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| CMS-PAS-SUS-16-048 | ||
| Search for new physics in events with two low momentum opposite-sign leptons and missing transverse energy at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| March 2017 | ||
| Abstract: A search for new physics in events with low-momentum opposite-sign leptons and missing transverse momentum, using 35.9 fb$^{-1}$ of integrated luminosity collected by CMS experiment at $\sqrt{s}= $ 13 TeV, is presented. The data observed are consistent with expectations from the standard model. The results are interpreted in terms of pair production of charginos and neutralinos ($\tilde{\chi}_1^{\pm}$, $\tilde{\chi}_2^{0}$) with nearly degenerate masses, as expected in natural compressed higgsino models, and in terms of the pair production of top squarks for the case that the neutralino and the top squark have similar masses. At 95% confidence level, $\tilde{\chi}_1^{\pm}$/$\tilde{\chi}_2^{0}$ are excluded for masses up to 230 GeV for a mass difference of 20 GeV with respect to the lightest neutralino, which is a region constrained so far by the LEP experiments only. An interpretation is also provided in terms of top squark ($\tilde{t}$) pair production processes with degenerate mass spectra and chargino-mediated decays. Masses of the $\tilde{t}$ up to 450 GeV are excluded for a mass difference of 40 GeV with respect to the lightest neutralino. | ||
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Links:
CDS record (PDF) ;
inSPIRE record ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, PLB 782 (2018) 440. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures & Tables | References | CMS Publications |
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| Additional information on efficiencies needed for reinterpretation of these results are available here. Additional technical material for CMS speakers can be found here |
| Figures | |
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Figure 1:
Left: electroweakino pair production and decay. Right: chargino-mediated $\tilde{ \mathrm{ t } }$ pair production and decay. |
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Figure 1-a:
Electroweakino pair production and decay. |
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Figure 1-b:
Chargino-mediated $\tilde{ \mathrm{ t } }$ pair production and decay. |
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Figure 2:
Same-sign control region for $ \tilde{ \mathrm{ t } } $ selection and $ {E_{\mathrm {T}}^{\text {miss}}} > $ 200 GeV. The shape of of the leading lepton pT is used as input to the final signal extraction. The superimposed signal is from $ \tilde{ \mathrm{ t } } $ pair production where the mass of the $ \tilde{ \mathrm{ t } } $ is 350 GeV and the difference in mass with the lightest neutralino is 20 GeV (T2tt350/20). |
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Figure 3:
Left: Electroweakino search region for 125 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 200 GeV (muon only channel) for 33.2 fb$^{-1}$ ; Middle: 200 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 250 GeV (muon and electron channel) for 35.9 fb$^{-1}$ ; Right: $ {E_{\mathrm {T}}^{\text {miss}}} > $ 250 GeV (muon and electron channel) for 35.9 fb$^{-1}$. The superimposed signal is from neutralino-chargino ($ \tilde{ \chi }^0 _2$-$ \tilde{ \chi }^{\pm} _1$) pair production where the mass of the chargino is 150 GeV and the difference in mass with the lightest neutralino is 20 GeV (TChi150/20). |
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Figure 3-a:
Electroweakino search region for 125 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 200 GeV (muon only channel) for 33.2 fb$^{-1}$. The superimposed signal is from neutralino-chargino ($ \tilde{ \chi }^0 _2$-$ \tilde{ \chi }^{\pm} _1$) pair production where the mass of the chargino is 150 GeV and the difference in mass with the lightest neutralino is 20 GeV (TChi150/20). |
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Figure 3-b:
Electroweakino search region for 200 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 250 GeV (muon and electron channel) for 35.9 fb$^{-1}$. The superimposed signal is from neutralino-chargino ($ \tilde{ \chi }^0 _2$-$ \tilde{ \chi }^{\pm} _1$) pair production where the mass of the chargino is 150 GeV and the difference in mass with the lightest neutralino is 20 GeV (TChi150/20). |
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Figure 3-c:
Electroweakino search region for $ {E_{\mathrm {T}}^{\text {miss}}} > $ 250 GeV (muon and electron channel) for 35.9 fb$^{-1}$. The superimposed signal is from neutralino-chargino ($ \tilde{ \chi }^0 _2$-$ \tilde{ \chi }^{\pm} _1$) pair production where the mass of the chargino is 150 GeV and the difference in mass with the lightest neutralino is 20 GeV (TChi150/20). |
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Figure 4:
Left: $ \tilde{ \mathrm{ t } } $ search region for 125 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 200 GeV (muon only channel) for 33.2 fb$^{-1}$ ; Middle: 200 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 300 GeV (muon and electron channel) for 35.9 fb$^{-1}$ ; Right: $ {E_{\mathrm {T}}^{\text {miss}}} > $ 300 GeV (muon and electron channel) for 35.9 fb$^{-1}$. The superimposed signal is from $ \tilde{ \mathrm{ t } } $ pair production where the mass of the $ \tilde{ \mathrm{ t } } $ is 350 GeV and the difference in mass with the lightest neutralino is 20 GeV (T2tt350/20). |
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Figure 4-a:
$ \tilde{ \mathrm{ t } } $ search region for 125 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 200 GeV (muon only channel) for 33.2 fb$^{-1}$. The superimposed signal is from $ \tilde{ \mathrm{ t } } $ pair production where the mass of the $ \tilde{ \mathrm{ t } } $ is 350 GeV and the difference in mass with the lightest neutralino is 20 GeV (T2tt350/20). |
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Figure 4-b:
$ \tilde{ \mathrm{ t } } $ search region for 200 $ < {E_{\mathrm {T}}^{\text {miss}}} < $ 300 GeV (muon and electron channel) for 35.9 fb$^{-1}$. The superimposed signal is from $ \tilde{ \mathrm{ t } } $ pair production where the mass of the $ \tilde{ \mathrm{ t } } $ is 350 GeV and the difference in mass with the lightest neutralino is 20 GeV (T2tt350/20). |
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Figure 4-c:
$ \tilde{ \mathrm{ t } } $ search region for $ {E_{\mathrm {T}}^{\text {miss}}} > $ 300 GeV (muon and electron channel) for 35.9 fb$^{-1}$. The superimposed signal is from $ \tilde{ \mathrm{ t } } $ pair production where the mass of the $ \tilde{ \mathrm{ t } } $ is 350 GeV and the difference in mass with the lightest neutralino is 20 GeV (T2tt350/20). |
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Figure 5:
The observed exclusion contours (black curves) assuming the NLO+NNL cross sections, with the corresponding 1 standard deviation uncertainties for electroweakino (left) and $ \tilde{ \mathrm{ t } } $ (right) search. The dashed (red) curves present the expected limits with 1 standard deviation experimental uncertainties. For the electroweakino search, results are based on a simplified model of $ \tilde{ \chi }^0 _2 \tilde{ \chi }^{\pm} _0 \rightarrow \tilde{ \chi }^0 _1 \tilde{ \chi }^0 _1 Z^{*}W^{*}$ process with a pure Wino production cross section, while a simplified model of the $ \tilde{ \mathrm{ t } } $ pair production, followed by the $ \tilde{ \mathrm{ t } } \rightarrow \tilde{ \chi }^{\pm} _1 \mathrm {b}$ and the subsequent $ \tilde{ \chi }^{\pm} _1 \rightarrow \tilde{ \chi }^0 _1 W^{*}$ decay is used for the $ \tilde{ \mathrm{ t } } $ search. In this last model, the mass of the $ \tilde{ \chi }^{\pm} _1$ is set to be $ (M_{ \tilde{ \mathrm{ t } } } + M_{ \tilde{ \chi }^0 _1})/2$. Data corresponds to an integrated luminosity ranging from 33.2 fb$^{-1}$ to 35.9 fb$^{-1}$. |
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Figure 5-a:
The observed exclusion contours (black curves) assuming the NLO+NNL cross sections, with the corresponding 1 standard deviation uncertainties for the electroweakino search. The dashed (red) curves present the expected limits with 1 standard deviation experimental uncertainties. Results are based on a simplified model of $ \tilde{ \chi }^0 _2 \tilde{ \chi }^{\pm} _0 \rightarrow \tilde{ \chi }^0 _1 \tilde{ \chi }^0 _1 Z^{*}W^{*}$ process with a pure Wino production cross section. Data corresponds to an integrated luminosity ranging from 33.2 fb$^{-1}$ to 35.9 fb$^{-1}$. |
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Figure 5-b:
The observed exclusion contours (black curves) assuming the NLO+NNL cross sections, with the corresponding 1 standard deviation uncertainties for the $ \tilde{ \mathrm{ t } } $ search. The dashed (red) curves present the expected limits with 1 standard deviation experimental uncertainties. A simplified model of the $ \tilde{ \mathrm{ t } } $ pair production, followed by the $ \tilde{ \mathrm{ t } } \rightarrow \tilde{ \chi }^{\pm} _1 \mathrm {b}$ and the subsequent $ \tilde{ \chi }^{\pm} _1 \rightarrow \tilde{ \chi }^0 _1 W^{*}$ decay is used. In this last model, the mass of the $ \tilde{ \chi }^{\pm} _1$ is set to be $ (M_{ \tilde{ \mathrm{ t } } } + M_{ \tilde{ \chi }^0 _1})/2$. Data corresponds to an integrated luminosity ranging from 33.2 fb$^{-1}$ to 35.9 fb$^{-1}$. |
| Tables | |
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Table 1:
Selection requirements for the signal regions. The subleading lepton ${p_{\mathrm {T}}}$ threshold is reduced to 3.5 GeV for muons in the high ${E_{\mathrm {T}}^{\text {miss}}} \tilde{ \mathrm{ t } } $-like signal region. $Iso_{\text {rel}}$ and $Iso_{\text {abs}}$ are relative and absolute isolation variables. |
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Table 2:
Summary of selection of control regions and the WW validation region (VR). |
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Table 3:
Data and simulation yields for the DY and ${\mathrm{ t } {}\mathrm{ \bar{t} } } $ control regions corresponding to a integrated luminosity of 35.9 fb$^{-1}$ (high ${E_{\mathrm {T}}^{\text {miss}}}$ region) and 33.2 fb$^{-1}$ (low ${E_{\mathrm {T}}^{\text {miss}}}$ region). Uncertainties are statistical. |
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Table 4:
Relative uncertainties on the final predictions for each individual systematic uncertainty source. |
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Table 5:
Predicted and data yields with uncertainty from the fit for the electroweakino search region for 33.2 fb$^{-1}$ and 35.9 fb$^{-1}$ of integrated luminosity. |
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Table 6:
Predicted and data yields with uncertainty from the fit for the $ \tilde{ \mathrm{ t } } $ search region for 33.2 fb$^{-1}$ and 35.9 fb$^{-1}$ of integrated luminosity. |
| Summary |
| A search for new physics in events with two low momentum opposite-sign leptons and missing transverse energy is presented using the data collected by CMS at a a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of up to 35.9 fb$^{-1}$. The data are found to be consistent with the standard model expectations. The results are interpreted in the framework of supersymmetric simplified models targeting electroweakino mass-degenerate spectra and $\tilde{\mathrm{t}}$-$\tilde{\chi}^0_1$ mass-degenerate benchmark scenarios. The search probes the $\tilde{\chi}^0_2\tilde{\chi}^{\pm}_1\rightarrow \tilde{\chi}^0_1\tilde{\chi}^0_1 Z^{*}W^{*}$ process for mass differences ($\Delta$m) between $\tilde{\chi}^0_2$ and $\tilde{\chi}^0_1$ of less than 20 GeV. Assuming Wino production cross sections, $\tilde{\chi}^0_2$ masses up to 230 GeV are excluded for $\Delta$m of 20 GeV. For the $\tilde{\mathrm{t}}$ chargino-mediated decay into $\tilde{\chi}^0_1 W^{*}$, $\tilde{\mathrm{t}}$ masses of up to 450 GeV are excluded for a $\Delta$m$(\tilde{\mathrm{t}},\tilde{\chi}^0_1)$ = 40 GeV assuming a simplified description of the model. |
| Additional Figures | |
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Additional Figure 1:
Covariance matrix for the expected backgrounds from the predictive fit in the electroweakino search region. |
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Additional Figure 2:
Covariance matrix for the expected backgrounds from the predictive fit in the electroweakino search region (text version). |
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Additional Figure 3:
Covariance matrix for the expected backgrounds from the predictive fit in the $\tilde{t}$ search region. |
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Additional Figure 4:
Covariance matrix for the expected backgrounds from the predictive fit in the $\tilde{t}$ search region (text version). |
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Additional Figure 5:
Observed significance in the electroweakino plane, based on a simplified model of $\tilde{\chi }_2^{0} \tilde{\chi }_1^{\pm } \rightarrow \tilde{\chi }_1^{0}\tilde{\chi }_1^{0} Z^{*}W^{*}$ with a pure Wino production cross section. Data corresponds to an integrated luminosity ranging from 33.2 fb$^{-1}$ to 35.9 fb$^{-1}$ . |
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Additional Figure 6:
Observed significance in the $\tilde{t}$ plane, based on a simplified model of $\tilde{t}$ pair production, followed by the $\tilde{t} \rightarrow \tilde{\chi }_1^{\pm } \mathrm {b}$ and the subsequent $ \tilde{\chi }_1^{\pm } \rightarrow \tilde{\chi }_1^{0} W^{*}$ decay. The mass of the $ \tilde{\chi }_1^{\pm }$ is set to be $ (M_{\tilde{t}} + M_{\tilde{\chi }_1^{0}})/2$. Data corresponds to an integrated luminosity ranging from 33.2 fb$^{-1}$ to 35.9 fb$^{-1}$. |
| Additional Tables | |
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Additional Table 1:
Cut-flow of yields for one of the search regions, corresponding to 33.2 fb$^{-1}$ of integrated luminosity for various signal model points, given as the mass pair ($\tilde{\chi }_2^{0}$,$\tilde{\chi }_1^{0}$) or ($\tilde{t}$,$\tilde{\chi }_1^{0}$). The yields are normalized to the theoretical cross sections. The "All events'' category below starts from all generated events where the Z boson decays leptonically. |
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