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LHC, CERN

CMS-B2G-19-004 ; CERN-EP-2021-115
Search for single production of a vector-like T quark decaying to a top quark and a Z boson in the final state with jets and missing transverse momentum at $\sqrt{s} = $ 13 TeV
JHEP 05 (2022) 093
Abstract: A search is presented for single production of a vector-like T quark with charge 2/3e, in the decay channel featuring a top quark and a Z boson, with the top quark decaying hadronically and the Z boson decaying to neutrinos. The search uses data collected by the CMS experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 137 fb$^{-1}$ recorded at the CERN LHC in 2016-2018. The search is sensitive to a T quark mass between 0.6 and 1.8 TeV with decay widths ranging from negligibly small up to 30% of the T quark mass. Reconstruction strategies for the top quark are based on the degree of Lorentz boosting of its final state. At 95% confidence level, the upper limit on the product of the cross section and branching fraction for a T quark of small decay width varies between 15 and 602 fb, depending on its mass. For a T quark with decay widths between 10 and 30% of its mass, this upper limit ranges between 16 and 836 fb. For most of the studied range, the results provide the best limits to date. This is the first search for single T quark production based on the full Run 2 data set of the LHC.
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Figures

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Figure 1:
A representative leading-order Feynman diagram for the production of a single vector-like quark T decaying into a Z boson and a top quark.

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Figure 2:
The product of the signal acceptance and efficiency, averaged over the three data-taking years, for the final event selection in the resolved (upper), partially merged (central), and merged (lower) categories, for the zero forward jets (left), and at least one forward jet (right) categories. The numerator is the number of events passing the respective selection, the denominator is the total expected number of events. All possible decays of the top quark and Z boson are considered in both numerator and denominator of the efficiency computation, therefore the values are not corrected for the branching fraction of the final state.

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Figure 2-a:
The product of the signal acceptance and efficiency, averaged over the three data-taking years, for the final event selection in the resolved (upper), partially merged (central), and merged (lower) categories, for the zero forward jets (left), and at least one forward jet (right) categories. The numerator is the number of events passing the respective selection, the denominator is the total expected number of events. All possible decays of the top quark and Z boson are considered in both numerator and denominator of the efficiency computation, therefore the values are not corrected for the branching fraction of the final state.

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Figure 2-b:
The product of the signal acceptance and efficiency, averaged over the three data-taking years, for the final event selection in the resolved (upper), partially merged (central), and merged (lower) categories, for the zero forward jets (left), and at least one forward jet (right) categories. The numerator is the number of events passing the respective selection, the denominator is the total expected number of events. All possible decays of the top quark and Z boson are considered in both numerator and denominator of the efficiency computation, therefore the values are not corrected for the branching fraction of the final state.

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Figure 2-c:
The product of the signal acceptance and efficiency, averaged over the three data-taking years, for the final event selection in the resolved (upper), partially merged (central), and merged (lower) categories, for the zero forward jets (left), and at least one forward jet (right) categories. The numerator is the number of events passing the respective selection, the denominator is the total expected number of events. All possible decays of the top quark and Z boson are considered in both numerator and denominator of the efficiency computation, therefore the values are not corrected for the branching fraction of the final state.

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Figure 2-d:
The product of the signal acceptance and efficiency, averaged over the three data-taking years, for the final event selection in the resolved (upper), partially merged (central), and merged (lower) categories, for the zero forward jets (left), and at least one forward jet (right) categories. The numerator is the number of events passing the respective selection, the denominator is the total expected number of events. All possible decays of the top quark and Z boson are considered in both numerator and denominator of the efficiency computation, therefore the values are not corrected for the branching fraction of the final state.

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Figure 2-e:
The product of the signal acceptance and efficiency, averaged over the three data-taking years, for the final event selection in the resolved (upper), partially merged (central), and merged (lower) categories, for the zero forward jets (left), and at least one forward jet (right) categories. The numerator is the number of events passing the respective selection, the denominator is the total expected number of events. All possible decays of the top quark and Z boson are considered in both numerator and denominator of the efficiency computation, therefore the values are not corrected for the branching fraction of the final state.

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Figure 2-f:
The product of the signal acceptance and efficiency, averaged over the three data-taking years, for the final event selection in the resolved (upper), partially merged (central), and merged (lower) categories, for the zero forward jets (left), and at least one forward jet (right) categories. The numerator is the number of events passing the respective selection, the denominator is the total expected number of events. All possible decays of the top quark and Z boson are considered in both numerator and denominator of the efficiency computation, therefore the values are not corrected for the branching fraction of the final state.

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Figure 3:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the merged categories, but with the AK8 jet SD mass outside the interval 105-220 GeV, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties after the fit to data in the validation regions are shown.

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Figure 3-a:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the merged categories, but with the AK8 jet SD mass outside the interval 105-220 GeV, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties after the fit to data in the validation regions are shown.

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Figure 3-b:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the merged categories, but with the AK8 jet SD mass outside the interval 105-220 GeV, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties after the fit to data in the validation regions are shown.

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Figure 3-c:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the merged categories, but with the AK8 jet SD mass outside the interval 105-220 GeV, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties after the fit to data in the validation regions are shown.

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Figure 3-d:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the merged categories, but with the AK8 jet SD mass outside the interval 105-220 GeV, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties after the fit to data in the validation regions are shown.

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Figure 3-e:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the merged categories, but with the AK8 jet SD mass outside the interval 105-220 GeV, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties after the fit to data in the validation regions are shown.

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Figure 3-f:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the merged categories, but with the AK8 jet SD mass outside the interval 105-220 GeV, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties after the fit to data in the validation regions are shown.

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Figure 4:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the resolved categories, but without the requirement of at least one b jet in the event, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties before the fit to data in the validation regions are shown.

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Figure 4-a:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the resolved categories, but without the requirement of at least one b jet in the event, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties before the fit to data in the validation regions are shown.

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Figure 4-b:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the resolved categories, but without the requirement of at least one b jet in the event, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties before the fit to data in the validation regions are shown.

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Figure 4-c:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the resolved categories, but without the requirement of at least one b jet in the event, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties before the fit to data in the validation regions are shown.

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Figure 4-d:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the resolved categories, but without the requirement of at least one b jet in the event, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties before the fit to data in the validation regions are shown.

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Figure 4-e:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the resolved categories, but without the requirement of at least one b jet in the event, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties before the fit to data in the validation regions are shown.

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Figure 4-f:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for events in validation samples selected as in the resolved categories, but without the requirement of at least one b jet in the event, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The predictions of the main background components have been determined in simulation with scale factors applied to match data extracted from control regions. The uncertainties before the fit to data in the validation regions are shown.

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Figure 5:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Illustrative signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 5-a:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Illustrative signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 5-b:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Illustrative signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 5-c:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Illustrative signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 5-d:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Illustrative signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 5-e:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Illustrative signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 5-f:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Illustrative signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 6:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the partially merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 6-a:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the partially merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 6-b:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the partially merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 6-c:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the partially merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 6-d:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the partially merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 6-e:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the partially merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 6-f:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the partially merged categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The overflow is included in the last bin. The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 7:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the resolved categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 7-a:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the resolved categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

png pdf
Figure 7-b:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the resolved categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 7-c:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the resolved categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 7-d:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the resolved categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 7-e:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the resolved categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 7-f:
Distributions of the transverse mass ${M_{{\mathrm {T}}}}$ of the reconstructed top quark and ${\vec{p}}_{\mathrm {T}}^{\,\text {miss}}$ system, for the selected events in the resolved categories, for events with no forward jet (left) and at least one forward jet (right), and for 2016 (upper), 2017 (central), and 2018 (lower). The distributions for the main background components have been determined in simulation with scale factors extracted from control regions. All background processes and the respective uncertainties are derived from the fit to data, while the distributions of signal processes are represented according to the expectation before the fit. The lines show the signal predictions for three benchmark mass values (0.8, 1.2, and 1.6 TeV) of a T quark of negligible resonance width. Signal yields are multiplied by a factor of 50 to improve their visibility.

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Figure 8:
Observed and expected 95% CL upper limits on the product of the single production cross section for a singlet VLQ T quark and the T $ \to $ tZ branching fraction, as functions of the T quark mass $m_{{\mathrm {T}}}$ for a narrow-width resonance (upper left), and a width of 10% (upper right), 20% (lower left), and 30% (lower right) of the T quark mass. A singlet T quark is assumed, produced in association with a bottom quark. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The solid curves show the theoretical expectation at NLO. In the case of a narrow-width resonance, the width of 1 (5)% of the resonance mass is indicated with a red (blue) curve.

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Figure 8-a:
Observed and expected 95% CL upper limits on the product of the single production cross section for a singlet VLQ T quark and the T $ \to $ tZ branching fraction, as functions of the T quark mass $m_{{\mathrm {T}}}$ for a narrow-width resonance (upper left), and a width of 10% (upper right), 20% (lower left), and 30% (lower right) of the T quark mass. A singlet T quark is assumed, produced in association with a bottom quark. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The solid curves show the theoretical expectation at NLO. In the case of a narrow-width resonance, the width of 1 (5)% of the resonance mass is indicated with a red (blue) curve.

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Figure 8-b:
Observed and expected 95% CL upper limits on the product of the single production cross section for a singlet VLQ T quark and the T $ \to $ tZ branching fraction, as functions of the T quark mass $m_{{\mathrm {T}}}$ for a narrow-width resonance (upper left), and a width of 10% (upper right), 20% (lower left), and 30% (lower right) of the T quark mass. A singlet T quark is assumed, produced in association with a bottom quark. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The solid curves show the theoretical expectation at NLO. In the case of a narrow-width resonance, the width of 1 (5)% of the resonance mass is indicated with a red (blue) curve.

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Figure 8-c:
Observed and expected 95% CL upper limits on the product of the single production cross section for a singlet VLQ T quark and the T $ \to $ tZ branching fraction, as functions of the T quark mass $m_{{\mathrm {T}}}$ for a narrow-width resonance (upper left), and a width of 10% (upper right), 20% (lower left), and 30% (lower right) of the T quark mass. A singlet T quark is assumed, produced in association with a bottom quark. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The solid curves show the theoretical expectation at NLO. In the case of a narrow-width resonance, the width of 1 (5)% of the resonance mass is indicated with a red (blue) curve.

png pdf
Figure 8-d:
Observed and expected 95% CL upper limits on the product of the single production cross section for a singlet VLQ T quark and the T $ \to $ tZ branching fraction, as functions of the T quark mass $m_{{\mathrm {T}}}$ for a narrow-width resonance (upper left), and a width of 10% (upper right), 20% (lower left), and 30% (lower right) of the T quark mass. A singlet T quark is assumed, produced in association with a bottom quark. The inner (green) band and the outer (yellow) band indicate the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The solid curves show the theoretical expectation at NLO. In the case of a narrow-width resonance, the width of 1 (5)% of the resonance mass is indicated with a red (blue) curve.

png pdf
Figure 9:
Observed 95% CL upper limit on the product of the single production cross section for a singlet VLQ T quark and the T $ \to $ tZ branching fraction, as a function of the T quark mass $m_{{\mathrm {T}}}$ and width $\Gamma $, for widths from 5 to 30% of the mass. A singlet T quark that is produced in association with a bottom quark is assumed. The solid red line indicates the boundary of the excluded region (on the hatched side) of theoretical cross sections, as reported in Table 1.
Tables

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Table 1:
Theoretical cross sections, in fb, for single production of a vector-like singlet T quark in association with a bottom quark, with the T quark decaying to tZ, for mass hypotheses between 0.6 and 1.8 TeV in steps of 0.1 TeV, and for resonance widths that are 1, 10, 20, and 30% of its mass. The framework for the computation is described in Refs. [42,43]. The uncertainties, given in parentheses, are obtained by halving and doubling the values of QCD renormalization and factorization scales.

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Table 2:
Summary of the systematic uncertainties. The maximum range of change in the pre-fit event rate of signals and backgrounds across all years and categories for one standard deviation change in the systematic uncertainty is reported in the "Effect'' column. All uncertainties are considered fully correlated across the three years of data-taking, except for those corresponding to the ECAL L1 trigger inefficiency and the background scale factors. The third column indicates whether the uncertainty affects both the rate and the shape of the distributions or the rate only. Except for the background scale factors, all the uncertainties affect both signal and background inputs to the fit.
Summary
A search for the single production of a vector-like quark T with charge 2/3e decaying to a top quark and a Z boson has been presented. The analysis is based on LHC proton-proton collision data collected by the CMS experiment, corresponding to an integrated luminosity of 137 fb$^{-1}$. Upper limits at 95% confidence level are set on the product of the production cross section and the T $\to$ tZ channel branching fraction. Values greater than 602 to 15 fb for T quark masses between 0.6 and 1.8 TeV are excluded at 95% confidence level for a T quark of negligible resonance width produced in association with a bottom quark. Values greater than 836 to 16 fb for masses between 0.6 and 1.8 TeV are excluded at 95% confidence level for a T quark of resonance width from 10 to 30% of its mass. These results provide the best exclusion limits on the production of single vector-like T quarks in the tZ decay channel over the mass range from 0.6 to 1.2 TeV and from 1.5 to 1.8 TeV. An interpretation of these results within a theoretical framework in which the T quark is a singlet, and assuming a resonance width of 5% of the mass, leads to the exclusion of a T quark of mass below 0.98 TeV. The excluded mass range extends up to 1.4 TeV for a resonance width 30% of the mass. This is the first search for single T quark production based on the full LHC Run 2 data set.
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