CMS-PAS-SUS-18-003

CMS-PAS-SUS-18-003
Search for the pair production of light top squarks in the $\mathrm{e}\mu$ final state in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
September 2018

Abstract: A search for the production of a pair of top squarks in proton-proton collisions at the CERN LHC is presented. This search targets a region of the parameter space where the kinematics of top squark pair production and top quark pair production are very similar, due to the mass of the neutralino being small. The search is performed with 36.9 fb$^{-1}$ of proton-proton collisions collected by the CMS detector, using events containing one opposite-sign electron-muon pair. The search is based on a precise estimation of the $\mathrm{t}\bar{\mathrm{t}}$ background, and the use of the $M_{\textrm{T2}}$ variable that combines the transverse mass of each lepton and the missing transverse energy. No excess of events is found over the standard model predictions and exclusion limits are placed at 95% confidence level on the production of top squarks up to masses of 200 GeV for models with $m_{\widetilde{\mathrm{t}_1}} - m_{\widetilde{\chi}^{0}_{1}} \simeq m_{\mathrm{top}}$.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 03 (2019) 101. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Diagram of the top squark-antisquark production with further decay into a top quark and the lightest neutralino.
png pdf	Figure 2: Normalized ${M_{\textrm {T2}}}$ distributions using variables at generator level for ${{\mathrm {t}\overline {\mathrm {t}}}}$ and simulated signal events with two generated leptons with ${p_{\mathrm {T}}}$ of at least 20 GeV and $\| \eta \| \leq $ 2.4. The difference in the high values of ${M_{\textrm {T2}}}$ increases significantly for points with $\Delta m = {{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}$ is different from the top quark mass (left), or for signal models with higher neutralino masses when keeping $\Delta m = {{m}_{\mathrm {top}}} $ (right).
png pdf	Figure 2-a: Normalized ${M_{\textrm {T2}}}$ distribution using variables at generator level for ${{\mathrm {t}\overline {\mathrm {t}}}}$ and simulated signal events with two generated leptons with ${p_{\mathrm {T}}}$ of at least 20 GeV and $\| \eta \| \leq $ 2.4. The difference in the high values of ${M_{\textrm {T2}}}$ increases significantly for points with $\Delta m = {{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}$ is different from the top quark mass.
png pdf	Figure 2-b: Normalized ${M_{\textrm {T2}}}$ distribution using variables at generator level for ${{\mathrm {t}\overline {\mathrm {t}}}}$ and simulated signal events with two generated leptons with ${p_{\mathrm {T}}}$ of at least 20 GeV and $\| \eta \| \leq $ 2.4. The difference in the high values of ${M_{\textrm {T2}}}$ increases significantly for signal models with higher neutralino masses when keeping $\Delta m = {{m}_{\mathrm {top}}} $.
png pdf	Figure 3: Distribution 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 Sec. 7. The last bin contains the overflow events.
png pdf	Figure 3-a: Distribution for leading lepton ${p_{\mathrm {T}}}$. The uncertainty band includes statistical and all systematic uncertainties described in Sec. 7. The last bin contains the overflow events.
png pdf	Figure 3-b: Distribution for subleading lepton ${p_{\mathrm {T}}}$. The uncertainty band includes statistical and all systematic uncertainties described in Sec. 7. The last bin contains the overflow events.
png pdf	Figure 3-c: Distribution for $\Delta \phi ({\mathrm {e}}, {{\mu}})$. The uncertainty band includes statistical and all systematic uncertainties described in Sec. 7. The last bin contains the overflow events.
png pdf	Figure 3-d: Distribution for ${{p_{\mathrm {T}}} ^\text {miss}}$. The uncertainty band includes statistical and all systematic uncertainties described in Sec. 7. The last bin contains the overflow events.
png pdf	Figure 4: Expected and observed ${M_{\textrm {T2}}}$ distribution for expected background and data. 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 hashed bands correspond to total systematic and statistical uncertainties.
png pdf	Figure 5: Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for ${{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}= {{m}_{\mathrm {top}}}$ (top left), ${{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}= {{m}_{\mathrm {top}}} +$ 7.5 GeV (top right) and ${{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}= {{m}_{\mathrm {top}}} -$ 7.5 GeV (bottom). The green and yellow bands correspond to the 1$\sigma $ and 2$\sigma $ ranges of the expected upper limits.
png pdf	Figure 5-a: Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for ${{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}= {{m}_{\mathrm {top}}}$. The green and yellow bands correspond to the 1$\sigma $ and 2$\sigma $ ranges of the expected upper limits.
png pdf	Figure 5-b: Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for ${{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}= {{m}_{\mathrm {top}}} +$ 7.5 GeV. The green and yellow bands correspond to the 1$\sigma $ and 2$\sigma $ ranges of the expected upper limits.
png pdf	Figure 5-c: Expected and observed upper limits at 95% CL on the signal strength as a function of the top squark mass for ${{m}_{\tilde{\mathrm {t}}_1}}- {{m}_{{\tilde{\chi}^{0}_{1}}}}= {{m}_{\mathrm {top}}} -$ 7.5 GeV. The green and yellow bands correspond to the 1$\sigma $ and 2$\sigma $ ranges of the expected upper limits.

Summary

A search for a top squark with mass close to the top quark mass is presented, using events with one opposite-sign electron-muon pair, at least two jets, and at least one b jet. The $\tilde{\mathrm{t}} \rightarrow \mathrm{t}\chi$ decay mode is considered, and different top squark masses are explored up to 240 GeV with neutralino masses of $m_{\chi} \simeq m_{\tilde{\mathrm{t}}} - m_{\mathrm{t}} $.
An accurate estimation of the $\mathrm{t \bar{t} }$ background, accounting for uncertainties due to all known theory and experimental effects, is crucial to be able to distinguish a signal that is expected to appear as an excess over the background expectation. The $m_{\mathrm{T}}(\ell\ell)$ variable is used in a binned profile likelihood fit to increase the sensitivity thanks to the slightly different kinematics between the signal and the $\mathrm{t \bar{t} }$ background. Further sensitivity is gained due to the absence of a kinematic endpoint in this variable for the signal, when top squark and neutralino mass difference is greater than the top quark mass or when there are massive neutralinos in the event.
No excess is observed and upper limits are set at 95% CL on the top squark production cross section for top squark masses of up to 210 GeV in models with $ m_{\tilde{\mathrm{t}}} - m_{\mathrm{t}} \simeq m_{\chi} $ and masses up to 240 GeV in models with a mass difference of 7.5 GeV. This result extends the exclusion limits of top squark searches at the LHC to higher top quark masses in the nearly degenerate region that was previously unexplored.

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