrm{ Z } $ bosons on-shell. Points represent the data, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and luminosity uncertainties. |
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Figure 4-b:
Distributions of (a) the four-lepton invariant mass $m_{\ell \ell \ell '\ell '}$ and (b) dilepton candidate mass for four-lepton events selected with both $\mathrm{ Z } $ bosons on-shell. Points represent the data, shaded histograms represent the standard model prediction and reducible background estimate. Hatched regions around the predicted yield represent combined statistical, systematic, theoretical, and 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 }} $. |
| Tables | |
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Table 1:
The contributions of each signal systematic uncertainty source on the cross section measurements. Luminosity uncertainty, and PDF and scale uncertainties, are considered separately. 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 '\ell '} < $ 100 GeV, and estimated yields of background events evaluated from data, are shown for each final state and are summed in the total expected yield. The first uncertainty is statistical, the second systematic. |
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Table 3:
The observed and expected yields of $\mathrm{ Z } \mathrm{ Z } $ events, and estimated yields of background events evaluated from data, are shown for each final state and are summed in the total expected yield. The first uncertainty is statistical, the second systematic. |
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Table 4:
Fiducial definitions for 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. In particular, the $\mathrm{ p p \to ZZ } $ cross section was measured to be $\sigma(\mathrm{ pp \to ZZ} ) =$ 14.6 $^{+1.9}_{-1.8}$ (stat) $^{+0.5}_{-0.3}$ (syst) $\pm$ 0.2 (theo) $\pm$ 0.4 (lumi) pb for both Z bosons produced in the mass range 60 $ < m_{\ell\ell} < $ 120 GeV, and the branching fraction for Z decays to four leptons was measured to be $\mathcal{B}(\mathrm{Z} \to \ell\ell\ell'\ell') =$ (4.9 $_{-0.7}^{+0.8}$ (stat) $_{-0.2}^{+0.3}$ (syst) $_{-0.1}^{+0.2}$ (theo) $\pm$ 0.1 (lumi)) $\times 10^{-6}$, for four-lepton mass in the range 80 $ < m_{\ell\ell\ell'\ell'} < $ 100 GeV and dilepton mass $m_{\ell\ell'} > $ 4 GeV for all opposite-charge lepton pairs. The results are consistent with standard model predictions. |
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