CMSPASTOP22013  
Observation of four top quark production in protonproton collisions at $ \sqrt{s}= $ 13 TeV  
CMS Collaboration  
23 March 2023  
Abstract: The first 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 2016 to 2018 at the CERN LHC and corresponding to an integrated luminosity of 138 fb$ ^{1} $. Events with two samesign, three, and four charged leptons (electrons and muons) and additional jets are analyzed with multivariate discriminants to distinguish the signal process from the main backgrounds. The signal cross section is measured with a profile likelihood fit to be 17.9 $^{+3.7}_{3.5}$ (stat) $^{+2.4}_{2.1}$ (syst) fb, in agreement with the best available theoretical predictions. The observed (expected) significance of the signal is 5.5 (4.9) standard deviations above the backgroundonly hypothesis.  
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These preliminary results are superseded in this paper, PLB 847 (2023) 138290. The superseded preliminary plots can be found here. 
Figures & Tables  Summary  Additional Figures  References  CMS Publications 

Figures  
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Figure 1:
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 for the ID criteria applied in Ref. [34], where red circles and blue squares are used for electron and muon criteria, respectively. 
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Figure 1a:
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 for the ID criteria applied in Ref. [34], where red circles and blue squares are used for electron and muon criteria, respectively. 
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Figure 1b:
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 for the ID criteria applied in Ref. [34], where red circles and blue squares are used for electron and muon criteria, respectively. 
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Figure 2:
Schematic representation of the event selection and categorization. 
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Figure 3:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 3a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 3b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 3c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 3d:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 3e:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 3f:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 3g:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (upper row), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle row), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (lower row) of SR$2\ell $. The $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ and $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ classes are shown separately in the $ \mathrm{e}\mathrm{e} $ (left), $ \mathrm{e}\mu $ (middle) and $ \mu\mu $ (right) categories. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 4:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (left), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (right) of SR$3\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 4a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (left), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (right) of SR$3\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 4b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (left), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (right) of SR$3\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 4c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (left), for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ class (middle), and for the $ \mathrm{t} \bar{\mathrm{t}} $ background in the $ \mathrm{t} \bar{\mathrm{t}} $ class (right) of SR$3\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 5:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (left) and for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the combined $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ and $ \mathrm{t} \bar{\mathrm{t}} $ classes (right) of SR$4\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. No data events are observed in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR$4\ell $. 
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Figure 5a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (left) and for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the combined $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ and $ \mathrm{t} \bar{\mathrm{t}} $ classes (right) of SR$4\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. No data events are observed in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR$4\ell $. 
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Figure 5b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class (left) and for the $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ background in the combined $ {\mathrm{t}\bar{\mathrm{t}}} \mathrm{X} $ and $ \mathrm{t} \bar{\mathrm{t}} $ classes (right) of SR$4\ell $. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. No data events are observed in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ class of SR$4\ell $. 
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Figure 6:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ \mathrm{t} \bar{\mathrm{t}} $ background, shown for CR2$\ell$23j1b (left) and CR2$\ell$45j2b (middle), and in the event yields with positive and negative sum of lepton charges in CR3$\ell$2j1b (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 6a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ \mathrm{t} \bar{\mathrm{t}} $ background, shown for CR2$\ell$23j1b (left) and CR2$\ell$45j2b (middle), and in the event yields with positive and negative sum of lepton charges in CR3$\ell$2j1b (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 6b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ \mathrm{t} \bar{\mathrm{t}} $ background, shown for CR2$\ell$23j1b (left) and CR2$\ell$45j2b (middle), and in the event yields with positive and negative sum of lepton charges in CR3$\ell$2j1b (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 6c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ \mathrm{t} \bar{\mathrm{t}} $ background, shown for CR2$\ell$23j1b (left) and CR2$\ell$45j2b (middle), and in the event yields with positive and negative sum of lepton charges in CR3$\ell$2j1b (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 7:
Comparison of the number of observed (points) and predicted (colored histograms) events in the jet multiplicity distribution, shown for CR3$\ell$Z (left), CR3$\ell$Z with the additional requirement of $ N_{\mathrm{b}}\geq $ 1 (middle), and CR4$\ell$Z (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 7a:
Comparison of the number of observed (points) and predicted (colored histograms) events in the jet multiplicity distribution, shown for CR3$\ell$Z (left), CR3$\ell$Z with the additional requirement of $ N_{\mathrm{b}}\geq $ 1 (middle), and CR4$\ell$Z (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 7b:
Comparison of the number of observed (points) and predicted (colored histograms) events in the jet multiplicity distribution, shown for CR3$\ell$Z (left), CR3$\ell$Z with the additional requirement of $ N_{\mathrm{b}}\geq $ 1 (middle), and CR4$\ell$Z (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 7c:
Comparison of the number of observed (points) and predicted (colored histograms) events in the jet multiplicity distribution, shown for CR3$\ell$Z (left), CR3$\ell$Z with the additional requirement of $ N_{\mathrm{b}}\geq $ 1 (middle), and CR4$\ell$Z (right). The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
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Figure 8:
Comparison of the number of observed (points) and predicted (colored histograms) events as a function of $ \log_{10}(\mathrm{S}/\mathrm{B}) $, where S and B are the predicted signal and background yields, respectively, evaluated for each bin in the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ classes of the signal regions before the fit to data. Only bins with $ \log_{10}(\mathrm{S}/\mathrm{B}) >  $ 1 are 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 systematic uncertainty in the predictions. The predictions are shown ``postfit'', i.e.,, with the values of the signal and background normalizations and nuisance parameters obtained in the fit to the data applied. 
Tables  
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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. [44,45]. 
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Table 2:
List of the input variables to the event multiclassification BDTs, sorted by importance in the 2$ \ell $ BDT. Observables not (only) used in the 3 $ \ell $+4 $ \ell $ BDT are marked with \textdagger (\textdaggerdbl ). The $ m_{\mathrm{T2}} $ variable is defined in Ref. [93,94] and constructed from $ {\vec p}_{\mathrm{T}}^{\kern1pt\text{miss}} $ and two fourmomenta of particle (systems) specified in the table. 
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Table 3:
Comparison of fit results in the channels individually and in their combination. 
Summary 
In summary, we have reported the first observation for the production of four top quarks ($ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $) in protonproton (pp) collisions, using events with two samesign, three, and four charged leptons (electrons and muons) and additional jets. The observed (expected) significance of the $ {\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} $ signal above the backgroundonly hypothesis is 5.5 (4.9) standard deviations. The signal cross section is measured to be $ \sigma(\mathrm{p}\mathrm{p}\to{\mathrm{t}\bar{\mathrm{t}}} {\mathrm{t}\bar{\mathrm{t}}} )= $ 17.9 $^{+3.7}_{3.5} $ (stat) $ ^{+2.4}_{2.1} $ (syst) $ fb, in agreement with the standard model prediction. 
Additional Figures  
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Additional Figure 1:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
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Additional Figure 2:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
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Additional Figure 3:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
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Additional Figure 4:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
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Additional Figure 5:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
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Additional Figure 6:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
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Additional Figure 7:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution using the "medium" working point for the b jet identification, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
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Additional Figure 8:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution using the "medium" working point for the b jet identification, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 9:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution using the "medium" working point for the b jet identification, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 10:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 11:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 12:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 13:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ p_{\mathrm{T}}^{\mathrm{miss}} $ distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 14:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ p_{\mathrm{T}}^{\mathrm{miss}} $ distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 15:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ p_{\mathrm{T}}^{\mathrm{miss}} $ distribution, shown for 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 systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 16:
Comparison of the number of observed (points) and predicted (colored histograms) events in the BDT output distributions for the $ \mathrm{t\bar{t}} $ background of the combined CR2$ \ell $23j1b and CR3$ \ell $45j2b. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 17:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of jets distribution, shown for the CR3$ \ell $Z with the additional requirement of at least one b jet using the "medium" working point for the b jet identification. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 18:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution, shown for the CR3$ \ell $Z with the additional requirement of at least one b jet using the "medium" working point for the b jet identification. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 19:
Comparison of the number of observed (points) and predicted (colored histograms) events in the number of b jets distribution using the "medium" working point for the b jet identification, shown for the CR3$ \ell $Z with the additional requirement of at least one b jet using the "medium" working point for the b jet identification. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 20:
Comparison of the number of observed (points) and predicted (colored histograms) events in the $ H_{\mathrm{T}} $ distribution, shown for the CR3$ \ell $Z with the additional requirement of at least one b jet using the "medium" working point for the b jet identification. The vertical bars on the points represent the statistical uncertainties in the data, and the hatched bands the systematic uncertainty in the predictions. The predicted signal and background yields are shown with their best fit normalizations from the simultaneous fit to the data. 
png pdf 
Additional Figure 21:
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 standard model prediction. The right panel shows the expected and observed significances, with the printed values rounded to the first decimal. 
png pdf 
Additional Figure 22:
For the nuisance parameters listed in the left column, the pulls $ (\hat{\theta}\theta_0)/\Delta\theta $ (middle column) and impacts $ \Delta\hat{\mu} $ (right column) are displayed. The 20 nuisance parameters with the largest impacts in the fit used to determine the $ \mathrm{t\bar{t}t\bar{t}} $ cross section are shown. The impact $ \Delta\hat{\mu} $ is defined as the shift induced in the signal strength $ \mu $ when the nuisance parameter $ \theta $ is varied by $ \pm $ 1 standard deviation ($ \sigma $). The pull $ (\hat{\theta}\theta_0)/\Delta\theta $ is calculated from the values $ \hat{\theta} $ and $ \theta_0 $ after and before the fit of $ \theta $, respectively, and from its uncertainty $ \Delta\theta $ before the fit. The label "correlated" and the peryear labels indicate nuisance parameters associated with the correlated and uncorrelated parts of a systematic uncertainty. The uncertainty for additional jets in $ \mathrm{t\bar{t}W} $ production has a onesided template before the fit, and thus a onesided impact after the fit is expected. 
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