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| CMS-B2G-23-009 ; CERN-EP-2025-228 | ||
| Search for single production of a vector-like T quark decaying to a top quark and a neutral scalar boson in the lepton+jets final state in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 29 October 2025 | ||
| Submitted to J. High Energy Phys. | ||
| Abstract: A search for single production of a vector-like T quark with charge 2$ e$/3, decaying to a top quark and a neutral scalar boson is presented. The boson can be a standard model Higgs boson (H) or a new scalar boson ($ \phi $). In the first case, a branching fraction of 25% is assumed for the decay $ \mathrm{T} \to\mathrm{t} \mathrm{H} $, while in the second case the T quark is assumed to decay exclusively to $ \mathrm{t} \phi $. The top quark is identified via its lepton+jets decay, and the neutral boson via its decay into a bottom quark-antiquark pair. Final states with Lorentz-boosted topologies are considered and machine-learning techniques are exploited for optimal event classification. The analysis uses data collected by the CMS experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $ recorded at the CERN LHC in 2016-2018. Upper limits at 95% confidence level are set on the product of cross section and branching fraction for a T quark in a narrow-width approximation. They vary between 14.8 and 0.1 fb, for T quark masses in the range 1-3 TeV and $ \phi $ boson masses in the range 25-250 GeV. These are the first exclusion limits set on the production of a single T quark decaying into a top quark and a new neutral scalar boson. For the decay channel into a top quark and a standard model Higgs boson, the results provide the best limits on production cross sections to date, for T quark masses above 2 TeV. | ||
| Links: e-print arXiv:2510.25874 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
A representative leading-order Feynman diagram for the production of a single vector-like quark T decaying into a top quark and a neutral scalar boson, which may be a new scalar boson or an SM Higgs boson. Associated production with a bottom quark in the final state is shown. |
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Figure 2:
The receiver operating characteristic curve for the TopVsQCD (left) and TopVsOther (right) top quark taggers. The signal efficiency and background misidentification rate are evaluated on top quark candidates with an associated muon, with $ p_{\mathrm{T}} > $ 500 GeV and in the merged configuration in simulated $ \mathrm{t} \overline{\mathrm{T}} $ events. The red triangle in the left plot corresponds to the requirement on the TopVsQCD variable providing a signal efficiency of 99%; the blue triangle and the green square in the right plot correspond to the loose and tight WP of the TopVsOther tagger, respectively. A comparison with a cut-based selection (red point) is also shown in the TopVsOther case. |
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Figure 2-a:
The receiver operating characteristic curve for the TopVsQCD (left) and TopVsOther (right) top quark taggers. The signal efficiency and background misidentification rate are evaluated on top quark candidates with an associated muon, with $ p_{\mathrm{T}} > $ 500 GeV and in the merged configuration in simulated $ \mathrm{t} \overline{\mathrm{T}} $ events. The red triangle in the left plot corresponds to the requirement on the TopVsQCD variable providing a signal efficiency of 99%; the blue triangle and the green square in the right plot correspond to the loose and tight WP of the TopVsOther tagger, respectively. A comparison with a cut-based selection (red point) is also shown in the TopVsOther case. |
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Figure 2-b:
The receiver operating characteristic curve for the TopVsQCD (left) and TopVsOther (right) top quark taggers. The signal efficiency and background misidentification rate are evaluated on top quark candidates with an associated muon, with $ p_{\mathrm{T}} > $ 500 GeV and in the merged configuration in simulated $ \mathrm{t} \overline{\mathrm{T}} $ events. The red triangle in the left plot corresponds to the requirement on the TopVsQCD variable providing a signal efficiency of 99%; the blue triangle and the green square in the right plot correspond to the loose and tight WP of the TopVsOther tagger, respectively. A comparison with a cut-based selection (red point) is also shown in the TopVsOther case. |
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Figure 3:
Left: block diagram showing the event classification based on top quark tagging. Right: schematic view of the signal, validation, and control regions defined in the analysis. The labels TopT (Top Tight) and TopL (Top Loose) refer to events where there is a top quark passing a tight working point for the TopVsOther tagger with a background rejection of 99%, or a loose working point with a background rejection of 90% while vetoing the tight working point to ensure statistical independence of the categories. The label LepT refers to events that are neither in the TopT or TopL categories, but where a top quark candidate is built via a lepton and b-jet pair. The XbbT, XbbL, and XbbV labels refer to the fact that the largest score of the XbbVsQCD tagger evaluated on AK8 jets selected in the event is in the XbbVsQCD range of ($ {>} $ 0.98), (0.80-0.98), or ($ {<} $ 0.80), respectively. Additionally, for the event to be included in the XbbV category, the mass of the jet with the largest score is required to be in the 60-220 GeV range. |
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Figure 3-a:
Left: block diagram showing the event classification based on top quark tagging. Right: schematic view of the signal, validation, and control regions defined in the analysis. The labels TopT (Top Tight) and TopL (Top Loose) refer to events where there is a top quark passing a tight working point for the TopVsOther tagger with a background rejection of 99%, or a loose working point with a background rejection of 90% while vetoing the tight working point to ensure statistical independence of the categories. The label LepT refers to events that are neither in the TopT or TopL categories, but where a top quark candidate is built via a lepton and b-jet pair. The XbbT, XbbL, and XbbV labels refer to the fact that the largest score of the XbbVsQCD tagger evaluated on AK8 jets selected in the event is in the XbbVsQCD range of ($ {>} $ 0.98), (0.80-0.98), or ($ {<} $ 0.80), respectively. Additionally, for the event to be included in the XbbV category, the mass of the jet with the largest score is required to be in the 60-220 GeV range. |
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Figure 3-b:
Left: block diagram showing the event classification based on top quark tagging. Right: schematic view of the signal, validation, and control regions defined in the analysis. The labels TopT (Top Tight) and TopL (Top Loose) refer to events where there is a top quark passing a tight working point for the TopVsOther tagger with a background rejection of 99%, or a loose working point with a background rejection of 90% while vetoing the tight working point to ensure statistical independence of the categories. The label LepT refers to events that are neither in the TopT or TopL categories, but where a top quark candidate is built via a lepton and b-jet pair. The XbbT, XbbL, and XbbV labels refer to the fact that the largest score of the XbbVsQCD tagger evaluated on AK8 jets selected in the event is in the XbbVsQCD range of ($ {>} $ 0.98), (0.80-0.98), or ($ {<} $ 0.80), respectively. Additionally, for the event to be included in the XbbV category, the mass of the jet with the largest score is required to be in the 60-220 GeV range. |
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Figure 4:
Distribution of the $ M_{\mathrm{X}_{\mathrm{b}\mathrm{b}}} $ mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) (upper), in the (TopL, XbbL) (lower) VRs, for the muon (left) and electron channels (right). The distributions are shown before the final fit for signal extraction. |
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Figure 4-a:
Distribution of the $ M_{\mathrm{X}_{\mathrm{b}\mathrm{b}}} $ mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) (upper), in the (TopL, XbbL) (lower) VRs, for the muon (left) and electron channels (right). The distributions are shown before the final fit for signal extraction. |
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Figure 4-b:
Distribution of the $ M_{\mathrm{X}_{\mathrm{b}\mathrm{b}}} $ mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) (upper), in the (TopL, XbbL) (lower) VRs, for the muon (left) and electron channels (right). The distributions are shown before the final fit for signal extraction. |
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Figure 4-c:
Distribution of the $ M_{\mathrm{X}_{\mathrm{b}\mathrm{b}}} $ mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) (upper), in the (TopL, XbbL) (lower) VRs, for the muon (left) and electron channels (right). The distributions are shown before the final fit for signal extraction. |
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Figure 4-d:
Distribution of the $ M_{\mathrm{X}_{\mathrm{b}\mathrm{b}}} $ mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) (upper), in the (TopL, XbbL) (lower) VRs, for the muon (left) and electron channels (right). The distributions are shown before the final fit for signal extraction. |
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Figure 5:
Invariant mass distribution of the $ \mathrm{T} $ quark candidates selected in the $ \mathrm{T} \to\mathrm{t}\mathrm{H} $ channel, for events with at least one forward jet and in the SRs for (TopT, XbbT) (upper) and (TopL, XbbT) (lower). For these events, the reconstructed mass of the Higgs boson candidate is required to be between 110 and 140 GeV. Distributions are shown for events in the muon (left) and electron (right) channel. The first (last) bin of each distribution also includes underflow (overflow) events. The lower panels show the difference between the number of events in data and that expected, normalized to the statistical uncertainty of the data. The orange band represents the systematic uncertainties, also normalized to the statistical uncertainty of the data. |
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Figure 5-a:
Invariant mass distribution of the $ \mathrm{T} $ quark candidates selected in the $ \mathrm{T} \to\mathrm{t}\mathrm{H} $ channel, for events with at least one forward jet and in the SRs for (TopT, XbbT) (upper) and (TopL, XbbT) (lower). For these events, the reconstructed mass of the Higgs boson candidate is required to be between 110 and 140 GeV. Distributions are shown for events in the muon (left) and electron (right) channel. The first (last) bin of each distribution also includes underflow (overflow) events. The lower panels show the difference between the number of events in data and that expected, normalized to the statistical uncertainty of the data. The orange band represents the systematic uncertainties, also normalized to the statistical uncertainty of the data. |
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Figure 5-b:
Invariant mass distribution of the $ \mathrm{T} $ quark candidates selected in the $ \mathrm{T} \to\mathrm{t}\mathrm{H} $ channel, for events with at least one forward jet and in the SRs for (TopT, XbbT) (upper) and (TopL, XbbT) (lower). For these events, the reconstructed mass of the Higgs boson candidate is required to be between 110 and 140 GeV. Distributions are shown for events in the muon (left) and electron (right) channel. The first (last) bin of each distribution also includes underflow (overflow) events. The lower panels show the difference between the number of events in data and that expected, normalized to the statistical uncertainty of the data. The orange band represents the systematic uncertainties, also normalized to the statistical uncertainty of the data. |
png pdf |
Figure 5-c:
Invariant mass distribution of the $ \mathrm{T} $ quark candidates selected in the $ \mathrm{T} \to\mathrm{t}\mathrm{H} $ channel, for events with at least one forward jet and in the SRs for (TopT, XbbT) (upper) and (TopL, XbbT) (lower). For these events, the reconstructed mass of the Higgs boson candidate is required to be between 110 and 140 GeV. Distributions are shown for events in the muon (left) and electron (right) channel. The first (last) bin of each distribution also includes underflow (overflow) events. The lower panels show the difference between the number of events in data and that expected, normalized to the statistical uncertainty of the data. The orange band represents the systematic uncertainties, also normalized to the statistical uncertainty of the data. |
png pdf |
Figure 5-d:
Invariant mass distribution of the $ \mathrm{T} $ quark candidates selected in the $ \mathrm{T} \to\mathrm{t}\mathrm{H} $ channel, for events with at least one forward jet and in the SRs for (TopT, XbbT) (upper) and (TopL, XbbT) (lower). For these events, the reconstructed mass of the Higgs boson candidate is required to be between 110 and 140 GeV. Distributions are shown for events in the muon (left) and electron (right) channel. The first (last) bin of each distribution also includes underflow (overflow) events. The lower panels show the difference between the number of events in data and that expected, normalized to the statistical uncertainty of the data. The orange band represents the systematic uncertainties, also normalized to the statistical uncertainty of the data. |
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Figure 6:
Observed 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel, as a function of $ m_{\mathrm{T} } $ and $ m_{\phi} $ masses. |
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Figure 7:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 25 up to 100 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
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Figure 7-a:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 25 up to 100 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 7-b:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 25 up to 100 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 7-c:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 25 up to 100 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 7-d:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 25 up to 100 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 8:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 125 up to 250 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 8-a:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 125 up to 250 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 8-b:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 125 up to 250 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 8-c:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 125 up to 250 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 8-d:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 125 up to 250 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 8-e:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the single T quark product of cross section and branching ratio for the $ \mathrm{T} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ m_{\mathrm{T} } $, for fixed values of $ m_{\phi} $, from 125 up to 250 GeV. The inner (green) and outer (yellow) bands represent the regions containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. |
png pdf |
Figure 9:
Observed (solid black line) and expected (dashed line) 95% CL upper limit on the single T quark production cross section times branching fraction for the $ \mathrm{T} \to\mathrm{t}\mathrm{H} $ channel as a function of the $ m_{\mathrm{T} } $ mass. The inner (green) and outer (yellow) bands represent the region containing 68 and 95%, respectively, of the limit values expected under the background-only hypothesis. The solid blue (dashed red) curve shows the theoretical expectation at NLO for a singlet T quark assuming a narrow resonance with width 5% (1%) of the resonance mass [6,7]. |
| Tables | |
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Table 1:
List of the variables employed for each of the eight trainings of the multiclass BDT algorithm for top quark candidate identification. The trainings are different for each lepton channel, for the merged or resolved configuration of the top quark candidate, and for the $ p_{\mathrm{T}} $ range of the top quark candidate with a threshold of 500 GeV. |
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Table 2:
List of systematic uncertainties, showing whether a source modifies the event rate or the distribution shape, and whether the effect is correlated or uncorrelated across the years of data taking. All the listed sources affect both signal and background processes. The relative impact on the postfit signal strength is shown for a representative set of signals with a T mass $ (m_{\mathrm{T} }) $ hypothesis of 1.8 TeV and varying the mass of $ \phi $ boson. |
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Table 3:
Requirement on the reconstructed $ M_{\mathrm{X}_{\mathrm{b}\mathrm{b}}} $ mass of the $ \phi $ boson candidate, as a function of the $ m_{\phi} $ boson mass hypothesis being tested. The $ M_{\mathrm{X}_{\mathrm{b}\mathrm{b}}} $ mass must fall within the specified interval. |
| Summary |
| A search for the single production of a vector-like quark T with charge 2$ e$/3 decaying to a top quark and a neutral scalar boson, which can be a standard model Higgs boson (H) or a new scalar boson ($ \phi $), has been presented. In the first case, a branching fraction of 25% is assumed for the decay $ \mathrm{T} \to\mathrm{t} \mathrm{H} $, while in the second case the T quark is assumed to decay exclusively to $ \mathrm{t} \phi $. Final states where the top quark decays in the lepton+jets channel and the neutral scalar boson decays into a bottom quark-antiquark pair are considered. The analysis is based on LHC proton-proton collision data, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. No significant deviation is observed between the data and the expected background. Upper limits at 95% confidence level (CL) are set on the product of the production cross section and $ \mathrm{T} \to \mathrm{t}\phi \to \mathrm{b}\ell\nu\mathrm{b}\overline{\mathrm{b}} $ branching fraction as functions of the masses of T quark ($ m_{\mathrm{T} } $) and of $ \phi $ boson ($ m_{\phi} $), and assuming a T quark in a narrow-width approximation produced in association with a bottom quark. For an $ m_{\phi} $ of 25 (250) GeV, values greater than 2.3 to 0.1 (14.8 to 0.3) fb are excluded at 95% CL for $ m_{\mathrm{T} } $ between 1.0 and 3.0 (1.3 and 3.0) TeV. These are the first exclusion limits set on the production of a single T quark decaying into a top quark and a new neutral scalar boson. The case of a vector-like quark T decaying to a top quark and a standard model Higgs boson has been studied as well, and upper limits at 95% CL are set on the product of the production cross section and $ \mathrm{T} \to\mathrm{t}\mathrm{H} $ branching fraction: values greater than 100 to 1.0 fb are excluded at 95% CL for $ m_{\mathrm{T} } $ between 1.0 and 3.0 TeV. These results provide the best exclusion limits to date for $ m_{\mathrm{T} } > $ 2 TeV. |
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