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CMS-PAS-SUS-16-050
Search for supersymmetry using hadronic top quark tagging in 13 TeV pp collisions
Abstract: A search is presented for supersymmetry in all-hadronic events with missing transverse momentum and tagged top quarks. The data sample was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV. Search regions are defined using the multiplicity of bottom and top quark candidates, the presence of an imbalance in transverse momentum, and the hadronic energy in the event. With no statistically significant excess of events observed beyond the expected contributions from the standard model, we set exclusion limits at the 95% confidence level on the masses of new particles in the context of simplified models of direct and gluino-mediated top squark production. For direct top squark production with decays to a top quark and a neutralino, top squark masses up to 1020 GeV and neutralino masses up to 430 GeV are excluded. Gluino masses up to 2040 GeV and neutralino masses up to 1150 GeV are excluded for models of gluino pair production where each gluino decays to a top-antitop quark pair and a neutralino.
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) with top squark decay via a top quark, and the T1tttt model (top right) where the gluino decays to top quarks and the LSP via an off-shell top squark, and the T5ttcc model (bottom) where the gluino decays to an on-shell top squark, which decays to a charm quark and the LSP.

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Figure 1-a:
Diagram representing one of the simplified models of direct and gluino-mediated top squark production considered in this study: the T2tt model, with top squark decay via a top quark.

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Figure 1-b:
Diagram representing one of the simplified models of direct and gluino-mediated top squark production considered in this study: the T1tttt model, where the gluino decays to top quarks and the LSP via an off-shell top squark.

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Figure 1-c:
Diagram representing one of the simplified models of direct and gluino-mediated top squark production considered in this study: the T5ttcc model, where the gluino decays to an on-shell top squark, which decays to a charm quark and the LSP.

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Figure 2:
The tagging efficiency of the top quark tagger as a function of the generator-level hadronically decaying top quark $ {p_{\mathrm {T}}} $. The efficiencies of the monojet (red boxes), dijet (magenta upper-triangles), and trijet (green lower-triangles) categories are shown together with the efficiency of their combination (blue circles). The efficiency is computed using the T2tt signal model with $m_{\tilde{ \mathrm{ t } } } = $ 850 GeV and $m_{\tilde{\chi}^0_1 } = $ 100 GeV, and it is similar for ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ events. The vertical bars depict the statistical uncertainty.

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Figure 3:
Search region definitions for bin numbers 0-83. The highest ${E_{\mathrm {T}}^{\text {miss}}}$ and ${M_{\mathrm {T2}}} / {H_{\mathrm {T}}}$ bins are open-ended, e.g., bin 20 requires $ {E_{\mathrm {T}}^{\text {miss}}} > $ 750 GeV and $ {M_{\mathrm {T2}}} > $ 750 GeV.

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Figure 3-a:
Search region definitions for bin numbers 0-20.

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Figure 3-b:
Search region definitions for bin numbers 21-36.

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Figure 3-c:
Search region definitions for bin numbers 37-47.

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Figure 3-d:
Search region definitions for bin numbers 48-57.

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Figure 3-e:
Search region definitions for bin numbers 58-67.

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Figure 3-f:
Search region definitions for bin numbers 70-77.

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Figure 3-g:
Search region definitions for bin numbers 78 and 79.

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Figure 3-h:
Search region definitions for bin numbers 80 and 81.

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Figure 3-i:
Search region definitions for bin numbers 82 and 83.

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Figure 4:
Validation of the translation factor method in the control sample selected by requiring $ {N_{\mathrm{ t } }} = $ 0 and $ {N_{\mathrm{ b } }} \geq $ 2 from the muon channel (left) and electron channel (right). The black points are observed data. The light blue histogram shows the prediction from ${\mathrm{ t } {}\mathrm{ \bar{t} } } $, ${{\mathrm{ W } }\text {+jets}} $, and single top events using the translation factor method. All other backgrounds come directly from simulated event yields. The shaded area includes only statistical uncertainties in the background estimate.

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Figure 4-a:
Validation of the translation factor method in the control sample selected by requiring $ {N_{\mathrm{ t } }} = $ 0 and $ {N_{\mathrm{ b } }} \geq $ 2 from the muon channel. The black points are observed data. The light blue histogram shows the prediction from ${\mathrm{ t } {}\mathrm{ \bar{t} } } $, ${{\mathrm{ W } }\text {+jets}} $, and single top events using the translation factor method. All other backgrounds come directly from simulated event yields. The shaded area includes only statistical uncertainties in the background estimate.

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Figure 4-b:
Validation of the translation factor method in the control sample selected by requiring $ {N_{\mathrm{ t } }} = $ 0 and $ {N_{\mathrm{ b } }} \geq $ 2 from the electron channel. The black points are observed data. The light blue histogram shows the prediction from ${\mathrm{ t } {}\mathrm{ \bar{t} } } $, ${{\mathrm{ W } }\text {+jets}} $, and single top events using the translation factor method. All other backgrounds come directly from simulated event yields. The shaded area includes only statistical uncertainties in the background estimate.

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Figure 5:
The ${N_{\mathrm{ b } }}$ (left) and $ {E_{\mathrm {T}}^{\text {miss}}} $ (right) distributions from data and simulation in the loose $\mathrm{ Z } \rightarrow \mu \mu $ control region, after applying the $S_\text {DY}(N_\textrm {j})$ scale factor to the simulation. The lower panels show the ratio between data and simulation. Only statistical uncertainties are shown. The values in parentheses in the legend indicate the integrated yield for each given process.

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Figure 5-a:
The ${N_{\mathrm{ b } }}$ distribution from data and simulation in the loose $\mathrm{ Z } \rightarrow \mu \mu $ control region, after applying the $S_\text {DY}(N_\textrm {j})$ scale factor to the simulation. The lower panel shows the ratio between data and simulation. Only statistical uncertainties are shown. The values in parentheses in the legend indicate the integrated yield for each given process.

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Figure 5-b:
The $ {E_{\mathrm {T}}^{\text {miss}}} $ distribution from data and simulation in the loose $\mathrm{ Z } \rightarrow \mu \mu $ control region, after applying the $S_\text {DY}(N_\textrm {j})$ scale factor to the simulation. The lower panel shows the ratio between data and simulation. Only statistical uncertainties are shown. The values in parentheses in the legend indicate the integrated yield for each given process.

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Figure 6:
Observed event yields in data (black points) and predicted SM background (filled solid area) for the 84 search bins. The lower panel shows the ratio of data over total background prediction in each search bin. For both panels, the error bars show the statistical uncertainty associated with the observed data counts, and the grey (blue) hatched bands indicate the statistical (systematic) uncertainties in the total predicted background.

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Figure 7:
Exclusion limit at 95% CL for the simplified model of direct top squark pair production with $\tilde{ \mathrm{ t } } \rightarrow \mathrm{ t } \tilde{\chi}^0_1 $ decays (T2tt model). The solid black curves represent the observed exclusion contour with respect to NLO+NLL signal cross section calculations [51] and the corresponding $\pm$1 standard deviation uncertainties. The dashed red curves indicate the expected exclusion contour and the $\pm$1 standard deviation uncertainties including experimental uncertainties. 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 of significant differences between the fast simulation and the GEANT4-based simulation for these low-$ {E_{\mathrm {T}}^{\text {miss}}} $ scenarios.

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Figure 8:
Exclusion limit at 95% CL for the simplified model of top squarks produced via decays of gluino pairs in the T1tttt (left) and T5ttcc (right) scenarios. The solid black curves represent the observed exclusion contour with respect to NLO+NLL signal cross section calculations [51] and the corresponding $\pm$1 standard deviation uncertainties. The dashed red curves indicate the expected exclusion contour and the $\pm$1 standard deviation uncertainties including experimental uncertainties.

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Figure 8-a:
Exclusion limit at 95% CL for the simplified model of top squarks produced via decays of gluino pairs in the T1tttt scenario. The solid black curves represent the observed exclusion contour with respect to NLO+NLL signal cross section calculations [51] and the corresponding $\pm$1 standard deviation uncertainties. The dashed red curves indicate the expected exclusion contour and the $\pm$1 standard deviation uncertainties including experimental uncertainties.

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Figure 8-b:
Exclusion limit at 95% CL for the simplified model of top squarks produced via decays of gluino pairs in the T5ttcc scenario. The solid black curves represent the observed exclusion contour with respect to NLO+NLL signal cross section calculations [51] and the corresponding $\pm$1 standard deviation uncertainties. The dashed red curves indicate the expected exclusion contour and the $\pm$1 standard deviation uncertainties including experimental uncertainties.
Tables

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Table 1:
The observed yields from data compared to the total background predictions for the first 48 search bins. The quoted uncertainties on the predicted background yields are statistical and systematic, respectively.

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Table 2:
The observed yields from data compared to the total background predictions for the remaining search bins. The quoted uncertainties on the predicted background yields are statistical and systematic, respectively.
Summary
Results have been presented from a search for direct and gluino-mediated top squark production in final states that include tagged top quark decays. The search uses all-hadronic events with at least four jets and a large imbalance in transverse momentum ($E_{\mathrm{T}}^{\text{miss}}$), selected from data corresponding to an integrated luminosity of 35.9 fb$^{-1}$ collected in proton-proton collisions at a center-of-mass energy of 13 TeV with the CMS detector. A set of search regions is defined based on $E_{\mathrm{T}}^{\text{miss}}$, ${m_{\mathrm{T2}}}$, $H_{\mathrm{T}}$, the number of top quark tagged objects, and the number of $\mathrm{b }$-tagged jets. No statistically significant excess of events is observed above the expected standard model background. Exclusion limits are set at 95% confidence level for simplified models of direct top squark pair production and of gluino pair production, where the gluinos decay to final states that include top quarks. For simplified models of pair production of top squarks, which decay to a top quark and a neutralino, top squark masses up to 1020 GeV and neutralino masses up to 430 GeV are excluded at 95% confidence level. For simplified models of gluino pair production, gluino masses up to 2040 GeV and neutralino masses up to 1150 GeV are excluded for T1tttt models. For T5ttcc models, gluino masses up to 1810 GeV and neutralino masses up to 1100 GeV are excluded.
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 $\tilde{\text{t}} \to \text{t} \tilde{\chi}_1^0$ decays (T2tt).

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

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

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Additional Figure 6:
Observed event yields in data (black points) and predicted SM background (filled solid area) for the aggregate search bins. The lower panel shows the ratio of data over total background prediction in each search bin. For both panels, the error bars show the statistical uncertainty associated with the observed data counts, and the grey (blue) hatched bands indicate the statistical (systematic) uncertainties in the total predicted background.
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:
Definition of aggregate search bins.

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Additional Table 4:
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 5:
Selected T2tt signal yields for the aggregate search bins.

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Additional Table 6:
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 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 simplified model and each search region bin 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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Compact Muon Solenoid
LHC, CERN