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CMS-TOP-17-003 ; CERN-EP-2017-309
Search for the flavor-changing neutral current interactions of the top quark and the Higgs boson which decays into a pair of b quarks at $\sqrt{s} = $ 13 TeV
JHEP 06 (2018) 102
Abstract: A search for flavor-changing neutral currents (FCNC) in events with the top quark and the Higgs boson is presented. The Higgs boson decay to a pair of b quarks is considered. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ recorded by the CMS experiment at the LHC in proton-proton collisions at $ \sqrt{s} = $ 13 TeV. Two channels are considered: single top quark FCNC production in association with the Higgs boson (${\mathrm{p}}{\mathrm{p}} \to \mathrm{t}\mathrm{H}$), and top quark pair production with FCNC decay of the top quark ($\mathrm{t} \to \mathrm{q}\mathrm{H}$). Final states with one isolated lepton and at least three reconstructed jets, among which at least two are associated with b quarks, are studied. No significant deviation is observed from the predicted background. Observed (expected) upper limits at 95% confidence level are set on the branching fractions of top quark decays, $\mathcal{B}(\mathrm{t} \to \mathrm{u}\mathrm{H}) < $ 0.47% (0.34%) and $\mathcal{B}(\mathrm{t} \to \mathrm{c}\mathrm{H}) < $ 0.47% (0.44%), assuming a single nonzero FCNC coupling.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Representative Feynman diagrams for FCNC $\mathrm{t} \mathrm{H} $ processes: associated production of the top quark with the Higgs boson (left), and FCNC decay of the top antiquark in $ {\mathrm{t} {}\mathrm{\bar{t}}} $ events (right). The FCNC vertex is indicated by the bullet.

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Figure 1-a:
Representative Feynman diagrams for FCNC $\mathrm{t} \mathrm{H} $ processes: associated production of the top quark with the Higgs boson (left), and FCNC decay of the top antiquark in $ {\mathrm{t} {}\mathrm{\bar{t}}} $ events (right). The FCNC vertex is indicated by the bullet.

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Figure 1-b:
Representative Feynman diagrams for FCNC $\mathrm{t} \mathrm{H} $ processes: associated production of the top quark with the Higgs boson (left), and FCNC decay of the top antiquark in $ {\mathrm{t} {}\mathrm{\bar{t}}} $ events (right). The FCNC vertex is indicated by the bullet.

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Figure 2:
Comparison between data and simulation for the most discriminating BDT input variables in the b3j3 category: lepton charge (upper left), CSVv2 discriminant value for one of the reconstructed b jets assigned to Higgs boson decay (upper right), reconstructed invariant mass of two b jets associated with the Higgs boson decay (lower left), and the maximum BDT discriminant value from the b jet assignment procedure (lower right). The last bin in the distribution for the reconstructed mass of the Higgs boson includes the overflows. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 2-a:
Comparison between data and simulation for the most discriminating BDT input variables in the b3j3 category: lepton charge (upper left), CSVv2 discriminant value for one of the reconstructed b jets assigned to Higgs boson decay (upper right), reconstructed invariant mass of two b jets associated with the Higgs boson decay (lower left), and the maximum BDT discriminant value from the b jet assignment procedure (lower right). The last bin in the distribution for the reconstructed mass of the Higgs boson includes the overflows. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 2-b:
Comparison between data and simulation for the most discriminating BDT input variables in the b3j3 category: lepton charge (upper left), CSVv2 discriminant value for one of the reconstructed b jets assigned to Higgs boson decay (upper right), reconstructed invariant mass of two b jets associated with the Higgs boson decay (lower left), and the maximum BDT discriminant value from the b jet assignment procedure (lower right). The last bin in the distribution for the reconstructed mass of the Higgs boson includes the overflows. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 2-c:
Comparison between data and simulation for the most discriminating BDT input variables in the b3j3 category: lepton charge (upper left), CSVv2 discriminant value for one of the reconstructed b jets assigned to Higgs boson decay (upper right), reconstructed invariant mass of two b jets associated with the Higgs boson decay (lower left), and the maximum BDT discriminant value from the b jet assignment procedure (lower right). The last bin in the distribution for the reconstructed mass of the Higgs boson includes the overflows. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 2-d:
Comparison between data and simulation for the most discriminating BDT input variables in the b3j3 category: lepton charge (upper left), CSVv2 discriminant value for one of the reconstructed b jets assigned to Higgs boson decay (upper right), reconstructed invariant mass of two b jets associated with the Higgs boson decay (lower left), and the maximum BDT discriminant value from the b jet assignment procedure (lower right). The last bin in the distribution for the reconstructed mass of the Higgs boson includes the overflows. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 3:
The BDT discriminant distributions for different jet categories for Hut training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 3-a:
The BDT discriminant distributions for different jet categories for Hut training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 3-b:
The BDT discriminant distributions for different jet categories for Hut training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 3-c:
The BDT discriminant distributions for different jet categories for Hut training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 3-d:
The BDT discriminant distributions for different jet categories for Hut training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 4:
The BDT discriminant distributions for different jet categories for Hct training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 4-a:
The BDT discriminant distributions for different jet categories for Hct training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 4-b:
The BDT discriminant distributions for different jet categories for Hct training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 4-c:
The BDT discriminant distributions for different jet categories for Hct training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 4-d:
The BDT discriminant distributions for different jet categories for Hct training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 4-e:
The BDT discriminant distributions for different jet categories for Hct training after the fit to data. All background processes are constrained to the SM expectation in the fit. The shaded area corresponds to the total uncertainty in the predicted background. The data-to-simulation ratio is also shown.

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Figure 5:
Excluded signal cross section at 95% CL per event category for Hut (left) and Hct (right).

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Figure 5-a:
Excluded signal cross section at 95% CL per event category for Hut (left) and Hct (right).

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Figure 5-b:
Excluded signal cross section at 95% CL per event category for Hut (left) and Hct (right).

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Figure 6:
Upper limits on $\mathcal {B}(\mathrm{t} \to \mathrm{u} \mathrm{H})$ and $\mathcal {B}(\mathrm{t} \to \mathrm{c} \mathrm{H})$ at 95% CL.

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Figure 7:
The best fit signal strength ($\mu $) for Hut (left) and Hct (right), which is restricted to positive values in the fit.

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Figure 7-a:
The best fit signal strength ($\mu $) for Hut (left) and Hct (right), which is restricted to positive values in the fit.

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Figure 7-b:
The best fit signal strength ($\mu $) for Hut (left) and Hct (right), which is restricted to positive values in the fit.
Tables

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Table 1:
Number of events in each category together with its total relative uncertainty as obtained from the fit to data for Hut.

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Table 2:
Number of events in each category together with its total relative uncertainty as obtained from the fit to data for Hct.
Summary
A search for flavor-changing neutral currents in events with a top quark and the Higgs boson, corresponding to a data sample of 35.9 fb$^{-1}$ collected in proton-proton collisions at $\sqrt{s} = $ 13 TeV, is presented. This is the first search to probe $\mathrm{t}\mathrm{H}$ flavor-changing neutral current couplings in both associated production of a top quark with the Higgs boson and in top quark decays. Observed (expected) upper limits at 95% confidence level are set on the branching fractions of top quark decays, $\mathcal{B}(\mathrm{t} \to \mathrm{u}\mathrm{H}) < $ 0.47% (0.34%) and $\mathcal{B}(\mathrm{t} \to \mathrm{c}\mathrm{H}) < $ 0.47% (0.44%). These results provide a significant improvement over the previous limits set by CMS in the $\mathrm{H} \to \mathrm{b\bar{b}}$ channel, as well as represent the best limits for $\mathcal{B}(\mathrm{t} \to \mathrm{u}\mathrm{H})$ at CMS.
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Compact Muon Solenoid
LHC, CERN