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| CMS-B2G-16-005 ; CERN-EP-2016-290 | ||
| Search for electroweak production of a vector-like quark decaying to a top quark and a Higgs boson using boosted topologies in fully hadronic final states | ||
| CMS Collaboration | ||
| 16 December 2016 | ||
| JHEP 04 (2017) 136 | ||
| Abstract: A search is performed for electroweak production of a vector-like 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 at the CERN LHC. The search targets T quarks decaying to a top quark and a Higgs boson in fully hadronic final states. For a T quark with mass above 1 TeV the daughter top quark and Higgs boson are highly Lorentz-boosted and can each appear 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. An excess of events is searched for in the T quark candidate mass distribution in the data, which is found to be consistent with the expected backgrounds. Upper limits at 95% confidence level are set on the product of the single T quark production cross sections and the branching fraction $\mathcal{B}({\mathrm{T}} \to \mathrm{ t }\mathrm{ H })$, and these vary between 0.31 and 0.93 pb for T quark masses in the range 1000-1800 GeV. This is the first search for single electroweak production of a vector-like T quark in fully hadronic final states. | ||
| Links: e-print arXiv:1612.05336 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Example production diagrams for the processes ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$ via the charged current (left) and ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ t } \mathrm{ q } }$ via the neutral current (right). |
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Figure 1-a:
Example production diagram for the process ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$ via the charged current. |
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Figure 1-b:
Example production diagram for the process ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ t } \mathrm{ q } }$ via the neutral current. |
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Figure 2:
The simulated ${M(\mathrm {T})}$ distributions for the ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } } $ and the ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ t } \mathrm{ q } } $ production modes with left-handed coupling for the T quark masses 1000, 1200, 1500, and 1800 GeV, after all selection criteria have been applied. The values of product of the signal cross sections and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ are taken to be 1 pb. |
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Figure 3:
The ${H_{\mathrm {T}}}$ (left) and ${M(\mathrm {T})}$ (right) 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 } {}\mathrm{ \bar{t} } } \text {+jets}}$ and ${\mathrm{ W } \text {+jets}} $) and the data (non-$ {{\mathrm{ t } {}\mathrm{ \bar{t} } } \text {+jets}}$ and non-$ {\mathrm{ W } \text {+jets}}$ multijets component). The simulated T quark signal distributions for two T quark masses are also shown. The values of product of the signal cross sections and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ are taken to be 1 pb. |
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Figure 3-a:
The ${H_{\mathrm {T}}}$ distribution 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 } {}\mathrm{ \bar{t} } } \text {+jets}}$ and ${\mathrm{ W } \text {+jets}} $) and the data (non-$ {{\mathrm{ t } {}\mathrm{ \bar{t} } } \text {+jets}}$ and non-$ {\mathrm{ W } \text {+jets}}$ multijets component). The simulated T quark signal distributions for two T quark masses are also shown. The values of product of the signal cross sections and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ are taken to be 1 pb. |
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Figure 3-b:
The ${M(\mathrm {T})}$ distribution 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 } {}\mathrm{ \bar{t} } } \text {+jets}}$ and ${\mathrm{ W } \text {+jets}} $) and the data (non-$ {{\mathrm{ t } {}\mathrm{ \bar{t} } } \text {+jets}}$ and non-$ {\mathrm{ W } \text {+jets}}$ multijets component). The simulated T quark signal distributions for two T quark masses are also shown. The values of product of the signal cross sections and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ are taken to be 1 pb. |
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Figure 4:
The expected and observed 95% confidence level upper limits on the product of the signal cross section and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ for the processes ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$ (upper figures) and ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ t } \mathrm{ q } }$ (lower figures), for different assumed values of the T quark mass, and with left-handed (left figures) and right-handed (right figures) couplings to the standard model third-generation quarks. The expected 1 and 2 standard deviation (s. d.) uncertainty bands are also shown. The limits are obtained assuming a resonance width of 10 GeV for the T quark. The dot-dashed curves in the upper left and lower right figures correspond to those predicted by the Simplest Simplified Model of Refs. [18,59], which predicts the existence of a left-handed and right-handed coupling for a singlet and doublet T quark, respectively. The benchmark coupling parameter values of $ {c^{{\mathrm{ b } \mathrm{ W } }}} _\mathrm {L} =$ 0.5 and $ {c^{\mathrm{ t } {\mathrm{ Z } } }} _\mathrm {R} = $ 0.5 are chosen for the comparison. |
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Figure 4-a:
The expected and observed 95% confidence level upper limit on the product of the signal cross section and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ for the processes ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$, for different assumed values of the T quark mass, and with left-handed couplings to the standard model third-generation quarks. The expected 1 and 2 standard deviation (s. d.) uncertainty bands are also shown. The limits are obtained assuming a resonance width of 10 GeV for the T quark. The dot-dashed curve corresponds to that predicted by the Simplest Simplified Model of Refs. [18,59], which predicts the existence of a left-handed coupling for a singlet T quark. The benchmark coupling parameter value of $ {c^{{\mathrm{ b } \mathrm{ W } }}} _\mathrm {L} =$ 0.5 is chosen for the comparison. |
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Figure 4-b:
The expected and observed 95% confidence level upper limit on the product of the signal cross section and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ for the processes ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$, for different assumed values of the T quark mass, and with right-handed couplings to the standard model third-generation quarks. The expected 1 and 2 standard deviation (s. d.) uncertainty bands are also shown. The limits are obtained assuming a resonance width of 10 GeV for the T quark. |
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Figure 4-c:
The expected and observed 95% confidence level upper limit on the product of the signal cross section and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ for the processes ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$, for different assumed values of the T quark mass, and with left-handed couplings to the standard model third-generation quarks. The expected 1 and 2 standard deviation (s. d.) uncertainty bands are also shown. The limits are obtained assuming a resonance width of 10 GeV for the T quark. |
png pdf |
Figure 4-d:
The expected and observed 95% confidence level upper limit on the product of the signal cross section and the branching fraction $\mathcal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$ for the processes ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ t } \mathrm{ q } }$, for different assumed values of the T quark mass, and with lright-handed couplings to the standard model third-generation quarks. The expected 1 and 2 standard deviation (s. d.) uncertainty bands are also shown. The limits are obtained assuming a resonance width of 10 GeV for the T quark. The dot-dashed curve corresponds to that predicted by the Simplest Simplified Model of Refs. [18,59], which predicts the existence of a right-handed coupling for a doublet T quark. The benchmark coupling parameter value of $ {c^{\mathrm{ t } {\mathrm{ Z } } }} _\mathrm {R} = $ 0.5 is chosen for the comparison. |
| Tables | |
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Table 1:
The signal efficiencies for successive event selections for the ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$ and ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ t } \mathrm{ q } }$ models with left-handed couplings. The preselection criteria are more efficient for the ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ t } \mathrm{ q } }$ process compared to the ${\mathrm{ p } \mathrm{ p } \to {\mathrm {T}} \mathrm{ b } \mathrm{ q } }$ process owing to the larger number of jets, and hence a higher $ {H_{\mathrm {T}}} $ per event, in the former. This is more pronounced for low T quark mass samples, where the ${H_{\mathrm {T}}}$ is close to the trigger threshold of 800 GeV. |
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Table 2:
Number of events for the control regions A, B, and C in the data, and in the non-multijet backgrounds. The difference between the data and the ${{\mathrm{ t } {}\mathrm{ \bar{t} } } \text {+jets}} $, ${\mathrm{ W } \text {+jets}} $, and the $\mathrm{ t } \mathrm{ W } $ backgrounds is attributed to the multijets background component. The uncertainties are statistical only. |
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Table 3:
Estimated background and the number of observed events in the signal region after all selection criteria. The combined statistical and systematic uncertainty is shown. |
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Table 4:
The observed and expected 95% confidence level upper limits on the product of the signal cross sections and the branching fraction $\cal {B}( { {\mathrm {T}} \to \mathrm{ t } \mathrm{ H } } )$, for various masses of the T quark. |
| Summary |
| A search for a vector-like top quark partner T in the single production mode is performed using proton-proton collision events at $ \sqrt{s} = $ 13 TeV collected by the CMS experiment in 2015. The T quarks are assumed to couple only to the standard model third-generation quarks. The decay channel exploited is ${{\mathrm{T}} \to \mathrm{t } \mathrm{ H }} $, with hadronic top quark decay and ${\mathrm{ H }\to\mathrm{ b \bar{b} }} $. Boosted H and t tagging techniques are used to identify the Higgs boson and the top quark decays in the final state, and the invariant mass of the two gives the T quark candidate mass. The background is mostly due to the standard model ${\mathrm{ t \bar{t} }\text{+jets}} $, with some contribution from multijet and W+jets processes. No significant excess of data above the background is observed in the T quark candidate mass distribution. The 95% confidence level upper limits on the product of the signal cross sections and the branching fraction $\mathcal{B}({{\mathrm{T}} \to \mathrm{t } \mathrm{ H }} )$ are set using Bayesian statistics. These vary between 0.31-0.93 pb for a T quark of mass ranging from 1000 to 1800 GeV, in the ${\mathrm{ p }\mathrm{ p }\to{\mathrm{T}} \mathrm{b }\mathrm{ q }} $ and ${\mathrm{ p }\mathrm{ p }\to{\mathrm{T}} \mathrm{t }\mathrm{ q }} $ production channels with left-handed and right-handed couplings to the standard model third-generation quarks. In the mass range considered for this analysis, the search sensitivity is essentially the same as that using leptonic final states [17]. The use of boosted techniques has led to an extension of the search region beyond those of previous analyses. This is the first time fully hadronic final states have been exploited in the search for single electroweak production of vector-like quarks at a hadron collider. |
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