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CMS-B2G-17-014 ; CERN-EP-2018-258
Search for top quark partners with charge 5/3 in the same-sign dilepton and single-lepton final states in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 03 (2019) 082
Abstract: A search for the pair production of heavy fermionic partners of the top quark with charge 5/3 (${X_{5/3}} $) is performed in proton-proton collisions at a center-of-mass energy of 13 TeV with the CMS detector at the CERN LHC. The data sample analyzed corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The ${X_{5/3}} $ quark is assumed always to decay into a top quark and a W boson. Both the right-handed and left-handed ${X_{5/3}} $ couplings to the W boson are considered. Final states with either a pair of same-sign leptons or a single lepton are studied. No significant excess of events is observed above the expected standard model background. Lower limits at 95% confidence level on the ${X_{5/3}} $ quark mass are set at 1.33 and 1.30 TeV respectively for the case of right-handed and left-handed couplings to W bosons in a combination of the same-sign dilepton and single-lepton final states.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Leading order Feynman diagrams showing pair production and decays of ${X_{5/3}} $ particles via QCD processes.

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Figure 1-a:
Leading order Feynman diagram showing pair production and decays of ${X_{5/3}} $ particles via a QCD process.

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Figure 1-b:
Leading order Feynman diagram showing pair production and decays of ${X_{5/3}} $ particles via a QCD process.

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Figure 2:
The $ {{H_{\mathrm {T}}} ^{\text {lep}}}$ distributions after the same-sign dilepton requirement, Z boson and quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event, for dielectron (upper left), dimuon (upper right), electron-muon (lower left) final states, and their combination (lower right). The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. The last bin includes overflow events. The lower panel in each plot shows the difference between the observed and the predicted numbers of events divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the uncertainty in the background, including both statistical and systematic components. Also shown are the expected signal distributions for a 1 TeV ${X_{5/3}} $ with LH (solid line) and RH (dashed line) couplings.

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Figure 2-a:
The $ {{H_{\mathrm {T}}} ^{\text {lep}}}$ distribution after the same-sign dilepton requirement, Z boson and quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event, for the dielectron final state. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. The last bin includes overflow events. The lower panel shows the difference between the observed and the predicted numbers of events divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the uncertainty in the background, including both statistical and systematic components. Also shown are the expected signal distributions for a 1 TeV ${X_{5/3}} $ with LH (solid line) and RH (dashed line) couplings.

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Figure 2-b:
The $ {{H_{\mathrm {T}}} ^{\text {lep}}}$ distribution after the same-sign dilepton requirement, Z boson and quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event, for the dimuon final state. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. The last bin includes overflow events. The lower panel shows the difference between the observed and the predicted numbers of events divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the uncertainty in the background, including both statistical and systematic components. Also shown are the expected signal distributions for a 1 TeV ${X_{5/3}} $ with LH (solid line) and RH (dashed line) couplings.

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Figure 2-c:
The $ {{H_{\mathrm {T}}} ^{\text {lep}}}$ distribution after the same-sign dilepton requirement, Z boson and quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event, for the electron-muon final state. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. The last bin includes overflow events. The lower panel shows the difference between the observed and the predicted numbers of events divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the uncertainty in the background, including both statistical and systematic components. Also shown are the expected signal distributions for a 1 TeV ${X_{5/3}} $ with LH (solid line) and RH (dashed line) couplings.

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Figure 2-d:
The $ {{H_{\mathrm {T}}} ^{\text {lep}}}$ distribution after the same-sign dilepton requirement, Z boson and quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event, for the combination of dielectron, dimuon and electron-muon final states. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. The last bin includes overflow events. The lower panel shows the difference between the observed and the predicted numbers of events divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the uncertainty in the background, including both statistical and systematic components. Also shown are the expected signal distributions for a 1 TeV ${X_{5/3}} $ with LH (solid line) and RH (dashed line) couplings.

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Figure 3:
Distributions of ${\text{min}[M(\ell, {\mathrm {b}})]}$ (left) and ${\Delta R(\ell, \mathrm {j_{2})}}$ (right) in data and simulation for events with at least three AK4 jets, including a leading (subleading) jet with $ {p_{\mathrm {T}}} > $ 250 (150) GeV, after combining the electron and muon channels. Example signal distributions are also shown, scaled by a factor of 120 (70) in the ${\text{min}[M(\ell, {\mathrm {b}})]}$ (${\Delta R(\ell, \mathrm {j_{2})}}$) distribution. The last bin includes overflow events. The lower panel in each plot shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background.

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Figure 3-a:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in data and simulation for events with at least three AK4 jets, including a leading (subleading) jet with $ {p_{\mathrm {T}}} > $ 250 (150) GeV, after combining the electron and muon channels. Example signal distributions are also shown, scaled by a factor of 120 in the ${\text{min}[M(\ell, {\mathrm {b}})]}$ distribution. The last bin includes overflow events. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background.

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Figure 3-b:
Distribution of ${\Delta R(\ell, \mathrm {j_{2})}}$ in data and simulation for events with at least three AK4 jets, including a leading (subleading) jet with $ {p_{\mathrm {T}}} > $ 250 (150) GeV, after combining the electron and muon channels. Example signal distributions are also shown, scaled by a factor of 70 in the ${\Delta R(\ell, \mathrm {j_{2})}}$ distribution. The last bin includes overflow events. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background.

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Figure 4:
Distributions of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in the $ {{\mathrm {t}\overline {\mathrm {t}}}} $ control region, for 1 b-tagged jet (upper left) and $\ge $2 b-tagged jets (upper right) categories, and of ${\text{min}[M(\ell, {\mathrm {j}})]}$ in the W+jets control region, for 0 W-tagged jets (lower left) and $\ge $1 W-tagged jets (lower right) categories. Example signal distributions are also shown. The background distributions correspond to background-only fit to data while signal distributions are before the fit to data. Electron and muon event samples are combined. The last bin includes overflow events and its content is divided by the bin width. The distributions in each category have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel in each plot shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 4-a:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in the $ {{\mathrm {t}\overline {\mathrm {t}}}} $ control region, for the 1 b-tagged jet category. Example signal distributions are also shown. The background distributions correspond to background-only fit to data while signal distributions are before the fit to data. Electron and muon event samples are combined. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 4-b:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in the $ {{\mathrm {t}\overline {\mathrm {t}}}} $ control region, for the $\ge $2 b-tagged jets category. Example signal distributions are also shown. The background distributions correspond to background-only fit to data while signal distributions are before the fit to data. Electron and muon event samples are combined. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 4-c:
Distribution of ${\text{min}[M(\ell, {\mathrm {j}})]}$ in the W+jets control region, for the 0 W-tagged jet category. Example signal distributions are also shown. The background distributions correspond to background-only fit to data while signal distributions are before the fit to data. Electron and muon event samples are combined. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 4-d:
Distribution of ${\text{min}[M(\ell, {\mathrm {j}})]}$ in the W+jets control region, for the $\ge $1 W-tagged jet category. Example signal distributions are also shown. The background distributions correspond to background-only fit to data while signal distributions are before the fit to data. Electron and muon event samples are combined. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 5:
Distributions of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with 0 t-tagged jets, 0 (upper) or $\geq $1 (lower) W-tagged jets, and 1 (left) or $\geq $2 (right) b-tagged jets for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while signal distributions are before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions in each category have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel in each plot shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 5-a:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with 0 t-tagged jets, 0 W-tagged jet and 1 b-tagged jet, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 5-b:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with 0 t-tagged jets, 0 W-tagged jet and $\geq $2 b-tagged jets, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 5-c:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with 0 t-tagged jets, $\geq $1 W-tagged jet and1 b-tagged jet, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 5-d:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with 0 t-tagged jets, $\geq $1 W-tagged jet and $\geq $2 b-tagged jets, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 6:
Distributions of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with $\geq $1 t-tagged jets, 0 (upper) or $\geq $1 (lower) W-tagged jets, and 1 (left) or $\geq $2 (right) b-tagged jets for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while signal distributions are before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions in each category have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel in each plot shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 6-a:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with $\geq $1 t-tagged jets, 0 W-tagged jet and 1 b-tagged jet, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while the signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 6-b:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with $\geq $1 t-tagged jets, 0 W-tagged jet and $\geq $2 b-tagged jets, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while the signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 6-c:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with $\geq $1 t-tagged jets, $\geq $1 W-tagged jet and 1 b-tagged jet, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while the signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 6-d:
Distribution of ${\text{min}[M(\ell, {\mathrm {b}})]}$ in events with $\geq $1 t-tagged jets, $\geq $1 W-tagged jet and $\geq $2 b-tagged jets, for the combined electron and muon samples in the signal region. Example signal distributions are also shown. The background distributions correspond to the background-only fit to data, while the signal distribution is before the fit to data. The last bin includes overflow events and its content is divided by the bin width. The distributions have variable-size bins, chosen so that the statistical uncertainty in the total background in each bin is less than 30%. The lower panel shows the difference between the observed and the predicted numbers of events in that bin divided by the total uncertainty. The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty in the observed measurement and the statistical and systematic uncertainties in the background-only fit to data.

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Figure 7:
Expected and observed limits at 95% CL for an LH (left) and RH (right) ${X_{5/3}} $ after combining all categories for the same-sign dilepton (upper row) and the single-lepton (lower row) final states. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.

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Figure 7-a:
Expected and observed limits at 95% CL for an LH ${X_{5/3}} $ after combining all categories for the same-sign dilepton final state. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.

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Figure 7-b:
Expected and observed limits at 95% CL for an RH ${X_{5/3}} $ after combining all categories for the same-sign dilepton final state. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.

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Figure 7-c:
Expected and observed limits at 95% CL for an LH ${X_{5/3}} $ after combining all categories for the single-lepton final state. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.

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Figure 7-d:
Expected and observed limits at 95% CL for an RH ${X_{5/3}} $ after combining all categories for the single-lepton final state. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.

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Figure 8:
Expected and observed limits at 95% CL for an LH (left) and RH (right) ${X_{5/3}} $ after combining the same-sign dilepton and single-lepton final states. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.

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Figure 8-a:
Expected and observed limits at 95% CL for an LH ${X_{5/3}} $ after combining the same-sign dilepton and single-lepton final states. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.

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Figure 8-b:
Expected and observed limits at 95% CL for an RH ${X_{5/3}} $ after combining the same-sign dilepton and single-lepton final states. The theoretical uncertainty in the signal cross section is shown as a narrow band around the theoretical prediction.
Tables

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Table 1:
Summary of yields from simulated prompt same-sign dilepton (SSP MC), same-sign nonprompt (Nonprompt), and opposite-sign prompt (ChargeMisID) backgrounds after the full analysis selection. Also shown are the number of expected events for an RH ${X_{5/3}} $ particle with a mass of 1 TeV. The uncertainties include both statistical and all systematic components (as described in Section 8). The number of events and uncertainties correspond to the background-only fit to data for the background, while for the signal they are based on the yields before the fit to data.

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Table 2:
Expected (observed) numbers of background (data) events passing the final selection requirements, in the $ {{\mathrm {t}\overline {\mathrm {t}}}} $ and W+jets control region (0.4 $ < {\Delta R(\ell, \mathrm {j_{2})}} < $ 1.0) categories, after combining the single-electron and single-muon channels. The numbers of events expected from two example signals are also shown. The event yields and their uncertainties correspond to the background-only fit to data for the background, while for the signal they are based on the values before the fit to data.

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Table 3:
Expected (observed) numbers of background (data) events passing the final selection requirements, in the signal region ($ {\Delta R(\ell, \mathrm {j_{2})}} > $ 1.0) categories, after combining the single-electron and single-muon channels. The numbers of events expected from two example signals are also shown. The event yields and their uncertainties correspond to the background-only fit to data for the background, while for the signal they are based on the values before the fit to data.

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Table 4:
Systematic uncertainties in percentage (%) in the same-sign dilepton final state, associated with the simulated processes. The "Normalization'' column refers to the uncertainties from the cross section normalization and the choice of PDF set.

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Table 5:
Summary of systematic uncertainties in the single-lepton final state. These uncertainties are included in both signal and all background processes, except for the top ${p_{\mathrm {T}}}$ systematic uncertainty, which is included only in $ {{\mathrm {t}\overline {\mathrm {t}}}} $. The range of uncertainty values in percentage (%) corresponds to the effect on the yields before the fit to data and is given across the relevant background processes and channels for each systematic uncertainty.
Summary
A search has been performed for a heavy top quark partner with an exotic 5/3 charge (${X_{5/3}} $) using proton-proton collision data collected by the CMS experiment in 2016 at a center-of-mass energy of 13 TeV and corresponding to 35.9 fb$^{-1}$. The ${X_{5/3}} $ quark is assumed always to decay into a top quark and a W boson. Two different final states, same-sign dilepton and single-lepton, are analyzed separately and then combined. No significant excess over the expected standard model backgrounds is seen in data. Lower limits are set on the mass of the ${X_{5/3}} $ particle. The observed (expected) limit is 1.33 (1.30) TeV for an ${X_{5/3}} $ particle with right-handed couplings to W bosons and 1.30 (1.28) TeV for an ${X_{5/3}} $ particle with left-handed couplings to W bosons in a combination of the same-sign dilepton and single-lepton final states.
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