CMS-PAS-SUS-19-003

CMS-PAS-SUS-19-003
Search for top squark pair production in a di-tau final state in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
May 2019

Abstract: A search for the pair production of the supersymmetric partner of the top quark, the top squark, produced in proton-proton collision events at $ \sqrt{s} = $ 13 TeV is presented in final states containing hadronically decaying tau leptons and large missing transverse momentum. These final states are highly sensitive to high $ \tan{\beta} $ or higgsino-like scenarios in which decays of electroweak gauginos to tau leptons are dominant. The search uses a dataset corresponding to an integrated luminosity of 77 fb$^{-1}$ which was recorded by the CMS detector during 2016 and 2017. No significant excess is observed with respect to the background prediction, and exclusion limits at 95% confidence level are presented in the top squark and lightest neutralino mass plane within the framework of simplified models. It is found that top squark masses up to 1100 GeV are excluded for a nearly massless neutralino.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 02 (2020) 015. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Top squark pair production in proton-proton collisions at the LHC, producing pairs of b quarks and taus accompanied with neutrinos and LSPs in the final state.
png pdf	Figure 1-a: Top squark pair production in proton-proton collisions at the LHC, producing pairs of b quarks and taus accompanied with neutrinos and LSPs in the final state.
png pdf	Figure 1-b: Top squark pair production in proton-proton collisions at the LHC, producing pairs of b quarks and taus accompanied with neutrinos and LSPs in the final state.
png pdf	Figure 1-c: Top squark pair production in proton-proton collisions at the LHC, producing pairs of b quarks and taus accompanied with neutrinos and LSPs in the final state.
png pdf	Figure 1-d: Top squark pair production in proton-proton collisions at the LHC, producing pairs of b quarks and taus accompanied with neutrinos and LSPs in the final state.
png pdf	Figure 2: Distributions of the search variables ${{p_{\mathrm {T}}} ^\text {miss}}$, ${m_\text {T2}}$, and ${H_{\mathrm {T}}}$ after baseline selections for data and the predicted background. The distributions for a few representative signal points corresponding to $ x = $ 0.5 and [$ m_{\tilde{\mathrm{t}}_{1}} $, $ m_{\tilde{\chi}^0_1} $] = [300, 100] GeV, [500, 350] GeV, and [800, 300] GeV are overlaid.
png pdf	Figure 2-a: Distribution of the search variable ${{p_{\mathrm {T}}} ^\text {miss}}$ after baseline selections for data and the predicted background. The distributions for a few representative signal points corresponding to $ x = $ 0.5 and [$ m_{\tilde{\mathrm{t}}_{1}} $, $ m_{\tilde{\chi}^0_1} $] = [300, 100] GeV, [500, 350] GeV, and [800, 300] GeV are overlaid.
png pdf	Figure 2-b: Distribution of the search variable ${m_\text {T2}}$ after baseline selections for data and the predicted background. The distributions for a few representative signal points corresponding to $ x = $ 0.5 and [$ m_{\tilde{\mathrm{t}}_{1}} $, $ m_{\tilde{\chi}^0_1} $] = [300, 100] GeV, [500, 350] GeV, and [800, 300] GeV are overlaid.
png pdf	Figure 2-c: Distribution of the search variable ${H_{\mathrm {T}}}$ after baseline selections for data and the predicted background. The distributions for a few representative signal points corresponding to $ x = $ 0.5 and [$ m_{\tilde{\mathrm{t}}_{1}} $, $ m_{\tilde{\chi}^0_1} $] = [300, 100] GeV, [500, 350] GeV, and [800, 300] GeV are overlaid.
png pdf	Figure 3: The 15 search regions defined in bins of ${{p_{\mathrm {T}}} ^\text {miss}}$, ${m_\text {T2}}$, and ${H_{\mathrm {T}}}$.
png pdf	Figure 4: Plots showing the purities (top), scale-factors (middle), and $ \text {SF}^{e \mu} - \text {SF}^{\mu \mu} $ (bottom) in the different bins of the ${\mathrm{t} {}\mathrm{\bar{t}}}$ CR for 2016 (left) and 2017 (right) data.
png pdf	Figure 4-a: Plots showing the purities (top), scale-factors (middle), and $ \text {SF}^{e \mu} - \text {SF}^{\mu \mu} $ (bottom) in the different bins of the ${\mathrm{t} {}\mathrm{\bar{t}}}$ CR for 2016 data.
png pdf	Figure 4-b: Plots showing the purities (top), scale-factors (middle), and $ \text {SF}^{e \mu} - \text {SF}^{\mu \mu} $ (bottom) in the different bins of the ${\mathrm{t} {}\mathrm{\bar{t}}}$ CR for 2017 data.
png pdf	Figure 5: Event yields in the 15 search bins as defined in Figure 3. The yields for a few representative signal points corresponding to $ x = $ 0.5 and [$ m_{\tilde{\mathrm{t}}_{1}} $, $ m_{\tilde{\chi}^0_1} $] = [300, 100] GeV, [500, 350] GeV, and [800, 300] GeV are overlaid.
png pdf	Figure 6: The exclusion limits at 95% CL for the pair production of the top squark decaying in a di-tau final state are displayed in the $ m_{\tilde{\mathrm{t}}_{1}} \text {-} m_{\tilde{\chi}^0_1} $ plane for $ x = $ 0.25 (top left), 0.5 (top right) and 0.75 (bottom), as described in Equation 1. The black (red) lines represent the observed (expected) limits. The solid lines represent the central values, and the dashed lines the one standard deviation variation due to the theoretical or experimental uncertainties around the central values. The colour shading corresponds to the observed limit on the cross section.
png pdf	Figure 6-a: The exclusion limits at 95% CL for the pair production of the top squark decaying in a di-tau final state are displayed in the $ m_{\tilde{\mathrm{t}}_{1}} \text {-} m_{\tilde{\chi}^0_1} $ plane for $ x = $ 0.25, as described in Equation 1. The black (red) lines represent the observed (expected) limits. The solid lines represent the central values, and the dashed lines the one standard deviation variation due to the theoretical or experimental uncertainties around the central values. The colour shading corresponds to the observed limit on the cross section.
png pdf	Figure 6-b: The exclusion limits at 95% CL for the pair production of the top squark decaying in a di-tau final state are displayed in the $ m_{\tilde{\mathrm{t}}_{1}} \text {-} m_{\tilde{\chi}^0_1} $ plane for $ x = $ 0.5, as described in Equation 1. The black (red) lines represent the observed (expected) limits. The solid lines represent the central values, and the dashed lines the one standard deviation variation due to the theoretical or experimental uncertainties around the central values. The colour shading corresponds to the observed limit on the cross section.
png pdf	Figure 6-c: The exclusion limits at 95% CL for the pair production of the top squark decaying in a di-tau final state are displayed in the $ m_{\tilde{\mathrm{t}}_{1}} \text {-} m_{\tilde{\chi}^0_1} $ plane for $ x = $ 0.75, as described in Equation 1. The black (red) lines represent the observed (expected) limits. The solid lines represent the central values, and the dashed lines the one standard deviation variation due to the theoretical or experimental uncertainties around the central values. The colour shading corresponds to the observed limit on the cross section.

Tables
png pdf	Table 1: Relative systematic uncertainties from different sources on signal and background yields in 2016 and 2017 combined. These values are the weighted (by the yields in the respective bins) averages of the relative uncertainties in the different search bins. For the asymmetric uncertainties, the upper (lower) entry is the uncertainty due to the upward (downward) variation. The numbers in box brackets in the heading indicate the top squark and LSP masses in GeV, respectively.
png pdf	Table 2: Event yields along with statistical and systematic uncertainties in 2016 and 2017 combined, for different background sources in the 15 search bins as defined in Figure 3. The notation used here is, $ \text {yield} ^{+\text {stat} +\text {syst}}_{-\text {stat}-\text {syst}} $.

Summary

The signature of top squark pair production in final states with two tau leptons has been explored in data collected by the CMS detector during 2016 and 2017, corresponding to integrated luminosities of 35.9 fb$^{-1}$ and 41.3 fb$^{-1}$, respectively. The search was performed in a final state containing an oppositely charged hadronic tau lepton pair, at least one jet identified as likely to contain a b hadron, and missing transverse momentum. The dominant standard model backgrounds were found to originate from top quark pair production and processes where jets were misidentified as hadronic tau lepton decays. Data driven techniques were followed to estimate these backgrounds, while other backgrounds were estimated using simulations.

No significant excess was observed, and exclusion limits on top squark masses in terms of neutralino masses were set at 95% confidence level, within the framework of simplfied models where the top squark decays via a chargino to final states including tau leptons. In such models, top squark masses have been excluded up to 1100 GeV for an almost massless LSP, and LSP masses up to 450 GeV have been excluded for a top squark mass of 900 GeV. These results are very useful in probing a region of the supersymmetric parameter space corresponding to high $\tan\beta$ and higgsino-like scenarios.

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