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| CMS-SUS-20-002 ; CERN-EP-2021-120 | ||
| Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 22 July 2021 | ||
| Eur. Phys. J. C 81 (2021) 970 | ||
| Abstract: A combination of searches for top squark pair production using proton-proton collision data at a center-of-mass energy of 13 TeV at the CERN LHC, corresponding to an integrated luminosity of 137 fb$^{-1}$ collected by the CMS experiment, is presented. Signatures with at least 2 jets and large missing transverse momentum are categorized into events with 0, 1, or 2 leptons. New results for regions of parameter space where the kinematical properties of top squark pair production and top quark pair production are very similar are presented. Depending on the model, the combined result excludes a top squark mass up to 1325 GeV for a massless neutralino, and a neutralino mass up to 700 GeV for a top squark mass of 1150 GeV. Top squarks with masses from 145 to 295 GeV, for neutralino masses from 0 to 100 GeV, with a mass difference between the top squark and the neutralino in a window of 30 GeV around the mass of the top quark, are excluded for the first time with CMS data. The results of theses searches are also interpreted in an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair. Upper limits are set on the cross section for mediator particle masses of up to 420 GeV. | ||
| Links: e-print arXiv:2107.10892 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Diagrams of top squark pair production with further decay of each top squark into a top quark and a neutralino (left), of each top squark into a chargino and a neutralino, with the chargino decaying then into a bottom quark and a W boson (center), and with a combination of the two top squark decay scenarios (right). |
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Figure 1-a:
Diagram of top squark pair production with further decay of each top squark into a top quark and a neutralino. |
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Figure 1-b:
Diagram of top squark pair production with further decay of each top squark into a chargino and a neutralino, with the chargino decaying then into a bottom quark and a W boson. |
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Figure 1-c:
Diagram of top squark pair production with further decay with a combination of the two top squark decay scenarios. |
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Figure 2:
Feynman diagram of direct DM production through a scalar ($\phi $) or pseudoscalar (a) mediator particle, in association with a top quark pair. |
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Figure 3:
Distributions of data and MC events in the signal region with the signal stacked on above the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm{t}}_{1}}} = $ 225 GeV and $ {{m}_{\tilde{\chi}^0_1}} = $ 50 GeV. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-SM prediction ratio. The uncertainty band includes statistical, background normalization and all systematic uncertainties described in Section 6.4. From upper left to lower right: leading lepton $ {p_{\mathrm {T}}} $, $ {{m_{\mathrm {T2}}} (\ell \ell)} $, ${H_{\mathrm {T}}}$, and $ {{p_{\mathrm {T}}} ^\text {miss}} $. |
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Figure 3-a:
Distribution of the leading lepton $ {p_{\mathrm {T}}} $ for data and MC events in the signal region with the signal stacked on above the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm{t}}_{1}}} = $ 225 GeV and $ {{m}_{\tilde{\chi}^0_1}} = $ 50 GeV. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-SM prediction ratio. The uncertainty band includes statistical, background normalization and all systematic uncertainties described in Section 6.4. |
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Figure 3-b:
Distribution of $ {{m_{\mathrm {T2}}} (\ell \ell)} $ for data and MC events in the signal region with the signal stacked on above the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm{t}}_{1}}} = $ 225 GeV and $ {{m}_{\tilde{\chi}^0_1}} = $ 50 GeV. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-SM prediction ratio. The uncertainty band includes statistical, background normalization and all systematic uncertainties described in Section 6.4. |
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Figure 3-c:
Distribution of ${H_{\mathrm {T}}}$ for data and MC events in the signal region with the signal stacked on above the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm{t}}_{1}}} = $ 225 GeV and $ {{m}_{\tilde{\chi}^0_1}} = $ 50 GeV. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-SM prediction ratio. The uncertainty band includes statistical, background normalization and all systematic uncertainties described in Section 6.4. |
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Figure 3-d:
Distribution of $ {{p_{\mathrm {T}}} ^\text {miss}} $ for data and MC events in the signal region with the signal stacked on above the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm{t}}_{1}}} = $ 225 GeV and $ {{m}_{\tilde{\chi}^0_1}} = $ 50 GeV. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-SM prediction ratio. The uncertainty band includes statistical, background normalization and all systematic uncertainties described in Section 6.4. |
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Figure 4:
Normalized distributions for some of the training variables in the baseline selection. Distributions for signal points with different top squark and neutralino masses and SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ events are compared. From upper left to lower right: ${{p_{\mathrm {T}}} ^\text {miss}}$, $ {m_{\mathrm {T2}}} ({\mathrm{e} \mu})$, $\Delta \eta ({\mathrm{e} \mu})$, and $\Delta \phi ({\mathrm{e} \mu})$. |
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Figure 4-a:
Normalized distribution of ${{p_{\mathrm {T}}} ^\text {miss}}$. Distributions for signal points with different top squark and neutralino masses and SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ events are compared. |
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Figure 4-b:
Normalized distribution of $ {m_{\mathrm {T2}}} ({\mathrm{e} \mu})$. Distributions for signal points with different top squark and neutralino masses and SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ events are compared. |
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Figure 4-c:
Normalized distribution of $\Delta \eta ({\mathrm{e} \mu})$. Distributions for signal points with different top squark and neutralino masses and SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ events are compared. |
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Figure 4-d:
Normalized distribution of $\Delta \phi ({\mathrm{e} \mu})$. Distributions for signal points with different top squark and neutralino masses and SM ${\mathrm{t} {}\mathrm{\bar{t}}}$ events are compared. |
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Figure 5:
Normalized DNN score distribution comparing the signal and the ${\mathrm{t} {}\mathrm{\bar{t}}}$ background in the signal region for two mass hypotheses: $ {{m}_{\tilde{\chi}^0_1}} =$ 50 (100) GeV and $ {{m}_{\tilde{\mathrm{t}}_{1}}} =$ 225 (275) GeV. |
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Figure 6:
Post-fit DNN score distributions in the signal region for different mass hypotheses of, from upper left to lower right, $({{m}_{\tilde{\mathrm{t}}_{1}}}, {{m}_{\tilde{\chi}^0_1}})=$ (225, 50); (275, 100); (275, 70); and (245, 100) GeV. The superimposed signal prediction is scaled by the post-fit signal strength and, in the upper panels, it is also multiplied by a factor 20 for better visibility. The post-fit uncertainty band (crosses) includes statistical, background normalization, and all systematic uncertainties described in Section 6.4. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-prediction ratio before the fit (dotted brown line) and after (dots), each of them with their corresponding band of uncertainties (blue band for the pre-fit and crosses band for the post-fit). The ratio between the sum of the signal and background predictions and the background prediction (purple line) is also shown. The masses of the signal model correspond to the values of the DNN mass parameters in each distribution. |
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Figure 6-a:
Post-fit DNN score distributions in the signal region for mass hypothesis of $({{m}_{\tilde{\mathrm{t}}_{1}}}, {{m}_{\tilde{\chi}^0_1}})=$ (225, 50) GeV. The superimposed signal prediction is scaled by the post-fit signal strength and, in the upper panels, it is also multiplied by a factor 20 for better visibility. The post-fit uncertainty band (crosses) includes statistical, background normalization, and all systematic uncertainties described in Section 6.4. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-prediction ratio before the fit (dotted brown line) and after (dots), each of them with their corresponding band of uncertainties (blue band for the pre-fit and crosses band for the post-fit). The ratio between the sum of the signal and background predictions and the background prediction (purple line) is also shown. The masses of the signal model correspond to the values of the DNN mass parameters in each distribution. |
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Figure 6-b:
Post-fit DNN score distributions in the signal region for mass hypothesis of $({{m}_{\tilde{\mathrm{t}}_{1}}}, {{m}_{\tilde{\chi}^0_1}})=$ (275, 100) GeV. The superimposed signal prediction is scaled by the post-fit signal strength and, in the upper panels, it is also multiplied by a factor 20 for better visibility. The post-fit uncertainty band (crosses) includes statistical, background normalization, and all systematic uncertainties described in Section 6.4. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-prediction ratio before the fit (dotted brown line) and after (dots), each of them with their corresponding band of uncertainties (blue band for the pre-fit and crosses band for the post-fit). The ratio between the sum of the signal and background predictions and the background prediction (purple line) is also shown. The masses of the signal model correspond to the values of the DNN mass parameters in each distribution. |
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Figure 6-c:
Post-fit DNN score distributions in the signal region for mass hypothesis of $({{m}_{\tilde{\mathrm{t}}_{1}}}, {{m}_{\tilde{\chi}^0_1}})=$ (275, 70) GeV. The superimposed signal prediction is scaled by the post-fit signal strength and, in the upper panels, it is also multiplied by a factor 20 for better visibility. The post-fit uncertainty band (crosses) includes statistical, background normalization, and all systematic uncertainties described in Section 6.4. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-prediction ratio before the fit (dotted brown line) and after (dots), each of them with their corresponding band of uncertainties (blue band for the pre-fit and crosses band for the post-fit). The ratio between the sum of the signal and background predictions and the background prediction (purple line) is also shown. The masses of the signal model correspond to the values of the DNN mass parameters in each distribution. |
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Figure 6-d:
Post-fit DNN score distributions in the signal region for mass hypothesis of $({{m}_{\tilde{\mathrm{t}}_{1}}}, {{m}_{\tilde{\chi}^0_1}})=$ (245, 100) GeV. The superimposed signal prediction is scaled by the post-fit signal strength and, in the upper panels, it is also multiplied by a factor 20 for better visibility. The post-fit uncertainty band (crosses) includes statistical, background normalization, and all systematic uncertainties described in Section 6.4. Events from ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, DY, nonprompt leptons, and diboson processes are grouped into the 'Other' category. The lower panel contains the data-to-prediction ratio before the fit (dotted brown line) and after (dots), each of them with their corresponding band of uncertainties (blue band for the pre-fit and crosses band for the post-fit). The ratio between the sum of the signal and background predictions and the background prediction (purple line) is also shown. The masses of the signal model correspond to the values of the DNN mass parameters in each distribution. |
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Figure 7:
Upper limit at 95% CL on the signal cross section as a function of the top squark and neutralino masses in the top quark corridor region. The model is excluded for all of the colored region inside the black boundary. |
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Figure 8:
Upper limit at 95% CL on the signal cross section as a function of the top squark mass for ${\Delta m\left (\tilde{\mathrm{t}}_{1},\,\tilde{\chi}^0_1 \right)}$ of 175 GeV (upper left), 185 GeV (upper right) and 165 GeV (lower). The green and yellow bands represent the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The purple dotted line indicates the approximate NNLO+NNLL production cross section. |
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Figure 8-a:
Upper limit at 95% CL on the signal cross section as a function of the top squark mass for ${\Delta m\left (\tilde{\mathrm{t}}_{1},\,\tilde{\chi}^0_1 \right)}$ of 175 GeV.The green and yellow bands represent the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The purple dotted line indicates the approximate NNLO+NNLL production cross section. |
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Figure 8-b:
Upper limit at 95% CL on the signal cross section as a function of the top squark mass for ${\Delta m\left (\tilde{\mathrm{t}}_{1},\,\tilde{\chi}^0_1 \right)}$ of 185 GeV.The green and yellow bands represent the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The purple dotted line indicates the approximate NNLO+NNLL production cross section. |
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Figure 8-c:
Upper limit at 95% CL on the signal cross section as a function of the top squark mass for ${\Delta m\left (\tilde{\mathrm{t}}_{1},\,\tilde{\chi}^0_1 \right)}$ of 165 GeV.The green and yellow bands represent the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The purple dotted line indicates the approximate NNLO+NNLL production cross section. |
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Figure 9:
Expected and observed limits in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane, for the $\tilde{\mathrm{t}}_{1} \to \mathrm{t} \tilde{\chi}^0_1 $ model (upper left), the $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \chi^{+}_{1} \to \mathrm{b} \mathrm{W^{+}} \tilde{\chi}^0_1 $ model (upper right) and a model with a branching fraction of 50% for each of these top squark decay modes (lower), assuming a mass difference between the neutralino and chargino of 5 GeV. The color indicates the 95% CL upper limit on the cross section at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL, while the dashed red lines indicate the expected limits at 95% CL and the region containing 68% of the distribution of limits expected under the background-only hypothesis of the combined analyses. The thin black lines show the effect of the theoretical uncertainties in the signal cross section. |
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Figure 9-a:
Expected and observed limits in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane, for the $\tilde{\mathrm{t}}_{1}, assuming a mass difference between the neutralino and chargino of 5 GeV. The color indicates the 95% CL upper limit on the cross section at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL, while the dashed red lines indicate the expected limits at 95% CL and the region containing 68% of the distribution of limits expected under the background-only hypothesis of the combined analyses. The thin black lines show the effect of the theoretical uncertainties in the signal cross section. |
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Figure 9-b:
Expected and observed limits in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane, for the $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \chi^{+}_{1} \to \mathrm{b} \mathrm{W^{+}} \tilde{\chi}^0_1 $ model, assuming a mass difference between the neutralino and chargino of 5 GeV. The color indicates the 95% CL upper limit on the cross section at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL, while the dashed red lines indicate the expected limits at 95% CL and the region containing 68% of the distribution of limits expected under the background-only hypothesis of the combined analyses. The thin black lines show the effect of the theoretical uncertainties in the signal cross section. |
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Figure 9-c:
Expected and observed limits in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane, for a model with a branching fraction of 50% for each of these top squark decay modes, assuming a mass difference between the neutralino and chargino of 5 GeV. The color indicates the 95% CL upper limit on the cross section at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL, while the dashed red lines indicate the expected limits at 95% CL and the region containing 68% of the distribution of limits expected under the background-only hypothesis of the combined analyses. The thin black lines show the effect of the theoretical uncertainties in the signal cross section. |
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Figure 10:
The 95% CL expected (dashed line) and observed limits (solid line) on $\sigma /\sigma _{\mathrm {theory}}$ for a fermionic DM particle with $m_{\chi} = $ 1 GeV, as a function of the mediator mass for a scalar (left) and pseudoscalar (right). The green and yellow bands represent the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The horizontal gray line indicates $\sigma /\sigma _{\mathrm {theory}}=$ 1. The mediator couplings are set to $g_\mathrm{q} =g_{\mathrm {DM}}=$ 1. |
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Figure 10-a:
The 95% CL expected (dashed line) and observed limits (solid line) on $\sigma /\sigma _{\mathrm {theory}}$ for a fermionic DM particle with $m_{\chi} = $ 1 GeV, as a function of the mediator mass for a scalar. The green and yellow bands represent the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The horizontal gray line indicates $\sigma /\sigma _{\mathrm {theory}}=$ 1. The mediator couplings are set to $g_\mathrm{q} =g_{\mathrm {DM}}=$ 1. |
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Figure 10-b:
The 95% CL expected (dashed line) and observed limits (solid line) on $\sigma /\sigma _{\mathrm {theory}}$ for a fermionic DM particle with $m_{\chi} = $ 1 GeV, as a function of the mediator mass for a pseudoscalar. The green and yellow bands represent the regions containing 68 and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The horizontal gray line indicates $\sigma /\sigma _{\mathrm {theory}}=$ 1. The mediator couplings are set to $g_\mathrm{q} =g_{\mathrm {DM}}=$ 1. |
| Tables | |
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Table 1:
Summary of the contributions of the experimental uncertainties in the DNN score distribution for signal and the ${\mathrm{t} {}\mathrm{\bar{t}}}$ background. The values represent the relative variation in the number of expected events across different signal models in the signal region. |
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Table 2:
Summary of the contribution of each modeling uncertainty source to the DNN score distribution for the ${\mathrm{t} {}\mathrm{\bar{t}}}$ background. |
| Summary |
| Four searches for top squark pair production and their statistical combination are presented. The searches use a data set of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS detector and corresponding to an integrated luminosity of 137 fb$^{-1}$. A dedicated analysis is presented that is sensitive to signal models where the mass splitting between the top squark and the lightest supersymmetric particle (LSP) is close to the top quark mass. A deep neural network algorithm is used to separate the signal from the top quark background using events containing an opposite-sign dilepton pair, at least two jets, at least one b-tagged jet, ${p_{\mathrm{T}}}miss > $ 50 GeV, and stransverse mass greater than 80 GeV. No excess of data over the standard model prediction is observed, and upper limits are set at 95% confidence level on the top squark production cross section. Top squarks with mass from 145 to 275 GeV, for LSP mass from 0 to 100 GeV, with a mass difference between the top squarks and LSP of up to 30 GeV deviation around the mass of the top quark, are excluded for the first time in CMS. Previously published searches in final states with 0, 1, or 2 leptons are combined to extend the exclusion limits of top squarks with masses up to 1325 GeV for a massless LSP and an LSP mass up to 700 GeV for a top squark mass of 1150 GeV, for certain models of top squark production. In an alternative signal model of dark matter production via a spin-0 mediator in association with a top quark pair, mediator particle masses up to 400 and 420 GeV are excluded for scalar or pseudoscalar mediators, respectively, assuming a dark matter particle mass of 1 GeV. |
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