CMSTOP22013 ; CERNEP2023090  
Observation of four top quark production in protonproton collisions at $ \sqrt{s}= $ 13 TeV  
CMS Collaboration  
22 May 2023  
Phys. Lett. B 847 (2023) 138290  
Abstract: The observation of the production of four top quarks in protonproton collisions is reported, based on a data sample collected by the CMS experiment at a centerofmass energy of 13 TeV in 20162018 at the CERN LHC and corresponding to an integrated luminosity of 138 fb$ ^{1} $. Events with two samesign, three, or four charged leptons (electrons and muons) and additional jets are analyzed. Compared to previous results in these channels, updated identification techniques for charged leptons and jets originating from the hadronization of b quarks, as well as a revised multivariate analysis strategy to distinguish the signal process from the main backgrounds, lead to an improved expected signal significance of 4.9 standard deviations above the backgroundonly hypothesis. Four top quark production is observed with a significance of 5.6 standard deviations, and its cross section is measured to be 17.7 $ ^{+3.7}_{3.5} $ (stat) $ ^{+2.3}_{1.9} $ (syst) fb, in agreement with the available standard model predictions.  
Links: eprint arXiv:2305.13439 [hepex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; 
Figures & Tables  Summary  Additional Figures & Tables  References  CMS Publications 

Figures  
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Figure 1:
Examples of Feynman diagrams that provide important contributions to $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ production. The first diagram (left) involves only the strong interaction, while the other two involve both strong and electroweak interactions with the exchange of a Z boson or virtual photon (middle), or a Higgs boson (right). 
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Figure 1a:
Example of Feynman diagram that provides important contribution to $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ production. The diagram involves only the strong interaction. 
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Figure 1b:
Example of Feynman diagram that provides important contribution to $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ production. The diagram involves both strong and electroweak interactions with the exchange of a Z boson or virtual photon. 
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Figure 1c:
Example of Feynman diagram that provides important contribution to $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ production. The diagram involves both strong and electroweak interactions with the exchange of a Higgs boson. 
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Figure 2:
Efficiency of selecting prompt leptons as a function of the misidentification probability for nonprompt leptons evaluated in simulated $ \mathrm{t} \bar{\mathrm{t}} $ events for the electron (red solid line) and muon (blue dashed line) ID BDT, shown for leptons with 10 $ < p_{\mathrm{T}} < $ 25 GeV (left) and $ p_{\mathrm{T}} > $ 25 GeV (right). Indicated with filled markers are the efficiencies for the ID criteria applied in this measurement and with empty markers those for the ID criteria applied in Ref. [41], where red circles and blue squares are used for electron and muon criteria, respectively. 
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Figure 2a:
Efficiency of selecting prompt leptons as a function of the misidentification probability for nonprompt leptons evaluated in simulated $ \mathrm{t} \bar{\mathrm{t}} $ events for the electron (red solid line) and muon (blue dashed line) ID BDT, shown for leptons with 10 $ < p_{\mathrm{T}} < $ 25 GeV. Indicated with filled markers are the efficiencies for the ID criteria applied in this measurement and with empty markers those for the ID criteria applied in Ref. [41], where red circles and blue squares are used for electron and muon criteria, respectively. 
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Figure 2b:
Efficiency of selecting prompt leptons as a function of the misidentification probability for nonprompt leptons evaluated in simulated $ \mathrm{t} \bar{\mathrm{t}} $ events for the electron (red solid line) and muon (blue dashed line) ID BDT, shown for leptons with $ p_{\mathrm{T}} > $ 25 GeV. Indicated with filled markers are the efficiencies for the ID criteria applied in this measurement and with empty markers those for the ID criteria applied in Ref. [41], where red circles and blue squares are used for electron and muon criteria, respectively. 
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Figure 3:
Schematic representation of the event selection and categorization. 
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Figure 4:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR2$\ell $ in the $ \mu\mu $ category (left), of SR3$\ell $ (middle), and of SR4$\ell $ (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 4a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR2$\ell $ in the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 4b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 4c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR4$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 5:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in CR3$\ell$Z (left), and in the number of b jets distribution in CR3$\ell$Z (middle) and CR4$\ell$Z (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 5a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in CR3$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 5b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in CR3$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 5c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in CR4$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 6:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t} \bar{\mathrm{t}} $ in the combined CR2$\ell$23j1b and CR2$\ell$45j2b (left), in the event yields with negative and positive sum of lepton charges in CR3$\ell$2j1b (middle), and in the number of jets distribution in the $ \mathrm{t} \bar{\mathrm{t}} $ class of the combined SR2$\ell $ and SR3$\ell $ (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 6a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t} \bar{\mathrm{t}} $ in the combined CR2$\ell$23j1b and CR2$\ell$45j2b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 6b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the event yields with negative and positive sum of lepton charges in CR3$\ell$2j1b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 6c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in the $ \mathrm{t} \bar{\mathrm{t}} $ class of the combined SR2$\ell $ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 7:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of SR2$\ell $, shown for the $ \mathrm{e}\mathrm{e} $ (upper left), $ \mathrm{e}\mu $ (upper middle), and $ \mu\mu $ (upper right) categories, of SR3$\ell $ (lower left) and of SR4$\ell $ (lower middle). Additionally, the comparison is shown for all SRs combined as a function of $ \log_{10}(\mathrm{S}/\mathrm{B}) $ (lower right), where S and B are evaluated for each bin of the fitted distributions as the predicted signal and background yields before the fit to data. Bins with $ \log_{10}(\mathrm{S}/\mathrm{B}) <  $1 are not included, and bins with $ \log_{10}(\mathrm{S}/\mathrm{B}) > $ 0.5 are included in the last bin. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 7a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR2$\ell $, shown for the $ \mathrm{e}\mathrm{e} $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 7b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR2$\ell $, shown for the $ \mathrm{e}\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 7c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR2$\ell $, shown for the $ \mu\mu $ category. 
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Figure 7d:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 7e:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR4$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 7f:
Comparison for all SRs combined as a function of $ \log_{10}(\mathrm{S}/\mathrm{B}) $, where S and B are evaluated for each bin of the fitted distributions as the predicted signal and background yields before the fit to data. Bins with $ \log_{10}(\mathrm{S}/\mathrm{B}) <  $1 are not included, and bins with $ \log_{10}(\mathrm{S}/\mathrm{B}) > $ 0.5 are included in the last bin. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data (``postfit''). 
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Figure 8:
Comparison of fit results in the channels individually and in their combination. The left panel shows the values of the measured cross section relative to the SM prediction from Ref. [6], where the displayed uncertainty does not include the uncertainty in the SM prediction. The right panel shows the expected and observed significance, with the printed values rounded to the first decimal. 
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Figure 9:
For the nuisance parameters listed in the left column, the pulls $ (\widehat{\theta}\theta_0)/\Delta\theta $ (middle column) and impacts $ \Delta\widehat{r} $ (right column) are displayed. The 20 nuisance parameters with the largest impacts in the fit used to determine the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ cross section are shown. The impact $ \Delta\widehat{r} $ is obtained from varying the nuisance parameter $ \theta $ by $ \pm $ 1 SD and evaluating the induced shift in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal strength $ r $. The pull $ (\widehat{\theta}\theta_0)/\Delta\theta $ is calculated from the values $ \widehat{\theta} $ and $ \theta_0 $ after and before the fit of $ \theta $, respectively, and from its uncertainty $ \Delta\theta $ before the fit. The label ``corr.'' and the peryear labels indicate nuisance parameters associated with the correlated and uncorrelated parts of a systematic uncertainty, respectively. The nuisance parameters labeled ``MC stat.''\ refer to the perbin statistical uncertainties in the predicted yields. The uncertainty associated with additional jets in $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{W} $ production corresponds to a onesided variation of the nominal template before the fit, and thus a onesided impact after the fit is expected. 
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Figure A1:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR2$\ell $ in the $ \mu\mu $ category (left), of SR3$\ell $ (middle), and of SR4$\ell $ (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A1a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR2$\ell $ in the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A1b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A1c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{}$X classes of SR4$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A2:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in CR3$\ell$Z (left), and in the number of b jets distribution in CR3$\ell$Z (middle) and CR4$\ell$Z (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A2a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in CR3$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A2b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in CR3$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A2c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in CR4$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A3:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t} \bar{\mathrm{t}} $ in the combined CR2$\ell$23j1b and CR2$\ell$45j2b (left), in the event yields with negative and positive sum of lepton charges in CR3$\ell$2j1b (middle), and in the number of jets distribution in the $ \mathrm{t} \bar{\mathrm{t}} $ class of the combined SR2$\ell $ and SR3$\ell $ (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A3a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t} \bar{\mathrm{t}} $ in the combined CR2$\ell$23j1b and CR2$\ell$45j2b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A3b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the event yields with negative and positive sum of lepton charges in CR3$\ell$2j1b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A3c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in the $ \mathrm{t} \bar{\mathrm{t}} $ class of the combined SR2$\ell $ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A4:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of SR2$\ell $, shown for the $ \mathrm{e}\mathrm{e} $ (upper left), $ \mathrm{e}\mu $ (upper middle), and $ \mu\mu $ (upper right) categories, of SR3$\ell $ (lower left) and of SR4$\ell $ (lower right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A4a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of SR2$\ell $, shown for the $ \mathrm{e}\mathrm{e} $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A4b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of SR2$\ell $, shown for the $ \mathrm{e}\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A4c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of SR2$\ell $, shown for the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A4d:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
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Figure A4e:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of SR4$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data (``prefit''). 
Tables  
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Table 1:
List of the input variables to the event multiclassification BDTs. The last two columns indicate the importance rank of the observables in the 2$ \ell $ and 3$ \ell $+4$ \ell $ BDT trainings, respectively, and a dash indicates that the observable is not used in that training. The $ m_{\mathrm{T2}} $ variable, defined in Refs. [96,97], is constructed from $ {\vec p}_{\mathrm{T}}^{\,\text{miss}} $ and two fourmomenta of the particles or particle systems specified in the table. 
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Table 2:
Number of predicted and observed events in the SR2$\ell $ and SR3$\ell {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes, both before the fit to the data (``prefit'') and with their best fit normalizations (``postfit''). The uncertainties in the predicted number of events include both the statistical and systematic components. The uncertainties in the total number of predicted background and background plus signal events are also given. A dash indicates that the corresponding background does not contribute. 
Summary 
A measurement of the production of four top quarks ($ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $) in protonproton collisions at $ \sqrt{s}= $ 13 TeV has been presented, using events with two samesign, three, and four charged leptons (electrons and muons) and additional jets from a data set corresponding to an integrated luminosity of 138 fb$ ^{1} $ recorded with the CMS detector at the LHC. Multivariate discriminants are employed in the identification of prompt leptons and jets originating from the decay of b hadrons, and to distinguish between selected events from the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal and the main background contributions. A profile likelihood fit is performed to the data in signal and control regions for the extraction of the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ cross section. The improvements in object identification and analysis strategy bring the sensitivity of the analysis to the observation level, with an observed (expected) significance of the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal above the backgroundonly hypothesis of 5.6 (4.9) standard deviations. The signal cross section is measured to be $ \sigma({\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} )= $ 17.7 $^{+3.7}_{3.5} $ (stat) $ ^{+2.3}_{1.9} $ (syst) fb, in agreement with the available standard model predictions. This result marks a significant milestone in the top quark physics program of the LHC. 
Additional Figures  
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Additional Figure 1:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}}$X in the $ \mathrm{t\bar{t}}$X class of SR2$\ell$ in the ee category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data ("postfit"). 
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Additional Figure 2:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}}$X in the $ \mathrm{t\bar{t}}$X class of SR2$\ell$ in the e$ \mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data ("postfit"). 
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Additional Figure 3:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the $ \mathrm{t\bar{t}} $ class of SR2$\ell$. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data ("postfit"). 
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Additional Figure 4:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the $ \mathrm{t\bar{t}} $ class of SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data ("postfit"). 
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Additional Figure 5:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the CR2$\ell$45j2b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data ("postfit"). 
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Additional Figure 6:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the CR2$\ell$23j1b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data ("postfit"). 
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Additional Figure 7:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in CR4$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data ("postfit"). 
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Additional Figure 8:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}}$X in the $ \mathrm{t\bar{t}}$X class of SR2$\ell$ in the ee category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). 
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Additional Figure 9:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}}$X in the $ \mathrm{t\bar{t}}$X class of SR2$\ell$ in the e$ \mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). 
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Additional Figure 10:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the $ \mathrm{t\bar{t}} $ class of SR2$\ell$. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). 
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Additional Figure 11:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the $ \mathrm{t\bar{t}} $ class of SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). 
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Additional Figure 12:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the CR2$\ell$45j2b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). 
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Additional Figure 13:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT score $ \mathrm{t\bar{t}} $ in the CR2$\ell$23j1b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). 
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Additional Figure 14:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in CR4$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). 
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Additional Figure 15:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution in CR3$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 16:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of medium b jets distribution in CR3$\ell$Z. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 17:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution in the $ \mathrm{t\bar{t}t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
png pdf 
Additional Figure 18:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ p_{\mathrm{T}}^{\mathrm{miss}} $ distribution in the $ \mathrm{t\bar{t}t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 19:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in the $ \mathrm{t\bar{t}t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 20:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of medium b jets distribution in the $ \mathrm{t\bar{t}t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 21:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in the $ \mathrm{t\bar{t}t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 22:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution in the $ \mathrm{t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 23:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ p_{\mathrm{T}}^{\mathrm{miss}} $ distribution in the $ \mathrm{t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 24:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in the $ \mathrm{t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 25:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of medium b jets distribution in the $ \mathrm{t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 26:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in the $ \mathrm{t\bar{t}} $ class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 27:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution in the $ \mathrm{t\bar{t}}$X class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 28:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ p_{\mathrm{T}}^{\mathrm{miss}} $ distribution in the $ \mathrm{t\bar{t}}$X class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
png pdf 
Additional Figure 29:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in the $ \mathrm{t\bar{t}}$X class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
png pdf 
Additional Figure 30:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of medium b jets distribution in the $ \mathrm{t\bar{t}}$X class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 31:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in the $ \mathrm{t\bar{t}}$X class of the combined SR2$\ell$ and SR3$\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
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Additional Figure 32:
Twodimensional scan of the $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{ttt} $ cross sections. The shading quantified by the color scale on the right reflects the negative loglikelihood difference with respect to the best fit value that is indicated by the white filled star. The 68% (solid line) and 95% (dashed line) CL contours are shown for the observed result. The white empty cross indicates the SM prediction. The correlation $ \rho $ between the two measured cross sections is $$0.94. 
png pdf 
Additional Figure 33:
Shape comparison of the predicted $ \mathrm{t\bar{t}t\bar{t}} $ (solid red line), $ \mathrm{ttt} $ (dashed green line), and combined other background (dotted black line) contributions in the BDT score $ \mathrm{t\bar{t}t\bar{t}} $ in the $ \mathrm{t\bar{t}t\bar{t}} $ class of SR2$\ell$. 
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Additional Figure 34:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR2$\ell$ distribution of $ \Delta R $ between the leading and subleading lepton. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The last bin includes the overflow contribution. 
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Additional Figure 35:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR3$\ell $ distribution of $ \Delta R $ between the leading and subleading lepton. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The last bin includes the overflow contribution. 
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Additional Figure 36:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR2$\ell$ distribution of the smallest $ \Delta R $ between any two b jets. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The last bin includes the overflow contribution. No values smaller than 0.4 are found since this is the jet radius. 
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Additional Figure 37:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR3$\ell $ distribution of the smallest $ \Delta R $ between any two b jets. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The last bin includes the overflow contribution. No values smaller than 0.4 are found since this is the jet radius. 
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Additional Figure 38:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR2$\ell$ distribution of the threejet mass closest to the top quark mass. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The first and last bins include the under and overflow contributions, respectively. 
png pdf 
Additional Figure 39:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR3$\ell $ distribution of the threejet mass closest to the top quark mass. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The first and last bins include the under and overflow contributions, respectively. 
png pdf 
Additional Figure 40:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR2$\ell$ distribution of the secondhighest DEEPJET score of any jet. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The last bin includes the overflow contribution. No values smaller than 0.04 are found because of the event selection requirement of two b jets. 
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Additional Figure 41:
Comparison of the number of observed (points) and predicted (colored histograms) events in the SR3$\ell $ distribution of the secondhighest DEEPJET score of any jet. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown before the fit to the data ("prefit"). The last bin includes the overflow contribution. No values smaller than 0.04 are found because of the event selection requirement of two b jets. 
png pdf 
Additional Figure 42:
Twodimensional scan of the $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{t\bar{t}W} $ cross sections as obtained from the fit with the $ \mathrm{t\bar{t}t\bar{t}} $, $ \mathrm{t\bar{t}W} $, and $ \mathrm{t\bar{t}W} $ cross sections as free parameters. The shading quantified by the color scale on the right reflects the negative loglikelihood difference with respect to the best fit value that is indicated by the white filled star. The 68% (solid line) and 95% (dashed line) CL contours are shown for the observed result. The white empty cross indicates the SM prediction. The correlation $ \rho $ between the two measured cross sections is $ + $0.07. 
png pdf 
Additional Figure 43:
Twodimensional scan of the $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{t\bar{t}Z} $ cross sections as obtained from the fit with the $ \mathrm{t\bar{t}t\bar{t}} $, $ \mathrm{t\bar{t}W} $, and $ \mathrm{t\bar{t}W} $ cross sections as free parameters. The shading quantified by the color scale on the right reflects the negative loglikelihood difference with respect to the best fit value that is indicated by the white filled star. The 68% (solid line) and 95% (dashed line) CL contours are shown for the observed result. The white empty cross indicates the SM prediction. The correlation $ \rho $ between the two measured cross sections is $ + $0.11. 
png pdf 
Additional Figure 44:
Twodimensional scan of the $ \mathrm{t\bar{t}W} $ and $ \mathrm{t\bar{t}Z} $ cross sections as obtained from the fit with the $ \mathrm{t\bar{t}t\bar{t}} $, $ \mathrm{t\bar{t}W} $, and $ \mathrm{t\bar{t}W} $ cross sections as free parameters. The shading quantified by the color scale on the right reflects the negative loglikelihood difference with respect to the best fit value that is indicated by the white filled star. The 68% (solid line) and 95% (dashed line) CL contours are shown for the observed result. The white empty cross indicates the SM prediction. The correlation $ \rho $ between the two measured cross sections is $ + $0.12. 
png pdf 
Additional Figure 45:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution in the combined $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{t\bar{t}X} $ class of SR2$\ell$ in the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
png pdf 
Additional Figure 46:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ p_{\mathrm{T}}^{\mathrm{miss}} $ distribution in the combined $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{t\bar{t}X} $ class of SR2$\ell$ in the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
png pdf 
Additional Figure 47:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution in the combined $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{t\bar{t}X} $ class of SR2$\ell$ in the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
png pdf 
Additional Figure 48:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of medium b jets distribution in the combined $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{t\bar{t}X} $ class of SR2$\ell$ in the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
png pdf 
Additional Figure 49:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution in the combined $ \mathrm{t\bar{t}t\bar{t}} $ and $ \mathrm{t\bar{t}X} $ class of SR2$\ell$ in the $ \mu\mu $ category. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the total uncertainty in the predictions. The signal and background yields are shown with the best fit normalizations from the simultaneous fit of the BDT score distributions to the data ("postfit"). The last bin includes the overflow contribution. 
Additional Tables  
png pdf 
Additional Table 1:
List of the input variables to the promptlepton ID BDTs. The nearest jet ($ \mathrm{j}_{\text{near}} $) is defined as the jet that includes the PF particle corresponding to the reconstructed lepton, and its momentum is recalibrated after subtracting the contribution from the lepton. The last two rows list input variables only used in the electron or muon ID BDTs, respectively, and are defined in Refs. [80, 83]. 
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