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| CMS-PAS-B2G-23-009 | ||
| Search for single production of a vector-like T quark decaying to a top quark and a neutral scalar boson in lepton+jets final states at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 17 May 2025 | ||
| Abstract: A search for single production of a vector-like T quark with charge 2/3$ e $, in the decay channel with a top quark and a neutral scalar boson $ \phi $ is presented. The $ \phi $ boson can be a standard model Higgs boson or a new particle beyond the standard model. The top quark is identified in its leptonic decay, and the neutral boson decays into a bottom quark-antiquark pair. Final states with boosted topologies are considered and machine learning techniques are exploited for optimal 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. Limits at 95% confidence levels are set on the product of the cross section and branching fraction for a T quark of small decay width. They are in the range between 15 and 0.15 fb, depending on T quark and $ \phi $ boson masses. In the case of the decay channel with a top quark and a standard model Higgs boson, for most of the studied range the analysis provides limits which are better or comparable with previous searches performed in CMS. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
A representative leading-order Feynman diagram for the production of a single vector-like quarkTdecaying into a top quark and a scalar boson, which may be an SM Higgs boson or a BSM particle. The associated production with a bottom quark in the final state is shown. |
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Figure 2:
ROC curves for the TopVsQCD (left) and TopVsOther (right) top taggers. The signal efficiency and background fake 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 and the yellow triangles in the right plot correspond to the loose and tight WP cuts on the TopVsOther variable, respectively. A comparison with a cut-based selection (red point) is also shown in the TopVsOther case. The cut-based selection requires the lepton to be isolated and applies a b tagging score threshold with a background efficiency of 1%; the top quark candidate is then chosen as the one with the lowest difference in mass with respect to the nominal top quark mass value taken from Ref. [76]. |
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Figure 2-a:
ROC curves for the TopVsQCD (left) and TopVsOther (right) top taggers. The signal efficiency and background fake 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 and the yellow triangles in the right plot correspond to the loose and tight WP cuts on the TopVsOther variable, respectively. A comparison with a cut-based selection (red point) is also shown in the TopVsOther case. The cut-based selection requires the lepton to be isolated and applies a b tagging score threshold with a background efficiency of 1%; the top quark candidate is then chosen as the one with the lowest difference in mass with respect to the nominal top quark mass value taken from Ref. [76]. |
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Figure 2-b:
ROC curves for the TopVsQCD (left) and TopVsOther (right) top taggers. The signal efficiency and background fake 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 and the yellow triangles in the right plot correspond to the loose and tight WP cuts on the TopVsOther variable, respectively. A comparison with a cut-based selection (red point) is also shown in the TopVsOther case. The cut-based selection requires the lepton to be isolated and applies a b tagging score threshold with a background efficiency of 1%; the top quark candidate is then chosen as the one with the lowest difference in mass with respect to the nominal top quark mass value taken from Ref. [76]. |
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Figure 3:
Left: block diagram showing the event classification based on top tagging. Right: schematic view of the signal, validation, and control regions defined in the analysis. |
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Figure 3-a:
Left: block diagram showing the event classification based on top tagging. Right: schematic view of the signal, validation, and control regions defined in the analysis. |
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Figure 3-b:
Left: block diagram showing the event classification based on top tagging. Right: schematic view of the signal, validation, and control regions defined in the analysis. |
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Figure 4:
Distribution of the mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) validation region (top), in the (TopL, XbbL) validation region (bottom), and for the muon (left) and electron channels (right). The distributions are shown before the final fit for the signal extraction. |
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Figure 4-a:
Distribution of the mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) validation region (top), in the (TopL, XbbL) validation region (bottom), and for the muon (left) and electron channels (right). The distributions are shown before the final fit for the signal extraction. |
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Figure 4-b:
Distribution of the mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) validation region (top), in the (TopL, XbbL) validation region (bottom), and for the muon (left) and electron channels (right). The distributions are shown before the final fit for the signal extraction. |
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Figure 4-c:
Distribution of the mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) validation region (top), in the (TopL, XbbL) validation region (bottom), and for the muon (left) and electron channels (right). The distributions are shown before the final fit for the signal extraction. |
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Figure 4-d:
Distribution of the mass of the AK8 jet selected as the $ \phi $ boson candidate for data and simulated background events in the (TopT, XbbL) validation region (top), in the (TopL, XbbL) validation region (bottom), and for the muon (left) and electron channels (right). The distributions are shown before the final fit for the signal extraction. |
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Figure 5:
Invariant mass distribution of the $ {\mathrm{T}} $ quark candidates for events with at least one forward jet in the signal regions (TopT, XbbT) (top) and (TopL, XbbT) (bottom). 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 panel reports the data minus the expected number of events 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 for events with at least one forward jet in the signal regions (TopT, XbbT) (top) and (TopL, XbbT) (bottom). 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 panel reports the data minus the expected number of events 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 for events with at least one forward jet in the signal regions (TopT, XbbT) (top) and (TopL, XbbT) (bottom). 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 panel reports the data minus the expected number of events 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-c:
Invariant mass distribution of the $ {\mathrm{T}} $ quark candidates for events with at least one forward jet in the signal regions (TopT, XbbT) (top) and (TopL, XbbT) (bottom). 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 panel reports the data minus the expected number of events 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-d:
Invariant mass distribution of the $ {\mathrm{T}} $ quark candidates for events with at least one forward jet in the signal regions (TopT, XbbT) (top) and (TopL, XbbT) (bottom). 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 panel reports the data minus the expected number of events 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 singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $ and $ \mathrm{m}_{\phi} $ masses. |
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Figure 7:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 25 GeV up to $ \mathrm{m}_{\phi}= $ 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 singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 25 GeV up to $ \mathrm{m}_{\phi}= $ 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-b:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 25 GeV up to $ \mathrm{m}_{\phi}= $ 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-c:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 25 GeV up to $ \mathrm{m}_{\phi}= $ 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-d:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 25 GeV up to $ \mathrm{m}_{\phi}= $ 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 8:
Observed (solid lines) and expected (dashed lines) 95% CL upper limits on the singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 125 GeV up to $ \mathrm{m}_{\phi}= $ 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 singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 125 GeV up to $ \mathrm{m}_{\phi}= $ 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 singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 125 GeV up to $ \mathrm{m}_{\phi}= $ 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 singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 125 GeV up to $ \mathrm{m}_{\phi}= $ 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 singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 125 GeV up to $ \mathrm{m}_{\phi}= $ 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 singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\phi \to \mathrm{b}\ell\nu \mathrm{b}\overline{\mathrm{b}} $ channel as a function of $ \mathrm{m}_{{\mathrm{T}} } $, for fixed values of $ \mathrm{m}_{\phi} $, from $ \mathrm{m}_{\phi}= $ 125 GeV up to $ \mathrm{m}_{\phi}= $ 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. |
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Figure 9:
Observed (solid black line) and expected (dashed line) 95% CL upper limits on the singleTquark production cross section times the branching fraction of the $ {\mathrm{T}} \to\mathrm{t}\mathrm{H} $ channel as a function of the $ \mathrm{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 red (blue) curve shows the theoretical expectation at NLO for a singletTquark assuming a narrow-width resonance of width 1% (5%) 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. |
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Table 2:
List of systematic uncertainties. For each source, it is described if it modifies the event rate or the distribution shape, and if 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 $ \mathrm{m}_{{\mathrm{T}} } = $ 1.8 TeV and varying $ \mathrm{m}_{\phi} $. |
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Table 3:
Applied requirement on the reconstructed $ \mathrm{M_{X_{bb}}} $ mass of the $ \phi $ boson candidate, as a function of the $ \mathrm{m}_{\phi} $ boson mass hypothesis being tested. The $ \mathrm{M_{X_{bb}}} $ mass must fall within the specified interval. |
| Summary |
| A search for the single production of a vector-like quarkTwith charge 2/3$e$ decaying to a top quark and a neutral scalar boson $ \phi $ has been presented. Final states where the top quark decays leptonically and the neutral scalar boson decays into a bottom quark-antiquark pair are considered. The analysis is based on LHC proton-proton collision data collected by the CMS experiment, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Upper limits at 95% confidence level are set on the product of the production cross section and the $ {\mathrm{T}} \to \mathrm{t}\phi \to \mathrm{b}\ell\nu\mathrm{b}\overline{\mathrm{b}} $ branching fraction as a function of theTquark mass and the $ \phi $ boson mass, and assuming aTquark of negligible resonance width produced in association with a bottom quark. For a $ \phi $ boson mass of 25 GeV, values greater than 2.0 to 0.15 fb are excluded at 95% confidence level forTquark masses between 1 and 3 TeV. For a $ \phi $ boson mass of 250 GeV andTquark masses between 1.3 and 3 TeV, values greater than 15 to 0.25 fb are excluded at 95% confidence level. The case of a vector-like quarkTdecaying to a top quark and a standard model Higgs boson has been studied as well, and upper limits at 95% confidence level are set on the product of the production cross section and the $ {\mathrm{T}} \to\mathrm{t}\mathrm{H} $ branching fraction: values greater than 100 to 1.0 fb are excluded at 95% CL forTquark masses between 1 and 3 TeV. Assuming a theoretical framework in which theTquark is a singlet, and a resonance width of 5% of its mass, the analysis is near the level of excludingTquark masses around 1.2 TeV at 95% confidence level. |
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