CMS-PAS-B2G-24-005

CMS-PAS-B2G-24-005
Search for heavy top quark partners with charge 5/3 in the single-lepton final state in proton-proton collisions at $ \sqrt{s}= $ 13 TeV
CMS Collaboration
2026-05-15

Abstract: A search for the pair production of a vector-like quark ($ X_{5/3} $), a heavy fermionic partner of the top quark with an exotic electric charge of 5/3 times the charge of the positron, is presented. The search is performed in the single-lepton final state, where the lepton is either an electron or a muon, using proton-proton collision data collected by the CMS experiment at the LHC from 2016 to 2018 at a center-of-mass energy of $ \sqrt{s}= $ 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Two decay modes are considered. In the standard model (SM) decay mode, the $ X_{5/3} $ is assumed to decay exclusively into a top quark and a W boson. In an alternative scenario involving decays to beyond SM (BSM) particles, the $ X_{5/3} $ is assumed to decay exclusively into a top quark and a charged Higgs boson ($ H^\pm $), with the $ H^\pm $ subsequently decaying into a top quark and a bottom quark. A multivariate discriminant is used to separate signal from background events and to extract any potential $ X_{5/3} $ signal. No excess of data events over the SM expectation is observed. Expected and observed upper limits on the pair production cross section of the $ X_{5/3} $ are set at 95% confidence level as a function of the $ X_{5/3} $ mass. The expected and observed exclusion ranges of the $ X_{5/3} $ mass extend up to 1600 and 1630 GeV, respectively, in the SM decay mode, and up to 1660 and 1710 GeV, respectively, in the BSM decay mode with a charged Higgs boson mass of 1000 GeV.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Pair production of $ \mathrm{X}_{5/3} $ VLQs and their subsequent decays.
png pdf	Figure 1-a: Pair production of $ \mathrm{X}_{5/3} $ VLQs and their subsequent decays.
png pdf	Figure 1-b: Pair production of $ \mathrm{X}_{5/3} $ VLQs and their subsequent decays.
png pdf	Figure 1-c: Pair production of $ \mathrm{X}_{5/3} $ VLQs and their subsequent decays.
png pdf	Figure 1-d: Pair production of $ \mathrm{X}_{5/3} $ VLQs and their subsequent decays.
png pdf	Figure 2: Distribution of $ S_{\mathrm{T}} $ after the initial selection for data and simulated background processes. The background uncertainty includes both statistical and systematic components.
png pdf	Figure 3: Angular separation between the lepton $ \ell $ and the jet subleading in $ p_{\mathrm{T}} $ after the initial selection for data and simulated background processes. The background uncertainty includes both statistical and systematic components.
png pdf	Figure 4: Post-fit distribution of the XGBOOST output for the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{W} $ model in data and simulation in the signal region after rebinning. The background contributions are shown after the fit to data, while the signal distribution is shown using the prefit prediction. The lower panel shows the difference between the data and the background prediction, divided by the total uncertainty in the background prediction.
png pdf	Figure 5: Post-fit distributions of the XGBOOST outputs for the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{\widetilde{H}^{\pm_j}} $ model with the lowest and highest $ \mathrm{\widetilde{H}^{\pm_j}} $ mass hypotheses, 200 and 1000 GeV, respectively, in data and simulation in the signal region after rebinning. The background contributions are shown after the fit to data, while the signal distributions are shown using the prefit predictions. The lower panels show the differences between the data and the background prediction, divided by the total uncertainty in the background prediction.
png pdf	Figure 5-a: Post-fit distributions of the XGBOOST outputs for the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{\widetilde{H}^{\pm_j}} $ model with the lowest and highest $ \mathrm{\widetilde{H}^{\pm_j}} $ mass hypotheses, 200 and 1000 GeV, respectively, in data and simulation in the signal region after rebinning. The background contributions are shown after the fit to data, while the signal distributions are shown using the prefit predictions. The lower panels show the differences between the data and the background prediction, divided by the total uncertainty in the background prediction.
png pdf	Figure 5-b: Post-fit distributions of the XGBOOST outputs for the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{\widetilde{H}^{\pm_j}} $ model with the lowest and highest $ \mathrm{\widetilde{H}^{\pm_j}} $ mass hypotheses, 200 and 1000 GeV, respectively, in data and simulation in the signal region after rebinning. The background contributions are shown after the fit to data, while the signal distributions are shown using the prefit predictions. The lower panels show the differences between the data and the background prediction, divided by the total uncertainty in the background prediction.
png pdf	Figure 6: Expected and observed upper limits on the $ \mathrm{X}_{5/3} $ pair production cross section for the combined 2016--2018 data set with an integrated luminosity of 138 fb$ ^{-1} $. The inner green and outer yellow bands indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis.
png pdf	Figure 7: Expected (left) and observed (right) 95% CL upper limits on the production cross section for pair-produced $ \mathrm{X}_{5/3} $ particles in the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{\widetilde{H}^{\pm_j}} $ model, shown as a function of the $ \mathrm{X}_{5/3} $ and $ \mathrm{\widetilde{H}^{\pm_j}} $ masses, for the combined 2016--2018 data set with an integrated luminosity of 138 fb$ ^{-1} $. The expected and observed lower limits on the $ \mathrm{X}_{5/3} $ mass are 1620--1690 and 1620--1740 GeV, respectively, for $ \mathrm{\widetilde{H}^{\pm_j}} $ masses between 200 and 1000 GeV.
png pdf	Figure 7-a: Expected (left) and observed (right) 95% CL upper limits on the production cross section for pair-produced $ \mathrm{X}_{5/3} $ particles in the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{\widetilde{H}^{\pm_j}} $ model, shown as a function of the $ \mathrm{X}_{5/3} $ and $ \mathrm{\widetilde{H}^{\pm_j}} $ masses, for the combined 2016--2018 data set with an integrated luminosity of 138 fb$ ^{-1} $. The expected and observed lower limits on the $ \mathrm{X}_{5/3} $ mass are 1620--1690 and 1620--1740 GeV, respectively, for $ \mathrm{\widetilde{H}^{\pm_j}} $ masses between 200 and 1000 GeV.
png pdf	Figure 7-b: Expected (left) and observed (right) 95% CL upper limits on the production cross section for pair-produced $ \mathrm{X}_{5/3} $ particles in the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{\widetilde{H}^{\pm_j}} $ model, shown as a function of the $ \mathrm{X}_{5/3} $ and $ \mathrm{\widetilde{H}^{\pm_j}} $ masses, for the combined 2016--2018 data set with an integrated luminosity of 138 fb$ ^{-1} $. The expected and observed lower limits on the $ \mathrm{X}_{5/3} $ mass are 1620--1690 and 1620--1740 GeV, respectively, for $ \mathrm{\widetilde{H}^{\pm_j}} $ masses between 200 and 1000 GeV.

Tables
png pdf	Table 1: Ranking of input variables for the BDT trained on the 2017 data set using signal samples with an $ \mathrm{X}_{5/3} $ mass of 1300 GeV. The ranking is based on the gain metric from XGBOOST.
png pdf	Table 2: Ranking of input variables for the BDT trained on the 2017 data set using signal samples with an $ \mathrm{X}_{5/3} $ mass of 1200 GeV and a $ \mathrm{\widetilde{H}^{\pm_j}} $ mass of 200 GeV. The ranking is based on the gain metric from XGBOOST.
png pdf	Table 3: Signal selection efficiencies for the SM and BSM decay modes, defined as the ratio of the number of signal events passing the final event selection to the total number of generated events in the simulated signal sample.
png pdf	Table 4: Summary of the systematic uncertainties considered, where one standard deviation is denoted by $ \sigma $. Uncertainties labeled as ``Shape'' correspond to variations whose templates are normalized to the total yield and therefore affect only the distribution shape.

Summary

An analysis setting the most stringent limits to date on the pair production of heavy partners of the top quark with an exotic electric charge of 5/3 times the charge of the positron ($ \mathrm{X}_{5/3} $) has been presented. The $ \mathrm{X}_{5/3} $ is assumed to decay either into a top quark and a W boson, or into a top quark and a charged Higgs boson ($ \mathrm{\widetilde{H}^{\pm_j}} $). The $ \mathrm{\widetilde{H}^{\pm_j}} $ subsequently decays into a top quark and a bottom quark. The search uses proton-proton collision data collected during 2016--2018 at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Only the single-lepton final state is considered, where the lepton is either an electron or a muon. Upper limits on the $ \mathrm{X}_{5/3} $ pair production cross section are set at 95% confidence level using a multivariate discriminant. No significant excess of data events over the standard model expectation is observed. The expected and observed exclusion ranges of the $ \mathrm{X}_{5/3} $ mass extend up to 1600 and 1630 GeV, respectively, in the decay mode to a top quark and a W boson. In the decay mode to a top quark and a $ \mathrm{\widetilde{H}^{\pm_j}} $, $ \mathrm{X}_{5/3} $ masses in the ranges 1620--1690 and 1620--1740 GeV are excluded at 95% confidence level for the expected and observed limits, respectively, for $ \mathrm{\widetilde{H}^{\pm_j}} $ masses between 200 and 1000 GeV. The interpretation in the $ \mathrm{X}_{5/3} \to \mathrm{t}\mathrm{\widetilde{H}^{\pm_j}} $ decay chain is restricted to simulated mass points satisfying the kinematic condition $ m_{\mathrm{X}_{5/3}} > m_{\mathrm{\widetilde{H}^{\pm_j}}} + m_{\mathrm{t}} $, and no interpolation between mass hypotheses is performed. This work also presents the first set of limits for the search for the $ \mathrm{X}_{5/3} $ decaying into a top quark and a $ \mathrm{\widetilde{H}^{\pm_j}} $. \newpage

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