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| CMS-PAS-TOP-24-017 | ||
| Four-top-quark production in final states with hadronically decaying tau leptons and search for vector-like leptons at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 2026-03-21 | ||
| Abstract: The first study of four top quark production in final states with hadronically decaying tau leptons ($ \tau_\mathrm{h} $) is presented using proton-proton collision data collected by the CMS experiment at a center-of-mass energy of 13 TeV during the 2016--2018 period at the CERN LHC. This dataset corresponds to an integrated luminosity of 138 fb$ ^{-1} $. Tau lepton final states provide sensitivity to BSM scenarios with enhanced third-generation couplings and complement established multilepton searches. The analysis is divided into subchannels with one $ \tau_\mathrm{h} $ and zero, one, or two additional electrons and muons to optimize sensitivity. Combining the three channels, the observed (expected) significance of the measured $ \mathrm{t\bar{t}t\bar{t}} $ signal with respect to the standard model background-only hypothesis is 1.1 $ (1.0) $ standard deviations. The production cross section of four top quarks is measured to be 16 $ ^{+14}_{-12} $ ($ \mathrm{stat} $)^+12_-8 ($ \mathrm{syst} $) $ \mathrm{fb} $. Additionally, a search is performed for vector-like leptons within the framework of the 4321 model in the same $ \tau_\mathrm{h} $ phase space. No significant excess is observed, and a lower limit of 830 GeV is set at 95% confidence level on the vector-like lepton mass, providing the first constraints from final states containing electrons and muons. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Examples of leading Feynman diagrams contributing to $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production. The first diagram (left ) involves only the strong interaction. The other two diagrams involve both strong and electroweak interactions with the exchange of a virtual Z boson or photon (middle ), or a virtual Higgs boson (right ). |
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Figure 1-a:
Examples of leading Feynman diagrams contributing to $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production. The first diagram (left ) involves only the strong interaction. The other two diagrams involve both strong and electroweak interactions with the exchange of a virtual Z boson or photon (middle ), or a virtual Higgs boson (right ). |
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Figure 1-b:
Examples of leading Feynman diagrams contributing to $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production. The first diagram (left ) involves only the strong interaction. The other two diagrams involve both strong and electroweak interactions with the exchange of a virtual Z boson or photon (middle ), or a virtual Higgs boson (right ). |
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Figure 1-c:
Examples of leading Feynman diagrams contributing to $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production. The first diagram (left ) involves only the strong interaction. The other two diagrams involve both strong and electroweak interactions with the exchange of a virtual Z boson or photon (middle ), or a virtual Higgs boson (right ). |
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Figure 2:
Branching ratios for the decay channels of $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $. The decay channels are characterized by the decay modes of individual top quarks and the charge of the resulting leptons in the final state. For instance, 1 $ \tau_{\text{h}}0\ell $ indicates that one top quark decays into hadronic tau leptons, zero top quarks decay leptonically, and three top quarks decay hadronically. Conversely, 0 $ \tau_{\text{h}}2\ell $SS signifies that two top quarks decay leptonically with same-sign leptons, whereas the other two top quarks decay hadronically. |
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Figure 2-a:
Branching ratios for the decay channels of $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $. The decay channels are characterized by the decay modes of individual top quarks and the charge of the resulting leptons in the final state. For instance, 1 $ \tau_{\text{h}}0\ell $ indicates that one top quark decays into hadronic tau leptons, zero top quarks decay leptonically, and three top quarks decay hadronically. Conversely, 0 $ \tau_{\text{h}}2\ell $SS signifies that two top quarks decay leptonically with same-sign leptons, whereas the other two top quarks decay hadronically. |
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Figure 2-b:
Branching ratios for the decay channels of $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $. The decay channels are characterized by the decay modes of individual top quarks and the charge of the resulting leptons in the final state. For instance, 1 $ \tau_{\text{h}}0\ell $ indicates that one top quark decays into hadronic tau leptons, zero top quarks decay leptonically, and three top quarks decay hadronically. Conversely, 0 $ \tau_{\text{h}}2\ell $SS signifies that two top quarks decay leptonically with same-sign leptons, whereas the other two top quarks decay hadronically. |
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Figure 3:
Examples of Feynman diagrams for VLL pair production and decay. The diagrams on the left show electroweak pair production of VLLs through $ s $-channel $ \mathrm{Z}/\gamma $ and W bosons at the LHC, where L represents either the neutral component N or the charged component E of the VLL doublet. The diagrams on the right illustrate the VLL decay process, mediated by a virtual vector leptoquark U, which proceeds primarily to third-generation leptons and quarks. |
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Figure 3-a:
Examples of Feynman diagrams for VLL pair production and decay. The diagrams on the left show electroweak pair production of VLLs through $ s $-channel $ \mathrm{Z}/\gamma $ and W bosons at the LHC, where L represents either the neutral component N or the charged component E of the VLL doublet. The diagrams on the right illustrate the VLL decay process, mediated by a virtual vector leptoquark U, which proceeds primarily to third-generation leptons and quarks. |
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Figure 3-b:
Examples of Feynman diagrams for VLL pair production and decay. The diagrams on the left show electroweak pair production of VLLs through $ s $-channel $ \mathrm{Z}/\gamma $ and W bosons at the LHC, where L represents either the neutral component N or the charged component E of the VLL doublet. The diagrams on the right illustrate the VLL decay process, mediated by a virtual vector leptoquark U, which proceeds primarily to third-generation leptons and quarks. |
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Figure 3-c:
Examples of Feynman diagrams for VLL pair production and decay. The diagrams on the left show electroweak pair production of VLLs through $ s $-channel $ \mathrm{Z}/\gamma $ and W bosons at the LHC, where L represents either the neutral component N or the charged component E of the VLL doublet. The diagrams on the right illustrate the VLL decay process, mediated by a virtual vector leptoquark U, which proceeds primarily to third-generation leptons and quarks. |
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Figure 3-d:
Examples of Feynman diagrams for VLL pair production and decay. The diagrams on the left show electroweak pair production of VLLs through $ s $-channel $ \mathrm{Z}/\gamma $ and W bosons at the LHC, where L represents either the neutral component N or the charged component E of the VLL doublet. The diagrams on the right illustrate the VLL decay process, mediated by a virtual vector leptoquark U, which proceeds primarily to third-generation leptons and quarks. |
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Figure 4:
Region definitions for the 1 $ \tau_{\text{h}}0\ell $ (left), 1 $ \tau_{\text{h}}1\ell $ (middle), and 1 $ \tau_{\text{h}}2\ell $ (right) channels. The baseline selection for 1 $ \tau_{\text{h}}0\ell $ and 1 $ \tau_{\text{h}}1\ell $ is different from that of 1 $ \tau_{\text{h}}2\ell $. |
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Figure 5:
Comparison of observed data (points) and predicted events (colored histograms) for selected variables in the analysis regions: the $ \tau_{\text{h}} $ charged-hadron multiplicity in 1 $ \tau_{\text{h}}0\ell \text{MR} $ (upper left), the jet multiplicity in 1 $ \tau_{\text{h}}0\ell \text{SR} $ (upper right), the $ p_{\mathrm{T}} $ of the jet associated with the $ \tau_{\text{h}} $ candidate in 1 $ \tau_{\text{h}}0\ell \text{VR} $ (middle left), the third-leading medium b-tagged jet $ p_{\mathrm{T}} $ in 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle right), the $ p_{\mathrm{T}}^\text{miss} $ in 1 $ \tau_{\text{h}}2\ell \text{CR2} $ (lower left), and the $ \Delta R $ between the $ \tau_{\text{h}} $ and the leading lepton in 1 $ \tau_{\text{h}}2\ell \text{SR} $ (lower right). The vertical bars on the data points represent statistical uncertainties, and the hatched bands indicate the total uncertainty in the predictions. |
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Figure 5-a:
Comparison of observed data (points) and predicted events (colored histograms) for selected variables in the analysis regions: the $ \tau_{\text{h}} $ charged-hadron multiplicity in 1 $ \tau_{\text{h}}0\ell \text{MR} $ (upper left), the jet multiplicity in 1 $ \tau_{\text{h}}0\ell \text{SR} $ (upper right), the $ p_{\mathrm{T}} $ of the jet associated with the $ \tau_{\text{h}} $ candidate in 1 $ \tau_{\text{h}}0\ell \text{VR} $ (middle left), the third-leading medium b-tagged jet $ p_{\mathrm{T}} $ in 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle right), the $ p_{\mathrm{T}}^\text{miss} $ in 1 $ \tau_{\text{h}}2\ell \text{CR2} $ (lower left), and the $ \Delta R $ between the $ \tau_{\text{h}} $ and the leading lepton in 1 $ \tau_{\text{h}}2\ell \text{SR} $ (lower right). The vertical bars on the data points represent statistical uncertainties, and the hatched bands indicate the total uncertainty in the predictions. |
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Figure 5-b:
Comparison of observed data (points) and predicted events (colored histograms) for selected variables in the analysis regions: the $ \tau_{\text{h}} $ charged-hadron multiplicity in 1 $ \tau_{\text{h}}0\ell \text{MR} $ (upper left), the jet multiplicity in 1 $ \tau_{\text{h}}0\ell \text{SR} $ (upper right), the $ p_{\mathrm{T}} $ of the jet associated with the $ \tau_{\text{h}} $ candidate in 1 $ \tau_{\text{h}}0\ell \text{VR} $ (middle left), the third-leading medium b-tagged jet $ p_{\mathrm{T}} $ in 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle right), the $ p_{\mathrm{T}}^\text{miss} $ in 1 $ \tau_{\text{h}}2\ell \text{CR2} $ (lower left), and the $ \Delta R $ between the $ \tau_{\text{h}} $ and the leading lepton in 1 $ \tau_{\text{h}}2\ell \text{SR} $ (lower right). The vertical bars on the data points represent statistical uncertainties, and the hatched bands indicate the total uncertainty in the predictions. |
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Figure 5-c:
Comparison of observed data (points) and predicted events (colored histograms) for selected variables in the analysis regions: the $ \tau_{\text{h}} $ charged-hadron multiplicity in 1 $ \tau_{\text{h}}0\ell \text{MR} $ (upper left), the jet multiplicity in 1 $ \tau_{\text{h}}0\ell \text{SR} $ (upper right), the $ p_{\mathrm{T}} $ of the jet associated with the $ \tau_{\text{h}} $ candidate in 1 $ \tau_{\text{h}}0\ell \text{VR} $ (middle left), the third-leading medium b-tagged jet $ p_{\mathrm{T}} $ in 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle right), the $ p_{\mathrm{T}}^\text{miss} $ in 1 $ \tau_{\text{h}}2\ell \text{CR2} $ (lower left), and the $ \Delta R $ between the $ \tau_{\text{h}} $ and the leading lepton in 1 $ \tau_{\text{h}}2\ell \text{SR} $ (lower right). The vertical bars on the data points represent statistical uncertainties, and the hatched bands indicate the total uncertainty in the predictions. |
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Figure 5-d:
Comparison of observed data (points) and predicted events (colored histograms) for selected variables in the analysis regions: the $ \tau_{\text{h}} $ charged-hadron multiplicity in 1 $ \tau_{\text{h}}0\ell \text{MR} $ (upper left), the jet multiplicity in 1 $ \tau_{\text{h}}0\ell \text{SR} $ (upper right), the $ p_{\mathrm{T}} $ of the jet associated with the $ \tau_{\text{h}} $ candidate in 1 $ \tau_{\text{h}}0\ell \text{VR} $ (middle left), the third-leading medium b-tagged jet $ p_{\mathrm{T}} $ in 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle right), the $ p_{\mathrm{T}}^\text{miss} $ in 1 $ \tau_{\text{h}}2\ell \text{CR2} $ (lower left), and the $ \Delta R $ between the $ \tau_{\text{h}} $ and the leading lepton in 1 $ \tau_{\text{h}}2\ell \text{SR} $ (lower right). The vertical bars on the data points represent statistical uncertainties, and the hatched bands indicate the total uncertainty in the predictions. |
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Figure 5-e:
Comparison of observed data (points) and predicted events (colored histograms) for selected variables in the analysis regions: the $ \tau_{\text{h}} $ charged-hadron multiplicity in 1 $ \tau_{\text{h}}0\ell \text{MR} $ (upper left), the jet multiplicity in 1 $ \tau_{\text{h}}0\ell \text{SR} $ (upper right), the $ p_{\mathrm{T}} $ of the jet associated with the $ \tau_{\text{h}} $ candidate in 1 $ \tau_{\text{h}}0\ell \text{VR} $ (middle left), the third-leading medium b-tagged jet $ p_{\mathrm{T}} $ in 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle right), the $ p_{\mathrm{T}}^\text{miss} $ in 1 $ \tau_{\text{h}}2\ell \text{CR2} $ (lower left), and the $ \Delta R $ between the $ \tau_{\text{h}} $ and the leading lepton in 1 $ \tau_{\text{h}}2\ell \text{SR} $ (lower right). The vertical bars on the data points represent statistical uncertainties, and the hatched bands indicate the total uncertainty in the predictions. |
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Figure 5-f:
Comparison of observed data (points) and predicted events (colored histograms) for selected variables in the analysis regions: the $ \tau_{\text{h}} $ charged-hadron multiplicity in 1 $ \tau_{\text{h}}0\ell \text{MR} $ (upper left), the jet multiplicity in 1 $ \tau_{\text{h}}0\ell \text{SR} $ (upper right), the $ p_{\mathrm{T}} $ of the jet associated with the $ \tau_{\text{h}} $ candidate in 1 $ \tau_{\text{h}}0\ell \text{VR} $ (middle left), the third-leading medium b-tagged jet $ p_{\mathrm{T}} $ in 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle right), the $ p_{\mathrm{T}}^\text{miss} $ in 1 $ \tau_{\text{h}}2\ell \text{CR2} $ (lower left), and the $ \Delta R $ between the $ \tau_{\text{h}} $ and the leading lepton in 1 $ \tau_{\text{h}}2\ell \text{SR} $ (lower right). The vertical bars on the data points represent statistical uncertainties, and the hatched bands indicate the total uncertainty in the predictions. |
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Figure 6:
Distributions of the binned BDT discriminant for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement in the three signal regions: 1 $ \tau_{\text{h}}0\ell \text{SR} $ (top), 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle), and 1 $ \tau_{\text{h}}2\ell \text{SR} $ (bottom). Pre-fit distributions are shown on the left and post-fit on the right. Each bin corresponds to a range of the BDT output score, with increasing bin number corresponding to higher signal purity. The points represent the observed data, and the stacked histograms show the predicted backgrounds and $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ signal. The hatched bands indicate the total uncertainty in the predictions. |
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Figure 6-a:
Distributions of the binned BDT discriminant for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement in the three signal regions: 1 $ \tau_{\text{h}}0\ell \text{SR} $ (top), 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle), and 1 $ \tau_{\text{h}}2\ell \text{SR} $ (bottom). Pre-fit distributions are shown on the left and post-fit on the right. Each bin corresponds to a range of the BDT output score, with increasing bin number corresponding to higher signal purity. The points represent the observed data, and the stacked histograms show the predicted backgrounds and $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ signal. The hatched bands indicate the total uncertainty in the predictions. |
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Figure 6-b:
Distributions of the binned BDT discriminant for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement in the three signal regions: 1 $ \tau_{\text{h}}0\ell \text{SR} $ (top), 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle), and 1 $ \tau_{\text{h}}2\ell \text{SR} $ (bottom). Pre-fit distributions are shown on the left and post-fit on the right. Each bin corresponds to a range of the BDT output score, with increasing bin number corresponding to higher signal purity. The points represent the observed data, and the stacked histograms show the predicted backgrounds and $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ signal. The hatched bands indicate the total uncertainty in the predictions. |
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Figure 6-c:
Distributions of the binned BDT discriminant for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement in the three signal regions: 1 $ \tau_{\text{h}}0\ell \text{SR} $ (top), 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle), and 1 $ \tau_{\text{h}}2\ell \text{SR} $ (bottom). Pre-fit distributions are shown on the left and post-fit on the right. Each bin corresponds to a range of the BDT output score, with increasing bin number corresponding to higher signal purity. The points represent the observed data, and the stacked histograms show the predicted backgrounds and $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ signal. The hatched bands indicate the total uncertainty in the predictions. |
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Figure 6-d:
Distributions of the binned BDT discriminant for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement in the three signal regions: 1 $ \tau_{\text{h}}0\ell \text{SR} $ (top), 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle), and 1 $ \tau_{\text{h}}2\ell \text{SR} $ (bottom). Pre-fit distributions are shown on the left and post-fit on the right. Each bin corresponds to a range of the BDT output score, with increasing bin number corresponding to higher signal purity. The points represent the observed data, and the stacked histograms show the predicted backgrounds and $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ signal. The hatched bands indicate the total uncertainty in the predictions. |
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Figure 6-e:
Distributions of the binned BDT discriminant for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement in the three signal regions: 1 $ \tau_{\text{h}}0\ell \text{SR} $ (top), 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle), and 1 $ \tau_{\text{h}}2\ell \text{SR} $ (bottom). Pre-fit distributions are shown on the left and post-fit on the right. Each bin corresponds to a range of the BDT output score, with increasing bin number corresponding to higher signal purity. The points represent the observed data, and the stacked histograms show the predicted backgrounds and $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ signal. The hatched bands indicate the total uncertainty in the predictions. |
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Figure 6-f:
Distributions of the binned BDT discriminant for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement in the three signal regions: 1 $ \tau_{\text{h}}0\ell \text{SR} $ (top), 1 $ \tau_{\text{h}}1\ell \text{SR} $ (middle), and 1 $ \tau_{\text{h}}2\ell \text{SR} $ (bottom). Pre-fit distributions are shown on the left and post-fit on the right. Each bin corresponds to a range of the BDT output score, with increasing bin number corresponding to higher signal purity. The points represent the observed data, and the stacked histograms show the predicted backgrounds and $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ signal. The hatched bands indicate the total uncertainty in the predictions. |
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Figure 7:
Results of the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production measurement for the individual channels (1 $ \tau_{\text{h}}0\ell $, 1 $ \tau_{\text{h}}1\ell $, and 1 $ \tau_{\text{h}}2\ell $) and their combination. The left panel shows the observed signal strength $ r = \sigma_{{\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} }/\sigma^{\text{th.}}_{{\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} } $ with $ \pm $ 1 standard deviation uncertainties, compared with the SM expectation (dashed line). The right panel shows the observed and expected significances for rejecting the background-only hypothesis. |
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Figure 8:
Post-fit (background-only) BDT discriminant distributions for the VLL search at $ m_{\mathrm{VLL}} = $ 600 GeV, for all regions used in the extraction of limit on mass across the 1 $ \tau_{\text{h}}0\ell $ (top and middle-left), 1 $ \tau_{\text{h}}1\ell $ (middle-right and bottom-left), and 1 $ \tau_{\text{h}}2\ell $ (bottom-right) channels. The stacked histograms represent the SM background predictions, with the expected VLL signal overlaid as a magenta line normalized to the 4321 model cross section. Hatched bands indicate the total uncertainty. Lower panels show the data-to-prediction ratio. |
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Figure 8-a:
Post-fit (background-only) BDT discriminant distributions for the VLL search at $ m_{\mathrm{VLL}} = $ 600 GeV, for all regions used in the extraction of limit on mass across the 1 $ \tau_{\text{h}}0\ell $ (top and middle-left), 1 $ \tau_{\text{h}}1\ell $ (middle-right and bottom-left), and 1 $ \tau_{\text{h}}2\ell $ (bottom-right) channels. The stacked histograms represent the SM background predictions, with the expected VLL signal overlaid as a magenta line normalized to the 4321 model cross section. Hatched bands indicate the total uncertainty. Lower panels show the data-to-prediction ratio. |
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Figure 8-b:
Post-fit (background-only) BDT discriminant distributions for the VLL search at $ m_{\mathrm{VLL}} = $ 600 GeV, for all regions used in the extraction of limit on mass across the 1 $ \tau_{\text{h}}0\ell $ (top and middle-left), 1 $ \tau_{\text{h}}1\ell $ (middle-right and bottom-left), and 1 $ \tau_{\text{h}}2\ell $ (bottom-right) channels. The stacked histograms represent the SM background predictions, with the expected VLL signal overlaid as a magenta line normalized to the 4321 model cross section. Hatched bands indicate the total uncertainty. Lower panels show the data-to-prediction ratio. |
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Figure 8-c:
Post-fit (background-only) BDT discriminant distributions for the VLL search at $ m_{\mathrm{VLL}} = $ 600 GeV, for all regions used in the extraction of limit on mass across the 1 $ \tau_{\text{h}}0\ell $ (top and middle-left), 1 $ \tau_{\text{h}}1\ell $ (middle-right and bottom-left), and 1 $ \tau_{\text{h}}2\ell $ (bottom-right) channels. The stacked histograms represent the SM background predictions, with the expected VLL signal overlaid as a magenta line normalized to the 4321 model cross section. Hatched bands indicate the total uncertainty. Lower panels show the data-to-prediction ratio. |
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Figure 8-d:
Post-fit (background-only) BDT discriminant distributions for the VLL search at $ m_{\mathrm{VLL}} = $ 600 GeV, for all regions used in the extraction of limit on mass across the 1 $ \tau_{\text{h}}0\ell $ (top and middle-left), 1 $ \tau_{\text{h}}1\ell $ (middle-right and bottom-left), and 1 $ \tau_{\text{h}}2\ell $ (bottom-right) channels. The stacked histograms represent the SM background predictions, with the expected VLL signal overlaid as a magenta line normalized to the 4321 model cross section. Hatched bands indicate the total uncertainty. Lower panels show the data-to-prediction ratio. |
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Figure 8-e:
Post-fit (background-only) BDT discriminant distributions for the VLL search at $ m_{\mathrm{VLL}} = $ 600 GeV, for all regions used in the extraction of limit on mass across the 1 $ \tau_{\text{h}}0\ell $ (top and middle-left), 1 $ \tau_{\text{h}}1\ell $ (middle-right and bottom-left), and 1 $ \tau_{\text{h}}2\ell $ (bottom-right) channels. The stacked histograms represent the SM background predictions, with the expected VLL signal overlaid as a magenta line normalized to the 4321 model cross section. Hatched bands indicate the total uncertainty. Lower panels show the data-to-prediction ratio. |
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Figure 8-f:
Post-fit (background-only) BDT discriminant distributions for the VLL search at $ m_{\mathrm{VLL}} = $ 600 GeV, for all regions used in the extraction of limit on mass across the 1 $ \tau_{\text{h}}0\ell $ (top and middle-left), 1 $ \tau_{\text{h}}1\ell $ (middle-right and bottom-left), and 1 $ \tau_{\text{h}}2\ell $ (bottom-right) channels. The stacked histograms represent the SM background predictions, with the expected VLL signal overlaid as a magenta line normalized to the 4321 model cross section. Hatched bands indicate the total uncertainty. Lower panels show the data-to-prediction ratio. |
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Figure 9:
Observed and expected 95% confidence level upper limits on the VLL pair production cross section as a function of the VLL mass in the context of the 4321 model. The solid black line shows the observed limit, while the dashed black line shows the expected limit. The green and yellow bands represent the $ \pm1\sigma $ and $ \pm2\sigma $ uncertainty ranges on the expected limit. The red line shows the theoretical prediction for VLL pair production. |
| Tables | |
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Table 1:
Decay modes of vector-like lepton (VLL) pairs and the resulting final-state particles in the 1 $ \tau_{\text{h}}0\ell $, 1 $ \tau_{\text{h}}1\ell $, and 1 $ \tau_{\text{h}}2\ell $ channels. Each VLL decays via an off-shell leptoquark: $ {E} \to \mathrm{b} (\mathrm{b}\tau) $ or b (t}\nu_{\!\tau) and $ {N} \to \mathrm{t} (\mathrm{b}\tau) $ or t (t}\nu_{\!\tau), where the decay products in parentheses originate from the intermediate vector leptoquark U. No distinction is made between particles and antiparticles. In the final-state column, b\ denotes bottom quarks, $ \mathrm{q} $\ denotes quarks from hadronic W boson decays, $ \nu_{\!\tau} $\ denotes tau neutrinos (from the leptoquark or W boson decay), and $ \nu_{\ell} $ denotes electron or muon neutrinos. |
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Table 2:
Selection criteria for the hadronic triggers in the 1 $ \tau_{\text{h}}0\ell $ and 1 $ \tau_{\text{h}}1\ell $ channels. Multiple criteria, each represented by one row, are used per year and combined with a logical OR. In the case of the four-jet trigger, the minimum jet $ p_{\mathrm{T}} $ is different for each jet and separated by a slash (/). |
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Table 3:
Region definition for the 1 $ \tau_{\text{h}}0\ell $, 1 $ \tau_{\text{h}}1\ell $ and 1 $ \tau_{\text{h}}2\ell $ channels. Values in parentheses in the $ H_{\mathrm{T}} $ and $ p_{\mathrm{T}}^{\text{6th jet}} $ columns apply to the $ N_{\mathrm{b}} \geq $ 3 region. For 1 $ \tau_{\text{h}}1\ell \text{CR1} $ and 1 $ \tau_{\text{h}}2\ell \text{CR2} $, the selection of $ N_{{{j}} } $ and $ N_{\mathrm{b}} $ of corresponding signal regions is vetoed to ensure no overlap with signal regions. |
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Table 4:
Summary of the most important BDT input variables for the $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ and VLL searches. Variables are ranked by their importance, defined as the weighted fraction of training events in the nodes where that variable is used for splitting, as implemented in the TMVA toolkit [none-none]. Only variables ranked in the top 10 for at least one channel are shown. Rankings are shown as ``$ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ (VLL)'', where the VLL ranking corresponds to the 600 GeV mass point; a dash ($ \text{---} $) indicates the variable is not in the top 10 for that channel. |
| Summary |
| A measurement of $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production and a search for vector-like lepton pairs in the context of the 4321 model are performed using proton-proton collision data at $ \sqrt{s}=13 \text{Te\hspace{-.08em}V} $, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The analysis uses final states with hadronically decaying tau leptons, categorized into 1 $ \tau_{\text{h}}0\ell $, 1 $ \tau_{\text{h}}1\ell $, and 1 $ \tau_{\text{h}}2\ell $ channels, representing the first exploration of $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ in tau-enriched topologies. The $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production cross section is measured to be $16^{+18}_{-15} $ fb, consistent with the SM prediction. An observed (expected) significance of 1.1 (1.0) SDs with respect to the background-only hypothesis is obtained, demonstrating that the $ \tau_{\text{h}} $ channel provides complementary sensitivity to $ {\mathrm{t}\overline{\mathrm{t}}} {\mathrm{t}\overline{\mathrm{t}}} $ production. The VLL search finds no significant excess, establishing observed (expected) exclusion limits of 830 (830) GeV at 95% confidence level. This constitutes the first search for VLL pairs in the 4321 model in final states containing light leptons, complementing the hadronic searches by CMS [48] and ATLAS [49]. Together with the ATLAS result excluding VLL masses below 910 GeV, these results are consistent with standard model predictions and do not confirm the 2.8 SD excess previously observed in the CMS all-hadronic channel. |
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