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| CMS-TOP-25-002 ; CERN-EP-2026-239 | ||
| Observation of an excess at the top quark pair production threshold in the single-lepton channel | ||
| CMS Collaboration | ||
| 18 August 2026 | ||
| Submitted to Reports on Progress in Physics | ||
| Abstract: A search is presented for top quark-antiquark ( $ \mathrm{t} \overline{\mathrm{t}} $) quasi-bound ``toponium'' states, near the kinematic $ \mathrm{t} \overline{\mathrm{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} \overline{\mathrm{t}} $ system. A significant excess of events is observed relative to the standard model prediction for $ \mathrm{t} \overline{\mathrm{t}} $ production calculated at next-to-next-to-leading order in perturbative quantum chromodynamics (QCD). The excess corresponds to an observed total cross section of 5.1 $ \pm $ 0.9 pb and is consistent with a simplified model of a color singlet pseudoscalar toponium motivated by nonrelativistic QCD, with a resulting observed significance of 6.1 standard deviations. The result provides an independent confirmation of the excess reported in the dilepton channel. | ||
| Links: e-print arXiv:2608.19546 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; 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}}^{\ast} $ (upper right), $ c_{\text{hel}} $ (lower left), and $ c_{\text{han}} $ (lower right) for the $ \eta_\mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{GFRW}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The $ c_{\text{hel}} $ and $ c_{\text{han}} $ distributions are restricted to events with $ \beta_{\mathrm{t}}^{\ast} < $ 0.4 at the parton level to focus on the threshold region. 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}}^{\ast} $ (upper right), $ c_{\text{hel}} $ (lower left), and $ c_{\text{han}} $ (lower right) for the $ \eta_\mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{GFRW}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The $ c_{\text{hel}} $ and $ c_{\text{han}} $ distributions are restricted to events with $ \beta_{\mathrm{t}}^{\ast} < $ 0.4 at the parton level to focus on the threshold region. 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}}^{\ast} $ (upper right), $ c_{\text{hel}} $ (lower left), and $ c_{\text{han}} $ (lower right) for the $ \eta_\mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{GFRW}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The $ c_{\text{hel}} $ and $ c_{\text{han}} $ distributions are restricted to events with $ \beta_{\mathrm{t}}^{\ast} < $ 0.4 at the parton level to focus on the threshold region. 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}}^{\ast} $ (upper right), $ c_{\text{hel}} $ (lower left), and $ c_{\text{han}} $ (lower right) for the $ \eta_\mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{GFRW}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The $ c_{\text{hel}} $ and $ c_{\text{han}} $ distributions are restricted to events with $ \beta_{\mathrm{t}}^{\ast} < $ 0.4 at the parton level to focus on the threshold region. 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}}^{\ast} $ (upper right), $ c_{\text{hel}} $ (lower left), and $ c_{\text{han}} $ (lower right) for the $ \eta_\mathrm{t} $ model (blue), the $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{GFRW}} $ model (orange), and the continuum $ \mathrm{t} \overline{\mathrm{t}} $ background (red). The $ c_{\text{hel}} $ and $ c_{\text{han}} $ distributions are restricted to events with $ \beta_{\mathrm{t}}^{\ast} < $ 0.4 at the parton level to focus on the threshold region. 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:
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). 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 excluding $ \eta_\mathrm{t} $. |
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Figure 2-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). 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 excluding $ \eta_\mathrm{t} $. |
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Figure 2-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). 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 excluding $ \eta_\mathrm{t} $. |
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Figure 3:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{\ast} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_{\text{correct}} $) relative to the reconstructible events ($ N_{\text{reconstructible}} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_{\text{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 3-a:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{\ast} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_{\text{correct}} $) relative to the reconstructible events ($ N_{\text{reconstructible}} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_{\text{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 3-b:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{\ast} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_{\text{correct}} $) relative to the reconstructible events ($ N_{\text{reconstructible}} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_{\text{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 3-c:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{\ast} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_{\text{correct}} $) relative to the reconstructible events ($ N_{\text{reconstructible}} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_{\text{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 3-d:
Reconstruction performance as a function of $ \beta_{\mathrm{t}}^{\ast} $ at the parton level. The plots in the left column show the fraction of correctly reconstructed events ($ N_{\text{correct}} $) relative to the reconstructible events ($ N_{\text{reconstructible}} $), while the plots in the right column display those relative to all generated events for each respective process ($ N_{\text{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:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{\ast} $ (upper, left), $ c_{\text{hel}} $ (upper, right), and $ c_{\text{han}} $ (lower) distributions from different sources. The upper panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \text{MC}_{\text{CR}} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties in the DD prediction are shown as dotted lines for $ \mathrm{W}_{\text{add}} $ (yellow), $ \text{DY}_{\text{add}} $ (turquoise), and $ \text{MC}_{\text{diff}} $ (dark red). All distributions are normalized to the simulated SR event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of the DD prediction and CR simulation to the SR simulation. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant mass requirements are not applied. |
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Figure 4-a:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{\ast} $ (upper, left), $ c_{\text{hel}} $ (upper, right), and $ c_{\text{han}} $ (lower) distributions from different sources. The upper panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \text{MC}_{\text{CR}} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties in the DD prediction are shown as dotted lines for $ \mathrm{W}_{\text{add}} $ (yellow), $ \text{DY}_{\text{add}} $ (turquoise), and $ \text{MC}_{\text{diff}} $ (dark red). All distributions are normalized to the simulated SR event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of the DD prediction and CR simulation to the SR simulation. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant mass requirements are not applied. |
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Figure 4-b:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{\ast} $ (upper, left), $ c_{\text{hel}} $ (upper, right), and $ c_{\text{han}} $ (lower) distributions from different sources. The upper panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \text{MC}_{\text{CR}} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties in the DD prediction are shown as dotted lines for $ \mathrm{W}_{\text{add}} $ (yellow), $ \text{DY}_{\text{add}} $ (turquoise), and $ \text{MC}_{\text{diff}} $ (dark red). All distributions are normalized to the simulated SR event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of the DD prediction and CR simulation to the SR simulation. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant mass requirements are not applied. |
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Figure 4-c:
Comparison of multijet/EW $ \beta_{\mathrm{t}}^{\ast} $ (upper, left), $ c_{\text{hel}} $ (upper, right), and $ c_{\text{han}} $ (lower) distributions from different sources. The upper panels show the distributions for the DD prediction (Multijet/EW DD, red line), the CR simulation ($ \text{MC}_{\text{CR}} $, pink line), and the SR simulation (stacked histogram). The gray shaded band represents the statistical uncertainty of the SR simulation. Systematic uncertainties in the DD prediction are shown as dotted lines for $ \mathrm{W}_{\text{add}} $ (yellow), $ \text{DY}_{\text{add}} $ (turquoise), and $ \text{MC}_{\text{diff}} $ (dark red). All distributions are normalized to the simulated SR event yield. The middle panels display the ratios of these systematic variations to the DD prediction, while the lower panels show the ratios of the DD prediction and CR simulation to the SR simulation. To reduce statistical fluctuations, the $ S_{\text{NN}} $ and invariant mass requirements are not applied. |
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Figure 5:
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 using the DD method. 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 5-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 using the DD method. 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 5-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 using the DD method. 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 5-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 using the DD method. 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 5-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 using the DD method. 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:
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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 $ 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 using the DD method. 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 (upper) and post-fit (lower) $ \beta_{\mathrm{t}}^{\ast}\times c_{\text{hel}}\times c_{\text{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 8-a:
Pre-fit (upper) and post-fit (lower) $ \beta_{\mathrm{t}}^{\ast}\times c_{\text{hel}}\times c_{\text{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 8-b:
Pre-fit (upper) and post-fit (lower) $ \beta_{\mathrm{t}}^{\ast}\times c_{\text{hel}}\times c_{\text{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:
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 9-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 9-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 10:
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 SD 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 11:
Impact of the 15 most significant systematic uncertainties on the measured $ \eta_\mathrm{t} $ cross section $ \widehat{\sigma}({\eta}{\mathrm{t}}} ) $. The left panel shows the impact $ \Delta\widehat{\sigma}({\eta}{\mathrm{t}}} ) $ on the measurement when varying each nuisance parameter by $ \pm $ 1 SD of its pre-fit uncertainty, represented by green and red bars, respectively. The right panel displays the pulls of the nuisance parameters, defined as $ (\widehat{\theta}-\theta_0)/\Delta\theta $, where $ \widehat{\theta} $ is the post-fit value, $ \theta_0 $ is the pre-fit value, and $ \Delta\theta $ is the pre-fit uncertainty. |
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Figure 12:
Post-fit distributions of $ \beta_{\mathrm{t}}^{\ast} $ (upper left), $ c_{\text{hel}} $ (upper right), and $ c_{\text{han}} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{\ast} $ distribution is integrated over $ c_{\text{hel}} $ and $ c_{\text{han}} $, while the $ c_{\text{hel}} $ ($ c_{\text{han}} $) distribution is integrated over $ c_{\text{han}} $ ($ c_{\text{hel}} $) and the first three bins of $ \beta_{\mathrm{t}}^{\ast} $ ($ \beta_{\mathrm{t}}^{\ast} < $ 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. |
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Figure 12-a:
Post-fit distributions of $ \beta_{\mathrm{t}}^{\ast} $ (upper left), $ c_{\text{hel}} $ (upper right), and $ c_{\text{han}} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{\ast} $ distribution is integrated over $ c_{\text{hel}} $ and $ c_{\text{han}} $, while the $ c_{\text{hel}} $ ($ c_{\text{han}} $) distribution is integrated over $ c_{\text{han}} $ ($ c_{\text{hel}} $) and the first three bins of $ \beta_{\mathrm{t}}^{\ast} $ ($ \beta_{\mathrm{t}}^{\ast} < $ 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. |
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Figure 12-b:
Post-fit distributions of $ \beta_{\mathrm{t}}^{\ast} $ (upper left), $ c_{\text{hel}} $ (upper right), and $ c_{\text{han}} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{\ast} $ distribution is integrated over $ c_{\text{hel}} $ and $ c_{\text{han}} $, while the $ c_{\text{hel}} $ ($ c_{\text{han}} $) distribution is integrated over $ c_{\text{han}} $ ($ c_{\text{hel}} $) and the first three bins of $ \beta_{\mathrm{t}}^{\ast} $ ($ \beta_{\mathrm{t}}^{\ast} < $ 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 12-c:
Post-fit distributions of $ \beta_{\mathrm{t}}^{\ast} $ (upper left), $ c_{\text{hel}} $ (upper right), and $ c_{\text{han}} $ (lower) for all four signal categories combined. The $ \beta_{\mathrm{t}}^{\ast} $ distribution is integrated over $ c_{\text{hel}} $ and $ c_{\text{han}} $, while the $ c_{\text{hel}} $ ($ c_{\text{han}} $) distribution is integrated over $ c_{\text{han}} $ ($ c_{\text{hel}} $) and the first three bins of $ \beta_{\mathrm{t}}^{\ast} $ ($ \beta_{\mathrm{t}}^{\ast} < $ 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:
Pre-fit comparison of the $ \eta_\mathrm{t} $ (blue) and $ {\mathrm{t}\overline{\mathrm{t}}} _{\text{GFRW}} $ (orange) models in the $ \beta_{\mathrm{t}}^{\ast}\times c_{\text{hel}}\times c_{\text{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 color singlet pseudoscalar toponium quasi-bound states has been conducted using proton-proton collision data collected by the CMS experiment from 2016 to 2018 at $ \sqrt{s}= $ 13 TeV. While previous studies utilized the dilepton channel, this analysis performs the search in the $ \mathrm{e}/\mu $+jets channel for the first time. The strategy leverages two angular observables sensitive to the spin correlations and parity of the top quark-antiquark ( $ \mathrm{t} \overline{\mathrm{t}} $) system, together with $ \beta_{\mathrm{t}}^{\ast} $, defined as the magnitude of the velocity of one top quark in the rest frame of the other. Compared to the invariant mass of the $ \mathrm{t} \overline{\mathrm{t}} $ system $ m({\mathrm{t}\overline{\mathrm{t}}} ) $, the $ \beta_{\mathrm{t}}^{\ast} $ variable is more sensitive to the signal and more robust against specific experimental and modeling uncertainties. The background-only hypothesis is excluded with an observed (expected) significance of 6.1 (7.3) standard deviations. When using a simplified model simulating a pseudoscalar color singlet particle ($ \eta_\mathrm{t} $), the total signal production cross section is measured to be 5.1 $ \pm $ 0.9 pb with the main uncertainties from the modeling of continuum $ \mathrm{t} \overline{\mathrm{t}} $ and $ \eta_\mathrm{t} $ production. This result provides an independent confirmation of the excess reported in the dilepton channel. In the $ m({\mathrm{t}\overline{\mathrm{t}}} ) < $ 350 GeV mass range, results obtained with both the $ \eta_\mathrm{t} $ model and an alternative model based on a Green's function reweighting of continuum $ \mathrm{t} \overline{\mathrm{t}} $ yield a consistent enhancement of roughly 3\unitpb. It is important to note that this study relies solely on approximate models, and further theoretical investigations are needed to more precisely describe the $ \mathrm{t} \overline{\mathrm{t}} $ threshold region. |
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