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CMS-SUS-17-001 ; CERN-EP-2017-252

Search for top squarks and dark matter particles in opposite-charge dilepton final states at $\sqrt{s} = $ 13 TeV

CMS Collaboration

2 November 2017

Phys. Rev. D 97 (2018) 032009

Abstract: A search for new physics is presented in final states with two oppositely charged leptons (electrons or muons), jets identified as originating from b quarks, and missing transverse momentum ($ p_{\mathrm{T}}^{\text miss} $). The search uses proton-proton collision data at $\sqrt{s} = $ 13 TeV amounting to 35.9 fb$^{-1}$ of integrated luminosity collected using the CMS detector in 2016. Hypothetical signal events are efficiently separated from the dominant $ \mathrm{t\bar{t}} $ background with requirements on $ p_{\mathrm{T}}^{\text miss} $ and transverse mass variables. No significant deviation is observed from the expected background. Exclusion limits are set in the context of simplified supersymmetric models with pair-produced top squarks. For top squarks, decaying exclusively to a top quark and a neutralino, exclusion limits are placed at 95% confidence level on the mass of the lightest top squark up to 800 GeV and on the lightest neutralino up to 360 GeV. These results, combined with searches in the single-lepton and all-jet final states, raise the exclusion limits up to 1050 GeV for the lightest top squark and up to 500 GeV for the lightest neutralino. For top squarks undergoing a cascade decay through charginos and sleptons, the mass limits reach up to 1300 GeV for top squarks and up to 800 GeV for the lightest neutralino. The results are also interpreted in a simplified model with a dark matter (DM) particle coupled to the top quark through a scalar or pseudoscalar mediator. For light DM, mediator masses up to 100 (50) GeV are excluded for scalar (pseudoscalar) mediators. The result for the scalar mediator achieves some of the most stringent limits to date in this model.

Links: e-print arXiv:1711.00752 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Additional information on efficiencies needed for reinterpretation of these results are available here. Additional technical material for CMS speakers can be found here.

Figures
png pdf	Figure 1: Diagrams for simplified SUSY models and for direct DM production: strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \overline{\tilde{\mathrm{t}}} _{1}$, where each top squark decays to a top quark and a $\tilde{\chi}^0_1$ (T2tt model, upper left), or where each top squark decays into a b quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a W boson and a $\tilde{\chi}^0_1 $ (T2bW model, upper right), or to a neutrino and an intermediate slepton $\nu \tilde{\ell}^{\pm}$ that yield $\nu \tilde{\chi}^0_1 $ and an $\ell ^\pm $ from the virtual slepton decay (T8bb$\ell\ell\nu\nu$ model, lower left). Direct DM production through scalar or pseudoscalar mediators in association with top quarks is shown at the lower right.
png pdf	Figure 1-a: Diagram for a simplified SUSY model of direct DM production: strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \overline{\tilde{\mathrm{t}}} _{1}$, where each top squark decays to a top quark and a $\tilde{\chi}^0_1$ (T2tt model).
png pdf	Figure 1-b: Diagram for a simplified SUSY model of direct DM production: strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \overline{\tilde{\mathrm{t}}} _{1}$, where each top squark decays into a b quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a W boson and a $\tilde{\chi}^0_1 $ (T2bW model).
png pdf	Figure 1-c: Diagram for a simplified SUSY model of direct DM production: strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \overline{\tilde{\mathrm{t}}} _{1}$, where each top squark decays into a b quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a neutrino and an intermediate slepton $\nu \tilde{\ell}^{\pm}$ that yield $\nu \tilde{\chi}^0_1 $ and an $\ell ^\pm $ from the virtual slepton decay (T8bb$\ell\ell\nu\nu$ model).
png pdf	Figure 1-d: Diagram for a simplified SUSY model of direct DM production: direct DM production through scalar or pseudoscalar mediators in association with top quarks.
png pdf	Figure 2: Distributions of ${M_{\text {T2}}(\ell \ell)} $ (left), $ {M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ (center), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (right) in simulation after preselection and requiring $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. A T2tt signal is shown with masses $m_{\tilde{\mathrm{t}}} = $ 750 GeV and $m_{\tilde{\chi}^0_1} = $ 1 GeV, as well as a more compressed signal with $m_{\tilde{\mathrm{t}}} = $ 600 GeV and $m_{\tilde{\chi}^0_1} = $ 300 GeV.
png pdf root	Figure 2-a: Distribution of ${M_{\text {T2}}(\ell \ell)} $ in simulation after preselection and requiring $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. A T2tt signal is shown with masses $m_{\tilde{\mathrm{t}}} = $ 750 GeV and $m_{\tilde{\chi}^0_1} = $ 1 GeV, as well as a more compressed signal with $m_{\tilde{\mathrm{t}}} = $ 600 GeV and $m_{\tilde{\chi}^0_1} = $ 300 GeV.
png pdf root	Figure 2-b: Distribution of $ {M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ in simulation after preselection and requiring $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. A T2tt signal is shown with masses $m_{\tilde{\mathrm{t}}} = $ 750 GeV and $m_{\tilde{\chi}^0_1} = $ 1 GeV, as well as a more compressed signal with $m_{\tilde{\mathrm{t}}} = $ 600 GeV and $m_{\tilde{\chi}^0_1} = $ 300 GeV.
png pdf root	Figure 2-c: Distribution of ${{p_{\mathrm {T}}} ^\text {miss}}$ in simulation after preselection and requiring $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. A T2tt signal is shown with masses $m_{\tilde{\mathrm{t}}} = $ 750 GeV and $m_{\tilde{\chi}^0_1} = $ 1 GeV, as well as a more compressed signal with $m_{\tilde{\mathrm{t}}} = $ 600 GeV and $m_{\tilde{\chi}^0_1} = $ 300 GeV.
png pdf	Figure 3: Left : distribution of ${M_{\text {T2}}(\ell \ell)}$ in a control region enriched in ${\mathrm{t} {}\mathrm{\bar{t}}} $ events and defined by $ {N_\text {jets}} \geq $ 2, $ {N_\text {b jets}} \geq $ 1, and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 80 GeV. The hatched band shows the uncertainties from experimental effects, as described in Section 7. Right : distribution of ${M_{\text {T2}}(\ell \ell)}$ after swapping an isolated lepton with an additional non-isolated lepton, as described in the text. For both plots, simulated yields are normalized to data using the yields in the $ {M_{\text {T2}}(\ell \ell)} < $ 100 GeV region.
png pdf root	Figure 3-a: Distribution of ${M_{\text {T2}}(\ell \ell)}$ in a control region enriched in ${\mathrm{t} {}\mathrm{\bar{t}}} $ events and defined by $ {N_\text {jets}} \geq $ 2, $ {N_\text {b jets}} \geq $ 1, and $ {{p_{\mathrm {T}}} ^\text {miss}} < $ 80 GeV. The hatched band shows the uncertainties from experimental effects, as described in Section 7. Simulated yields are normalized to data using the yields in the $ {M_{\text {T2}}(\ell \ell)} < $ 100 GeV region.
png pdf	Figure 3-b: Distribution of ${M_{\text {T2}}(\ell \ell)}$ 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 in the $ {M_{\text {T2}}(\ell \ell)} < $ 100 GeV region.
png pdf	Figure 4: Expected and observed yields in the five ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z}}$ control regions, which are defined by different requirements on the number of reconstructed jets and b jets, before (left) and after the fit (right). The hatched band contains all uncertainties discussed in the text.
png pdf root	Figure 4-a: Expected and observed yields in the five ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z}}$ control regions, which are defined by different requirements on the number of reconstructed jets and b jets, before the fit. The hatched band contains all uncertainties discussed in the text.
png pdf root	Figure 4-b: Expected and observed yields in the five ${{\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z}}$ control regions, which are defined by different requirements on the number of reconstructed jets and b jets, after the fit. The hatched band contains all uncertainties discussed in the text.
png pdf	Figure 5: Distributions of ${M_{\text {T2}}(\ell \ell)}$ (left), ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ (center), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (right) for same-flavor (SF) events falling within the Z boson mass window ($ | m(\ell \ell)-m_{\mathrm{Z}} | < $ 15 GeV), with at least two jets and $ {N_\text {b jets}} = $ 0, $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 80 GeV, and $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. The hatched band shows the uncertainties from experimental effects, as described in Section 7.
png pdf root	Figure 5-a: Distribution of ${M_{\text {T2}}(\ell \ell)}$ for same-flavor (SF) events falling within the Z boson mass window ($ | m(\ell \ell)-m_{\mathrm{Z}} | < $ 15 GeV), with at least two jets and $ {N_\text {b jets}} = $ 0, $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 80 GeV, and $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. The hatched band shows the uncertainties from experimental effects, as described in Section 7.
png pdf root	Figure 5-b: Distribution of ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ for same-flavor (SF) events falling within the Z boson mass window ($ | m(\ell \ell)-m_{\mathrm{Z}} | < $ 15 GeV), with at least two jets and $ {N_\text {b jets}} = $ 0, $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 80 GeV, and $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. The hatched band shows the uncertainties from experimental effects, as described in Section 7.
png pdf root	Figure 5-c: Distribution of ${{p_{\mathrm {T}}} ^\text {miss}}$ for same-flavor (SF) events falling within the Z boson mass window ($ | m(\ell \ell)-m_{\mathrm{Z}} | < $ 15 GeV), with at least two jets and $ {N_\text {b jets}} = $ 0, $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 80 GeV, and $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV. The hatched band shows the uncertainties from experimental effects, as described in Section 7.
png pdf root	Figure 6: Event yields in the 13 Drell-Yan and multiboson control regions for events with same-flavor (SF) leptons falling within the Z boson mass window ($ | m(\ell \ell)-m_{\mathrm{Z}} | < $ 15 GeV) and $ {N_\text {b jets}} = $ 0, after renormalizing with the scale factors obtained from the fit procedure described in the text. The hatched band shows the uncertainties from the fit including the uncertainties from the experimental effects, as described in Section 7.
png pdf	Figure 7: Distributions of $ {M_{\text {T2}}(\ell \ell)} $ for observed events in the $\mu \mu $ (left), $\mathrm{e} \mathrm{e} $ (middle), and $\mathrm{e} \mu $ (right) channels compared to the predicted SM backgrounds for the selection defined in Table 1. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 7-a: Distribution of $ {M_{\text {T2}}(\ell \ell)} $ for observed events in the $\mu \mu $ channel compared to the predicted SM backgrounds for the selection defined in Table 1. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 7-b: Distribution of $ {M_{\text {T2}}(\ell \ell)} $ for observed events in the $\mathrm{e} \mathrm{e} $ channel compared to the predicted SM backgrounds for the selection defined in Table 1. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 7-c: Distribution of $ {M_{\text {T2}}(\ell \ell)} $ for observed events in the $\mathrm{e} \mu $ channel compared to the predicted SM backgrounds for the selection defined in Table 1. The hatched band shows the uncertainties discussed in the text.
png pdf	Figure 8: Distributions of ${M_{\text {T2}}(\ell \ell)}$ (left), ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ (middle), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (right) for all lepton flavors for the selection defined in Table 1. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${{p_{\mathrm {T}}} ^\text {miss}}$ distributions. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 8-a: Distribution of ${M_{\text {T2}}(\ell \ell)}$ for all lepton flavors for the selection defined in Table 1. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${{p_{\mathrm {T}}} ^\text {miss}}$ distributions. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 8-b: Distribution of ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ for all lepton flavors for the selection defined in Table 1. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${{p_{\mathrm {T}}} ^\text {miss}}$ distributions. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 8-c: Distribution of ${{p_{\mathrm {T}}} ^\text {miss}}$ for all lepton flavors for the selection defined in Table 1. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${{p_{\mathrm {T}}} ^\text {miss}}$ distributions. The hatched band shows the uncertainties discussed in the text.
png pdf	Figure 9: Predicted backgrounds and observed yields in the $\mathrm{e} \mathrm{e} $ and $\mu \mu $ search regions (left) and the $\mathrm{e} \mu $ search regions (right). The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 9-a: Predicted backgrounds and observed yields in the $\mathrm{e} \mathrm{e} $ and $\mu \mu $ search regions. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 9-b: Predicted backgrounds and observed yields in the $\mathrm{e} \mu $ search region. The hatched band shows the uncertainties discussed in the text.
png pdf root	Figure 10: Predicted backgrounds and observed yields in the $\mathrm{e} \mathrm{e} $, $\mu \mu $, and $\mathrm{e} \mu $ search regions combined. The hatched band shows the uncertainties discussed in the text.
png pdf	Figure 11: Expected and observed limits for the T2tt model with $\tilde{\mathrm{t}}_{1} \to \mathrm{t} \tilde{\chi}^0_1 $ decays (left) and for the T2bW model with $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \tilde{ \chi }^{+}_{1} \to \mathrm{b} \mathrm{W^{+}} \tilde{\chi}^0_1 $ decays (right) in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane. The color indicates the 95% CL upper limit on the cross section times the square of the branching fraction at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL assuming 100% branching fraction, 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. The thin black lines show the effect of the theoretical uncertainties in the signal cross section.
png pdf root	Figure 11-a: Expected and observed limits for the T2tt model with $\tilde{\mathrm{t}}_{1} \to \mathrm{t} \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane. The color indicates the 95% CL upper limit on the cross section times the square of the branching fraction at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL assuming 100% branching fraction, 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. The thin black lines show the effect of the theoretical uncertainties in the signal cross section.
png pdf root	Figure 11-b: Expected and observed limits for the T2bW model with $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \tilde{ \chi }^{+}_{1} \to \mathrm{b} \mathrm{W^{+}} \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane. The color indicates the 95% CL upper limit on the cross section times the square of the branching fraction at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL assuming 100% branching fraction, 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. The thin black lines show the effect of the theoretical uncertainties in the signal cross section.
png pdf	Figure 12: Expected and observed limits for the T8bb$\ell\ell\nu\nu$ model with $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \tilde{ \chi }^{+}_{1} \to \mathrm{b} \nu \tilde{\ell} \to \mathrm{b} \nu \ell \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane for three different mass configurations defined by $m_{\tilde{\ell}} = x + (m_{\tilde{ \chi }^{+}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.05 (upper left), $x=$ 0.5 (upper right), and $x=$ 0.95 (lower). The description of curves is the same as in the caption of Fig. 11.
png pdf root	Figure 12-a: Expected and observed limits for the T8bb$\ell\ell\nu\nu$ model with $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \tilde{ \chi }^{+}_{1} \to \mathrm{b} \nu \tilde{\ell} \to \mathrm{b} \nu \ell \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane for three different mass configurations defined by $m_{\tilde{\ell}} = x + (m_{\tilde{ \chi }^{+}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.05. The description of curves is the same as in the caption of Fig. 11.
png pdf root	Figure 12-b: Expected and observed limits for the T8bb$\ell\ell\nu\nu$ model with $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \tilde{ \chi }^{+}_{1} \to \mathrm{b} \nu \tilde{\ell} \to \mathrm{b} \nu \ell \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane for three different mass configurations defined by $m_{\tilde{\ell}} = x + (m_{\tilde{ \chi }^{+}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.5. The description of curves is the same as in the caption of Fig. 11.
png pdf root	Figure 12-c: Expected and observed limits for the T8bb$\ell\ell\nu\nu$ model with $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \tilde{ \chi }^{+}_{1} \to \mathrm{b} \nu \tilde{\ell} \to \mathrm{b} \nu \ell \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane for three different mass configurations defined by $m_{\tilde{\ell}} = x + (m_{\tilde{ \chi }^{+}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.95. The description of curves is the same as in the caption of Fig. 11.
png pdf	Figure 13: Expected and observed limits for the T2tt model with $\tilde{\mathrm{t}}_{1} \to \mathrm{t} \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane combining the dilepton final state with the single-lepton [19] and the all-hadronic [20] final states as described in the text. The color indicates the 95% CL upper limit on the cross section times the square of the branching fraction at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL assuming 100% branching fraction, 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. The thin black lines show the effect of the theoretical uncertainties in the signal cross section. The green short-dashed, blue dotted, and long-short-dashed orange curves show the expected individual limits for the all-hadronic, single-lepton, and dilepton analyses, respectively. The whited out area on the diagonal corresponds to configurations where the mass difference between $\tilde{\mathrm{t}}_{1}$ and $\tilde{\chi}^0_1$ is very close to the top quark mass. In this region the signal acceptance strongly depends on the $\tilde{\chi}^0_1$ mass and is therefore hard to model.
png pdf	Figure 14: Expected and observed limits for the T2bW model with $\tilde{\mathrm{t}}_{1} \to \mathrm{b} \tilde{ \chi }^{+}_{1} \to \mathrm{b} \mathrm{W^{+}} \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane combining the dilepton final state with the all-hadronic [20] and the single-lepton [19] final states as described in the text. The mass of the chargino is chosen to be $(m_{\tilde{\mathrm{t}}_{1}} + m_{\tilde{\chi}^0_1})/2$. The description of curves is the same as in the caption of Fig. 13.
png pdf	Figure 15: The 95% CL expected (dashed line) and observed limits (solid line) on $\mu =\sigma /\sigma _{\text {theory}}$ for a fermionic DM particle with $m_{\chi} = $ 1 GeV assuming different scalar (left) and pseudoscalar (right) 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 $g_{\mathrm{q}}=g_\mathrm {DM}=$ 1. The gray hashed band around the observed limit corresponds to a 30% theory uncertainty in the inclusive signal cross section.
png pdf	Figure 15-a: The 95% CL expected (dashed line) and observed limits (solid line) on $\mu =\sigma /\sigma _{\text {theory}}$ 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 $g_{\mathrm{q}}=g_\mathrm {DM}=$ 1. The gray hashed band around the observed limit corresponds to a 30% theory uncertainty in the inclusive signal cross section.
png pdf	Figure 15-b: The 95% CL expected (dashed line) and observed limits (solid line) on $\mu =\sigma /\sigma _{\text {theory}}$ 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 $g_{\mathrm{q}}=g_\mathrm {DM}=$ 1. The gray hashed band around the observed limit corresponds to a 30% theory uncertainty in the inclusive signal cross section.

Tables
png pdf	Table 1: Overview of the preselection requirements.
png pdf	Table 2: Definition of the signal regions. The regions are further split into different- and same-flavor regions.
png pdf	Table 3: Relative systematic uncertainties in the background yields in the signal regions. Where given, ranges represent the minimal and maximal changes in yield across all signal regions.
png pdf	Table 4: Total expected background and event yields in data in each of the signal regions for same-flavor ($\mathrm{e^{+}} \mathrm{e^{-}} /\mu^{+} \mu^{-} $), different-flavor ($\mathrm{e^{\pm}} \mu^{\mp} $), and all channels combined with all the systematic uncertainties included as described in Section 7.
png pdf	Table 5: Ratios $\mu =\sigma /\sigma _{\text {theory}}$ of the 95% CL expected and observed limits to simplified model expectations for different DM particle masses and mediator masses and for scalar ($\phi $) and pseudoscalar ($\mathrm {a}$) mediators under the assumption $g_{\mathrm{q}}=g_\mathrm {DM}=$ 1. The uncertainties reflect the 68% band of the expected limits.
png pdf	Table 6: Expected and observed event yields, summed over all lepton flavors, in the aggregate signal regions defined by the selection requirements in the table.
png pdf	Table 7: Covariance (top) and correlation matrix (bottom) for the background prediction in the aggregate signal regions described in Table 6.

Summary

A search was presented for top squark pair production and dark matter in final states with two leptons, b jets, and large missing transverse momentum in data corresponding to an integrated luminosity of 35.9 fb$^{-1}$ in pp collisions collected at a center-of-mass energy of 13 TeV in the CMS detector at the LHC. An efficient background reduction using dedicated kinematic variables was achieved, suppressing by several orders of magnitude the large background from standard model dilepton $ \mathrm{ t \bar{t} } $ events. With no evidence observed for a deviation from the expected background from the standard model, results were interpreted in several simplified models for supersymmetric top squark pair production as well as through the production of a spin-0 dark matter mediator in association with a $ \mathrm{ t \bar{t} } $ pair. In the T2tt model with $\tilde{ \mathrm{t} }_{1} \to \mathrm{t}\tilde{ \chi }^{0}_{1}$ decays, $\tilde{ \mathrm{t} }_{1}$ masses $<$800 GeV and $\tilde{ \chi }^{0}_{1}$ masses $<$360 GeV are excluded. In the T2bW model with $\tilde{ \mathrm{t} }_{1} \to \mathrm{b}\tilde{ \chi }^{+}_{1} \to \mathrm{b}\mathrm{W}^{+}\tilde{ \chi }^{0}_{1}$ decays, $\tilde{ \mathrm{t} }_{1}$ masses $<$750 GeV and $\tilde{ \chi }^{0}_{1}$ masses $<$320 GeV are excluded, assuming the chargino mass to be the mean of the $\tilde{ \mathrm{t} }_{1}$ and the $\tilde{ \chi }^{0}_{1}$ masses. In the newly considered T8bb$\ell\ell\nu\nu$ model with decays $\tilde{ \mathrm{t} }_{1} \to \mathrm{b}\tilde{ \chi }^{+}_{1} \to \mathrm{b}\nu\tilde{\ell} \to \mathrm{b}\nu\ell\tilde{ \chi }^{0}_{1}$, and therefore 100% branching to dilepton final states, the sensitivity depends on the intermediate particle masses. With the chargino mass again taken as the mean of the $\tilde{ \mathrm{t} }_{1}$ and the $\tilde{ \chi }^{0}_{1}$ masses, the strongest exclusion is obtained if the slepton mass is close to the chargino mass. In this case, excluded masses reach up to 1.3 TeV for $\tilde{ \mathrm{t} }_{1}$ and 800 GeV for $\tilde{ \chi }^{0}_{1}$. The T2tt and T2bW results were combined with complementary searches in the all-jet and single-lepton channels, providing exclusions in the T2tt model of $\tilde{ \mathrm{t} }_{1}$ mass $<$1050 GeV for a massless $\tilde{ \chi }^{0}_{1}$, and a $\tilde{ \chi }^{0}_{1}$ mass of $<$500 GeV for a $\tilde{ \mathrm{t} }_{1}$ mass of 900 GeV. In the same way, the T2bW model is excluded for $\tilde{ \mathrm{t} }_{1}$ mass $<$1000 GeV for a massless $\tilde{ \chi }^{0}_{1}$, and a $\tilde{ \chi }^{0}_{1}$ mass of $<$450 GeV for a $\tilde{ \mathrm{t} }_{1}$ mass of 900 GeV. The combination extends the sensitivity by $\approx$50 GeV in the masses of both $\tilde{ \mathrm{t} }_{1}$ and $\tilde{ \chi }^{0}_{1}$ in the T2bW model, and by similar values in the T2tt model, when the difference between these masses is $\approx$200 GeV. Aggregate search regions were presented that can be used to reinterpret the results in a wider range of theoretical models of new physics that give rise to the chosen final state. In addition, the results were interpreted in a simplified model with a dark matter candidate particle coupled to the top quark via a scalar or a pseudoscalar mediator. Within the assumptions of the model, a scalar mediator with a mass up to 100 GeV and a pseudoscalar mediator with a mass up to 50 GeV are excluded for a dark matter candidate mass of 1 GeV. The result for the scalar mediator achieves some of the most stringent limits to date in this model.

Additional Figures
png pdf root	Additional Figure 1: Correlation matrix of the 13 signal regions, split into same-flavor (SF) and different-flavor (DF) regions.
png pdf root	Additional Figure 2: Covariance matrix of the 13 signal regions, split into same-flavor (SF) and different-flavor (DF) regions.
png pdf root	Additional Figure 3: Expected and observed limits for the T2tt model with $\tilde{\mathrm{t}}_{1} \to \mathrm{t} \tilde{\chi}^0_1 $ decays in the $m_{\tilde{\mathrm{t}}_{1}}$-$m_{\tilde{\chi}^0_1}$ mass plane, using aggregate signal regions. The color indicates the 95% CL upper limit on the cross section times the square of the branching fraction at each point in the plane. The area below the thick black curve represents the observed exclusion region at 95% CL assuming 100% branching fraction, 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. The thin black lines show the effect of the theoretical uncertainties in the signal cross section.

Additional Tables
png pdf	Additional Table 1: Cutflow table for two different configurations of T2tt SUSY signal models (left) and DM signal models (right). Numbers are normalized to an integrated luminosity of 35.9 fb$^{-1}$ and 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.
png pdf	Additional Table 2: Total expected signal and statistical uncertainties for the signal benchmark points in the same-flavor regions, normalized to an integrated luminosity of 35.9 fb$^{-1}$. All mass parameters are given in GeV.
png pdf	Additional Table 3: Total expected signal and statistical uncertainties for the signal benchmark points in the different-flavor regions, normalized to an integrated luminosity of 35.9 fb$^{-1}$. All mass parameters are given in GeV.
png pdf	Additional Table 4: Total expected signal and statistical uncertainties for the signal benchmark points in the combined same-flavor and different-flavor regions, normalized to an integrated luminosity of 35.9 fb$^{-1}$. All mass parameters are given in GeV.
png pdf	Additional Table 5: Total expected signal and statistical uncertainties for the signal benchmark points in the aggregate signal regions, normalized to an integrated luminosity of 35.9 fb$^{-1}$. All mass parameters are given in GeV.
png pdf	Additional Table 6: Total expected signal and statistical uncertainties for additional signal benchmark points in the same-flavor regions, normalized to an integrated luminosity of 35.9 fb$^{-1}$. The chargino mass is always assumed as $m_{\tilde{\chi}^{+} _1} = (m_{\tilde{\mathrm{t}}_{1}} + m_{\tilde{\chi}^{0} _1})/2$, and the neutralino mass is set to $m_{\tilde{\chi}^{0} _1} = $ 1 GeV for all benchmark points.
png pdf	Additional Table 7: Total expected signal and statistical uncertainties for additional signal benchmark points in the different-flavor regions, normalized to an integrated luminosity of 35.9 fb$^{-1}$. The chargino mass is always assumed as $m_{\tilde{\chi}^{+} _1} = (m_{\tilde{\mathrm{t}}_{1}} + m_{\tilde{\chi}^{0} _1})/2$, and the neutralino mass is set to $m_{\tilde{\chi}^{0} _1} = $ 1 GeV for all benchmark points.
png pdf	Additional Table 8: Total expected signal and statistical uncertainties for additional signal benchmark points in the same-flavor and different-flavor regions combined, normalized to an integrated luminosity of 35.9 fb$^{-1}$. The chargino mass is always assumed as $m_{\tilde{\chi}^{+} _1} = (m_{\tilde{\mathrm{t}}_{1}} + m_{\tilde{\chi}^{0} _1})/2$, and the neutralino mass is set to $m_{\tilde{\chi}^{0} _1} = $ 1 GeV for all benchmark points.
png pdf	Additional Table 9: Total expected signal and statistical uncertainties for additional signal benchmark points in the aggregate signal regions, normalized to an integrated luminosity of 35.9 fb$^{-1}$. The chargino mass is always assumed as $m_{\tilde{\chi}^{+} _1} = (m_{\tilde{\mathrm{t}}_{1}} + m_{\tilde{\chi}^{0} _1})/2$, and the neutralino mass is set to $m_{\tilde{\chi}^{0} _1} = $ 1 GeV for all benchmark points.

UFO Model for ttbar+DM Pseudoscalar mediator UFO Model for ttbar+DM Scalar mediator Example LHE Header File for DM model

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