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CMS-PAS-HIG-18-009

Search for the associated production of a Higgs boson and a single top quark in pp collisions at $\sqrt{s} = $ 13 TeV

CMS Collaboration

July 2018

Abstract: A search for the production of Higgs boson in association with a single top quark is presented, based on data collected in 2016 with the CMS detector at the LHC at a center of mass energy of 13 TeV, which corresponds to an integrated luminosity of 35.9 fb$^{-1}$. The production cross section of this process is highly sensitive to the absolute values of the modifiers of the top quark-Higgs boson coupling, $\kappa_{\mathrm{t}}$, and the coupling of vector bosons to the Higgs boson, $\kappa_{\mathrm{V}}$, as well as their relative signs with respect to the standard model. Analyses using multilepton signatures due to $\mathrm{H}\rightarrow\mathrm{W}\mathrm{W}$, $\mathrm{H}\rightarrow\tau\tau$, and $\mathrm{H}\rightarrow\mathrm{Z}\mathrm{Z}$ decay modes and that with a single lepton for the decay $\mathrm{H}\rightarrow\mathrm{b\overline{b}}$ are combined with a reinterpretation of a measurement of $\mathrm{H}\to\gamma\gamma$ to constrain $\kappa_\mathrm{t}$. For $\kappa_\mathrm{V}= $ 1.0 the observed data favor positive values of $\kappa_\mathrm{t}$ and exclude values of $\kappa_\mathrm{t}$ below about 0.9.

Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRD 99 (2019) 092005. The superseded preliminary plots can be found here.

Figures
png pdf	Figure 1: Leading-order Feynman diagrams for the associated production of single top quark and Higgs boson in the $t$-channel where the Higgs boson couple either to the top quark or the W boson.
png pdf	Figure 1-a: Leading-order Feynman diagrams for the associated production of single top quark and Higgs boson in the $t$-channel where the Higgs boson couple either to the top quark or the W boson.
png pdf	Figure 1-b: Leading-order Feynman diagrams for the associated production of single top quark and Higgs boson in the $t$-channel where the Higgs boson couple either to the top quark or the W boson.
png pdf	Figure 2: Acceptance and selection efficiency for the $ {{\mathrm {t}} {\mathrm {H}} \mathrm {q}} $ (red) and $ {{\mathrm {t}} {\mathrm {H}} {\mathrm {W}}} $ (blue) signal processes as a function of $ {\kappa _ {\mathrm {t}}} / {\kappa _\text {V}} $, for the $ {{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}} $ Leptonic (solid lines) and ${{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}}$ Hadronic categories (dashed lines).
png pdf	Figure 3: Scan of $-2\Delta \ln{(\mathcal {L})}$ for the combined fit of the ${{\mathrm {t}} {\mathrm {H}} + {{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}}} $ signal strength on the data (black line) and the individual channels (blue, red, and green), compared to fits on an Asimov dataset corresponding to the SM expectations (dashed lines). In each point the hypothesis of signal strength equal to one is tested against a fit with floating signal strength. The $ {{\mathrm {t}} {\mathrm {H}}} $ and $ {{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}} $ components are varied with a common signal strength.
png pdf	Figure 4: Observed and expected 95% C.L. upper limit on the $ {{\mathrm {t}} {\mathrm {H}}} $ cross section times combined $ {\mathrm {H}} \to {\mathrm {W}} {\mathrm {W}} ^*+ {{\tau} {\tau}} + {\mathrm {Z}} {\mathrm {Z}} ^*+ {{\mathrm {b}} {\overline {\mathrm {b}}}} + {\gamma \gamma} $ branching fraction for different values of the coupling ratio ${\kappa _ {\mathrm {t}}}$. The expected limit is calculated on a background-only dataset, i.e. without $ {{\mathrm {t}} {\mathrm {H}}} $ contribution, but including a ${\kappa _ {\mathrm {t}}} $-dependent contribution from ${{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}}$. The $ {{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}} $ normalization is kept fixed in the fit, while the $ {{\mathrm {t}} {\mathrm {H}}} $ signal strength is allowed to float.

Tables
png pdf	Table 1: Summary of event selection for the multilepton channels.
png pdf	Table 2: Summary of event selection for analysis with single lepton.
png pdf	Table 3: Expected and observed 95% C.L. upper limits on the $ {{\mathrm {t}} {\mathrm {H}}} $ production cross section times $ {\mathrm {H}} \to {\mathrm {W}} {\mathrm {W}} ^*+ {{\tau} {\tau}} + {\mathrm {Z}} {\mathrm {Z}} ^*+ {{\mathrm {b}} {\overline {\mathrm {b}}}} + {\gamma \gamma} $ branching ratio for a scenario of inverted couplings ($ {\kappa _ {\mathrm {t}}} / {\kappa _\text {V}} =-1.0$, top rows) and for a standard model-like signal ($ {\kappa _ {\mathrm {t}}} / {\kappa _\text {V}} =1.0$, bottom rows), in pb. The expected limit is calculated on a background-only dataset, i.e. \ without $ {{\mathrm {t}} {\mathrm {H}}} $ contribution, but including a ${\kappa _ {\mathrm {t}}} $-dependent contribution from ${{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}}$. The $ {{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}} $ normalization is kept fixed in the fit, while the $ {{\mathrm {t}} {\mathrm {H}}} $ signal strength is allowed to float. Limits can be compared to the expected $ {{\mathrm {t}} {\mathrm {H}}} $ cross sections $\times $ branching ratios of 0.834 pb and 0.077 pb for inverted top couplings and for the SM, respectively.

Summary

Events in pp collisions at $\sqrt{s} = $ 13 TeV compatible with the production of Higgs bosons in association with a single top quark have been studied to derive constraints on the magnitude and relative sign of Higgs boson couplings to top quarks and vector bosons. Dedicated analyses in multilepton and $\mathrm{b\bar{b}}$ final states are combined with a reinterpretation of a $\mathrm{H}\to{\gamma\gamma} $ measurement for the final result. For standard model-like Higgs couplings to vector bosons, the observed data favor a positive value of the modifier of the Higgs-top coupling, ${\kappa_\mathrm{t}} $ by about 1.5 standard deviations and exclude values outside the ranges of about $[-0.9, -0.5]$ and $[1.0, 2.1]$ at 95% C.L. An excess of observed data events over non-Higgs boson backgrounds is compatible with the SM expectation of ${\mathrm{t}\mathrm{H}+{\mathrm{t\bar{t}} \mathrm{H}} } $ production within about two standard deviations.

Additional Figures
png pdf	Additional Figure 1: List of most important nuisance parameters, ordered by their impact on a fit with a common ${{\mathrm {t}} {\mathrm {H}} }$+$ {{{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}}}$ signal strength floating. The impact is defined as the shift in best-fit signal strength when fixing each nuisance to its post-fit value plus or minus one standard deviation. Also shown is the pull for each nuisance parameter, defined as post-fit minus pre-fit values divided by the pre-fit uncertainties, with the post-fit uncertainty indicated.
png pdf	Additional Figure 2: Scan of negative log-likelihood for a fit with two independently floating signal strengths for ${{\mathrm {t}} {\mathrm {H}}} $ ($\mu _ {{\mathrm {t}} {\mathrm {H}}} $) and ${{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}} $ ($\mu _ {{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}} $), for $ {\kappa _ {\mathrm {t}}} = $ 1.0 (SM). The best-fit value of $\mu _ {{\mathrm {t}} {\mathrm {H}}} = $ 10.20, $\mu _ {{{\mathrm {t}\overline {\mathrm {t}}}} \mathrm {H}} = $ 1.66 is indicated with a white cross. The SM expectation is indicated with a black cross and has a value of $-2\Delta \ln{\mathcal {L}}$ of 5.28, corresponding to a $p$-value of 7.15%, assuming it is distributed according to a $\chi ^2$ function with two degrees of freedom.

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