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| CMS-SUS-19-011 ; CERN-EP-2020-131 | ||
| Search for top squark pair production using dilepton final states in pp collision data collected at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 13 August 2020 | ||
| Eur. Phys. J. C 81 (2021) 3 | ||
| Abstract: A search is presented for supersymmetric partners of the top quark (top squarks) 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 with the CMS detector, corresponding to an integrated luminosity of 137 fb$^{-1}$. Hypothetical signal events are efficiently separated from the dominant top quark pair production background with requirements on the significance of the missing transverse momentum and on 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 lightest top squarks. For top squarks decaying exclusively to a top quark and a lightest neutralino, lower limits are placed at 95% confidence level on the masses of the top squark and the neutralino up to 925 and 450 GeV, respectively. If the decay proceeds via an intermediate chargino, the corresponding lower limits on the mass of the lightest top squark are set 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 and 900 GeV respectively for the top squark and the lightest neutralino. | ||
| Links: e-print arXiv:2008.05936 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures & Tables | Summary | Additional Tables | References | CMS Publications |
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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 bottom 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 $\tilde{\chi}^{\pm}_1$, which yields a $\nu$, plus a $\tilde{\chi}^0_1$ and a $\ell ^\pm $ from the decay of a virtual slepton $\tilde{\ell}^{\pm}$, is described by the T8bb$\ell\ell\nu\nu$ model (right). |
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Figure 1-a:
Diagram for the T2tt simplified SUSY model with strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \bar{\tilde{\mathrm{t}}} _{1}$: the top squark decays to a top quark and a $\tilde{\chi}^0_1$. |
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Figure 1-b:
Diagram for the T2bW simplified SUSY model with strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \bar{\tilde{\mathrm{t}}} _{1}$: the top squark decays into a bottom quark and an intermediate $\tilde{\chi}^{\pm}_1$ that further decays into a W boson and a $\tilde{\chi}^0_1$. |
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Figure 1-c:
Diagram for the T8bb$\ell\ell\nu\nu$ simplified SUSY model with strong production of top squark pairs $\tilde{\mathrm{t}}_{1} \bar{\tilde{\mathrm{t}}} _{1}$: the decay of the intermediate $\tilde{\chi}^{\pm}_1$, which yields a $\nu$, plus a $\tilde{\chi}^0_1$ and a $\ell ^\pm $ from the decay of a virtual slepton $\tilde{\ell}^{\pm}$. |
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Figure 2:
Distribution of ${{p_{\mathrm {T}}} ^\text {miss}}$ significance ${\mathcal {S}}$ in a $\mathrm{Z} \to \ell \ell $ selection, requiring an SF lepton pair. Points with error bars represent the data, and the stacked histograms the SM backgrounds predicted as described in Section 6, with uncertainty in the SM prediction indicated by the hatched area. The red line represents a $\chi ^2$ distribution with two degrees of freedom. The last bin includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds. The relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 3:
The ${M_{\text {T2}}(\ell \ell)}$, ${M_{\text {T2}}(\mathrm{b} \ell b \ell)}$, and ${\mathcal {S}}$ distributions in the validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_{\mathrm{b}}} =$ 0, combining the SF and DF channels. All other event selection requirements are applied. For the ${M_{\text {T2}}(\mathrm{b} \ell b \ell)}$ and ${\mathcal {S}}$ distributions, $ {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 hatched band represents the experimental systematic uncertainties and the uncertainties in the scale factors. The last bin in each distribution includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds. The relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 3-a:
The ${M_{\text {T2}}(\ell \ell)}$ distribution in the validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_{\mathrm{b}}} =$ 0, combining the SF and DF channels. All other event selection requirements are applied. The individual processes are scaled using their measured respective scale factors, as described in the text. The hatched band represents the experimental systematic uncertainties and the uncertainties in the scale factors. The last bin includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds. The relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 3-b:
The ${M_{\text {T2}}(\mathrm{b} \ell b \ell)}$ distribution in the validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_{\mathrm{b}}} =$ 0, combining the SF and DF channels. All other event selection requirements are applied. The condition $ {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 hatched band represents the experimental systematic uncertainties and the uncertainties in the scale factors. The last bin includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds. The relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 3-c:
The ${\mathcal {S}}$ distribution in the validation regions requiring $ {N_\text {jets}} \geq $ 2 and $ {N_{\mathrm{b}}} =$ 0, combining the SF and DF channels. All other event selection requirements are applied. The condition $ {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 hatched band represents the experimental systematic uncertainties and the uncertainties in the scale factors. The last bin includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds. The relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 4:
Distributions of ${M_{\text {T2}}(\ell \ell)}$ (left), ${M_{\text {T2}}(\mathrm{b} \ell b \ell)}$ (middle), and ${\mathcal {S}}$ (right) for all lepton flavors for the preselection defined in Table 2. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required for the ${M_{\text {T2}}(\mathrm{b} \ell b \ell)}$ and ${\mathcal {S}}$ distributions. The last bin in each distribution includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds and the relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 4-a:
Distribution of ${M_{\text {T2}}(\ell \ell)}$ for all lepton flavors for the preselection defined in Table 2. The last bin includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds and the relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 4-b:
Distribution of ${M_{\text {T2}}(\mathrm{b} \ell b \ell)}$ for all lepton flavors for the preselection defined in Table 2. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required. The last bin includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds and the relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 4-c:
Distribution of ${\mathcal {S}}$ for all lepton flavors for the preselection defined in Table 2. Additionally, $ {M_{\text {T2}}(\ell \ell)} > $ 100 GeV is required. The last bin includes the overflow events. The lower panel gives the ratio between the observation and the predicted SM backgrounds and the relative uncertainty in the SM background prediction is shown as a hatched band. |
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Figure 5:
Predicted and observed yields in the signal and control regions as defined in Tables 3 and 4. The control regions TTCRSF and TTCRDF 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 ${N_{\mathrm{b}}}$ bins. The dilepton invariant mass and ${N_{\mathrm{b}}}$ selections are inverted for CR0-CR12 in order to constrain the Drell-Yan and multiboson normalizations, using only the SF channel. The lower panel gives the ratio between the observation and the predicted SM backgrounds. The hatched 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 b \tilde{\chi}^{\pm}_{1} \to 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. The small white area on the diagonal in the left figure 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. |
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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 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. The small white 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. |
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Figure 6-b:
Expected and observed limits for the T2bW model with $\tilde{\mathrm{t}}_{1} \to b \tilde{\chi}^{\pm}_{1} \to 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 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 b \tilde{\chi}^{\pm}_{1} \to b \nu \tilde{\ell} \to 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 $m_{\tilde{\ell}} = x \, (m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.05 (upper left), $x=$ 0.50 (upper right), and $x=$ 0.95 (lower). The description of curves is the same as in the caption of Fig. 6. |
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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 b \tilde{\chi}^{\pm}_{1} \to b \nu \tilde{\ell} \to 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 $m_{\tilde{\ell}} = x \, (m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.05. The description of curves is the same as in the caption of Fig. 6. |
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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 b \tilde{\chi}^{\pm}_{1} \to b \nu \tilde{\ell} \to 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 $m_{\tilde{\ell}} = x \, (m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.50. The description of curves is the same as in the caption of Fig. 6. |
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Figure 7-c:
Expected and observed limits for the T8bb$\ell\ell\nu\nu$ model with $\tilde{\mathrm{t}}_{1} \to b \tilde{\chi}^{\pm}_{1} \to b \nu \tilde{\ell} \to 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 $m_{\tilde{\ell}} = x \, (m_{\tilde{\chi}^{\pm}_{1}} - m_{\tilde{\chi}^0_1}) + m_{\tilde{\chi}^0_1}$ with $x=$ 0.95. The description of curves is the same as in the caption of Fig. 6. |
| Tables | |
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Table 1:
Event generator and orders of accuracy for each simulated background process. |
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Table 2:
Overview of the event preselection requirements. |
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Table 3:
Definition of the signal regions. The regions are further split into SF and DF regions. The preselection in Table 2 is applied to all regions. |
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Table 4:
Definition of the control regions. The preselection in Table 2 is applied to all regions. |
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Table 5:
Typical values (90% quantiles) and maximum values of the systematic uncertainties in all signal regions. |
| Summary |
|
A search for top squark pair production in final states with two opposite-charge leptons, b jets, and significant missing transverse momentum (${p_{\mathrm{T}}}^{\text{miss}})$ is presented. The data set of proton-proton collisions corresponds to an integrated luminosity of 137 fb$^{-1}$ and was collected with the CMS detector at a center-of-mass energy of 13 TeV. Transverse mass variables and the significance of ${p_{\mathrm{T}}}^{\text{miss}}$ are used to efficiently suppress backgrounds from standard model processes. No evidence for a deviation from the expected background is observed. The results are interpreted in several simplified models for supersymmetric top squark pair production and decay. 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 $\tilde{\mathrm{t}}_{1} \to \mathrm{b}\tilde{\chi}^{\pm}_{1} \to \mathrm{b}\mathrm{W^{+}}\tilde{\chi}^0_1$ 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 $\tilde{\chi}^0_1$ masses. In the T8bb$\ell\ell\nu\nu$ model with decays $\tilde{\mathrm{t}}_{1} \to \mathrm{b}\tilde{\chi}^{\pm}_{1} \to \mathrm{b}\nu\tilde{\ell} \to \mathrm{b}\nu\ell\tilde{\chi}^0_1$, therefore 100% branching fraction 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 $\tilde{\chi}^0_1$ 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 is taken as the mean of the chargino and neutralino masses, these numbers decrease 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. |
| Additional Tables | |
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Additional Table 1:
Cutflow table for three different configurations of T2tt SUSY signal models. Numbers are normalized to an integrated luminosity of 137fb$^{-1}$ and shown for top squark and neutralino masses of 350 and 150 GeV, 600 and 300 GeV, and 800 and 100 GeV, respectively. |
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