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| CMS-TOP-19-002 ; CERN-EP-2021-241 | ||
| Search for flavor-changing neutral current interactions of the top quark and the Higgs boson decaying to a bottom quark-antiquark pair at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 17 December 2021 | ||
| JHEP 02 (2022) 169 | ||
| Abstract: A search for flavor-changing neutral current interactions of the top quark (t) and the Higgs boson (H) is presented. The search is based on a data sample corresponding to an integrated luminosity of 137 fb$^{-1}$ recorded by the CMS experiment at the LHC in proton-proton collisions at $\sqrt{s} = $ 13 TeV. Events containing exactly one lepton (muon or electron) and at least three jets, among which at least two are identified as originating from the hadronization of a bottom quark, are analyzed. A set of deep neural networks is used for kinematic event reconstruction, while boosted decision trees distinguish the signal from the background events. No significant excess over the background predictions is observed, and upper limits on the signal production cross sections are extracted. These limits are interpreted in terms of top quark decay branching fractions ($\mathcal{B}$) to the Higgs boson and an up (u) or a charm quark (c). Assuming one nonvanishing extra coupling at a time, the observed (expected) upper limits at 95% confidence level are $\mathcal{B}({\mathrm{t} \to \mathrm{H}\mathrm{u}} ) < $ 0.079 (0.11)% and $\mathcal{B}({\mathrm{t} \to \mathrm{H}\mathrm{c}} ) < $ 0.094 (0.086)%. | ||
| Links: e-print arXiv:2112.09734 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Illustrative Feynman diagrams for Hqt FCNC interactions: the production of a single top quark with the Higgs boson (left), and FCNC decay of the top quark in ${\mathrm{t} {}\mathrm{\bar{t}}}$ events (right), where q = u or c. |
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Figure 1-a:
Illustrative Feynman diagram for Hqt FCNC interactions: the production of a single top quark with the Higgs boson. |
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Figure 1-b:
Illustrative Feynman diagram for Hqt FCNC interactions: FCNC decay of the top quark in ${\mathrm{t} {}\mathrm{\bar{t}}}$ events, where q = u or c. |
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Figure 2:
The reconstructed $m_{\mathrm{b} {}\mathrm{\bar{b}}}$ (Higgs boson candidate mass) in the b3j3 category for the ST signal scenario (left), the distribution of the second largest DeepCSV value for the b-tagged jet from the Higgs boson decay in the b4j4 category for the TT signal scenario (middle), and the mass of the hadronically decaying top quark in the b2j4 category for the SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ background scenario (right) for the combined 2017+2018 data. The lower panel shows the ratio of observed data to the SM prediction. The shaded band corresponds to the total uncertainty in the predicted background. For the mass plots the last bin contains the overflow events. |
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Figure 2-a:
The reconstructed $m_{\mathrm{b} {}\mathrm{\bar{b}}}$ (Higgs boson candidate mass) in the b3j3 category for the ST signal scenario for the combined 2017+2018 data. The lower panel shows the ratio of observed data to the SM prediction. The shaded band corresponds to the total uncertainty in the predicted background. |
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Figure 2-b:
The distribution of the second largest DeepCSV value for the b-tagged jet from the Higgs boson decay in the b4j4 category for the TT signal scenario for the combined 2017+2018 data. The lower panel shows the ratio of observed data to the SM prediction. The shaded band corresponds to the total uncertainty in the predicted background. |
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Figure 2-c:
The mass of the hadronically decaying top quark in the b2j4 category for the SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ background scenario for the combined 2017+2018 data. The lower panel shows the ratio of observed data to the SM prediction. The shaded band corresponds to the total uncertainty in the predicted background. |
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Figure 3:
The BDT output distributions for the combined 2017+2018 data and simulation for the different jet categories, assuming the Hut coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 3-a:
The BDT output distributions for the combined 2017+2018 data and simulation for the b2j3 jet category, assuming the Hut coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 3-b:
The BDT output distributions for the combined 2017+2018 data and simulation for the b2j4 jet category, assuming the Hut coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 3-c:
The BDT output distributions for the combined 2017+2018 data and simulation for the b3j3 jet category, assuming the Hut coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 3-d:
The BDT output distributions for the combined 2017+2018 data and simulation for the b3j4 jet category, assuming the Hut coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 3-e:
The BDT output distributions for the combined 2017+2018 data and simulation for the b4j4 jet category, assuming the Hut coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 4:
The BDT output distributions for the combined 2017+2018 data and simulation for the different jet categories, assuming the Hct coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 4-a:
The BDT output distributions for the combined 2017+2018 data and simulation for the b2j3 jet category, assuming the Hct coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 4-b:
The BDT output distributions for the combined 2017+2018 data and simulation for the b2j4 jet category, assuming the Hct coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 4-c:
The BDT output distributions for the combined 2017+2018 data and simulation for the b3j3 jet category, assuming the Hct coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
png pdf |
Figure 4-d:
The BDT output distributions for the combined 2017+2018 data and simulation for the b3j4 jet category, assuming the Hct coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
png pdf |
Figure 4-e:
The BDT output distributions for the combined 2017+2018 data and simulation for the b4j4 jet category, assuming the Hct coupling. The lower panel shows the ratio of observed data to the SM prediction. The shaded bands correspond to the post-fit total uncertainty in the predicted background. The signal contributions are normalized to the total number of events in data. |
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Figure 5:
Excluded limits on the product of the cross section and branching fraction at 95% CL for the Hut (left) and Hct (right) couplings obtained using the BDT distributions. Each jet category and their combination are shown separately. |
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Figure 5-a:
Excluded limits on the product of the cross section and branching fraction at 95% CL for the Hut coupling obtained using the BDT distributions. Each jet category and their combination are shown separately. |
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Figure 5-b:
Excluded limits on the product of the cross section and branching fraction at 95% CL for the Hct coupling obtained using the BDT distributions. Each jet category and their combination are shown separately. |
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Figure 6:
Upper limits on the couplings ${\kappa _{{\mathrm{H} \mathrm{u} \mathrm{t}}}}$ and ${\kappa _{{\mathrm{H} \mathrm{c} \mathrm{t}}}}$ (left), and the branching fractions $\mathcal {B}({\mathrm{t} \to \mathrm{H} \mathrm{u}})$ and $\mathcal {B}({\mathrm{t} \to \mathrm{H} \mathrm{c}})$ (right) at 95% CL. |
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Figure 6-a:
Upper limits on the couplings ${\kappa _{{\mathrm{H} \mathrm{u} \mathrm{t}}}}$ and ${\kappa _{{\mathrm{H} \mathrm{c} \mathrm{t}}}}$ at 95% CL. |
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Figure 6-b:
Upper limits on the branching fractions $\mathcal {B}({\mathrm{t} \to \mathrm{H} \mathrm{u}})$ and $\mathcal {B}({\mathrm{t}\to\mathrm{H} \mathrm{c}})$ at 95% CL. |
| Tables | |
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Table 1:
Number of events in the combined 2017+2018 data and prediction for the backgrounds assuming absence of the signal events, shown separately for each jet category, with uncertainties obtained from the fit on the BDT distributions of Hct coupling. |
| Summary |
| A search for flavor-changing neutral current interactions in events with a top quark (t) decaying leptonically and a Higgs boson (H) decaying to a bottom quark-antiquark pair has been presented. The search uses the LHC data, collected at $\sqrt{s} = $ 13 TeV in 2016-2018 and corresponding to an integrated luminosity of 137 fb$^{-1}$ of proton-proton collisions. Events are analyzed, in the single-lepton channel containing a muon or electron in addition to the presence of at least three jets, where at least two of them are identified as originating from the hadronization of a bottom quark. No significant deviation from the standard model prediction has been observed and upper limits on the branching fractions $\mathcal{B}({\mathrm{t} \to \mathrm{H}\mathrm{q}} )$ have been set where q refers to the up (u) and charm quarks (c); their observed (expected) excluded values at 95% confidence level are $\mathcal{B}({\mathrm{t} \to \mathrm{H}\mathrm{u}} ) < $ 0.079(0.11)% and $\mathcal{B}({\mathrm{t} \to \mathrm{H}\mathrm{c}} ) < $ 0.094 (0.086)%. The observed limits reach the order of 10$^{-4}$ in terms of branching fraction compared to the prediction from several well-known extensions of standard model that predict the values as high as 10$^{-5}$ to 10$^{-3}$. This search substantially improves upon previous CMS results in the same final state by exploiting the larger integrated luminosity of 137 fb$^{-1}$ and by using advanced multivariate analysis techniques to perform the kinematic event reconstruction and signal extraction. |
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