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CMS-SUS-16-050 ; CERN-EP-2017-257
Search for supersymmetry in proton-proton collisions at 13 TeV using identified top quarks
Phys. Rev. D 97 (2018) 012007
Abstract: A search for supersymmetry is presented based on proton-proton collision events containing identified hadronically decaying top quarks, no leptons, and an imbalance $ {p_{\mathrm{T}}}^{\text{miss}} $ in transverse momentum. The data were collected with the CMS detector at the CERN LHC at a center-of-mass energy of 13 TeV, and correspond to an integrated luminosity of 35.9 fb$^{-1}$. Search regions are defined in terms of the multiplicity of bottom quark jet and top quark candidates, the $ {p_{\mathrm{T}}}^{\text{miss}} $, the scalar sum of jet transverse momenta, and the $m_{\mathrm{T2}}$ mass variable. No statistically significant excess of events is observed relative to the expectation from the standard model. Lower limits on the masses of supersymmetric particles are determined at 95% confidence level in the context of simplified models with top quark production. For a model with direct top squark pair production followed by the decay of each top squark to a top quark and a neutralino, top squark masses up to 1020 GeV and neutralino masses up to 430 GeV are excluded. For a model with pair production of gluinos followed by the decay of each gluino to a top quark-antiquark pair and a neutralino, gluino masses up to 2040 GeV and neutralino masses up to 1150 GeV are excluded. These limits extend previous results.
Figures & Tables Summary Additional Figures & Tables References CMS Publications
Additional information on efficiencies needed for reinterpretation of these results are available here
Additional technical material for CMS speakers can be found here
Figures

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Figure 1:
Diagrams representing the simplified models of direct and gluino-mediated top squark production considered in this study: the T2tt model (top left), the T1tttt model (top right), the T1ttbb model (middle left), the T5tttt (middle right), and the T5ttcc model (bottom).

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Figure 1-a:
Diagram representing a simplified model of direct and gluino-mediated top squark production considered in this study: the T2tt model.

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Figure 1-b:
Diagram representing a simplified model of direct and gluino-mediated top squark production considered in this study: the T1tttt model.

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Figure 1-c:
Diagram representing a simplified model of direct and gluino-mediated top squark production considered in this study: the T1ttbb model.

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Figure 1-d:
Diagram representing a simplified model of direct and gluino-mediated top squark production considered in this study: the T5tttt model.

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Figure 1-e:
Diagram representing a simplified model of direct and gluino-mediated top squark production considered in this study: the T5ttcc model.

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Figure 2:
Efficiency of the top quark tagger as a function of generator-level top quark $ {p_{\mathrm {T}}} $ for the monojet (red boxes), dijet (magenta triangles), and trijet (green upside-down triangles) categories and for their combination (blue circles), as determined using T2tt signal events with a top squark mass of 850 GeV and an LSP mass of 100 GeV. The vertical bars indicate the statistical uncertainties.

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Figure 3:
Search region definitions in the kinematic variables. The highest $ {{p_{\mathrm {T}}} ^\text {miss}} $, $ {m_{\mathrm {T2}}} $, and $ {H_{\mathrm {T}}} $ regions are open-ended, e.g., $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 750 GeV and $ {m_{\mathrm {T2}}} > $ 750 GeV for search region 21.

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Figure 3-a:
Search region definitions in the kinematic variables.

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Figure 3-b:
Search region definitions in the kinematic variables.

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Figure 3-c:
Search region definitions in the kinematic variables.

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Figure 3-d:
Search region definitions in the kinematic variables.

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Figure 3-e:
Search region definitions in the kinematic variables.

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Figure 3-f:
Search region definitions in the kinematic variables.

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Figure 3-g:
Search region definitions in the kinematic variables.

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Figure 3-h:
Search region definitions in the kinematic variables.

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Figure 3-i:
Search region definitions in the kinematic variables.

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Figure 4:
Distribution of $ {{p_{\mathrm {T}}} ^\text {miss}} $ in the sideband data sample in comparison to predictions for SM processes. The prediction for $ {\mathrm{t} {}\mathrm{\bar{t}}} $, single top quark, and ${{{\mathrm{W}}\text {+jets}}}$ events is obtained using translation factors applied to a single-electron control sample (left) or to a single-muon control sample (right). The hatched bands indicate the statistical uncertainties in the total SM prediction. Note that the data and the predictions for all backgrounds except that for $ {\mathrm{t} {}\mathrm{\bar{t}}} $, single top quark, and W+jets events are identical between the left and right plots.

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Figure 4-a:
Distribution of $ {{p_{\mathrm {T}}} ^\text {miss}} $ in the sideband data sample in comparison to predictions for SM processes. The prediction for $ {\mathrm{t} {}\mathrm{\bar{t}}} $, single top quark, and ${{{\mathrm{W}}\text {+jets}}}$ events is obtained using translation factors applied to a single-electron control sample. The hatched bands indicate the statistical uncertainties in the total SM prediction. Note that the data and the predictions for all backgrounds except that for $ {\mathrm{t} {}\mathrm{\bar{t}}} $, single top quark, and W+jets events are identical between the left and right plots.

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Figure 4-b:
Distribution of $ {{p_{\mathrm {T}}} ^\text {miss}} $ in the sideband data sample in comparison to predictions for SM processes. The prediction for $ {\mathrm{t} {}\mathrm{\bar{t}}} $, single top quark, and ${{{\mathrm{W}}\text {+jets}}}$ events is obtained using translation factors applied to a single-muon control sample. The hatched bands indicate the statistical uncertainties in the total SM prediction. Note that the data and the predictions for all backgrounds except that for $ {\mathrm{t} {}\mathrm{\bar{t}}} $, single top quark, and W+jets events are identical between the left and right plots.

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Figure 5:
The $ {{p_{\mathrm {T}}} ^\text {miss}} $ (left) and ${N_{\mathrm{b}}}$ (right) distributions of data and simulation in the loose dimuon control sample after applying a correction, as described in the text, to account for differences between the data and simulation for the ${N_{\text {j}}}$ distribution. The lower panels show the ratio between data and simulation. Only statistical uncertainties are shown. The values in parentheses indicate the integrated yields for each component.

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Figure 5-a:
The $ {{p_{\mathrm {T}}} ^\text {miss}} $ distribution of data and simulation in the loose dimuon control sample after applying a correction, as described in the text, to account for differences between the data and simulation for the ${N_{\text {j}}}$ distribution. The lower panel shows the ratio between data and simulation. Only statistical uncertainties are shown. The values in parentheses indicate the integrated yields for each component.

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Figure 5-b:
The ${N_{\mathrm{b}}}$ distribution of data and simulation in the loose dimuon control sample after applying a correction, as described in the text, to account for differences between the data and simulation for the ${N_{\text {j}}}$ distribution. The lower panel shows the ratio between data and simulation. Only statistical uncertainties are shown. The values in parentheses indicate the integrated yields for each component.

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Figure 6:
Observed event yields (black points) and prefit SM background predictions (filled solid areas) for the 84 search regions, where "prefit'' means there is no constraint from the likelihood fit. The lower panel shows the ratio of the data to the total background prediction. The hatched bands correspond to the total uncertainty in the background prediction.

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Figure 7:
Observed event yields (black points) and prefit SM background predictions (filled solid areas) for the 10 aggregate search regions, where "prefit'' means there is no constraint from the likelihood fit. The lower panel shows the ratio of the data to the total background prediction. The hatched bands correspond to the total uncertainty in the background prediction.

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Figure 8:
The 95% CL upper limit on the production cross section of the T2tt simplified model as a function of the top squark and LSP masses. The solid black curves represent the observed exclusion contour with respect to NLO+NLL signal cross sections and the change in this contour due to variation of these cross sections within their theoretical uncertainties[54]. The dashed red curves indicate the mean expected exclusion contour and the region containing 68% of the distribution of expected exclusion limits under the background-only hypothesis. No interpretation is provided for signal models for which $|m_{\tilde{\mathrm{t}}} - m_{\tilde{\chi}^0_1} - m_{\mathrm{t}}| \le $ 25 GeV and $m_{\tilde{\mathrm{t}}} \leq $ 275 GeV because signal events are essentially indistinguishable from SM $ {\mathrm{t} {}\mathrm{\bar{t}}} $ events in this region, rendering the signal event acceptance difficult to model.

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Figure 9:
The 95% CL upper limit on the production cross section of the T1tttt (upper left), T1ttbb (upper right), T5tttt (bottom left), and T5ttcc (bottom right) simplified models as a function of the gluino and LSP masses. The meaning of the curves is explained in the Fig. 8 caption. Limits are not given for the T5tttt model for $ {m_{\tilde{\chi}^0_1}} < $ 50 GeV for the reason stated in the text.

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Figure 9-a:
The 95% CL upper limit on the production cross section of the T1tttt simplified models as a function of the gluino and LSP masses. The meaning of the curves is explained in the Fig. 8 caption. Limits are not given for the T5tttt model for $ {m_{\tilde{\chi}^0_1}} < $ 50 GeV for the reason stated in the text.

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Figure 9-b:
The 95% CL upper limit on the production cross section of the T1ttbb simplified models as a function of the gluino and LSP masses. The meaning of the curves is explained in the Fig. 8 caption. Limits are not given for the T5tttt model for $ {m_{\tilde{\chi}^0_1}} < $ 50 GeV for the reason stated in the text.

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Figure 9-c:
The 95% CL upper limit on the production cross section of the T5tttt simplified models as a function of the gluino and LSP masses. The meaning of the curves is explained in the Fig. 8 caption. Limits are not given for the T5tttt model for $ {m_{\tilde{\chi}^0_1}} < $ 50 GeV for the reason stated in the text.

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Figure 9-d:
The 95% CL upper limit on the production cross section of the T5ttcc simplified models as a function of the gluino and LSP masses. The meaning of the curves is explained in the Fig. 8 caption. Limits are not given for the T5tttt model for $ {m_{\tilde{\chi}^0_1}} < $ 50 GeV for the reason stated in the text.
Tables

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Table 1:
Definition of the aggregate search regions.

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Table 2:
The observed number of events and the total background prediction for search regions with $ {N_{\mathrm{t}}} =$ 1 and $ {N_{\mathrm{b}}} =$ 1. The first uncertainty in the background prediction is statistical and the second is systematic.

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Table 3:
The observed number of events and the total background prediction for search regions with $ {N_{\mathrm{t}}} =$ 1 and $ {N_{\mathrm{b}}} \geq $ 2. The first uncertainty in the background prediction is statistical and the second is systematic.

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Table 4:
The observed number of events and the total background prediction for search regions with $ {N_{\mathrm{t}}} \geq $ 2. The first uncertainty in the background prediction is statistical and the second is systematic.

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Table 5:
The observed number of events and the total background prediction for the aggregate search regions. The first uncertainty in the background prediction is statistical and the second is systematic.
Summary
Results are presented from a search for direct and gluino-mediated top squark production in proton-proton collisions at a center-of-mass energy of 13 TeV. The centerpiece of the analysis is a top quark tagging algorithm that identifies hadronically decaying top quarks with high efficiency across a wide range of top quark transverse momentum ${p_{\mathrm{T}}}$. The search is based on all-hadronic events with at least four jets, at least one tagged top quark, at least one tagged bottom quark jet, and a large imbalance in transverse momentum $ {p_{\mathrm{T}}}^{\text{miss}} $. The data correspond to an integrated luminosity of 35.9 fb$^{-1}$ collected with the CMS detector at the LHC in 2016. A set of 84 search regions is defined based on $ {p_{\mathrm{T}}}^{\text{miss}} $, the mass variable $ {m_{\mathrm{T2}}} $, the scalar ${p_{\mathrm{T}}} $ sum of jets $ {H_{\mathrm{T}}}$, the number of tagged top quarks, and the number of tagged bottom quark jets. No statistically significant excess of events is observed relative to the expectation from the standard model.

Cross section upper limits at 95% confidence level are evaluated for a simplified model of direct top squark pair production, in which the top squarks decay to a top quark and the lightest supersymmetric particle (LSP) neutralino, and for simplified models of gluino pair production, in which the gluinos decay to final states containing top quarks and LSPs. Using the signal cross sections calculated with next-to-leading-order plus next-to-leading-logarithm accuracy, 95% confidence level lower limits are set on the masses of the top squark, the gluino, and the LSP. For the model of direct top squark pair production, top squark masses up to 1020 GeV and LSP masses up to 430 GeV are excluded. For the models of gluino pair production, gluinos with masses as large as 1810 to 2040 GeV are excluded, depending on the model, with corresponding exclusions for LSPs with masses as large as 1100 to 1150 GeV. These results significantly extend those of our previous study [19]. The use of top quark tagging provides a novel means to search for new phenomena at the LHC, yielding complementary sensitivity to other approaches.
Additional Figures

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Additional Figure 1:
Pre-fit background covariance matrix.

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Additional Figure 2:
Pre-fit background correlation matrix.

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Additional Figure 3:
SMS model significance for the SMS model of direct top squark production, with $\tilde{\mathrm{t}} \to \text {t} \tilde{\chi}_1^0$ decays (T2tt).

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Additional Figure 4:
SMS model significance for the SMS model of gluino production, with $\tilde{\mathrm{g}} \to \text {t} \overline {\text {t}} \tilde{\chi}_1^0$ decays (T1tttt).

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Additional Figure 5:
SMS model significance for the SMS model of gluino production, with $\tilde{\mathrm{g}} \to \text {t} \tilde{\mathrm{t}}$, $\tilde{\mathrm{t}} \to \text {c} \tilde{\chi}_1^0$ decays (T5ttcc).

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Additional Figure 6:
SMS model significance for the SMS model of gluino production, with $\tilde{\mathrm{g}} \to \text {t} \overline {\text {b}} \tilde{\chi}_1^0$ decays (T1tttt).

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Additional Figure 7:
SMS model significance for the SMS model of gluino production, with $\tilde{\mathrm{g}} \to \text {t} \tilde{\mathrm{t}}$, $\tilde{\mathrm{t}} \to \text {t} \tilde{\chi}_1^0$ decays (T5tttt).
Additional Tables

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Additional Table 1:
The cut flow for a few benchmark signal models of direct top squark production. For entries in the block labeled "Preselection requirements'', each efficiency is computed with respect to the previous one. For the other two blocks, all efficiencies are computed with respect to the last line of the "Preselection requirements'' block.

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Additional Table 2:
The cut flow for a few benchmark signal models of gluino mediated top squark production. For entries in the block labeled "Preselection requirements'', each efficiency is computed with respect to the previous one. For the other two blocks, all efficiencies are computed with respect to the last line of the "Preselection requirements'' block.

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Additional Table 3:
Observed yields from the full 35.9 fb$^{-1}$ luminosity of data compared to our background predictions for all the aggregate search bins.

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Additional Table 4:
Selected T2tt signal yields for the aggregate search bins.

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Additional Table 5:
Selected gluino mediated signal yields for the aggregate search bins.
Our simplified tagger code can be found in this github link. It can also be downloaded directly from here. We recommend a 3% uncertainty per tagged top to cover possible difference in efficiency between the simplified tagger and the tagger used in our main analysis.

The simplified tagger uses the same framework as the full tagger described in the paper with the exception that the random forest (RF) decision tree used to identify trijet top candiates is retrained without using the quark-gluon likelihood values and replacing the combined-secondary-vertex (CSV) b-jet tagger discriminator with a binary discriminator (0 for not a b-jet, 1 for b-jet). This binary b-jet tagging discriminator uses the CMS CSVS ("medium") working point.

ROOT files with efficiency maps for each search region bin for some of the simplified models are provided in the following files:
- T2tt: acc_maps_T2tt.root
- T1tttt: acc_maps_T1tttt.root
- T5ttcc: acc_maps_T5ttcc.root
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