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| CMS-PAS-SUS-19-011 | ||
| Search for top squark pair production in the dilepton final state using 137 fb$^{-1}$ of proton-proton collision integrated luminosity at $\sqrt{s} =$ 13 TeV | ||
| CMS Collaboration | ||
| May 2020 | ||
| Abstract: A search for supersymmetric partners of the top quark is presented in final states with two oppositely charged leptons (electrons or muons), jets identified as originating from b quarks, and missing transverse momentum. The search uses data from proton-proton collisions at $\sqrt{s}= $ 13 TeV collected from 2016 to 2018 using the CMS detector, amounting to 137 fb$^{-1}$ of integrated luminosity. Hypothetical signal events are efficiently separated from the dominant top-quark pair background with requirements on the significance of missing transverse momentum 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 925 GeV and on the lightest neutralino up to 450 GeV. If the decay proceeds via an intermediate chargino, the exclusion limit at 95% confidence level of the lightest top squark reaches up to 850 GeV for neutralino masses below 420 GeV. For top squarks undergoing a cascade decay through charginos and sleptons, the mass limits reach up to 1.4 TeV for the top squark and up to 900 GeV for the lightest neutralino. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, EPJC 81 (2021) 3. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Diagrams for simplified SUSY models with strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \bar{\tilde{\mathrm{t}}} _{1}$. In the T2tt model (left), the top squark decays to a top quark and a $\tilde{\chi}^0_1$. In the T2bW model (center), the top squark decays into a b quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a W boson and a $\tilde{\chi}^0_1$. The decay of the intermediate $\tilde{\chi}^{\pm}_1$ to a slepton $\ell^{\pm}$ that yields $\nu \tilde{\chi}^0_1 $ and a $\ell ^\pm $ from the virtual slepton decay is described by the T8bb$ \ell \ell \nu \nu$ model (right). |
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Figure 1-a:
Diagrams for simplified SUSY models with strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \bar{\tilde{\mathrm{t}}} _{1}$. In the T2tt model (left), the top squark decays to a top quark and a $\tilde{\chi}^0_1$. In the T2bW model (center), the top squark decays into a b quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a W boson and a $\tilde{\chi}^0_1$. The decay of the intermediate $\tilde{\chi}^{\pm}_1$ to a slepton $\ell^{\pm}$ that yields $\nu \tilde{\chi}^0_1 $ and a $\ell ^\pm $ from the virtual slepton decay is described by the T8bb$ \ell \ell \nu \nu$ model (right). |
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Figure 1-b:
Diagrams for simplified SUSY models with strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \bar{\tilde{\mathrm{t}}} _{1}$. In the T2tt model (left), the top squark decays to a top quark and a $\tilde{\chi}^0_1$. In the T2bW model (center), the top squark decays into a b quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a W boson and a $\tilde{\chi}^0_1$. The decay of the intermediate $\tilde{\chi}^{\pm}_1$ to a slepton $\ell^{\pm}$ that yields $\nu \tilde{\chi}^0_1 $ and a $\ell ^\pm $ from the virtual slepton decay is described by the T8bb$ \ell \ell \nu \nu$ model (right). |
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Figure 1-c:
Diagrams for simplified SUSY models with strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \bar{\tilde{\mathrm{t}}} _{1}$. In the T2tt model (left), the top squark decays to a top quark and a $\tilde{\chi}^0_1$. In the T2bW model (center), the top squark decays into a b quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a W boson and a $\tilde{\chi}^0_1$. The decay of the intermediate $\tilde{\chi}^{\pm}_1$ to a slepton $\ell^{\pm}$ that yields $\nu \tilde{\chi}^0_1 $ and a $\ell ^\pm $ from the virtual slepton decay is described by the T8bb$ \ell \ell \nu \nu$ model (right). |
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Figure 2:
Distribution of ${\mathcal {S}}$ in a $\mathrm{Z} \to \ell \ell $ selection, requiring a same-flavor (SF) lepton pair. Events with no genuine ${{p_{\mathrm {T}}} ^\text {miss}}$ such as Drell-Yan follow a $\chi ^2$ distribution with two degrees of freedom (red line). Processes with true ${{p_{\mathrm {T}}} ^\text {miss}}$ such as ${\mathrm{t} \mathrm{\bar{t}}}$ or production of two or more W or Z bosons populate high values of the ${\mathcal {S}}$ distribution. |
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Figure 3:
${M_{\text {T2}}(\ell \ell)}$, ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$ distributions in validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_\text {b jets}} =$ 0, combining the same- and opposite-flavor channel. All other event selection requirements are applied. For ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required. The individual processes are scaled using their measured respective scale factors, as described in the text. The hashed band represents the experimental systematic uncertainties and the uncertainties in the scale factors as discussed in the text. |
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Figure 3-a:
${M_{\text {T2}}(\ell \ell)}$, ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$ distributions in validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_\text {b jets}} =$ 0, combining the same- and opposite-flavor channel. All other event selection requirements are applied. For ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required. The individual processes are scaled using their measured respective scale factors, as described in the text. The hashed band represents the experimental systematic uncertainties and the uncertainties in the scale factors as discussed in the text. |
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Figure 3-b:
${M_{\text {T2}}(\ell \ell)}$, ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$ distributions in validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_\text {b jets}} =$ 0, combining the same- and opposite-flavor channel. All other event selection requirements are applied. For ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required. The individual processes are scaled using their measured respective scale factors, as described in the text. The hashed band represents the experimental systematic uncertainties and the uncertainties in the scale factors as discussed in the text. |
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Figure 3-c:
${M_{\text {T2}}(\ell \ell)}$, ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$ distributions in validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_\text {b jets}} =$ 0, combining the same- and opposite-flavor channel. All other event selection requirements are applied. For ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required. The individual processes are scaled using their measured respective scale factors, as described in the text. The hashed band represents the experimental systematic uncertainties and the uncertainties in the scale factors as discussed in the text. |
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Figure 4:
Distributions of ${M_{\text {T2}}(\ell \ell)}$ (left), ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ (middle), and ${\mathcal {S}}$ (right) for all lepton flavors for the selection defined in Table 2. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$. |
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Figure 4-a:
Distributions of ${M_{\text {T2}}(\ell \ell)}$ (left), ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ (middle), and ${\mathcal {S}}$ (right) for all lepton flavors for the selection defined in Table 2. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$. |
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Figure 4-b:
Distributions of ${M_{\text {T2}}(\ell \ell)}$ (left), ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ (middle), and ${\mathcal {S}}$ (right) for all lepton flavors for the selection defined in Table 2. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$. |
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Figure 4-c:
Distributions of ${M_{\text {T2}}(\ell \ell)}$ (left), ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ (middle), and ${\mathcal {S}}$ (right) for all lepton flavors for the selection defined in Table 2. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell \mathrm{b} \ell)}$ and ${\mathcal {S}}$. |
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Figure 5:
Predicted and observed yields in the signal and control regions as defined in Table 3 and 4. The control regions TTCRSF and TTCROF are defined by $ {M_{\text {T2}}(\ell \ell)} < $ 100 GeV and are used to constrain the ${\mathrm{t} \mathrm{\bar{t}}}$ normalization. The ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{Z}}$ control regions employ a 3 lepton requirement in different ${N_\text {jets}}$ and ${N_\text {b jets}}$ bins. The dilepton invariant mass and ${N_\text {b jets}}$ selections are inverted for CR0-CR12 in order to constrain the Drell-Yan and multiboson normalizations, using only the same-flavor channel. Good agreement between data and the post-fit SM prediction is observed in the control and signal regions. The hashed band reflects the post-fit systematic uncertainties. |
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Figure 6:
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 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 for the decays of the SUSY particles, 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. |
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Figure 6-a:
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 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 for the decays of the SUSY particles, 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. |
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Figure 6-b:
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 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 for the decays of the SUSY particles, 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. |
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Figure 7:
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. 7. |
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Figure 7-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 (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. 7. |
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Figure 7-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 $x=$ 0.05 (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. 7. |
png pdf |
Figure 7-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 $x=$ 0.05 (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. 7. |
| Tables | |
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Table 1:
Cross section normalization order, event generator, and perturbative order for each simulated background process. |
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Table 2:
Overview of the event selection requirements. |
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Table 3:
Definition of the signal regions. The regions are further split into different- and same-flavor regions. The preselection in Table 2 is applied in all regions. |
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Table 4:
Definition of control regions. The preselection in Table 2 is applied in all regions. |
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Table 5:
Typical values (90% quantiles) and maximal values of the systematic uncertainties in all signal regions. |
| Summary |
|
A search for top squark pair production in final states with two leptons with opposite charge, b jets, and significant missing transverse momentum is presented. The data set of pp collisions corresponds to an integrated luminosity of 137 fb$^{-1}$ is used that was collected with the CMS detector from 2016 to 2018 at a center-of-mass energy of 13 TeV. Transverse mass variables and the significance of missing transverse momentum are used to efficiently suppress backgrounds from SM processes. No evidence for a deviation from the expected background is observed, and results are interpreted in several simplified models for supersymmetric top squark pair production. In the T2tt model with $\tilde{\mathrm{t}}_{1} \to \mathrm{t}\tilde{\chi}^0_1$ decays, $\tilde{\mathrm{t}}_{1}$ masses up to 925 GeV and $\tilde{\chi}^0_1$ masses up to 450 GeV are excluded. In the TbW 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 up to 850 GeV and $\tilde{\chi}^0_1$ masses up to 420 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 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 1.4 TeV for $\tilde{\mathrm{t}}_{1}$ and 900 GeV for $\tilde{\chi}^0_1$. When the slepton mass and the chargino mass are similar, these numbers reduce to 1.3 TeV for $\tilde{\mathrm{t}}_{1}$ and 750 GeV for $\tilde{\chi}^0_1$. A further reduction to 1.2 TeV for $\tilde{\mathrm{t}}_{1}$ and to 100 GeV for $\tilde{\chi}^0_1$ is observed when the slepton mass is close to the neutralino mass. |
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