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CMS-PAS-B2G-16-005
Search for electroweak production of a vectorlike quark decaying to a top quark and a Higgs boson using boosted topologies in an all-hadronic final state
Abstract: A search is performed for the electroweak production of a vectorlike top quark partner T of charge +2/3 in association with a standard model top or bottom quark, using 2.3 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=$ 13 TeV collected by the CMS experiment. The search targets T quarks decaying to a top quark and a Higgs boson in a fully hadronic final state. For a T quark with mass above 1 TeV, the daughter top quark and the Higgs boson are highly Lorentz-boosted and each can be reconstructed as a single hadronic jet. Jet substructure and b tagging techniques are used to identify the top quark and Higgs boson jets, and to suppress the standard model backgrounds, which are found to be consistent with observations. Upper limits at 95% CL are set on the product of the single T quark production cross section and the $\mathcal{B} (\mathrm{T} \rightarrow \rm{tH} ) $ that vary between 0.31-0.93 pb for T quark masses in the range 1000-1800 GeV.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1-a:
Production diagrams for the processes ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ with the charged current (a) and ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}}$ with the neutral current (b).

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Figure 1-b:
Production diagrams for the processes ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ with the charged current (a) and ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}}$ with the neutral current (b).

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Figure 2:
The ${M(\mathrm T)}$ distributions for the ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ production mode with LH coupling for the T quark masses 1000, 1200, 1500, and 1800 GeV after all the selection criteria. The signal $\sigma \mathcal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ is set to 1 pb.

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Figure 3-a:
The ${H_{\mathrm {T}}}$ (a) and ${M(\mathrm T)}$ (b) distributions after full event selection. The black markers with error bars are the data. The various background components are shown as filled histograms, and are estimated using simulations ($ {\mathrm{ t } \overline {\mathrm{ t } }}$+jets and W+jets ) and the data (non-$ {\mathrm{ t } \overline {\mathrm{ t } }}$+jets and non-W+jets multijets component). The T quark signal distribution for two T quark masses are also shown. The signal $\sigma \mathcal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ is set to 1 pb.

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Figure 3-b:
The ${H_{\mathrm {T}}}$ (a) and ${M(\mathrm T)}$ (b) distributions after full event selection. The black markers with error bars are the data. The various background components are shown as filled histograms, and are estimated using simulations ($ {\mathrm{ t } \overline {\mathrm{ t } }}$+jets and W+jets ) and the data (non-$ {\mathrm{ t } \overline {\mathrm{ t } }}$+jets and non-W+jets multijets component). The T quark signal distribution for two T quark masses are also shown. The signal $\sigma \mathcal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ is set to 1 pb.

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Figure 4-a:
The expected and observed limits on the $\sigma \cal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ of the production processes ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ (a,b) and ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}}$ (c,d), for different assumed values of the T quark mass, and with left handed (a,c) and right-handed (b,d) coupling to the SM 3rd generation quarks. The theoretical lines in the top left and bottom right figures correspond to those predicted by the Simplest Simplified Model of Ref. [21,22], which predict the existence of a left and right handed coupling for a singlet and doublet T quark, respectively. No theoretical prediction of a right handed singlet or a left handed doublet is possible within the simplified model, and thus theoretical cross sections are not shown for the top right and the bottom left plots. It should however be noted that such couplings may still be possible in a non-minimal model.

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Figure 4-b:
The expected and observed limits on the $\sigma \cal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ of the production processes ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ (a,b) and ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}}$ (c,d), for different assumed values of the T quark mass, and with left handed (a,c) and right-handed (b,d) coupling to the SM 3rd generation quarks. The theoretical lines in the top left and bottom right figures correspond to those predicted by the Simplest Simplified Model of Ref. [21,22], which predict the existence of a left and right handed coupling for a singlet and doublet T quark, respectively. No theoretical prediction of a right handed singlet or a left handed doublet is possible within the simplified model, and thus theoretical cross sections are not shown for the top right and the bottom left plots. It should however be noted that such couplings may still be possible in a non-minimal model.

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Figure 4-c:
The expected and observed limits on the $\sigma \cal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ of the production processes ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ (a,b) and ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}}$ (c,d), for different assumed values of the T quark mass, and with left handed (a,c) and right-handed (b,d) coupling to the SM 3rd generation quarks. The theoretical lines in the top left and bottom right figures correspond to those predicted by the Simplest Simplified Model of Ref. [21,22], which predict the existence of a left and right handed coupling for a singlet and doublet T quark, respectively. No theoretical prediction of a right handed singlet or a left handed doublet is possible within the simplified model, and thus theoretical cross sections are not shown for the top right and the bottom left plots. It should however be noted that such couplings may still be possible in a non-minimal model.

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Figure 4-d:
The expected and observed limits on the $\sigma \cal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ of the production processes ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ (a,b) and ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}}$ (c,d), for different assumed values of the T quark mass, and with left handed (a,c) and right-handed (b,d) coupling to the SM 3rd generation quarks. The theoretical lines in the top left and bottom right figures correspond to those predicted by the Simplest Simplified Model of Ref. [21,22], which predict the existence of a left and right handed coupling for a singlet and doublet T quark, respectively. No theoretical prediction of a right handed singlet or a left handed doublet is possible within the simplified model, and thus theoretical cross sections are not shown for the top right and the bottom left plots. It should however be noted that such couplings may still be possible in a non-minimal model.

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Figure 5-a:
The expected and observed limits on the scaling factors $| {\rm c_L^{bW}} |$ (a) and $| {\rm c_R^{tZ}} |$ (b) of the Simplest Simplified Model of Ref. [21,22], wh ich predict the existence of a left and right handed coupling for a singlet and double Tquark, respectively. The left (a) plots are for a singlet (doublet) T quark.

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Figure 5-b:
The expected and observed limits on the scaling factors $| {\rm c_L^{bW}} |$ (a) and $| {\rm c_R^{tZ}} |$ (b) of the Simplest Simplified Model of Ref. [21,22], wh ich predict the existence of a left and right handed coupling for a singlet and double Tquark, respectively. The left (a) plots are for a singlet (doublet) T quark.
Tables

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Table 1:
The signal efficiencies for successive event selections in percent for the ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}}$ (LH) and ${\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}}$ (LH) models. The statistical size of the simulated signal samples are much larger than the total integrated luminosity and hence the statistical errors on the efficiencies are negligible.

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Table 2:
Numbers of events for the control region $A$, $B$,and $C$ in the data, and the non-multijets background. The difference between the data and the ${\mathrm{ t } \overline {\mathrm{ t } }}$+jets , W+jets , and the tW, is attributed to the multijets background component. The errors are statistical only.

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Table 3:
Estimated background and observed events in the signal region after all selection criteria. The combined statistical and systematic uncertainty is shown. The systematic uncertainty on the data-driven multijets background is anticorrelated with that on the MC-driven ${\mathrm{ t } \overline {\mathrm{ t } }}$+jets background. Hence the uncertainty on the total background is less than what one would obtain if the uncertainties on the individual backgrounds are added in quadrature.

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Table 4:
The observed and expected 95% CL upper limits on the cross sections times the branching ratio $\sigma ( {\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ b } {\rm q}} ) \cal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$ and $\sigma ( {\mathrm{ p } \mathrm{ p } \to {\rm T} \mathrm{ t } {\rm q}} ) \cal {B}( {\rm T} {\to } \mathrm{ t } \mathrm{ H } )$, for different masses of the T quark.
Summary
A search for a vectorlike top quark partner T in the single production mode is performed using pp collision events at $\sqrt{s} =$ 13 TeV collected by the CMS experiment in 2015. The T quarks are assumed to couple only to the SM 3rd generation quarks. The decay channel exploited is $\mathrm{ T \to tH }$, with hadronic top quark decay, and $\mathrm{ H \to bb }$. Boosted H and t tagging are used to identify the top quark and Higgs boson decays in the final state, and the invariant mass of the two gives the T quark candidate mass. The background is mostly due to SM $\mathrm{ t \bar{t} }$+jets, with some amount of multijet and W+jets processes. No excess of data above the background is observed and 95% CL upper limits on $\sigma \mathcal{B}(\mathrm{ T \to tH })$ are set varying between 0.31-0.93 pb for T quark masses ranging from 1000 GeV to 1800 GeV in the $ \mathrm{ T \to b q }$ and $ \mathrm{ T \to t q } $ production channels with left and right handed couplings to the SM third generation quarks.
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Compact Muon Solenoid
LHC, CERN