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Search for pair- and single-production of vector-like quarks in final states with at least one $Z$ boson decaying into a pair of electrons or muons in $pp$ collision data collected with the ATLAS detector at $\sqrt{s} = 13$ TeV

The collaboration
Phys.Rev.D 98 (2018) 112010, 2018.

Abstract (data abstract)
CERN-LHC, ATLAS. A search for vector-like quarks is presented, which targets their decay into a $Z$ boson and a third-generation Standard Model quark. In the case of a vector-like quark $T$ ($B$) with charge $+2/3e$ ($-1/3e$), the decay searched for is $T \rightarrow Zt$ ($B \rightarrow Zb$). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=13$ TeV. The final state used is characterized by the presence of $b$-tagged jets, as well as a $Z$ boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair- and single-production of vector-like quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transverse-momentum large-radius jets, as well as - in the case of the single-production selections - the presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vector-like quarks of $m_T > 1030$ GeV ($m_T > 1210$ GeV) and $m_B > 1010$ GeV ($m_B > 1140$ GeV) in the singlet (doublet) model. In the case of 100% branching ratio for $T\rightarrow Zt$ ($B\rightarrow Zb$), the limits are $m_T > 1340$ GeV ($m_B > 1220$ GeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vector-like quark masses.

• #### Figure 3(c)

Data from Figure 3(c)

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Comparison of the distribution of the scalar sum of small-$R$ jet transverse momenta, $H_T$, between data and the background prediction...

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Data from Figure 3(d)

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Comparison of the distribution of the scalar sum of small-$R$ jet transverse momenta, $H_T$, between data and the background prediction...

• #### Figure 5(c)

Data from Figure 5(c)

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Comparison of the distribution of the invariant mass of the $Z$ boson candidate and the highest-$p_T$ $b$-tagged jet, $m(Zb)$, between...

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Data from Figure 7(c)

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Comparison of the distribution of the scalar sum of small-$R$ jet and lepton transverse momenta, $S_T$, between data and the...

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Data from Figure 9(c)

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Comparison of the distribution of the invariant mass of the $Z$ boson candidate and the highest-$p_T$ top-tagged large-$R$ jet, $m_{Zt}$,...

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Data from Figure 11(c)

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Comparison of the distribution of the scalar sum of small-$R$ jet and lepton transverse momenta, $S_T$, between data and the...

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Data from Figure 13(a)

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Upper limits at 95$\%$ CL on the cross section of vector-like quark pair production (PP) for $T\bar T$ in the...

• #### Figure 13(b)

Data from Figure 13(b)

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Upper limits at 95$\%$ CL on the cross section of vector-like quark pair production (PP) for $B\bar B$ in the...

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Data from Figure 13(c)

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Upper limits at 95$\%$ CL on the cross section of vector-like quark pair production (PP) for $T\bar T$ in the...

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Data from Figure 13(d)

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Upper limits at 95$\%$ CL on the cross section of vector-like quark pair production (PP) for $B\bar B$ in the...

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Data from Figure 13(e)

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Upper limits at 95$\%$ CL on the cross section of vector-like quark pair production (PP) for $T\bar T$ with a...

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Data from Figure 13(f)

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Upper limits at 95$\%$ CL on the cross section of vector-like quark pair production (PP) for $B\bar B$ with a...

• #### Figure 14(a)

Data from Figure 14(a)

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Expected 95$\%$ CL lower limits from the combination of the pair-production channels on the mass of vector-like quarks for all...

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Data from Figure 14(b)

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Observed 95$\%$ CL lower limits from the combination of the pair-production channels on the mass of vector-like quarks for all...

• #### Figure 14(c)

Data from Figure 14(c)

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Expected 95$\%$ CL lower limits from the combination of the pair-production channels on the mass of vector-like quarks for all...

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Data from Figure 14(d)

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Observed 95$\%$ CL lower limits from the combination of the pair-production channels on the mass of vector-like quarks for all...

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Data from Figure 15

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Upper limits at 95$\%$ CL on the cross section times BR to $Zt$ of single production (SP) of a $T$-quark....

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Data from Figure 16(a)

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Expected and observed 95$\%$ CL limits from the combination of the single-production channels on the coupling of the $T$ quark...

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Data from Figure 16(b)

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Expected and observed 95$\%$ CL limits from the combination of the single-production channels on the mixing angle in the singlet...

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Data from Figure 17(a)

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Expected lower limit from the combination of the single-production channels on the mass of the $T$ quark as a function...

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Data from Figure 17(b)

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Observed lower limit from the combination of the single-production channels on the mass of the $T$ quark as a function...

• #### Auxiliary Figure 1(a)

Data from Figure 1(a) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 1(b)

Data from Figure 1(b) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

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Data from Figure 1(c) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 1(d)

Data from Figure 1(d) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 1(e)

Data from Figure 1(e) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 2(a)

Data from Figure 2(a) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 2(b)

Data from Figure 2(b) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 2(c)

Data from Figure 2(c) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 2(d)

Data from Figure 2(d) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 2(e)

Data from Figure 2(e) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 3(a)

Data from Figure 3(a) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 3(b)

Data from Figure 3(b) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 3(c)

Data from Figure 3(c) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 3(d)

Data from Figure 3(d) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 3(e)

Data from Figure 3(e) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow T\bar T)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 4(a)

Data from Figure 4(a) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 4(b)

Data from Figure 4(b) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 4(c)

Data from Figure 4(c) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 4(d)

Data from Figure 4(d) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 4(e)

Data from Figure 4(e) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 5(a)

Data from Figure 5(a) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 5(b)

Data from Figure 5(b) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 5(c)

Data from Figure 5(c) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 5(d)

Data from Figure 5(d) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 5(e)

Data from Figure 5(e) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 6(a)

Data from Figure 6(a) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 6(b)

Data from Figure 6(b) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 6(c)

Data from Figure 6(c) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 6(d)

Data from Figure 6(d) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Figure 6(e)

Data from Figure 6(e) of auxiliary material

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Observed upper limits at 95$\%$ CL on $\sigma(pp\rightarrow B\bar B)$ for the combination of the pair-production channels for all combinations...

• #### Auxiliary Table 1

Data from Table 1 of auxiliary material

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Observed upper limits at 95$\%$ CL on the cross section times BR to $Zt$ as a function of the vector-like...

• #### Auxiliary Table 2 (upper part)

Data from Table 2 (upper part) of auxiliary material

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Signal cutflows for the $B\bar B$ process in the singlet model in the PP $2\ell$ $0-1$J channel in the 0-large-$R$...

• #### Auxiliary Table 2 (lower part)

Data from Table 2 (lower part) of auxiliary material

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Signal cutflows for the $B\bar B$ process in the singlet model in the PP $2\ell$ $0-1$J channel in the 1-large-$R$...

• #### Auxiliary Table 3 (upper part)

Data from Table 3 (upper part) of auxiliary material

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Signal cutflows for the $B\bar B$ process in the doublet model in the PP $2\ell$ $0-1$J channel in the 0-large-$R$...

• #### Auxiliary Table 3 (lower part)

Data from Table 3 (lower part) of auxiliary material

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Signal cutflows for the $B\bar B$ process in the doublet model in the PP $2\ell$ $0-1$J channel in the 1-large-$R$...

• #### Auxiliary Table 4 (upper part)

Data from Table 4 (upper part) of auxiliary material

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Signal cutflows for the $B\bar B$ process in the case of 100$\%$ BR for $B \rightarrow Zb$ in the PP...

• #### Auxiliary Table 4 (lower part)

Data from Table 4 (lower part) of auxiliary material

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Signal cutflows for the $B\bar B$ process in the case of 100$\%$ BR for $B \rightarrow Zb$ in the PP...

• #### Auxiliary Table 5 (upper part)

Data from Table 5 (upper part) of auxiliary material

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Signal cutflows for the $T\bar T$ process in the singlet model in the PP $2\ell$ $0-1$J channel in the 0-large-$R$...

• #### Auxiliary Table 5 (lower part)

Data from Table 5 (lower part) of auxiliary material

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Signal cutflows for the $T\bar T$ process in the singlet model in the PP $2\ell$ $0-1$J channel in the 1-large-$R$...

• #### Auxiliary Table 6 (upper part)

Data from Table 6 (upper part) of auxiliary material

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Signal cutflows for the $T\bar T$ process in the doublet model in the PP $2\ell$ $0-1$J channel in the 0-large-$R$...

• #### Auxiliary Table 6 (lower part)

Data from Table 6 (lower part) of auxiliary material

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Signal cutflows for the $T\bar T$ process in the doublet model in the PP $2\ell$ $0-1$J channel in the 1-large-$R$...

• #### Auxiliary Table 7 (upper part)

Data from Table 7 (upper part) of auxiliary material

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Signal cutflows for the $T\bar T$ process in the case of 100$\%$ BR for $T \rightarrow Zt$ in the PP...

• #### Auxiliary Table 7 (lower part)

Data from Table 7 (lower part) of auxiliary material

10.17182/hepdata.83660.v1/t64

Signal cutflows for the $T\bar T$ process in the case of 100$\%$ BR for $T \rightarrow Zt$ in the PP...

• #### Auxiliary Table 8

Data from Table 8 of auxiliary material

10.17182/hepdata.83660.v1/t65

Signal cutflows for the $T\bar T$ process in the singlet model in the PP $2\ell$ $\geq 2$J channel. Only statistical...

• #### Auxiliary Table 9

Data from Table 9 of auxiliary material

10.17182/hepdata.83660.v1/t66

Signal cutflows for the $B\bar B$ process in the singlet model in the PP $2\ell$ $\geq 2$J channel. Only statistical...

• #### Auxiliary Table 10

Data from Table 10 of auxiliary material

10.17182/hepdata.83660.v1/t67

Signal cutflows for the $T\bar T$ process in the doublet model in the PP $2\ell$ $\geq 2$J channel. Only statistical...

• #### Auxiliary Table 11

Data from Table 11 of auxiliary material

10.17182/hepdata.83660.v1/t68

Signal cutflows for the $B\bar B$ process in the doublet model in the PP $2\ell$ $\geq 2$J channel. Only statistical...

• #### Auxiliary Table 12

Data from Table 12 of auxiliary material

10.17182/hepdata.83660.v1/t69

Signal cutflows for the $T\bar T$ process in the case of 100$\%$ BR for $T \rightarrow Zt$ in the PP...

• #### Auxiliary Table 13

Data from Table 13 of auxiliary material

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Signal cutflows for the $B\bar B$ process in the case of 100$\%$ BR for $B \rightarrow Zb$ in the PP...

• #### Auxiliary Table 14

Data from Table 14 of auxiliary material

10.17182/hepdata.83660.v1/t71

Signal cutflows for the $T\bar T$ process in the singlet model in the PP $\geq 3\ell$ channel. Only statistical uncertainties...

• #### Auxiliary Table 15

Data from Table 15 of auxiliary material

10.17182/hepdata.83660.v1/t72

Signal cutflows for the $B\bar B$ process in the singlet model in the PP $\geq 3\ell$ channel. Only statistical uncertainties...

• #### Auxiliary Table 16

Data from Table 16 of auxiliary material

10.17182/hepdata.83660.v1/t73

Signal cutflows for the $T\bar T$ process in the doublet model in the PP $\geq 3\ell$ channel. Only statistical uncertainties...

• #### Auxiliary Table 17

Data from Table 17 of auxiliary material

10.17182/hepdata.83660.v1/t74

Signal cutflows for the $B\bar B$ process in the doublet model in the PP $\geq 3\ell$ channel. Only statistical uncertainties...

• #### Auxiliary Table 18

Data from Table 18 of auxiliary material

10.17182/hepdata.83660.v1/t75

Signal cutflows for the $T\bar T$ process in the case of 100$\%$ BR for $T \rightarrow Zt$ in the PP...

• #### Auxiliary Table 19

Data from Table 19 of auxiliary material

10.17182/hepdata.83660.v1/t76

Signal cutflows for the $B\bar B$ process in the case of 100$\%$ BR for $B \rightarrow Zb$ in the PP...

• #### Auxiliary Table 20

Data from Table 20 of auxiliary material

10.17182/hepdata.83660.v1/t77

Signal cutflows for single-$T$-quark production with $\kappa_T = 0.5$ in the SP $2\ell$ channel. Only statistical uncertainties are shown.

• #### Auxiliary Table 21

Data from Table 21 of auxiliary material

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Signal cutflows for single-$T$-quark production with $\kappa_T = 0.5$ in the SP $\geq 3\ell$ channel. Only statistical uncertainties are shown.

• #### Auxiliary Table 22

Data from Table 22 of auxiliary material

10.17182/hepdata.83660.v1/t79

Signal efficiencies in $\%$ in the PP $2\ell$ $0-1$J channel in the 0-large-$R$ jet SR. Uncertainties are statistical only.

• #### Auxiliary Table 23

Data from Table 23 of auxiliary material

10.17182/hepdata.83660.v1/t80

Signal efficiencies in $\%$ in the PP $2\ell$ $0-1$J channel in the 1-large-$R$ jet SR. Uncertainties are statistical only.

• #### Auxiliary Table 24

Data from Table 24 of auxiliary material

10.17182/hepdata.83660.v1/t81

Signal efficiencies in $\%$ in the PP $2\ell$ $\geq 2$J channel. Uncertainties are statistical only.

• #### Auxiliary Table 25

Data from Table 25 of auxiliary material

10.17182/hepdata.83660.v1/t82

Signal efficiencies in $\%$ in the PP $\geq 3\ell$ channel. Uncertainties are statistical only.

• #### Auxiliary Table 26

Data from Table 26 of auxiliary material

10.17182/hepdata.83660.v1/t83

Signal efficiencies in $\%$ in the SP $2\ell$ channel for $\kappa_T = 0.5$. Uncertainties are statistical only.

• #### Auxiliary Table 27

Data from Table 27 of auxiliary material

10.17182/hepdata.83660.v1/t84

Signal efficiencies in $\%$ in the SP $\geq 3\ell$ channel for $\kappa_T = 0.5$. Uncertainties are statistical only.

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