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| CMS-PAS-SUS-13-023 | ||
| A Search for Scalar Top Quark Production and Decay to All Hadronic Final States in pp Collisions at $\sqrt{s} =$ 8 TeV | ||
| CMS Collaboration | ||
| August 2015 | ||
| Abstract: A search for scalar top quarks in final states with jets and missing transverse momentum is presented. The data sample of proton--proton collisions used corresponds to an integrated luminosity of 18.9 fb$^{-1}$ collected at $\sqrt{s} =$ 8 TeV with the CMS detector at the LHC. The search features novel background suppression and prediction methods, including a dedicated top-quark pair reconstruction algorithm. The data are found to be in agreement with the predicted backgrounds. Exclusion limits are set in simplified SUSY models with the top squark decaying to jets and an undetected neutralino, either through an on-shell top quark or through an intermediate chargino. Models with the top squark decaying via an on-shell top quark are excluded for top squark masses up to 755 GeV in the case of neutralino masses below 200 GeV. Models with an intermediate chargino are excluded in some scenarios for top squark masses up to 650 GeV. | ||
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Links:
CDS record (PDF) ;
Public twiki page ;
CADI line (restricted) ; Figures are also available from the CDS record. These preliminary results are superseded in this paper, EPJC 76 (2016) 460. |
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| Figures | |
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Figure 1-a:
Diagrams representing the two simplified models of direct top squark pair production considered in this study: T2tt with top squark decay via an on-shell top quark (a) and T2bW with top squark decay via an intermediate chargino (b). |
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Figure 1-b:
Diagrams representing the two simplified models of direct top squark pair production considered in this study: T2tt with top squark decay via an on-shell top quark (a) and T2bW with top squark decay via an intermediate chargino (b). |
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Figure 2-a:
a: The efficiency to select non-prompt versus the efficiency to select prompt candidates. The curves are produced by varying BDT discriminator values for electrons, muons, and taus. Prompt leptons are those matched to lepton candidates in semi-leptonic $ {\mathrm{ t \bar{t} }} $ events, whereas non-prompt leptons are those that are matched to lepton candidates in fully hadronic $ {\mathrm{ t \bar{t} }} $ in the case of electrons and muons, or fully hadronic T2tt signal events in the case of taus. Right: $ {m_{\mathrm {T}}} $ of $ {E}_{\mathrm{T}}^{\text{miss}} $ and the momentum of the visible tau decay products, for tau candidates matched to tau decays in semi-leptonic $ {\mathrm{ t \bar{t} }} $ events, and all tau candidates in a fully hadronic T2tt signal (MStop = 620, MLSP = 40). |
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Figure 2-b:
a: The efficiency to select non-prompt versus the efficiency to select prompt candidates. The curves are produced by varying BDT discriminator values for electrons, muons, and taus. Prompt leptons are those matched to lepton candidates in semi-leptonic $ {\mathrm{ t \bar{t} }} $ events, whereas non-prompt leptons are those that are matched to lepton candidates in fully hadronic $ {\mathrm{ t \bar{t} }} $ in the case of electrons and muons, or fully hadronic T2tt signal events in the case of taus. Right: $ {m_{\mathrm {T}}} $ of $ {E}_{\mathrm{T}}^{\text{miss}} $ and the momentum of the visible tau decay products, for tau candidates matched to tau decays in semi-leptonic $ {\mathrm{ t \bar{t} }} $ events, and all tau candidates in a fully hadronic T2tt signal (MStop = 620, MLSP = 40). |
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Figure 3-a:
The distributions of properties of reconstructed top pairs are shown for data together with signal and background MC data samples for three choices of MStop and MLSP after the baseline selection. The left plot shows the minimum angular separation between any two jets in the leading reconstructed top, defined as the top candidate of the pair with the highest discriminator value, while the right plot shows the minimum separation in $\phi $ between each jet and $ {E}_{\mathrm{T}}^{\text{miss}} $ in the sub-leading reconstructed top. |
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Figure 3-b:
The distributions of properties of reconstructed top pairs are shown for data together with signal and background MC data samples for three choices of MStop and MLSP after the baseline selection. The left plot shows the minimum angular separation between any two jets in the leading reconstructed top, defined as the top candidate of the pair with the highest discriminator value, while the right plot shows the minimum separation in $\phi $ between each jet and $ {E}_{\mathrm{T}}^{\text{miss}} $ in the sub-leading reconstructed top. |
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Figure 4-a:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed. Results are plotted as a function of the MVA discriminator output for the five T2bW validation regions. ``Data'' is the ratio of event yields in data to event yields in MC. The MC events have been reweighted. The last three bins along the abscissa in each plot contain the ratios for the two extended validation regions and the validation region itself. The histogram labelled ``MC without corr." plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC as used for the ``Data" histogram. The other histograms provide the relative contributions of the various background processes. |
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Figure 4-b:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed. Results are plotted as a function of the MVA discriminator output for the five T2bW validation regions. ``Data'' is the ratio of event yields in data to event yields in MC. The MC events have been reweighted. The last three bins along the abscissa in each plot contain the ratios for the two extended validation regions and the validation region itself. The histogram labelled ``MC without corr." plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC as used for the ``Data" histogram. The other histograms provide the relative contributions of the various background processes. |
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Figure 4-c:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed. Results are plotted as a function of the MVA discriminator output for the five T2bW validation regions. ``Data'' is the ratio of event yields in data to event yields in MC. The MC events have been reweighted. The last three bins along the abscissa in each plot contain the ratios for the two extended validation regions and the validation region itself. The histogram labelled ``MC without corr." plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC as used for the ``Data" histogram. The other histograms provide the relative contributions of the various background processes. |
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Figure 4-d:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed. Results are plotted as a function of the MVA discriminator output for the five T2bW validation regions. ``Data'' is the ratio of event yields in data to event yields in MC. The MC events have been reweighted. The last three bins along the abscissa in each plot contain the ratios for the two extended validation regions and the validation region itself. The histogram labelled ``MC without corr." plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC as used for the ``Data" histogram. The other histograms provide the relative contributions of the various background processes. |
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Figure 4-e:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed. Results are plotted as a function of the MVA discriminator output for the five T2bW validation regions. ``Data'' is the ratio of event yields in data to event yields in MC. The MC events have been reweighted. The last three bins along the abscissa in each plot contain the ratios for the two extended validation regions and the validation region itself. The histogram labelled ``MC without corr." plots the ratio whose numerator is the total MC event count before corrections and whose denominator is the event count for the corrected MC as used for the ``Data" histogram. The other histograms provide the relative contributions of the various background processes. |
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Figure 5-a:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 5-b:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 5-c:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 5-d:
Comparisons of BDT discriminator outputs for data and corrected MC for $ {\mathrm{ t \bar{t} }} $ and W+jets single-lepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 6-a:
Comparisons of BDT discriminator outputs for data and corrected MC for the Z+jets dilepton control samples, with leptons removed, for the five T2bW validation regions. All quantities are defined as in Fig. 4. |
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Figure 6-b:
Comparisons of BDT discriminator outputs for data and corrected MC for the Z+jets dilepton control samples, with leptons removed, for the five T2bW validation regions. All quantities are defined as in Fig. 4. |
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Figure 6-c:
Comparisons of BDT discriminator outputs for data and corrected MC for the Z+jets dilepton control samples, with leptons removed, for the five T2bW validation regions. All quantities are defined as in Fig. 4. |
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Figure 6-d:
Comparisons of BDT discriminator outputs for data and corrected MC for the Z+jets dilepton control samples, with leptons removed, for the five T2bW validation regions. All quantities are defined as in Fig. 4. |
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Figure 6-e:
Comparisons of BDT discriminator outputs for data and corrected MC for the Z+jets dilepton control samples, with leptons removed, for the five T2bW validation regions. All quantities are defined as in Fig. 4. |
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Figure 7-a:
Comparisons of BDT discriminator outputs for data and corrected MC in the Z+jets dilepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 7-b:
Comparisons of BDT discriminator outputs for data and corrected MC in the Z+jets dilepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 7-c:
Comparisons of BDT discriminator outputs for data and corrected MC in the Z+jets dilepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 7-d:
Comparisons of BDT discriminator outputs for data and corrected MC in the Z+jets dilepton control samples, with leptons removed, for the four T2tt validation regions. All quantities are defined as in Fig. 4. |
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Figure 8-a:
Observed and predicted event yields for each T2bW search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 8-b:
Observed and predicted event yields for each T2bW search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 8-c:
Observed and predicted event yields for each T2bW search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 8-d:
Observed and predicted event yields for each T2bW search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 8-e:
Observed and predicted event yields for each T2bW search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 9-a:
Observed and predicted event yields for each T2tt search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 9-b:
Observed and predicted event yields for each T2tt search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 9-c:
Observed and predicted event yields for each T2tt search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 9-d:
Observed and predicted event yields for each T2tt search region discriminator. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 10-a:
Observed and predicted event yields for each T2bW search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. The blue error band is the relative systematic error in the prediction yield. |
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Figure 10-b:
Observed and predicted event yields for each T2bW search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. The blue error band is the relative systematic error in the prediction yield. |
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Figure 10-c:
Observed and predicted event yields for each T2bW search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. The blue error band is the relative systematic error in the prediction yield. |
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Figure 10-d:
Observed and predicted event yields for each T2bW search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. The blue error band is the relative systematic error in the prediction yield. |
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Figure 10-e:
Observed and predicted event yields for each T2bW search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC event yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. The blue error band is the relative systematic error in the prediction yield. |
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Figure 11-a:
Observed and predicted event yields for each T2tt search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 11-b:
Observed and predicted event yields for each T2tt search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 11-c:
Observed and predicted event yields for each T2tt search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 11-d:
Observed and predicted event yields for each T2tt search region discriminator before lepton vetoes are applied. The bottom pane of each figure shows the ratio of observed to predicted yields where the error bars on data points only include the statistical uncertainties in the data and MC yields. The transparent violet bands represent the relative systematic uncertainties in the predictions. |
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Figure 12-a:
Search regions providing the most stringent limits in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane in T2tt (a) and T2bW signal topologies (for mass splitting parameter values $x$ = 0.25, 0.50, 0.75). For T2tt, T2tt+LM, T2tt+MM, T2tt+HM and T2tt+VHM search regions are number 1, 2, 3 and 4, respectively. In T2bW the T2bW+LX, T2bW+LM, T2bW+MXHM, T2bW+VHM, and T2bW+HXHM search regions are numbered 1, 2, 3, 4 and 5 respectively. In regions with $m_{ {\tilde{\chi}^{0}_{1}} }$ similar to $m_{ {\tilde{t}} }$, the different search regions can have similar sensitivity which leads to the fluctuations seen among neighboring bins. |
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Figure 12-b:
Search regions providing the most stringent limits in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane in T2tt (a) and T2bW signal topologies (for mass splitting parameter values $x$ = 0.25, 0.50, 0.75). For T2tt, T2tt+LM, T2tt+MM, T2tt+HM and T2tt+VHM search regions are number 1, 2, 3 and 4, respectively. In T2bW the T2bW+LX, T2bW+LM, T2bW+MXHM, T2bW+VHM, and T2bW+HXHM search regions are numbered 1, 2, 3, 4 and 5 respectively. In regions with $m_{ {\tilde{\chi}^{0}_{1}} }$ similar to $m_{ {\tilde{t}} }$, the different search regions can have similar sensitivity which leads to the fluctuations seen among neighboring bins. |
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Figure 12-c:
Search regions providing the most stringent limits in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane in T2tt (a) and T2bW signal topologies (for mass splitting parameter values $x$ = 0.25, 0.50, 0.75). For T2tt, T2tt+LM, T2tt+MM, T2tt+HM and T2tt+VHM search regions are number 1, 2, 3 and 4, respectively. In T2bW the T2bW+LX, T2bW+LM, T2bW+MXHM, T2bW+VHM, and T2bW+HXHM search regions are numbered 1, 2, 3, 4 and 5 respectively. In regions with $m_{ {\tilde{\chi}^{0}_{1}} }$ similar to $m_{ {\tilde{t}} }$, the different search regions can have similar sensitivity which leads to the fluctuations seen among neighboring bins. |
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Figure 12-d:
Search regions providing the most stringent limits in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane in T2tt (a) and T2bW signal topologies (for mass splitting parameter values $x$ = 0.25, 0.50, 0.75). For T2tt, T2tt+LM, T2tt+MM, T2tt+HM and T2tt+VHM search regions are number 1, 2, 3 and 4, respectively. In T2bW the T2bW+LX, T2bW+LM, T2bW+MXHM, T2bW+VHM, and T2bW+HXHM search regions are numbered 1, 2, 3, 4 and 5 respectively. In regions with $m_{ {\tilde{\chi}^{0}_{1}} }$ similar to $m_{ {\tilde{t}} }$, the different search regions can have similar sensitivity which leads to the fluctuations seen among neighboring bins. |
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Figure 13-a:
Observed and expected 95% CL limit on the $ {\tilde{t}} {\overline {\tilde{t}}} $ production cross section and exclusion areas in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane for the T2tt (a) and T2bW signal topologies (with $x$=0.25, 0.50, 0.75). |
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Figure 13-b:
Observed and expected 95% CL limit on the $ {\tilde{t}} {\overline {\tilde{t}}} $ production cross section and exclusion areas in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane for the T2tt (a) and T2bW signal topologies (with $x$=0.25, 0.50, 0.75). |
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Figure 13-c:
Observed and expected 95% CL limit on the $ {\tilde{t}} {\overline {\tilde{t}}} $ production cross section and exclusion areas in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane for the T2tt (a) and T2bW signal topologies (with $x$=0.25, 0.50, 0.75). |
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Figure 13-d:
Observed and expected 95% CL limit on the $ {\tilde{t}} {\overline {\tilde{t}}} $ production cross section and exclusion areas in the $m_{ {\tilde{t}} }-m_{ {\tilde{\chi}^{0}_{1}} }$ plane for the T2tt (a) and T2bW signal topologies (with $x$=0.25, 0.50, 0.75). |
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