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CMS-PAS-B2G-16-019
Search for heavy vector-like quarks decaying to same-sign dileptons
Abstract: A search for the production of heavy partners of the top quark with charge 5/3 decaying to same-sign dileptons is presented. The data sample was collected at a center of mass energy of $\sqrt{s} = $ 13 TeV by the CMS experiment at the Large Hadron Collider in 2016 and corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The same-sign dilepton final state has three main sources of background: background from prompt standard model processes, background from events where the charge of one of the leptons is misidentified, and background from non-prompt or fake leptons. No significant excess is seen above the expected background and lower limits are placed on the $\mathrm{X}_{5/3}$ mass of 1.16 (1.10) TeV for right (left) handed chirality particle while the expected limits are 1.20 (1.15) TeV for a right (left) handed $\mathrm{X}_{5/3}$.
Figures & Tables Summary References CMS Publications
Figures

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

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Figure 1-a:
Feynman diagram for the production of pairs of $\mathrm{X}_{5/3} $particles via QCD processes.

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

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Figure 2:
The $H^{\text {lep}}_{\text {T}}$ distributions after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and requiring at least two AK4 jets in the event. The bottom panel on all plots shows the difference between the observed and the predicted number of events in that bin divided by the total uncertainty ($\sigma $). The total uncertainty is calculated as the sum in quadrature of the statistical uncertainty on the observed measurement and the uncertainty, including both statistical and systematic, on the background.

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Figure 2-a:
The $H^{\text {lep}}_{\text {T}}$ all-channel distribution after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and requiring at least two AK4 jets in the event.

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Figure 2-b:
The $H^{\text {lep}}_{\text {T}}$ ee distribution after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and requiring at least two AK4 jets in the event.

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Figure 2-c:
The $H^{\text {lep}}_{\text {T}}$ e$\mu$ distribution after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and requiring at least two AK4 jets in the event.

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Figure 2-d:
The $H^{\text {lep}}_{\text {T}}$ $\mu\mu$ distribution after the same-sign dilepton selection, Z/quarkonia lepton invariant mass vetoes, and requiring at least two AK4 jets in the event.

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Figure 3:
95% CL observed and expected limits for a left-handed (left) and right-handed (right) $\mathrm{X}_{5/3} $ for all channels combined. The theoretical uncertainty on the signal cross section is shown as a red band around the central prediction.

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Figure 3-a:
95% CL observed and expected limits for a left-handed $\mathrm{X}_{5/3} $ for all channels combined. The theoretical uncertainty on the signal cross section is shown as a red band around the central prediction.

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Figure 3-b:
95% CL observed and expected limits for a right-handed $\mathrm{X}_{5/3} $ for all channels combined. The theoretical uncertainty on the signal cross section is shown as a red band around the central prediction.
Tables

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Table 1:
$\mathrm{X}_{5/3} $signal samples and their cross sections. Samples for both LH and RH chiralities were generated for the different mass points.

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Table 2:
Details of systematic uncertainties applied for lepton triggering, identification ("ID''), isolation ("ISO''), and luminosity.

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Table 3:
Systematic uncertainties associated with the background processes which we take from simulation. "JES'' refers to the uncertainty from the jet energy scale while "Theory'' refers to uncertainties from the cross section normalization and choice of PDF.

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Table 4:
Summary of yields from prompt same-sign dilepton Monte Carlo (PSS MC), same-sign non-prompt (NonPrompt), and opposite-sign prompt (ChargeMisID) backgrounds after the full analysis selection. Also shown are the number of expected events for a right handed 1000 GeV $\mathrm{X}_{5/3} $. The errors include both statistical and all systematic uncertainties.
Summary
In summary, we have performed a search for an exotic top partner with charge 5/3 decaying to same-sign dileptons using 35.9 fb$^{-1}$ of data collected by the CMS experiment at $\sqrt{s} = $ 13 TeV. No significant excess above the expected values is seen and observed (expected) limits are placed on the $\mathrm{X}_{5/3}$ mass of 1.16 (1.20) TeV for a right handed $\mathrm{X}_{5/3}$ and 1.10 (1.15) TeV for a left handed $\mathrm{X}_{5/3}$.
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