CMSPASEXO19015  
Search for singly and pairproduced leptoquarks coupling to thirdgeneration fermions in protonproton collisions at $\sqrt{s} = $ 13 TeV  
CMS Collaboration  
July 2020  
Abstract: A search for leptoquarks produced singly and in pairs in protonproton collisions is presented. The leptoquark (LQ) may couple to a top quark plus a $\tau$ lepton (t$\tau$) or a bottom quark plus a neutrino (b$\nu$, scalar LQ), or else to t$\nu$ or b$\tau$ (vector LQ), leading to the final states t$\tau\nu$b and t$\tau\nu$. The data were recorded by the CMS experiment at the CERN LHC at $\sqrt{s} = $ 13 TeV and correspond to an integrated luminosity of 137 fb$^{1}$. The data are found to be in agreement with the standard model background prediction. Lower limits at 95% CL are set on the LQ mass in the range 0.981.73 TeV, depending on the LQ spin and the LQleptonquark vertex coupling $\lambda$, and assuming equal branching fractions for the two LQ decay modes considered. These are the most stringent constraints on the existence of leptoquarks in this scenario.  
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These preliminary results are superseded in this paper, Accepted by PLB. The superseded preliminary plots can be found here. 
Figures & Tables  Summary  Additional Figures & Tables  References  CMS Publications 

Figures  
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Figure 1:
Main Feynman diagrams for LQ production: pairwise (left), and in combination with a lepton (right). 
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Figure 2:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection. Upperleft: boosted top (hadronically decaying top quark reconstructed in the fully or partially merged topology) and exactly one b jet; lowerleft: boosted top and at least two b jets; upperright: resolved top (hadronically decaying top quark reconstructed in the resolved topology) and exactly one b jet; lowerright: resolved top and at least two b jets. 
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Figure 2a:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, boosted top (hadronically decaying top quark reconstructed in the fully or partially merged topology) and exactly one b jet. 
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Figure 2b:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, resolved top (hadronically decaying top quark reconstructed in the resolved topology) and exactly one b jet. 
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Figure 2c:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, boosted top (hadronically decaying top quark reconstructed in the fully or partially merged topology) and at least two b jets. 
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Figure 2d:
Distribution of the variable ${S_{\rm {T}}}$ for events passing the signal selection, resolved top (hadronically decaying top quark reconstructed in the resolved topology) and at least two b jets. 
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Figure 3:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$ (upper), $\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5 (centerleft) and 2.5 (lowerleft), and $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$+$\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5 (centerright) and 2.5 (lowerright), as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pairproduced LQ$_{S}$, for which an NLO calculation [44] is shown. 
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Figure 3a:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pairproduced LQ$_{S}$, for which an NLO calculation [44] is shown. 
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Figure 3b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pairproduced LQ$_{S}$, for which an NLO calculation [44] is shown. 
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Figure 3c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pairproduced LQ$_{S}$, for which an NLO calculation [44] is shown. 
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Figure 3d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$+$\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pairproduced LQ$_{S}$, for which an NLO calculation [44] is shown. 
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Figure 3e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{S}} {\overline {{\text {LQ}}}_{S}})$+$\sigma (pp\rightarrow \nu {\text {LQ}_{S}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{S}$. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO, except for pairproduced LQ$_{S}$, for which an NLO calculation [44] is shown. 
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Figure 4:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$ (upper), $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (centerleft) and 2.5 (lowerleft), and $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (centerright) and 2.5 (lowerright), as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
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Figure 4a:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
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Figure 4b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
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Figure 4c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
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Figure 4d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
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Figure 4e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 0. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
png pdf 
Figure 5:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$ (upper), $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (centerleft) and 2.5 (lowerleft), and $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5 (centerright) and 2.5 (lowerright), as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
png pdf 
Figure 5a:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
png pdf 
Figure 5b:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
png pdf 
Figure 5c:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
png pdf 
Figure 5d:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 1.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
png pdf 
Figure 5e:
The observed and expected (solid and dotted black lines) 95% CL upper limits on $\sigma (pp\rightarrow {\text {LQ}_{V}} {\overline {{\text {LQ}}}_{V}})$+$\sigma (pp\rightarrow \tau {\text {LQ}_{V}})$ $\lambda =$ 2.5, as a function of the mass of the LQ$_{V}$, with $k = $ 1. The bands represent the expected variation of the limit to within one and two standard deviation(s). The solid blue curve indicates the theoretical predictions at LO. 
png pdf 
Figure 6:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQleptonquark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The upper plot pertains to an LQ$_{S}$ with equal couplings to t$\tau $, b$\nu $, while the lower plots are for an LQ$_{V}$ assuming $k = $ 0 (left) and 1 (right) and equal couplings to t$\nu $, b$\tau $. For LQ$_{V}$, the gray area shows the band preferred 95% CL) by Bphysics anomalies: $\lambda = \sqrt {0.7 \pm 0.2 } \times m_{\rm {LQ}}$ TeV [42]. 
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Figure 6a:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQleptonquark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The plot pertains to an LQ$_{S}$ with equal couplings to t$\tau $, b$\nu $. 
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Figure 6b:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQleptonquark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The plot pertains to an LQ$_{V}$ assuming $k = $ 0 and equal couplings to t$\nu $, b$\tau $. The gray area shows the band preferred 95% CL) by Bphysics anomalies: $\lambda = \sqrt {0.7 \pm 0.2 } \times m_{\rm {LQ}}$ TeV [42]. 
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Figure 6c:
The observed and expected (solid and dotted lines) 95% CL LQ exclusion limits in the plane of the LQleptonquark vertex coupling and the mass of the LQ for single (brown lines), pair (blue lines) production, and considering their sum (black lines). Regions to the left of the lines are excluded. The plot pertains to an LQ$_{V}$ assuming $k = $ 1 and equal couplings to t$\nu $, b$\tau $. The gray area shows the band preferred 95% CL) by Bphysics anomalies: $\lambda = \sqrt {0.7 \pm 0.2 } \times m_{\rm {LQ}}$ TeV [42]. 
Tables  
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Table 1:
Yields from the SM background estimation, expected signal, and data, for the selected events. 
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Table 2:
Lower limits on the mass in TeV of the leptoquarks LQ$_{S}$, LQ$_{V}$ $k = $ 0, and LQ$_{V}$ $k = $ 1, based on the pair and singleproduction mechanisms taken separately and together. The searches that depend on the $\lambda $ parameter are given for values of 1.5 and 2.5. The expected limits are given in parentheses. 
Summary 
A search for leptoquarks, coupling to thirdgeneration fermions, and produced in pairs and singly in association with a lepton, has been presented. The leptoquark LQ may couple to a top quark plus a $\tau$ lepton ($\mathrm{t}\tau$) or a bottom quark plus a neutrino ($\mathrm{b}\nu$, scalar LQ) or else to $\mathrm{t}\nu$ and $\mathrm{b}\tau$ (vector LQ), resulting in the final states $\mathrm{t}\tau\nu\mathrm{b}$ and $\mathrm{t}\tau\nu$. The channel in which both the top quark and the $\tau$ lepton decay hadronically is investigated, including the case of a large LQ$\mathrm{t}$ mass splitting giving rise to a boosted top quark whose decay daughters may not be separated on the scale of the spatial resolution of the jet. Such a signature has not been previously examined in searches for physics beyond the standard model. The data used corresponds to an integrated luminosity of 137 fb$^{1}$ collected with the CMS detector at the LHC in pp collisions at $\sqrt{s} = $ 13 TeV. The observations are found to be in agreement with the standard model prediction. Exclusion limits are given in the plane of the LQleptonquark vertex coupling $\lambda$ and the LQ mass for scalar and vector leptoquarks. The range of lower limits on the LQ mass, at 95% CL, is 0.981.73 TeV, depending on $\lambda$ and the leptoquark spin. These results represent the most stringent limits to date on the presence of such leptoquarks for the case of a decay branching fraction of 0.5 to each leptonquark pair. These results also probe the parameter space preferred by the Bphysics anomalies in several models ([41], [42]), and exclude relevant portions. 
Additional Figures  
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Additional Figure 1:
Pair leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton, having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
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Additional Figure 2:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for scalar leptoquark (LQ$_{S}$, left), vector leptoquark (LQ$_{V}$) with k = 1 (center), and LQ$_{V}$ with k = 0 (right). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
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Additional Figure 2a:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for scalar leptoquark (LQ$_{S}$). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
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Additional Figure 2b:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for vector leptoquark (LQ$_{V}$) with k = 1. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
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Additional Figure 2c:
Single leptoquark total signal selection efficiency, including all the events that enter the top and b jet categories defined in the search, for vector leptoquark (LQ$_{V}$) with k = 0. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
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Additional Figure 3:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). 
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Additional Figure 3a:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark. 
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Additional Figure 3b:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 1. 
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Additional Figure 3c:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 0. 
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Additional Figure 4:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). 
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Additional Figure 4a:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark. 
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Additional Figure 4b:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 1. 
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Additional Figure 4c:
Leptoquark mass distribution at generator level for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 0. 
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Additional Figure 5:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
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Additional Figure 5a:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for scalar leptoquark. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
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Additional Figure 5b:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 1. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
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Additional Figure 5c:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 0.8 TeV, for vector leptoquark with k = 0. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
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Additional Figure 6:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark (left), vector leptoquark with k = 1 (center), and vector leptoquark with k = 0 (right). The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
png pdf 
Additional Figure 6a:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for scalar leptoquark. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
png pdf 
Additional Figure 6b:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 1. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
png pdf 
Additional Figure 6c:
${S_{\rm {T}}}$ distribution for a singly produced leptoquark with mass equal to 1.7 TeV, for vector leptoquark with k = 0. The events are selected using the requirements sought in the search region specified in the main text, including all the events that enter the top and b jet categories. 
Additional Tables  
png pdf 
Additional Table 1:
Pair leptoquark signal selection efficiency relative to each requirement sought in the analysis, including all the events that enter the top and b jet categories defined in the search. The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton, having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
png pdf 
Additional Table 2:
Single leptoquark signal selection efficiency relative to each requirement sought in the analysis, including all the events that enter the top and b jet categories defined in the search, for a leptoquark mass of 0.8 TeV and different hypotheses: scalar leptoquark (LQ$_{S}$, upper), vector leptoquark (LQ$_{V}$) with k = 1 (center), and LQ$_{V}$ with k = 0 (lower). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
png pdf 
Additional Table 3:
Single leptoquark signal selection efficiency relative to each requirement sought in the analysis, including all the events that enter the top and b jet categories defined in the search, for a leptoquark mass of 1.7 TeV and different hypotheses: scalar leptoquark (LQ$_{S}$, upper), vector leptoquark (LQ$_{V}$) with k = 1 (center), and LQ$_{V}$ with k = 0 (lower). The signal samples contain all the final states that can originate from the leptoquark decays to a top quark plus a neutrino or a bottom quark plus a tau lepton (LQ$_{V}$), or else a top quark plus a tau lepton or a bottom quark plus a neutrino (LQ$_{S}$), having the leptoquark decay a branching fraction of 0.5 to each fermion pair. 
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