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| CMS-PAS-TOP-25-002 | ||
| Observation of a pseudoscalar excess at the top quark pair production threshold in the single lepton channel | ||
| CMS Collaboration | ||
| 2026-03-22 | ||
| Abstract: A search is presented for top quark-antiquark ($ \mathrm{t\bar{t}} $) bound states near the $ \mathrm{t\bar{t}} $ production threshold, in final states with a single electron or muon and jets. The study uses proton-proton collision data at $ \sqrt{s} = $ 13 TeV, collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. The analysis examines the relative velocity between the top quark and antiquark, along with two angular observables sensitive to the parity and spin of the $ \mathrm{t\bar{t}} $ system. A significant excess of events is observed relative to the standard model prediction for $ \mathrm{t\bar{t}} $ production calculated at next-to-next-to-leading order in perturbative quantum chromodynamics. The excess corresponds to an observed cross section of 5.1 $ \pm $ 0.9 $ \mathrm{pb} $ and is consistent with a simplified model of a color-singlet pseudoscalar toponium motivated by nonrelativistic quantum chromodynamics. The result provides an independent confirmation of the excess reported in the dilepton channel. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Parton-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 1-a:
Parton-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 1-b:
Parton-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 1-c:
Parton-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 1-d:
Parton-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 2:
Detector-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions are shown for all signal categories defined in Section 6 combined. The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 2-a:
Detector-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions are shown for all signal categories defined in Section 6 combined. The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 2-b:
Detector-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions are shown for all signal categories defined in Section 6 combined. The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 2-c:
Detector-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions are shown for all signal categories defined in Section 6 combined. The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 2-d:
Detector-level distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ (upper left), $ \beta_{\mathrm{t}}^{*} $ (upper right), $ c_\mathrm{hel} $ (lower left), and $ c_\mathrm{han} $ (lower right) for the $ \eta \mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The distributions are shown for all signal categories defined in Section 6 combined. The distributions for the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background include the NNLO QCD reweighting and the EW corrections. All distributions are normalized to unit area. |
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Figure 3:
The $ S_{\text{NN}} $ distribution before the mass window selections is presented for the 1b (left) and 2b (right) categories, with data points compared against the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ component is categorized into correctly reconstructed, incorrectly reconstructed, ``nonreconstructible'', and non $ \mathrm{e}/\mu $+jets events. The gray band represents the total statistical and systematic uncertainties in the prediction, while the vertical error bars on the data points indicate the statistical uncertainties of the data. The lower panels illustrate the ratio of observed data to predicted yields. |
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Figure 3-a:
The $ S_{\text{NN}} $ distribution before the mass window selections is presented for the 1b (left) and 2b (right) categories, with data points compared against the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ component is categorized into correctly reconstructed, incorrectly reconstructed, ``nonreconstructible'', and non $ \mathrm{e}/\mu $+jets events. The gray band represents the total statistical and systematic uncertainties in the prediction, while the vertical error bars on the data points indicate the statistical uncertainties of the data. The lower panels illustrate the ratio of observed data to predicted yields. |
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Figure 3-b:
The $ S_{\text{NN}} $ distribution before the mass window selections is presented for the 1b (left) and 2b (right) categories, with data points compared against the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ component is categorized into correctly reconstructed, incorrectly reconstructed, ``nonreconstructible'', and non $ \mathrm{e}/\mu $+jets events. The gray band represents the total statistical and systematic uncertainties in the prediction, while the vertical error bars on the data points indicate the statistical uncertainties of the data. The lower panels illustrate the ratio of observed data to predicted yields. |
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Figure 4:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{*} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_\mathrm{correct} $) relative to the reconstructible events ($ N_\mathrm{reconstructible} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_\mathrm{all} $). The plots in the upper row are based on the continuum $ \mathrm{t} \overline{\mathrm{t}} $ simulation, while those in the lower row use the $ \eta \mathrm{t} $ simulation. Results are presented separately for the 1b and 2b categories under the $ S_{\text{low}} $ and $ S_{\text{high}} $ selections. |
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Figure 4-a:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{*} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_\mathrm{correct} $) relative to the reconstructible events ($ N_\mathrm{reconstructible} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_\mathrm{all} $). The plots in the upper row are based on the continuum $ \mathrm{t} \overline{\mathrm{t}} $ simulation, while those in the lower row use the $ \eta \mathrm{t} $ simulation. Results are presented separately for the 1b and 2b categories under the $ S_{\text{low}} $ and $ S_{\text{high}} $ selections. |
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Figure 4-b:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{*} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_\mathrm{correct} $) relative to the reconstructible events ($ N_\mathrm{reconstructible} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_\mathrm{all} $). The plots in the upper row are based on the continuum $ \mathrm{t} \overline{\mathrm{t}} $ simulation, while those in the lower row use the $ \eta \mathrm{t} $ simulation. Results are presented separately for the 1b and 2b categories under the $ S_{\text{low}} $ and $ S_{\text{high}} $ selections. |
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Figure 4-c:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{*} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_\mathrm{correct} $) relative to the reconstructible events ($ N_\mathrm{reconstructible} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_\mathrm{all} $). The plots in the upper row are based on the continuum $ \mathrm{t} \overline{\mathrm{t}} $ simulation, while those in the lower row use the $ \eta \mathrm{t} $ simulation. Results are presented separately for the 1b and 2b categories under the $ S_{\text{low}} $ and $ S_{\text{high}} $ selections. |
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Figure 4-d:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{*} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_\mathrm{correct} $) relative to the reconstructible events ($ N_\mathrm{reconstructible} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_\mathrm{all} $). The plots in the upper row are based on the continuum $ \mathrm{t} \overline{\mathrm{t}} $ simulation, while those in the lower row use the $ \eta \mathrm{t} $ simulation. Results are presented separately for the 1b and 2b categories under the $ S_{\text{low}} $ and $ S_{\text{high}} $ selections. |
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Figure 5:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{*} $ (upper, left), $ c_\mathrm{hel} $ (upper, right) and $ c_\mathrm{han} $ (lower) distributions from different sources. The top panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \mathrm{MC}_\mathrm{CR} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties on the DD prediction are shown as dotted lines for $ \mathrm{W}_\mathrm{add} $ (yellow), $ \mathrm{DY}_\mathrm{add} $ (turquoise), and $ \mathrm{MC}_\mathrm{diff} $ (dark red). All distributions are normalized to the SR MC event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of all distributions to the SR MC. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant-mass requirements are not applied. |
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Figure 5-a:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{*} $ (upper, left), $ c_\mathrm{hel} $ (upper, right) and $ c_\mathrm{han} $ (lower) distributions from different sources. The top panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \mathrm{MC}_\mathrm{CR} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties on the DD prediction are shown as dotted lines for $ \mathrm{W}_\mathrm{add} $ (yellow), $ \mathrm{DY}_\mathrm{add} $ (turquoise), and $ \mathrm{MC}_\mathrm{diff} $ (dark red). All distributions are normalized to the SR MC event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of all distributions to the SR MC. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant-mass requirements are not applied. |
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Figure 5-b:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{*} $ (upper, left), $ c_\mathrm{hel} $ (upper, right) and $ c_\mathrm{han} $ (lower) distributions from different sources. The top panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \mathrm{MC}_\mathrm{CR} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties on the DD prediction are shown as dotted lines for $ \mathrm{W}_\mathrm{add} $ (yellow), $ \mathrm{DY}_\mathrm{add} $ (turquoise), and $ \mathrm{MC}_\mathrm{diff} $ (dark red). All distributions are normalized to the SR MC event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of all distributions to the SR MC. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant-mass requirements are not applied. |
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Figure 5-c:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{*} $ (upper, left), $ c_\mathrm{hel} $ (upper, right) and $ c_\mathrm{han} $ (lower) distributions from different sources. The top panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \mathrm{MC}_\mathrm{CR} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties on the DD prediction are shown as dotted lines for $ \mathrm{W}_\mathrm{add} $ (yellow), $ \mathrm{DY}_\mathrm{add} $ (turquoise), and $ \mathrm{MC}_\mathrm{diff} $ (dark red). All distributions are normalized to the SR MC event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of all distributions to the SR MC. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant-mass requirements are not applied. |
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Figure 6:
Pre-fit distributions of $ |y(\mathrm{t})| $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 6-a:
Pre-fit distributions of $ |y(\mathrm{t})| $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 6-b:
Pre-fit distributions of $ |y(\mathrm{t})| $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 6-c:
Pre-fit distributions of $ |y(\mathrm{t})| $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 6-d:
Pre-fit distributions of $ |y(\mathrm{t})| $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 7:
Pre-fit distributions of $ p_{\mathrm{T}}(\mathrm{t}) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 7-a:
Pre-fit distributions of $ p_{\mathrm{T}}(\mathrm{t}) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 7-b:
Pre-fit distributions of $ p_{\mathrm{T}}(\mathrm{t}) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 7-c:
Pre-fit distributions of $ p_{\mathrm{T}}(\mathrm{t}) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 7-d:
Pre-fit distributions of $ p_{\mathrm{T}}(\mathrm{t}) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 8:
Pre-fit distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 8-a:
Pre-fit distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 8-b:
Pre-fit distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 8-c:
Pre-fit distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 8-d:
Pre-fit distributions of $ m({\mathrm{t}\overline{\mathrm{t}}} ) $ in all four categories. The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ \mathrm{t} \overline{\mathrm{t}} $ and single top quark contributions are taken from simulation, while the multijet+EW background is derived from the CR. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the data points indicate their statistical uncertainties. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 9:
Pre-fit (upper) and post-fit (lower) $ \beta_{\mathrm{t}}^{*} \times c_\mathrm{hel} \times c_\mathrm{han} $ distributions with all signal categories and data-taking periods combined. The data (points) are compared with the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ x $-axis shows bins of the unrolled three-dimensional distribution. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the points indicate the statistical uncertainties in the data. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 9-a:
Pre-fit (upper) and post-fit (lower) $ \beta_{\mathrm{t}}^{*} \times c_\mathrm{hel} \times c_\mathrm{han} $ distributions with all signal categories and data-taking periods combined. The data (points) are compared with the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ x $-axis shows bins of the unrolled three-dimensional distribution. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the points indicate the statistical uncertainties in the data. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 9-b:
Pre-fit (upper) and post-fit (lower) $ \beta_{\mathrm{t}}^{*} \times c_\mathrm{hel} \times c_\mathrm{han} $ distributions with all signal categories and data-taking periods combined. The data (points) are compared with the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The $ x $-axis shows bins of the unrolled three-dimensional distribution. The gray uncertainty band represents the combined statistical and systematic uncertainties in the predictions, while the vertical bars on the points indicate the statistical uncertainties in the data. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 10:
Post-fit $ S_{\text{NN}} $ distributions for the 1b (left) and 2b (right) categories. The data (points) are compared with the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The gray uncertainty band represents the post-fit combined statistical and systematic uncertainties in the predictions, while the vertical bars on the points indicate the statistical uncertainties in the data. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 10-a:
Post-fit $ S_{\text{NN}} $ distributions for the 1b (left) and 2b (right) categories. The data (points) are compared with the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The gray uncertainty band represents the post-fit combined statistical and systematic uncertainties in the predictions, while the vertical bars on the points indicate the statistical uncertainties in the data. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 10-b:
Post-fit $ S_{\text{NN}} $ distributions for the 1b (left) and 2b (right) categories. The data (points) are compared with the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The gray uncertainty band represents the post-fit combined statistical and systematic uncertainties in the predictions, while the vertical bars on the points indicate the statistical uncertainties in the data. The ratios to the predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
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Figure 11:
Observed (black solid line) and expected (blue dashed line) negative log-likelihood ratio scan as a function of the total $ \eta \mathrm{t} $ cross section. The gray horizontal lines represent the confidence intervals at the indicated standard deviation ($ \sigma $) levels. The vertical dashed line marks the nominal signal hypothesis $ (\mu=1) $, corresponding to a cross section of 6.43\unitpb. |
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Figure 12:
Impact of the 15 most significant systematic uncertainties on the measured $ \eta \mathrm{t} $ cross section $ \hat{\sigma}({\eta}{\mathrm{t}} ) $. The left panel shows the impact $ \Delta\hat{\sigma}({\eta}{\mathrm{t}} ) $ on the measurement when varying each nuisance parameter by $ +1\sigma $ and $ -1\sigma $ of its pre-fit uncertainty, represented by green and red bars, respectively. The right panel displays the pulls of the nuisance parameters, defined as $ (\hat{\theta} - \theta_{0})/\Delta\theta $, where $ \hat{\theta} $ is the post-fit value, $ \theta_{0} $ is the pre-fit value, and $ \Delta\theta $ is the pre-fit uncertainty. |
png pdf |
Figure 13:
Post-fit distributions of $ \beta_{\mathrm{t}}^{*} $ (upper left), $ c_\mathrm{hel} $ (upper right), and $ c_\mathrm{han} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{*} $ distribution is integrated over $ c_\mathrm{hel} $ and $ c_\mathrm{han} $, while the $ c_\mathrm{hel} $ ($ c_\mathrm{han} $) distribution is integrated over $ c_\mathrm{han} $ ($ c_\mathrm{hel} $) and the first three bins of $ \beta_{\mathrm{t}}^{*} $ ($ \beta_{\mathrm{t}}^{*} < $ 0.75). The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The gray uncertainty band represents the combined statistical and systematic uncertainties in the prediction. The vertical bars on the points indicate the statistical uncertainty in the data. The ratios to predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
png pdf |
Figure 13-a:
Post-fit distributions of $ \beta_{\mathrm{t}}^{*} $ (upper left), $ c_\mathrm{hel} $ (upper right), and $ c_\mathrm{han} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{*} $ distribution is integrated over $ c_\mathrm{hel} $ and $ c_\mathrm{han} $, while the $ c_\mathrm{hel} $ ($ c_\mathrm{han} $) distribution is integrated over $ c_\mathrm{han} $ ($ c_\mathrm{hel} $) and the first three bins of $ \beta_{\mathrm{t}}^{*} $ ($ \beta_{\mathrm{t}}^{*} < $ 0.75). The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The gray uncertainty band represents the combined statistical and systematic uncertainties in the prediction. The vertical bars on the points indicate the statistical uncertainty in the data. The ratios to predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
png pdf |
Figure 13-b:
Post-fit distributions of $ \beta_{\mathrm{t}}^{*} $ (upper left), $ c_\mathrm{hel} $ (upper right), and $ c_\mathrm{han} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{*} $ distribution is integrated over $ c_\mathrm{hel} $ and $ c_\mathrm{han} $, while the $ c_\mathrm{hel} $ ($ c_\mathrm{han} $) distribution is integrated over $ c_\mathrm{han} $ ($ c_\mathrm{hel} $) and the first three bins of $ \beta_{\mathrm{t}}^{*} $ ($ \beta_{\mathrm{t}}^{*} < $ 0.75). The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The gray uncertainty band represents the combined statistical and systematic uncertainties in the prediction. The vertical bars on the points indicate the statistical uncertainty in the data. The ratios to predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
png pdf |
Figure 13-c:
Post-fit distributions of $ \beta_{\mathrm{t}}^{*} $ (upper left), $ c_\mathrm{hel} $ (upper right), and $ c_\mathrm{han} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{*} $ distribution is integrated over $ c_\mathrm{hel} $ and $ c_\mathrm{han} $, while the $ c_\mathrm{hel} $ ($ c_\mathrm{han} $) distribution is integrated over $ c_\mathrm{han} $ ($ c_\mathrm{hel} $) and the first three bins of $ \beta_{\mathrm{t}}^{*} $ ($ \beta_{\mathrm{t}}^{*} < $ 0.75). The data (points) are compared to the predictions without $ \eta \mathrm{t} $ (stacked histograms) and those including it (blue line). The gray uncertainty band represents the combined statistical and systematic uncertainties in the prediction. The vertical bars on the points indicate the statistical uncertainty in the data. The ratios to predicted yields excluding $ \eta \mathrm{t} $ are shown in the lower panels. |
png pdf |
Figure 14:
Pre-fit comparison of the $ \eta \mathrm{t} $ (blue) and $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{NRQCD}} $ (orange) models in the $ \beta_{\mathrm{t}}^{*} \times c_\mathrm{hel} \times c_\mathrm{han} $ binning used for the signal extraction for all four signal categories combined, shown as ratios to the continuum $ \mathrm{t} \overline{\mathrm{t}} $ at NNLO QCD accuracy. The continuum $ \mathrm{t} \overline{\mathrm{t}} $ at NLO QCD accuracy normalized to the NNLO total cross section (red) is also shown for reference. |
| Summary |
| A search for the color-singlet pseudoscalar toponium quasi-bound-state was conducted using pp collision data collected by the CMS experiment from 2016 to 2018 at $ \sqrt{s} = $ 13 TeV. While previous studies utilized the \text{dilepton} channel, this analysis performs the search in the $ \mathrm{e}/\mu $+jets channel for the first time. The strategy leverages the variables $ c_\mathrm{hel} $, $ c_\mathrm{han} $, and $ \beta_{\mathrm{t}}^{*} $, the latter of which is more sensitive to the $ \eta \mathrm{t} $ signal and more robust against certain uncertainties. The background-only hypothesis is excluded with an observed (expected) significance of 6.1(7.3) $ \sigma $. The total $ \eta \mathrm{t} $ production cross section is measured to be 5.1 $ \pm $ 0.9 pb with main uncertainties from the modeling of the $ \mathrm{t} \overline{\mathrm{t}} $ background. This result provides an independent confirmation of the excess reported in the dilepton channel. It is important to note that this study relies solely on a simplified model, and further theoretical investigations are needed to accurately model the $ \mathrm{t} \overline{\mathrm{t}} $ threshold region. |
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