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| CMS-PAS-SUS-17-001 | ||
| Search for direct stop pair production in the dilepton final state at $\sqrt{s}=$ 13 TeV | ||
| CMS Collaboration | ||
| March 2017 | ||
| Abstract: A search for direct top squark production in the opposite-sign dilepton channel is presented, using LHC pp collision data at $\sqrt{s}= $ 13 TeV amounting to 35.9 fb$^{-1}$ collected by the CMS detector in 2016. The search is performed in final states with two leptons, electrons or muons, jets, of which at least one is b-tagged, and missing transverse momentum. Signal regions are defined using transverse mass variables as well as missing transverse energy, which efficiently separate the signal from the dominant top-quark pair background. No significant deviation from the background prediction is observed. Exclusion limits are set in the context of two different simplified supersymmetric models with pair production of top squarks. For top squarks that each decay to a top quark and a neutralino, masses of the lightest top squark below 800 GeV are excluded at a confidence level of 95% for neutralino masses below 350 GeV. Interpreting the results within an alternative model where the top squarks undergo a cascade decay through charginos and sleptons, top squarks with masses up to 1300 GeV are excluded for neutralino masses of 400 GeV. | ||
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Links:
CDS record (PDF) ;
inSPIRE record ;
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These preliminary results are superseded in this paper, PRD 97 (2018) 032009. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures & Tables | References | CMS Publications |
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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:
Strong production of top squark pairs ($\tilde{ \mathrm{ t } } _{1}\overline{ \tilde{\mathrm{t}}}_{1}$) in simplified models. The top squarks either decay directly to a top quark and a neutralino ($\tilde{\chi}^0_1 $), shown in diagram (a), or undergo a cascade decay into a neutralino via an intermediate chargino ($\tilde{\chi}^{{\pm}}_1 $), shown in diagram (b). The decay via an intermediate slepton ($\tilde{\ell }$) to a lepton and the neutralino is shown in diagram (c). |
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Figure 1-a:
Strong production of top squark pairs ($\tilde{ \mathrm{ t } } _{1}\overline{ \tilde{\mathrm{t}}}_{1}$) in simplified models. In this diagram, the top squarks decay directly to a top quark and a neutralino ($\tilde{\chi}^0_1 $). |
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Figure 1-b:
Strong production of top squark pairs ($\tilde{ \mathrm{ t } } _{1}\overline{ \tilde{\mathrm{t}}}_{1}$) in simplified models. In this diagram, the top squarks undergo a cascade decay into a neutralino via an intermediate chargino ($\tilde{\chi}^{\pm}_1 $). |
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Figure 1-c:
Strong production of top squark pairs ($\tilde{ \mathrm{ t } } _{1}\overline{ \tilde{\mathrm{t}}}_{1}$) in simplified models. The diagram shows the decay via an intermediate slepton ($\tilde{\ell }$) to a lepton and the neutralino. |
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Figure 2:
Distributions of ${M_{T2}(\ell \ell )} $, ${M_{T2}(b\ell b\ell )} $, and ${E_{\mathrm {T}}^{\text {miss}}}$ in simulation after preselection and requiring $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 2-a:
Distributions of ${M_{T2}(\ell \ell )} $ in simulation after preselection and requiring $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 2-b:
Distributions of ${M_{T2}(b\ell b\ell )} $ in simulation after preselection and requiring $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 2-c:
Distributions of ${E_{\mathrm {T}}^{\text {miss}}}$ in simulation after preselection and requiring $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 3:
Distributions of ${M_{T2}(\ell \ell )}$ in a control region enriched with $\mathrm{ t \bar{t} }$ events and defined by $ {N_\text {jets}} \geq $ 2, $ {N_\text {bjets}} \geq $ 1 and $ {E_{\mathrm {T}}^{\text {miss}}} < $ 80 GeV (left). Distribution of ${M_{T2}(\ell \ell )}$ after swapping an isolated lepton with an additional non-isolated lepton as described in the text (right). Simulated yields are normalized to data using the yields at $ {M_{T2}(\ell \ell )} < $ 100 GeV. |
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Figure 3-a:
Distributions of ${M_{T2}(\ell \ell )}$ in a control region enriched with $\mathrm{ t \bar{t} }$ events and defined by $ {N_\text {jets}} \geq $ 2, $ {N_\text {bjets}} \geq $ 1 and $ {E_{\mathrm {T}}^{\text {miss}}} < $ 80 GeV. Simulated yields are normalized to data using the yields at $ {M_{T2}(\ell \ell )} < $ 100 GeV. |
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Figure 3-b:
Distribution of ${M_{T2}(\ell \ell )}$ after swapping an isolated lepton with an additional non-isolated lepton as described in the text. Simulated yields are normalized to data using the yields at $ {M_{T2}(\ell \ell )} < $ 100 GeV. |
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Figure 4:
Expected and observed yields in the 5 ttZ control regions, before (left) and after the fit (right) which are defined by different requirements on the number of reconstructed jets and b-tagged jets. The hatched band contains all uncertainties discussed in the text. |
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Figure 4-a:
Expected and observed yields in the 5 ttZ control regions, before the fit, which are defined by different requirements on the number of reconstructed jets and b-tagged jets. The hatched band contains all uncertainties discussed in the text. |
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Figure 4-b:
Expected and observed yields in the 5 ttZ control regions, after the fit which, are defined by different requirements on the number of reconstructed jets and b-tagged jets. The hatched band contains all uncertainties discussed in the text. |
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Figure 5:
Distributions of ${M_{T2}(\ell \ell )} $, ${E_{\rm T}^{\rm miss}}$ and ${M_{T2}(b\ell b\ell )}$ for same-flavor (ee/$\mu \mu $) events falling within the Z-mass window, with at least two jets and $ {N_\text {bjets}} = $ 0, $ {E_{\rm T}^{\rm miss}} > $ 80 GeV and $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 5-a:
Distribution of ${M_{T2}(\ell \ell )} $ for same-flavor (ee/$\mu \mu $) events falling within the Z-mass window, with at least two jets and $ {N_\text {bjets}} = $ 0, $ {E_{\rm T}^{\rm miss}} > $ 80 GeV and $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 5-b:
Distribution of ${E_{\rm T}^{\rm miss}}$ for same-flavor (ee/$\mu \mu $) events falling within the Z-mass window, with at least two jets and $ {N_\text {bjets}} = $ 0, $ {E_{\rm T}^{\rm miss}} > $ 80 GeV and $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 5-c:
Distribution of ${M_{T2}(b\ell b\ell )}$ for same-flavor (ee/$\mu \mu $) events falling within the Z-mass window, with at least two jets and $ {N_\text {bjets}} = $ 0, $ {E_{\rm T}^{\rm miss}} > $ 80 GeV and $ {M_{T2}(\ell \ell )} > $ 100 GeV. |
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Figure 6:
Event yields in the 13 Drell-Yan and diboson control regions for events with same-flavor leptons falling within the Z-mass window and $ {N_\text {bjets}} = $ 0 after renormalizing with the scale factors obtained from the fit procedure. Requirements of jet-multiplicity, ${E_{\mathrm {T}}^{\text {miss}}} $, ${M_{T2}(\ell \ell )}$ and ${M_{T2}(b\ell b\ell )}$ are aligned with the corresponding signal regions. |
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Figure 7:
$ {M_{T2}(\ell \ell )} $ distributions of observed events in $\mu \mu $, ee, e$\mu $ channels compared to the predicted SM backgrounds using simulation in the selection defined in Table 1. The shaded band covers all uncertainties discussed in the text. |
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Figure 7-a:
$ {M_{T2}(\ell \ell )} $ distributions of observed events in the $\mu \mu $ channel compared to the predicted SM backgrounds using simulation in the selection defined in Table 1. The shaded band covers all uncertainties discussed in the text. |
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Figure 7-b:
$ {M_{T2}(\ell \ell )} $ distributions of observed events in the ee channel compared to the predicted SM backgrounds using simulation in the selection defined in Table 1. The shaded band covers all uncertainties discussed in the text. |
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Figure 7-c:
$ {M_{T2}(\ell \ell )} $ distributions of observed events in the e$\mu $ channel compared to the predicted SM backgrounds using simulation in the selection defined in Table 1. The shaded band covers all uncertainties discussed in the text. |
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Figure 8:
Distributions of ${M_{T2}(\ell \ell )} $, ${M_{T2}(b\ell b\ell )}$ and ${E_{\mathrm {T}}^{\text {miss}}}$ in all flavor channels for the selection defined in Table 1. For ${M_{T2}(b\ell b\ell )}$ and ${E_{\mathrm {T}}^{\text {miss}}}$ additionally $ {M_{T2}(\ell \ell )} > $ 100 GeV is required. The shaded band covers all uncertainties discussed in the text. |
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Figure 8-a:
Distribution of ${M_{T2}(\ell \ell )} $ in all flavor channels for the selection defined in Table 1. The shaded band covers all uncertainties discussed in the text. |
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Figure 8-b:
Distribution of ${M_{T2}(b\ell b\ell )}$ in all flavor channels for the selection defined in Table 1. $ {M_{T2}(\ell \ell )} > $ 100 GeV is required. The shaded band covers all uncertainties discussed in the text. |
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Figure 8-c:
Distribution of ${E_{\mathrm {T}}^{\text {miss}}}$ in all flavor channels for the selection defined in Table 1. $ {M_{T2}(\ell \ell )} > $ 100 GeV is required. The shaded band covers all uncertainties discussed in the text. |
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Figure 9:
Predicted backgrounds and observed yields in each search region. The shaded band covers all uncertainties discussed in the text. |
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Figure 9-a:
Predicted backgrounds and observed yields in each search region, $\mu\mu$ and ee channels. The shaded band covers all uncertainties discussed in the text. |
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Figure 9-b:
Predicted backgrounds and observed yields in each search region, e$\mu$ channel. The shaded band covers all uncertainties discussed in the text. |
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Figure 10:
As Fig. 9, but combining all channels. |
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Figure 11:
Expected and observed limits at 95% CL for the "direct decay'' mode $\tilde{ \mathrm{ t } } \to \mathrm{t}\tilde{\chi}^0_1 $ in the $m_{\tilde{ \mathrm{ t } } }$, $m_{\tilde{\chi}^0_1 }$ mass plane (left) and for the "chargino decay'' mode $\tilde{ \mathrm{ t } } \to b\tilde{\chi}^{\pm} \to \mathrm{W}\tilde{\chi}^0_1 $ (right). |
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Figure 11-a:
Expected and observed limits at 95% CL for the "direct decay'' mode $\tilde{ \mathrm{ t } } \to \mathrm{t}\tilde{\chi}^0_1 $ in the $m_{\tilde{ \mathrm{ t } } }$, $m_{\tilde{\chi}^0_1 }$ mass plane. |
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Figure 11-b:
Expected and observed limits at 95% CL for the "chargino decay'' mode $\tilde{ \mathrm{ t } } \to b\tilde{\chi}^{\pm} \to \mathrm{W}\tilde{\chi}^0_1 $. |
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Figure 12:
95% CL expected and observed limits for the "cascade decay'' mode $\tilde{ \mathrm{ t } } \to \mathrm{b}\tilde{\chi}^{\pm} \to \nu \tilde{\ell } \to \ell \tilde{\chi}^0 $ in the $m_{\tilde{ \mathrm{ t } } }$, $m_{\tilde{\chi}^0_1 }$ mass plane. |
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Figure 12-a:
95% CL expected and observed limits for the "cascade decay'' mode $\tilde{ \mathrm{ t } } \to \mathrm{b}\tilde{\chi}^{\pm} \to \nu \tilde{\ell } \to \ell \tilde{\chi}^0 $ in the $m_{\tilde{ \mathrm{ t } } }$, $m_{\tilde{\chi}^0_1 }$ mass plane. |
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Figure 12-b:
95% CL expected and observed limits for the "cascade decay'' mode $\tilde{ \mathrm{ t } } \to \mathrm{b}\tilde{\chi}^{\pm} \to \nu \tilde{\ell } \to \ell \tilde{\chi}^0 $ in the $m_{\tilde{ \mathrm{ t } } }$, $m_{\tilde{\chi}^0_1 }$ mass plane. |
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Figure 12-c:
95% CL expected and observed limits for the "cascade decay'' mode $\tilde{ \mathrm{ t } } \to \mathrm{b}\tilde{\chi}^{\pm} \to \nu \tilde{\ell } \to \ell \tilde{\chi}^0 $ in the $m_{\tilde{ \mathrm{ t } } }$, $m_{\tilde{\chi}^0_1 }$ mass plane. |
| Tables | |
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Table 1:
Overview of the preselection. |
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Table 2:
Definition of the signal regions. The regions are furthermore split into opposite-flavor and same-flavor regions. |
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Table 3:
Minimal and maximal relative errors for the systematic uncertainties over all signal regions in Fig. 10. Numbers are given relative to the total background contribution per signal region. |
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Table 4:
Yields for data and total expected background in each of the signal regions for same-flavor (ee/$\mu \mu $), different-flavor (e$\mu $) and all channels combined with all systematic uncertainties as described in Section 6. |
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Table 5:
Post-fit covariance (left) and correlation matrix (right) for aggregated signal regions. |
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Table 6:
Yields and uncertainties in the aggregated signal regions. |
| Summary |
|
We presented a search for supersymmetry in a final state of two leptons, b jets, and large missing transverse momentum, originating from decays of pair-produced top squarks to two top quarks and neutralinos, with a subsequent fully leptonic decay of the top quarks. We used a data set corresponding to an integrated luminosity of 35.9 fb$^{-1}$ of pp collisions collected in 2016 at a center-of-mass energy of 13 TeV with the CMS detector at the LHC. An efficient background reduction using dedicated kinematical variables was achieved, with in particular the large background of SM dilepton ${\mathrm{ t \bar{t} } }$ events suppressed by several orders of magnitude. We observe no evidence for an excess above the expected background from standard model processes. In the "direct decay" mode, mass configurations with neutralino masses of $ m_{\tilde{\chi}^0_1} \leq $ 1350 GeV and top squark masses of $ m_{\tilde{t}} \leq $ 800 GeV are excluded at a confidence level of 95%. For the "chargino decay" mode, where the chargino mass is assumed to equal the arithmetic mean of the top squark and the lightest neutralino, the corresponding exclusion limit reaches $ m_{\tilde{\mathrm{t}}} \leq $ 750 GeV when the mass of the lightest neutralino is below $ m_{\tilde{\chi}^0_1} \leq $ 300. In the "cascade decay" mode, top squark masses up to $ m_{\tilde{\mathrm{t}}} \leq $ 1200 GeV are excluded for neutralino masses below $ m_{\tilde{\chi}^0_1} \leq $ 400 GeV when the chargino mass is chosen to be half way between stop and neutralino, and the slepton mass half way between chargino and neutralino. If the slepton mass is closer to the mass of the chargino, limits reach even higher top squark and neutralino masses. Otherwise, if the slepton mass is almost degenerate with the neutralino, the limit is considerably weaker. |
| Additional Figures | |
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Additional Figure 1:
95% CL expected (dashed line) and observed limits (solid line) on $\mu =\sigma /\sigma _{\textrm {theory}}$ for a fermionic DM particle with $m_{\chi }= $ 1 GeV assuming different scalar (left) and pseudoscalar mediator (right) masses. 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 red line indicates $\mu = $ 1. The mediator couplings are set to $g_{\rm q}=g_{\rm DM}= $ 1. The gray hashed band around the observed limit corresponds to a 30% theory uncertainty on the inclusive signal cross section. |
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Additional Figure 1-a:
95% CL expected (dashed line) and observed limits (solid line) on $\mu =\sigma /\sigma _{\textrm {theory}}$ for a fermionic DM particle with $m_{\chi }= $ 1 GeV assuming different scalar mediator (masses. 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 red line indicates $\mu = $ 1. The mediator couplings are set to $g_{\rm q}=g_{\rm DM}= $ 1. The gray hashed band around the observed limit corresponds to a 30% theory uncertainty on the inclusive signal cross section. |
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Additional Figure 1-b:
95% CL expected (dashed line) and observed limits (solid line) on $\mu =\sigma /\sigma _{\textrm {theory}}$ for a fermionic DM particle with $m_{\chi }= $ 1 GeV assuming different pseudoscalar mediator masses. 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 red line indicates $\mu = $ 1. The mediator couplings are set to $g_{\rm q}=g_{\rm DM}= $ 1. The gray hashed band around the observed limit corresponds to a 30% theory uncertainty on the inclusive signal cross section. |
| Additional Tables | |
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Additional Table 1:
Ratios $\mu =\sigma /\sigma _{\textrm {theory}}$ of the 95% CL expected and observed limits to simplified model expectations for different DM particle masses and mediator masses and for scalar ($\phi $) and pseudoscalar ($a$) mediators under the assumption $g_{\rm q}=g_{\rm DM}=$ 1. The uncertainties reflect the 68% band of the expected limits. |
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Additional Table 2:
Cutflow table for two different configurations of T2tt SUSY signal models (left) and DM signal models (right). Numbers are shown for T2tt signals with top squark masses of 750 (600) GeV and LSP masses of 1 (300) GeV, and scalar (pseudoscalar) mediator mass of 10 GeV and DM particle mass of 1 GeV. |
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