CMSPASTOP16016  
Search for standard model production of four top quarks in protonproton collisions at 13 TeV  
CMS Collaboration  
August 2016  
Abstract: A search for standard model four top quark production, combining the single lepton and dilepton channels, is presented. The analysis utilises the data recorded by the CMS experiment at $\sqrt{s} =$ 13 TeV in 2015, which corresponds to an integrated luminosity of 2.6 fb$^{1}$. A boosted decision tree algorithm is used to select signal and suppress background events. Upper limits on four top quark production of $ 10.2 \times \sigma_{t\overline{t}t\overline{t}}^{SM}$ observed and $ 10.8^{+6.7}_{3.8} \times \sigma_{t\overline{t}t\overline{t}}^{SM}$ expected are calculated at the 95% confidence level.  
Links:
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inSPIRE record ;
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These preliminary results are superseded in this paper, PLB 772 (2017) 336. The superseded preliminary plots can be found here. 
Figures  
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Figure 1:
The dominant diagram for ${\mathrm {t}\overline {\mathrm {t}}} {\mathrm {t}\overline {\mathrm {t}}}$ production in the SM at leading order. 
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Figure 2a:
The ${\textrm {BDT}_{\textrm {trijet1}}}$ discriminant for data and simulation in the dilepton channel for the $\mu \mu $ (a), $\mu $e (b) and ee (c) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}} $+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}} $, ${\mathrm {t}\overline {\mathrm {t}}} $+Z, ${\mathrm {t}\overline {\mathrm {t}}} $+W, and ${\mathrm {t}\overline {\mathrm {t}}} $+H production. 
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Figure 2b:
The ${\textrm {BDT}_{\textrm {trijet1}}}$ discriminant for data and simulation in the dilepton channel for the $\mu \mu $ (a), $\mu $e (b) and ee (c) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}} $+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}} $, ${\mathrm {t}\overline {\mathrm {t}}} $+Z, ${\mathrm {t}\overline {\mathrm {t}}} $+W, and ${\mathrm {t}\overline {\mathrm {t}}} $+H production. 
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Figure 2c:
The ${\textrm {BDT}_{\textrm {trijet1}}}$ discriminant for data and simulation in the dilepton channel for the $\mu \mu $ (a), $\mu $e (b) and ee (c) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}} $+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}} $, ${\mathrm {t}\overline {\mathrm {t}}} $+Z, ${\mathrm {t}\overline {\mathrm {t}}} $+W, and ${\mathrm {t}\overline {\mathrm {t}}} $+H production. 
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Figure 3a:
The ${\textrm {BDT}_{\textrm {trijet2}}}$ discriminant for data and simulation in the single lepton channel for the $\mu $+jets (a) and e+jets (b) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 3b:
The ${\textrm {BDT}_{\textrm {trijet2}}}$ discriminant for data and simulation in the single lepton channel for the $\mu $+jets (a) and e+jets (b) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 4a:
The $N {_{\textrm {j}}}$ distributions for data and simulation in the single lepton channel for the $\mu $+jets (a), e+jets (b), the dilepton channel for the $\mu \mu $ (c), $\mu $e (d) and ee (e) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 4b:
The $N {_{\textrm {j}}}$ distributions for data and simulation in the single lepton channel for the $\mu $+jets (a), e+jets (b), the dilepton channel for the $\mu \mu $ (c), $\mu $e (d) and ee (e) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 4c:
The $N {_{\textrm {j}}}$ distributions for data and simulation in the single lepton channel for the $\mu $+jets (a), e+jets (b), the dilepton channel for the $\mu \mu $ (c), $\mu $e (d) and ee (e) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 4d:
The $N {_{\textrm {j}}}$ distributions for data and simulation in the single lepton channel for the $\mu $+jets (a), e+jets (b), the dilepton channel for the $\mu \mu $ (c), $\mu $e (d) and ee (e) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 4e:
The $N {_{\textrm {j}}}$ distributions for data and simulation in the single lepton channel for the $\mu $+jets (a), e+jets (b), the dilepton channel for the $\mu \mu $ (c), $\mu $e (d) and ee (e) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 5a:
The $S$ distributions for data and simulation in the dilepton channel for the $\mu \mu $ (a), $\mu $e (b) and ee (bottom) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 5b:
The $S$ distributions for data and simulation in the dilepton channel for the $\mu \mu $ (a), $\mu $e (b) and ee (bottom) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 5c:
The $S$ distributions for data and simulation in the dilepton channel for the $\mu \mu $ (a), $\mu $e (b) and ee (bottom) final states with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 6a:
The $T$ distributions for data and simulation in the single lepton channel for the $\mu $+jets (a) and e+jets (b) final states are shown with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 6b:
The $T$ distributions for data and simulation in the single lepton channel for the $\mu $+jets (a) and e+jets (b) final states are shown with the dominant systematic uncertainty shown in hatched bands and the ${\mathrm {t}\overline {\mathrm {t}}}$+X sample representing the sum of ${\mathrm {t}\overline {\mathrm {t}}}$ , ${\mathrm {t}\overline {\mathrm {t}}}$+Z, ${\mathrm {t}\overline {\mathrm {t}}}$+W, and ${\mathrm {t}\overline {\mathrm {t}}}$+H production. 
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Figure 7a:
Dilepton BDT output discriminator summed across lepton species channels for 45 jets (upper left), 67 jets (upper right), and $\geq $8 jets (bottom). 
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Figure 7b:
Dilepton BDT output discriminator summed across lepton species channels for 45 jets (upper left), 67 jets (upper right), and $\geq $8 jets (bottom). 
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Figure 7c:
Dilepton BDT output discriminator summed across lepton species channels for 45 jets (upper left), 67 jets (upper right), and $\geq $8 jets (bottom). 
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Figure 8a:
BDT output distributions in the $\mu $+jets channel (a) and e+jets channel (b) for the $\geq 9 N {_{\textrm {j}}}$ and $3 N {_{\textrm {tags}}^{\textrm {M}}} $category. 
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Figure 8b:
BDT output distributions in the $\mu $+jets channel (a) and e+jets channel (b) for the $\geq 9 N {_{\textrm {j}}}$ and $3 N {_{\textrm {tags}}^{\textrm {M}}} $category. 
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Figure 9a:
BDT output distributions in the $\mu $ jets channel (a) and e+jets channel (b) for the $\geq 9 N {_{\textrm {j}}} $ and $\geq 4 N {_{\textrm {tags}}^{\textrm {M}}}$ category. 
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Figure 9b:
BDT output distributions in the $\mu $ jets channel (a) and e+jets channel (b) for the $\geq 9 N {_{\textrm {j}}} $ and $\geq 4 N {_{\textrm {tags}}^{\textrm {M}}}$ category. 
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Figure 10:
Expected and observed upper limits on ${\sigma _{ {\mathrm {t}\overline {\mathrm {t}}} {\mathrm {t}\overline {\mathrm {t}}} }^{SM}}$ for the single lepton, dilepton, and combined analysis in multiples of ${\sigma _{ {\mathrm {t}\overline {\mathrm {t}}} {\mathrm {t}\overline {\mathrm {t}}} }^{SM}} $. 
Tables  
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Table 1:
Expected and observed 95% CL upper limits on the standard model four top quark production as a multiple of ${\sigma _{ {\mathrm {t}\overline {\mathrm {t}}} {\mathrm {t}\overline {\mathrm {t}}} }^{SM}} $. The values quoted on the expected limits are the 1$\sigma $ uncertainties. 
Summary 
A search for events containing four top quarks has been performed using 2.6 fb$^{1}$ of data recorded by the CMS experiment at $\sqrt{s} =$ 13 TeV. The analysis focuses on a combination of the single lepton channel in the $\mu$+jets and e+jets final states and dilepton channel in the $\mu \mu$, $\mu$e, and ee final states. The analysis is comprised of three stages. Firstly, a baseline selection is defined for each final state that is used to broadly select signal events while suppressing backgrounds. Secondly, an event classification scheme based on a BDT algorithm is defined, further enhancing sensitivity to four top quark production. The BDT algorithm exploits the differences in event activity, event topology, b content and t content to discriminate between signal and background. Thirdly, upper limits on four top quark production of $10.2 \times {\sigma_{{t}\overline{{t}}} {{t}\overline{{t}}} }^{SM} $ observed and $10.8^{+6.7}_{3.8} \times \sigma^{SM}_{{{t}\overline{{t}}} {{t}\overline{{t}}} }$ expected are calculated at the 95% CL. 
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