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CMS-PAS-TOP-17-017
Search for flavour changing neutral currents in top quark production and decays with three-lepton final state using the data collected at $\sqrt{s} = $ 13 TeV
Abstract: A search for flavour changing neutral currents (FCNC) involving a top quark and a Z boson is presented. The analysis uses the 2016 proton collision data collected by the CMS experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search focuses on single top quark and top quark pair FCNC interactions observable in three lepton final states, where the FCNC interaction happens at the production or at the top quark decay. No significant deviation is observed from the predicted background. Observed (expected) upper limits at 95$%$ confidence level are set on the branching fractions of top quark decays: ${\mathcal{B}}(t \rightarrow uZ) < $ 0.024% (0.015%) and ${\mathcal{B}}(t \rightarrow cZ) < $ 0.045% (0.037%), assuming one non-vanishing coupling at a time.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Single top quark event Feynman diagrams at leading order. The vertex labelled tZq is the sought-for FCNC interaction.

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Figure 1-a:
Single top quark event Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.

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Figure 1-b:
Single top quark event Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.

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Figure 2:
Top quark pair Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.

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Figure 2-a:
Top quark pair Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.

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Figure 2-b:
Top quark pair Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.

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Figure 3:
The discriminating variable distribution after the fit for all different leptonic channels. Upper left: top quark pair tZu; upper right: top quark pair tZc; lower left: single top quark tZu; lower right: single top quark tZc.

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Figure 3-a:
The discriminating variable distribution after the fit for all different leptonic channels: top quark pair tZu.

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Figure 3-b:
The discriminating variable distribution after the fit for all different leptonic channels: top quark pair tZc.

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Figure 3-c:
The discriminating variable distribution after the fit for all different leptonic channels: single top quark tZu.

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Figure 3-d:
The discriminating variable distribution after the fit for all different leptonic channels: single top quark tZc.

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Figure 4:
Exclusion limits at 95% CL on the FCNC branching fractions as a function of the cross section of the FCNC process, considering only the tZc (right) or tZu (left) coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 4-a:
Exclusion limits at 95% CL on the FCNC branching fractions as a function of the cross section of the FCNC process, considering only the tZc coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 4-b:
Exclusion limits at 95% CL on the FCNC branching fractions as a function of the cross section of the FCNC process, considering only the tZu coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 5:
Exclusion limits at 95% CL on the FCNC couplings as a function of the cross section of the FCNC process, considering only the tZc (right) or tZu (left) coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 5-a:
Exclusion limits at 95% CL on the FCNC couplings as a function of the cross section of the FCNC process, considering only the tZc coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 5-b:
Exclusion limits at 95% CL on the FCNC couplings as a function of the cross section of the FCNC process, considering only the tZu coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 6:
Exclusion limits at 95% CL for each leptonic channel and signal region on the FCNC tZu (left) and tZc (right) branching fractions considering one non-vanishing coupling at a time. The CMS 8 TeV [7] observed (expected) limit of is given with a blue dashed line).

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Figure 6-a:
Exclusion limits at 95% CL for each leptonic channel and signal region on the FCNC tZu branching fractions considering one non-vanishing coupling at a time. The CMS 8 TeV [7] observed (expected) limit of is given with a blue dashed line).

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Figure 6-b:
Exclusion limits at 95% CL for each leptonic channel and signal region on the FCNC tZc branching fractions considering one non-vanishing coupling at a time. The CMS 8 TeV [7] observed (expected) limit of is given with a blue dashed line).

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Figure 7:
Exclusion regions at 95% CL on the FCNC branching fractions (left) and couplings (right) in the 2D plane of both the tZu and tZc variables. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 7-a:
Exclusion regions at 95% CL on the FCNC branching fractions in the 2D plane of both the tZu and tZc variables. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

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Figure 7-b:
Exclusion regions at 95% CL on the FCNC couplings in the 2D plane of both the tZu and tZc variables. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
Tables

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Table 1:
The statistically independent regions used in the analysis.

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Table 2:
Systematic uncertainties considered in this analysis. The column labelled type represents how the uncertainty is treated.
Summary
A search for flavour changing neutral currents (FCNC) involving a top quark and a Z boson is presented. The analysis uses the 2016 proton collision data collected by the CMS experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search focuses on single top quark and top quark pair FCNC interactions observable in three lepton final states, where the FCNC interaction happens at the production or at the top quark decay. No significant deviation is observed from the predicted background. Observed (expected) upper limits at 95% confidence level are set on the branching fractions of top quark decays: ${\mathcal{B}}(t \rightarrow uZ) < $ 0.024% (0.015%) and ${\mathcal{B}}(t \rightarrow cZ) < $ 0.045% (0.037%), assuming one non-vanishing coupling at a time. Significant improvements are developed with respect to previous searches, namely by using other kinematic variables as input into the BDT as well as a better handle on the NPL background. The expected limit for the FCNC $ tZu interaction is stronger than the current best observed (expected) limit of 0.017% (0.024%) set at 13 TeV by ATLAS [39]. The observed (expected) limit on the tZc interaction set by ATLAS is 0.023% (0.032%) and is comparable with the expected limit presented in this search.
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Compact Muon Solenoid
LHC, CERN