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CMS-B2G-15-006 ; CERN-EP-2017-102
Search for top quark partners with charge 5/3 in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
JHEP 08 (2017) 073
Abstract: A search for the production of heavy partners of the top quark with charge 5/3 ($\mathrm{X}_{5/3}$) decaying into a top quark and a W boson is performed with a data sample corresponding to an integrated luminosity of 2.3 fb$^{-1}$, collected in proton-proton collisions at a center-of-mass energy of 13 TeV with the CMS detector at the CERN LHC. Final states with either a pair of same-sign leptons or a single lepton, along with jets, are considered. No significant excess is observed in the data above the expected standard model background contribution and an $\mathrm{X}_{5/3}$ quark with right-handed (left-handed) couplings is excluded at 95% confidence level for masses below 1020 (990) GeV. These are the first limits based on a combination of the same-sign dilepton and the single-lepton final states, as well as the most stringent limits on the $\mathrm{X}_{5/3}$ mass to date.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Leading order Feynman diagrams for the production and decay of pairs of $ {\mathrm {X}_{5/3}} $ particles via QCD processes.

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Figure 1-a:
Leading order Feynman diagram for the production and decay of pairs of $ {\mathrm {X}_{5/3}} $ particles via a QCD gluon-gluon process.

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Figure 1-b:
Leading order Feynman diagrams for the production and decay of pairs of $ {\mathrm {X}_{5/3}} $ particles via QCD processes.

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Figure 2:
The $ {H^{\text {lep}}_{\mathrm {T}}}$ distributions after the same-sign dilepton selection, Z quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. The lower panel in all plots 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 distributions for a 700 GeV $ {\mathrm {X}_{5/3}} $ with right-handed (solid line) and left-handed (dashed line) couplings to W bosons.

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Figure 2-a:
$ {H^{\text {lep}}_{\mathrm {T}}}$ distribution (ee+e$\mu$+$\mu\mu$) after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. 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 distributions for a 700 GeV $ {\mathrm {X}_{5/3}} $ with right-handed (solid line) and left-handed (dashed line) couplings to W bosons.

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Figure 2-b:
$ {H^{\text {lep}}_{\mathrm {T}}}$ distribution (ee) after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. 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 distributions for a 700 GeV $ {\mathrm {X}_{5/3}} $ with right-handed (solid line) and left-handed (dashed line) couplings to W bosons.

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Figure 2-c:
$ {H^{\text {lep}}_{\mathrm {T}}}$ distribution (e$\mu$) after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. 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 distributions for a 700 GeV $ {\mathrm {X}_{5/3}} $ with right-handed (solid line) and left-handed (dashed line) couplings to W bosons.

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Figure 2-d:
$ {H^{\text {lep}}_{\mathrm {T}}}$ distribution ($\mu\mu$) after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and the requirement of at least two AK4 jets in the event. The hatched area shows the combined systematic and statistical uncertainty in the background prediction for each bin. 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 distributions for a 700 GeV $ {\mathrm {X}_{5/3}} $ with right-handed (solid line) and left-handed (dashed line) couplings to W bosons.

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Figure 3:
Distributions of the number of AK4 jets (upper left), the numbers of b-tagged (upper right), W-tagged (lower left), and t-tagged jets (lower right) in data and simulation for combined electron and muon event samples, at the preselection level. The lower panel in all plots 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 distributions of representative signal events, which are scaled by a factor of 100.

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Figure 3-a:
Distribution of the number of AK4 jets in data and simulation for combined electron and muon event samples, at the preselection level. 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 is the distribution of representative signal events, which are scaled by a factor of 100.

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Figure 3-b:
Distribution of the number of b-tagged jets in data and simulation for combined electron and muon event samples, at the preselection level. 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 is the distribution of representative signal events, which are scaled by a factor of 100.

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Figure 3-c:
Distribution of the number of W-tagged jets in data and simulation for combined electron and muon event samples, at the preselection level. 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 is the distribution of representative signal events, which are scaled by a factor of 100.

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Figure 3-d:
Distribution of the number of t-tagged jets in data and simulation for combined electron and muon event samples, at the preselection level. 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 is the distribution of representative signal events, which are scaled by a factor of 100.

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Figure 4:
Distributions of min [M($\ell, \mathrm{ b } $)] (left) and $\Delta R$($\ell $, $j_{2}$) (right) in data and simulation for selected events with at least four jets and lepton $ {p_{\mathrm {T}}} > $ 80 GeV. The lower panel in all plots 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 min [M($\ell $, $\mathrm{ b } $)] ($\Delta R$($\ell $, $j_{2}$)) distributions of representative signal events, which are scaled by a factor of 100 (50) so that the shape differences between signal and background are visible.

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Figure 4-a:
Distribution of min [M($\ell, \mathrm{ b } $)] in data and simulation for selected events with at least four jets and lepton $ {p_{\mathrm {T}}} > $ 80 GeV. 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 is the min [M($\ell $, $\mathrm{ b } $)] distribution of representative signal events, which are scaled by a factor of 100 (50) so that the shape differences between signal and background are visible.

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Figure 4-b:
Distribution of $\Delta R$($\ell $, $j_{2}$) in data and simulation for selected events with at least four jets and lepton $ {p_{\mathrm {T}}} > $ 80 GeV. 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 is the $\Delta R$($\ell $, $j_{2}$) distribution of representative signal events, which are scaled by a factor of 100 (50) so that the shape differences between signal and background are visible.

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Figure 5:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ control region, for 1 b-tagged jet (upper left) and $\ge $2 b-tagged jets (upper right) categories, and of min [M($\ell $, jet)] in the W+jets control region, for 0 W-tagged (lower left) and $\ge $1 W-tagged jet (lower right) categories for combined electron and muon event samples. The horizontal bars on the data points indicate the bin widths. The lower panel in all plots 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. A small QCD multijet contribution is displayed in the bottom left plot; in all other distributions, it is less than 0.5% and is not shown.

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Figure 5-a:
Distribution of min [M($\ell $, $\mathrm{ b } $)] in the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ control region, for the 1 b-tagged jet category, for combined electron and muon event samples. The horizontal bars on the data points indicate the bin widths. 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. The QCD contribution is less than 0.5% and is not shown.

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Figure 5-b:
Distribution of min [M($\ell $, $\mathrm{ b } $)] in the ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ control region, for the $\ge $2 b-tagged jets category, for combined electron and muon event samples. The horizontal bars on the data points indicate the bin widths. 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. The QCD contribution is less than 0.5% and is not shown.

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Figure 5-c:
Distribution of min [M($\ell $, jet)] in the W+jets control region, for the 0 W-tagged category for combined electron and muon event samples. The horizontal bars on the data points indicate the bin widths. 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. A small QCD multijet contribution is displayed.

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Figure 5-d:
Distribution of min [M($\ell $, jet)] in the W+jets control region, for the $\ge $1 W-tagged jet category for combined electron and muon event samples. The horizontal bars on the data points indicate the bin widths. 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. The QCD contribution is less than 0.5% and is not shown.

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Figure 6:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in (upper) 0 or (lower) $\ge $1 W-tagged jets and (left) 1 or (right) $\ge $2 b-tagged jets categories with 0 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. The lower panel in all plots 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 6-a:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the 0 W-tagged jet and 1 b-tagged jet category with 0 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 6-b:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the 0 W-tagged jet and $\ge $2 b-tagged jets category with 0 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 6-c:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the $\ge $1 W-tagged jets and 1 b-tagged jet category with 0 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 6-d:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the $\ge $1 W-tagged jets and $\ge $2 b-tagged jets category with 0 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 7:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in 0 (upper) and $\ge $1 (lower) W-tagged jets and 1 (left) and $\ge $2 (right) b-tagged jets categories with $\ge $1 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. The lower panel in all plots 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 7-a:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the 0 W-tagged jet and 1 b-tagged jet category with $\ge $1 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 7-b:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the 0 W-tagged jet and $\ge $2 b-tagged jets category with $\ge $1 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 7-c:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the $\ge $1 W-tagged jet and 1 b-tagged jet category with $\ge $1 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 7-d:
Distributions of min [M($\ell $, $\mathrm{ b } $)] in the $\ge $1 W-tagged jet and $\ge $2 b-tagged jets category with $\ge $1 t-tagged jets for combined electron and muon samples, at the final selection level. The horizontal bars on the data points indicate the bin widths. 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 distributions of representative signal events, which are scaled by a factor of 10.

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Figure 8:
The expected and observed upper limits at 95% CL for a left-handed (left) and right-handed (right) $ {\mathrm {X}_{5/3}} $ for the same-sign dilepton signature (upper) and the single-lepton signature (lower) after combining all channels in each signature. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.

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Figure 8-a:
The expected and observed upper limits at 95% CL for a left-handed $ {\mathrm {X}_{5/3}} $ for the same-sign dilepton signature after combining all channels in each signature. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.

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Figure 8-b:
The expected and observed upper limits at 95% CL for a right-handed $ {\mathrm {X}_{5/3}} $ for the same-sign dilepton signature after combining all channels in each signature. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.

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Figure 8-c:
The expected and observed upper limits at 95% CL for a left-handed $ {\mathrm {X}_{5/3}} $ for the single-lepton signature after combining all channels in each signature. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.

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Figure 8-d:
The expected and observed upper limits at 95% CL for a right-handed $ {\mathrm {X}_{5/3}} $ for the single-lepton signature after combining all channels in each signature. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.

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Figure 9:
The expected and observed upper limits at 95% CL after combining the same-sign dilepton and the single-lepton signatures for left-handed (left) and right-handed (right) $ {\mathrm {X}_{5/3}} $ scenarios. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.

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Figure 9-a:
The expected and observed upper limits at 95% CL after combining the same-sign dilepton and the single-lepton signatures for left-handed $ {\mathrm {X}_{5/3}} $ scenario. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.

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Figure 9-b:
The expected and observed upper limits at 95% CL after combining the same-sign dilepton and the single-lepton signatures for right-handed $ {\mathrm {X}_{5/3}} $ scenario. The theoretical prediction for the $ {\mathrm {X}_{5/3}} $ pair production cross section is shown as a band including its uncertainty.
Tables

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Table 1:
Summary of background yields from SM processes with two same-sign prompt leptons (SSP MC), same-sign non-prompt leptons (NonPrompt), and opposite-sign prompt leptons (ChargeMisID), as well as observed data events after the full analysis selection for the same-sign dilepton channel, with an integrated luminosity of 2.3 fb$^{-1}$. Also shown are the numbers of expected events for a right-handed $ {\mathrm {X}_{5/3}} $ with a mass of 800 GeV. The uncertainties include both statistical and systematic components, as discussed in Section 7.

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Table 2:
Expected (observed) numbers of background (data) events passing the final selection requirements, in the eight tagging categories after combining electron and muon categories, for the single-lepton channel, with an integrated luminosity of 2.3 fb$^{-1}$. Also shown are the numbers of expected events for a LH $ {\mathrm {X}_{5/3}} $ with a mass of 800 GeV and an RH $ {\mathrm {X}_{5/3}} $ with a mass of 1.1 TeV. Uncertainties quoted in the table include both statistical as well as the systematic components listed in Table 5. The Poisson uncertainty upper bound (${<}$1.8) is used for the categories where the QCD multijet event yield is zero.

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Table 3:
Details of systematic uncertainties applied for lepton triggering, identification (''ID''), isolation (''ISO''), and integrated luminosity.

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

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Table 5:
Summary of all systematic uncertainties considered in the single-lepton channel. Each uncertainty is included in both signal and all background processes unless noted otherwise.
Summary
A search has been performed for the production of heavy partners of the top quark with charge 5/3 decaying into a top quark and a W boson, using 2.3 fb$^{-1}$ of proton-proton collision data collected by the CMS experiment at 13 TeV. Events with two different signatures are analyzed: final states with either a pair of same-sign leptons or a single lepton, along with jets. No significant excess is observed in the data above the expected standard model background. Upper bounds at 95% confidence level are set on the production cross section of heavy top quark partners. The $\mathrm{X}_{5/3}$ masses with right-handed (left-handed) couplings below 1020 (990) GeV are excluded at 95% confidence level. These are the most stringent limits placed on the $\mathrm{X}_{5/3}$ mass and the first limits based on a combination of these two different final states.
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