CMS-SUS-18-003 ; CERN-EP-2018-312 | ||
Search for the pair production of light top squarks in the $\mathrm{e}^{\pm}\mu^{\mp}$ final state in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
CMS Collaboration | ||
4 January 2019 | ||
JHEP 03 (2019) 101 | ||
Abstract: A search for the production of a pair of top squarks at the LHC is presented. This search targets a region of parameter space where the kinematics of top squark pair production and top quark pair production are very similar, because of the mass difference between the top squark and the neutralino being close to the top quark mass. The search is performed with 35.9 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = $ 13 TeV, collected by the CMS detector in 2016, using events containing one electron-muon pair with opposite charge. The search is based on a precise estimate of the top quark pair background, and the use of the ${M_{\textrm{T2}}}$ variable, which combines the transverse mass of each lepton and the missing transverse momentum. No excess of events is found over the standard model predictions. Exclusion limits are placed at 95% confidence level on the production of top squarks up to masses of 200 GeV for models with a mass difference between the top squark and the lightest neutralino close to that of the top quark. | ||
Links: e-print arXiv:1901.01288 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
Diagram of the top squark pair production with further decay into a top (antitop) quark and the lightest neutralino. |
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Figure 2:
Normalized ${M_{\textrm {T2}}}$ distributions for various mass hypotheses for the top squark and for the neutralino. Variables at the generator level are used for ${{\mathrm {t}\overline {\mathrm {t}}}}$ and signal events with two generated leptons with ${p_{\mathrm {T}}}$ of at least 20 GeV and $ {| \eta |} \leq $ 2.4. The last bin includes the overflow. |
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Figure 2-a:
Normalized ${M_{\textrm {T2}}}$ distributions for various mass hypotheses for the top squark and for the neutralino. Variables at the generator level are used for ${{\mathrm {t}\overline {\mathrm {t}}}}$ and signal events with two generated leptons with ${p_{\mathrm {T}}}$ of at least 20 GeV and $ {| \eta |} \leq $ 2.4. The last bin includes the overflow. |
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Figure 2-b:
Normalized ${M_{\textrm {T2}}}$ distributions for various mass hypotheses for the top squark and for the neutralino. Variables at the generator level are used for ${{\mathrm {t}\overline {\mathrm {t}}}}$ and signal events with two generated leptons with ${p_{\mathrm {T}}}$ of at least 20 GeV and $ {| \eta |} \leq $ 2.4. The last bin includes the overflow. |
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Figure 3:
Distributions for leading and subleading lepton ${p_{\mathrm {T}}}$, $\Delta \phi ({\mathrm {e}}, {{\mu}})$, and ${{p_{\mathrm {T}}} ^\text {miss}}$. The uncertainty band includes statistical and all systematic uncertainties described in Section 7. The last bin contains the overflow events. The signal is stacked on top of the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm {t}}_1}} = $ 175 GeV and $ {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 1 GeV. |
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Figure 3-a:
Distribution for the leading lepton ${p_{\mathrm {T}}}$. The uncertainty band includes statistical and all systematic uncertainties described in Section 7. The last bin contains the overflow events. The signal is stacked on top of the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm {t}}_1}} = $ 175 GeV and $ {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 1 GeV. |
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Figure 3-b:
Distribution for the subleading lepton ${p_{\mathrm {T}}}$. The uncertainty band includes statistical and all systematic uncertainties described in Section 7. The last bin contains the overflow events. The signal is stacked on top of the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm {t}}_1}} = $ 175 GeV and $ {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 1 GeV. |
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Figure 3-c:
Distribution for $\Delta \phi ({\mathrm {e}} {{\mu}})$. The uncertainty band includes statistical and all systematic uncertainties described in Section 7. The last bin contains the overflow events. The signal is stacked on top of the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm {t}}_1}} = $ 175 GeV and $ {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 1 GeV. |
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Figure 3-d:
Distribution for ${{p_{\mathrm {T}}} ^\text {miss}}$. The uncertainty band includes statistical and all systematic uncertainties described in Section 7. The last bin contains the overflow events. The signal is stacked on top of the background prediction for a mass hypothesis of $ {{m}_{\tilde{\mathrm {t}}_1}} = $ 175 GeV and $ {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 1 GeV. |
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Figure 4:
${M_{\textrm {T2}}}$ distribution (prefit) for data and predicted background. The ${M_{\textrm {T2}}}$ distribution for a signal corresponding to a top squark mass of 205 GeV and a neutralino mass of 30 GeV is also shown, stacked on top of the background estimate. The hatched bands correspond to the combined systematic and statistical uncertainties on background rates. The last bin of the histogram includes the overflow events. The lower pane shows the ratio between the observed data and the predicted SM background. |
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Figure 5:
Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for $ {{m}_{\tilde{\mathrm {t}}_1}} - {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 175 GeV (upper left), $ {{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 167.5 GeV (upper right) and $ {{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 182.5 GeV (lower). The green dark and yellow light bands correspond to the 68 and 95% CL ranges of the expected upper limits. |
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Figure 5-a:
Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for $ {{m}_{\tilde{\mathrm {t}}_1}} - {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 175 GeV. The green dark and yellow light bands correspond to the 68 and 95% CL ranges of the expected upper limits. |
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Figure 5-b:
Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for $ {{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 167.5 GeV. The green dark and yellow light bands correspond to the 68 and 95% CL ranges of the expected upper limits. |
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Figure 5-c:
Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for $ {{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}} = $ 182.5 GeV. The green dark and yellow light bands correspond to the 68 and 95% CL ranges of the expected upper limits. |
Tables | |
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Table 1:
Summary of the uncertainties on the ${M_{\textrm {T2}}}$ distribution resulting from ${{\mathrm {t}\overline {\mathrm {t}}}}$ background modelling uncertainties. The ranges correspond to variations of the uncertainty along the ${M_{\textrm {T2}}}$ distribution. When only one number is shown, the uncertainty is approximately constant over the entire ${M_{\textrm {T2}}}$ range. |
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Table 2:
Summary of the uncertainties in ${{\mathrm {t}\overline {\mathrm {t}}}}$ background and signal simulation resulting from experimental uncertainties. The numbers represent typical values of the uncertainties in the signal and ${{\mathrm {t}\overline {\mathrm {t}}}}$ background yields or ranges for these uncertainties in different ${M_{\textrm {T2}}}$ bins and in different signal samples. |
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Table 3:
Number of expected and observed events after the selection, with $ {M_{\textrm {T2}}} > 0$ and $ {M_{\textrm {T2}}} > $ 90 GeV. The quoted uncertainties reflect both the statistical and systematic contributions. |
Summary |
A search is presented for a top squark with a mass difference from the neutralino mass close to the top quark mass, ${{m}_{\tilde{\mathrm{t}}_{1}}} - {{m}_{\tilde{\chi}^0_1}} \approx {m_{\mathrm{t}}} $, using events with one opposite-sign electron-muon pair, at least two jets, and at least one b jet. The $\tilde{\mathrm{t}}_{1}\to\mathrm{t}\tilde{\chi}^0_1$ decay mode is considered, and different top squark masses are explored up to 240 GeV with neutralino masses of ${{m}_{\tilde{\chi}^0_1}} \approx {{m}_{\tilde{\mathrm{t}}_{1}}} - {m_{\mathrm{t}}} $. The ${M_{\textrm{T2}}}$ variable is used in a binned profile likelihood fit to increase the sensitivity, owing to the different kinematic distributions between the signal and the $\mathrm{t\bar{t}}$ background. Further sensitivity is gained from the absence of a kinematic endpoint in this variable for the signal. No excess is observed and upper limits are set at 95% confidence level on the top squark production cross section for top squark masses up to 210 GeV in models with ${{m}_{\tilde{\mathrm{t}}_{1}}} - {{m}_{\tilde{\chi}^0_1}} \approx {m_{\mathrm{t}}} $ and masses up to 240 GeV in models with a mass difference of 7.5 GeV. This result significantly extends the exclusion limits of top squark searches at the LHC to higher top squark masses in the region where ${{m}_{\tilde{\mathrm{t}}_{1}}} - {{m}_{\tilde{\chi}^0_1}} \approx {m_{\mathrm{t}}} $, that was previously unexplored. |
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