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Search for top squarks in final states with one isolated lepton, jets, and missing transverse momentum in $\sqrt{s}=13$ TeV $pp$ collisions with the ATLAS detector

The ATLAS collaboration
Phys.Rev. D94 (2016) 052009, 2016

Abstract (data abstract)
CERN-LHC. The results of a search for the stop, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, jets, and missing transverse momentum are reported. The search uses the 2015 LHC $pp$ collision data at a center-of-mass energy of $\sqrt{s}=13$ TeV recorded by the ATLAS detector and corresponding to an integrated luminosity of 3.2 fb${}^{-1}$. The analysis targets two types of signal models: gluino-mediated pair production of stops with a nearly mass-degenerate stop and neutralino; and direct pair production of stops, decaying to the top quark and the lightest neutralino. The experimental signature in both signal scenarios is similar to that of a top quark pair produced in association with large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits on gluino and stop masses are set at 95% confidence level. The results extend the LHC Run-1 exclusion limit on the gluino mass up to 1460 GeV in the gluino-mediated scenario in the high gluino and low stop mass region, and add an excluded stop mass region from 745 to 780 GeV for the direct stop model with a massless lightest neutralino. The results are also reinterpreted to set exclusion limits in a model of vector-like top quarks.

  • Table 1

    Data from Figure 3a

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    Comparison of data with estimated backgrounds in the $am_\text{T2}$ distribution with the STCR1 event selection except for the requirement on...

  • Table 2

    Data from Figure 3b

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    Comparison of data with estimated backgrounds in the $b$-tagged jet multiplicity with the STCR1 event selection except for the requirement...

  • Table 3

    Data from Figure 3c

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    Comparison of data with estimated backgrounds in the $\Delta R(b_1,b_2)$ distribution with the STCR1 event selection except for the requirement...

  • Table 4

    Data from Figure 4a

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    Comparison of data with estimated backgrounds in the $\tilde{E}_\text{T}^\text{miss}$ distribution with the TZCR1 event selection except for the requirement on...

  • Table 5

    Data from Figure 4b

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    Comparison of data with estimated backgrounds in the $\tilde{m}_\text{T}$ distribution with the TZCR1 event selection except for the requirement on...

  • Table 6

    Data from Figure 5

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    Comparison of the observed data ($n_\text{obs}$) with the predicted background ($n_\text{exp}$) in the validation and signal regions. The background predictions...

  • Table 7

    Data from Figure 6a

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    Jet multiplicity distributions for events where exactly two signal leptons are selected. No correction factors are included in the background...

  • Table 8

    Data from Figure 6b

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    Jet multiplicity distributions for events where exactly one lepton plus one $\tau$ candidate are selected. No correction factors are included...

  • Table 9

    Data from Figure 7a

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    The $E_\text{T}^\text{miss}$ distribution in SR1. In the plot, the full event selection in the corresponding signal region is applied, except...

  • Table 10

    Data from Figure 7b

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    The $m_\text{T}$ distribution in SR1. In the plot, the full event selection in the corresponding signal region is applied, except...

  • Table 11

    Data from Figure 8a

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    Expected (black dashed) 95% excluded regions in the plane of $m_{\tilde{g}}$ versus $m_{\tilde{t}_1}$ for gluino-mediated stop production.

  • Table 12

    Data from Figure 8a

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    Observed (red solid) 95% excluded regions in the plane of $m_{\tilde{g}}$ versus $m_{\tilde{t}_1}$ for gluino-mediated stop production.

  • Table 13

    Data from Figure 8b

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    Expected (black dashed) 95% excluded regions in the plane of $m_{\tilde{t}_1}$ versus $m_{\tilde{\chi}_1^0}$ for direct stop production.

  • Table 14

    Data from Figure 8b

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    Observed (red solid) 95% excluded regions in the plane of $m_{\tilde{t}_1}$ versus $m_{\tilde{\chi}_1^0}$ for direct stop production.

  • Table 15

    Data from Figure 9

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    The expected upper limits on $T$ quark pair production times the squared branching ratio for $T \rightarrow tZ$ as a...

  • Table 16

    Data from Figure 9

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    The observed upper limits on $T$ quark pair production times the squared branching ratio for $T \rightarrow tZ$ as a...

  • Table 17

    Data from Auxiliary Figure 1

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    The expected limits on $T$ quarks as a function of the branching ratios $B\left(T \rightarrow bW\right)$ and $B\left(T \rightarrow tH\right)$...

  • Table 18

    Data from Auxiliary Figure 1

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    The observed limits on $T$ quarks as a function of the branching ratios $B\left(T \rightarrow bW\right)$ and $B\left(T \rightarrow tH\right)$...

  • Table 19

    Data from Auxiliary Figure 2a

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    The $m_\text{T}$ distribution in the WVR2-tail validation region which has the same preselection and jet $p_\text{T}$ requirements as SR2.

  • Table 20

    Data from Auxiliary Figure 2b

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    The $am_\text{T2}$ distribution in the WVR2-tail validation region which has the same preselection and jet $p_\text{T}$ requirements as SR2.

  • Table 21

    Data from Auxiliary Figure 3a

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    Large-radius jet mass ($R=1.2$), decomposed into the number of small-radius jet constituents. The lower panel shows the ratio of the...

  • Table 22

    Data from Auxiliary Figure 3b

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    Distribution of $m_\text{T2}^\tau$ in data for a selection enriched in $t\bar{t}$ events with one hadronically decaying $\tau$. Events that have...

  • Table 23

    Data from Auxiliary Figure 5a

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    Upper limits on the model cross-section in units of pb for the gluino-mediated stop models.

  • Table 24

    Data from Auxiliary Figure 5b

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    Upper limits on the model cross-section in units of pb for the models with direct stop pair production.

  • Table 25

    Data from Auxiliary Figure 6a

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    Illustration of the best expected signal region per signal grid point for the gluino-mediated stop models. This mapping is used...

  • Table 26

    Data from Auxiliary Figure 6b

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    Illustration of the best expected signal region per signal grid point for models with direct stop pair production. This mapping...

  • Table 27

    Data from Auxiliary Figure 7a

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    Expected $CL_s$ values for the gluino-mediated stop models.

  • Table 28

    Data from Auxiliary Figure 7b

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    Observed $CL_s$ values for the gluino-mediated stop models.

  • Table 29

    Data from Auxiliary Figure 7c

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    Expected $CL_s$ values for the direct stop pair production models.

  • Table 30

    Data from Auxiliary Figure 7d

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    Observed $CL_s$ values for the direct stop pair production models.

  • Table 31

    Data from Auxiliary Figure 8a

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    Expected limit using SR1 for models with direct stop pair production and an unpolarized stop (and bino LSP).

  • Table 32

    Data from Auxiliary Figure 8a

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    Expected limit using SR1 for models with direct stop pair production with $\tilde{t}_1=\tilde{t}_L$ (and bino LSP).

  • Table 33

    Data from Auxiliary Figure 8a

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    Expected limit using SR1 for models with direct stop pair production with $\tilde{t}_1\sim\tilde{t}_R$ (and bino LSP).

  • Table 34

    Data from Auxiliary Figure 8a

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    Observed limit using SR1 for models with direct stop pair production and an unpolarized stop (and bino LSP).

  • Table 35

    Data from Auxiliary Figure 8a

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    Observed limit using SR1 for models with direct stop pair production with $\tilde{t}_1=\tilde{t}_L$ (and bino LSP).

  • Table 36

    Data from Auxiliary Figure 8a

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    Observed limit using SR1 for models with direct stop pair production with $\tilde{t}_1\sim\tilde{t}_R$ (and bino LSP).

  • Table 37

    Data from Auxiliary Figure 8b

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    Expected limit using SR2 for models with direct stop pair production and an unpolarized stop (and bino LSP).

  • Table 38

    Data from Auxiliary Figure 8b

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    Expected limit using SR2 for models with direct stop pair production with $\tilde{t}_1=\tilde{t}_L$ (and bino LSP).

  • Table 39

    Data from Auxiliary Figure 8b

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    Expected limit using SR2 for models with direct stop pair production with $\tilde{t}_1\sim\tilde{t}_R$ (and bino LSP).

  • Table 40

    Data from Auxiliary Figure 8b

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    Observed limit using SR2 for models with direct stop pair production and an unpolarized stop (and bino LSP).

  • Table 41

    Data from Auxiliary Figure 8b

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    Observed limit using SR2 for models with direct stop pair production with $\tilde{t}_1=\tilde{t}_L$ (and bino LSP).

  • Table 42

    Data from Auxiliary Figure 8b

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    Observed limit using SR2 for models with direct stop pair production with $\tilde{t}_1\sim\tilde{t}_R$ (and bino LSP).

  • Table 43

    Data from Auxiliary Figure 8c

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    Expected limit using SR1+SR2 (best expected) for models with direct stop pair production and an unpolarized stop (and bino LSP).

  • Table 44

    Data from Auxiliary Figure 8c

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    Expected limit using SR1+SR2 (best expected) for models with direct stop pair production with $\tilde{t}_1=\tilde{t}_L$ (and bino LSP).

  • Table 45

    Data from Auxiliary Figure 8c

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    Expected limit using SR1+SR2 (best expected) for models with direct stop pair production with $\tilde{t}_1\sim\tilde{t}_R$ (and bino LSP).

  • Table 46

    Data from Auxiliary Figure 8c

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    Observed limit using SR1+SR2 (best expected) for models with direct stop pair production and an unpolarized stop (and bino LSP).

  • Table 47

    Data from Auxiliary Figure 8c

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    Observed limit using SR1+SR2 (best expected) for models with direct stop pair production with $\tilde{t}_1=\tilde{t}_L$ (and bino LSP).

  • Table 48

    Data from Auxiliary Figure 8c

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    Observed limit using SR1+SR2 (best expected) for models with direct stop pair production with $\tilde{t}_1\sim\tilde{t}_R$ (and bino LSP).

  • Table 49

    Data from Auxiliary Figure 9a

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    Acceptance for SR1 in the gluino-mediated stop models. The acceptance is defined as the fraction of signal events that pass...

  • Table 50

    Data from Auxiliary Figure 9b

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    Acceptance for SR1 in the direct stop pair production. The acceptance is defined as the fraction of signal events that...

  • Table 51

    Data from Auxiliary Figure 10a

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    Acceptance for SR2 in the gluino-mediated stop models. The acceptance is defined as the fraction of signal events that pass...

  • Table 52

    Data from Auxiliary Figure 10b

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    Acceptance for SR2 in the direct stop pair production. The acceptance is defined as the fraction of signal events that...

  • Table 53

    Data from Auxiliary Figure 11a

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    Acceptance for SR3 in the gluino-mediated stop models. The acceptance is defined as the fraction of signal events that pass...

  • Table 54

    Data from Auxiliary Figure 11b

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    Acceptance for SR3 in the direct stop pair production. The acceptance is defined as the fraction of signal events that...

  • Table 55

    Data from Auxiliary Figure 12a

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    Efficiency for SR1 in the gluino-mediated stop models. The efficiency is the ratio between the expected signal rate calculated with...

  • Table 56

    Data from Auxiliary Figure 12b

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    Efficiency for SR1 in the direct stop pair production. The efficiency is the ratio between the expected signal rate calculated...

  • Table 57

    Data from Auxiliary Figure 13a

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    Efficiency for SR2 in the gluino-mediated stop models. The efficiency is the ratio between the expected signal rate calculated with...

  • Table 58

    Data from Auxiliary Figure 13b

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    Efficiency for SR2 in the direct stop pair production. The efficiency is the ratio between the expected signal rate calculated...

  • Table 59

    Data from Auxiliary Figure 14a

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    Efficiency for SR3 in the gluino-mediated stop models. The efficiency is the ratio between the expected signal rate calculated with...

  • Table 60

    Data from Auxiliary Figure 14b

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    Efficiency for SR3 in the direct stop pair production. The efficiency is the ratio between the expected signal rate calculated...

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