CMS-PAS-TOP-22-002

CMS-PAS-TOP-22-002
Search for flavor-changing neutral current interactions of the top quark and Higgs boson in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
28 September 2023

Abstract: A search for flavor-changing neutral current interactions of the top quark (t) and Higgs boson (H) is presented. The search is based on Run 2 proton-proton collisions collected in 2016--2018 at a center-of-mass energy of 13 TeV with the CMS detector at the CERN LHC and corresponding to 138 fb$ ^{-1} $. Events containing a pair of leptons with the same-sign electric charge and at least one jet are considered. Two boosted decision trees are trained to distinguish the signal from the standard model background. No significant excess above the estimated background is observed. The results are used to constrain the branching ratio ($ \mathcal{B} $) of the top quark decaying to a Higgs boson and an up (u) or charm quark (c). The observed (expected) 95% confidence level upper limits are found to be 0.072% (0.059%) for $ \mathcal{B}\left(\mathrm{t\to Hu}\right) $ and 0.043% (0.062%) for $ \mathcal{B}\left(\mathrm{t\to Hc}\right) $. A combination of searches for flavor-changing neutral current interactions of top quarks and Higgs bosons in final states with a pair of photons or a pair of bottom quarks, with data collected by the CMS experiment, is performed. The resulting observed (expected) 95% confidence level upper limits are 0.019% (0.027%) for $ \mathcal{B}\left(\mathrm{t\to Hu}\right) $ and 0.037% (0.035%) for $ \mathcal{B}\left(\mathrm{t\to Hc}\right) $. This result constitutes the best exclusion limits to date.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagrams for the production modes considered: FCNC-associated production of a single top quark with a Higgs boson (ST, left), and $ \mathrm{t} \overline{\mathrm{t}} $ production with the FCNC decay of the top quark to a Higgs boson and an up or charm quark (TT, right).
png pdf	Figure 1-a: Representative Feynman diagrams for the production modes considered: FCNC-associated production of a single top quark with a Higgs boson (ST, left), and $ \mathrm{t} \overline{\mathrm{t}} $ production with the FCNC decay of the top quark to a Higgs boson and an up or charm quark (TT, right).
png pdf	Figure 1-b: Representative Feynman diagrams for the production modes considered: FCNC-associated production of a single top quark with a Higgs boson (ST, left), and $ \mathrm{t} \overline{\mathrm{t}} $ production with the FCNC decay of the top quark to a Higgs boson and an up or charm quark (TT, right).
png pdf	Figure 2: The prefit (upper) and postfit (lower) observed and expected distributions of BDT score in the signal region for Run 2 are shown. The $ \mathrm{t} \to \mathrm{H}\mathrm{u} $ signal is shown on the left and $ \mathrm{t} \to \mathrm{H}\mathrm{c} $ is shown on the right. The uncertainty bands include the statistical and systematic uncertainties of the analysis.
png pdf	Figure 2-a: The prefit (upper) and postfit (lower) observed and expected distributions of BDT score in the signal region for Run 2 are shown. The $ \mathrm{t} \to \mathrm{H}\mathrm{u} $ signal is shown on the left and $ \mathrm{t} \to \mathrm{H}\mathrm{c} $ is shown on the right. The uncertainty bands include the statistical and systematic uncertainties of the analysis.
png pdf	Figure 2-b: The prefit (upper) and postfit (lower) observed and expected distributions of BDT score in the signal region for Run 2 are shown. The $ \mathrm{t} \to \mathrm{H}\mathrm{u} $ signal is shown on the left and $ \mathrm{t} \to \mathrm{H}\mathrm{c} $ is shown on the right. The uncertainty bands include the statistical and systematic uncertainties of the analysis.
png pdf	Figure 2-c: The prefit (upper) and postfit (lower) observed and expected distributions of BDT score in the signal region for Run 2 are shown. The $ \mathrm{t} \to \mathrm{H}\mathrm{u} $ signal is shown on the left and $ \mathrm{t} \to \mathrm{H}\mathrm{c} $ is shown on the right. The uncertainty bands include the statistical and systematic uncertainties of the analysis.
png pdf	Figure 2-d: The prefit (upper) and postfit (lower) observed and expected distributions of BDT score in the signal region for Run 2 are shown. The $ \mathrm{t} \to \mathrm{H}\mathrm{u} $ signal is shown on the left and $ \mathrm{t} \to \mathrm{H}\mathrm{c} $ is shown on the right. The uncertainty bands include the statistical and systematic uncertainties of the analysis.
png pdf	Figure 3: The expected and observed limits on the anomalous coupling strength (left) and on the anomalous branching fraction (right) for Run 2 are shown.
png pdf	Figure 3-a: The expected and observed limits on the anomalous coupling strength (left) and on the anomalous branching fraction (right) for Run 2 are shown.
png pdf	Figure 3-b: The expected and observed limits on the anomalous coupling strength (left) and on the anomalous branching fraction (right) for Run 2 are shown.
png pdf	Figure 4: Summary plot which shows the observed and expected results from three individual analyses and their combination. The dotted line shows the expected limit and the solid, red line shows the observed limit in all cases. The green and yellow bands show the 1 and 2 sigma ranges on the expected limits.

Tables
png pdf	Table 1: Summary of the trigger thresholds used to select the analysis data set. The triggers require two isolated leptons, with different $ p_{\mathrm{T}} $ requirements based on the flavors of each. In the case of opposite flavor triggers, the flavor of the leading lepton is listed first.
png pdf	Table 2: Summary of the analysis preselection requirements.
png pdf	Table 3: Sources of systematic uncertainties in the yields of signal and background processes, as well as their impact on the yields in the signal regions. The impact is expressed as a one sigma range.
png pdf	Table 4: Observed (expected) upper limits on the $ \mathrm{t} \to \mathrm{H}\mathrm{u} $ and $ \mathrm{t} \to \mathrm{H}\mathrm{c} $ branching fractions for the three searches in different Higgs boson decay channels performed by the CMS Collaboration. A statistical combination of the these results is also reported.

Summary

This note presents the results of a search for flavor-changing neutral current interactions of the top quark, Higgs boson, and an up or charm quark. The search is performed in a final state with a pair of leptons of same-sign electric charge and at least one jet. Expected yields from backgrounds emerging from detector effects are estimated by extrapolating yields observed in control regions using transfer factors measured in orthogonal data samples. Expected yields from standard model processes producing a pair of prompt leptons with the same-sign electric charge are estimated from simulation. Two trained boosted decision trees are used to evaluate and classify each event. No excess above the estimated background from standard model processes is observed. The observed (expected) 95% confidence level upper limits on the branching ratio are found to be 0.072% (0.059%) for $ \mathcal{B}\left(\mathrm{t} \to \mathrm{H}\mathrm{u}\right) $ and 0.043% (0.062%) for $ \mathcal{B}\left(\mathrm{t} \to \mathrm{H}\mathrm{c}\right) $. These limits can be cast as constraints on the anomalous coupling strengths $ \kappa_{\mathrm{t}\mathrm{u}\mathrm{H}} < $ 0.071 (0.064) and $ \kappa_{\mathrm{t}\mathrm{c}\mathrm{H}} < $ 0.055 (0.065). A statistical combination of the results of this search with those of previous CMS publications searching for similar phenomena where the Higgs boson decays to a bottom quark-antiquark pair or to a pair of photons is performed. The results of this combination constrain $ \mathcal{B}\left(\mathrm{t} \to \mathrm{H}\mathrm{u}\right) < $ 0.019% (0.027%) and $ \mathcal{B}\left(\mathrm{t} \to \mathrm{H}\mathrm{c}\right) < $ 0.037% (0.035%) and represent the most stringent constraints on these interactions to date.

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