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| CMS-SUS-17-005 ; CERN-EP-2018-079 | ||
| Search for top squarks decaying via four-body or chargino-mediated modes in single-lepton final states in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 15 May 2018 | ||
| JHEP 09 (2018) 065 | ||
| Abstract: A search for the pair production of the lightest supersymmetric partner of the top quark ($ {\tilde{\mathrm{t}}_{1}} $) is presented. The search focuses on a compressed scenario where the mass difference between the top squark and the lightest supersymmetric particle, often considered to be the lightest neutralino ($ \tilde{\chi}^0_1 $), is smaller than the mass of the W boson. The proton-proton collision data were recorded by the CMS experiment at a centre-of-mass energy of 13 TeV, and correspond to an integrated luminosity of 35.9 fb$^{-1}$. In this search, two decay modes of the top squark are considered: a four-body decay into a bottom quark, two additional fermions, and a $ \tilde{\chi}^0_1 $; and a decay via an intermediate chargino. Events are selected using the presence of a high-momentum jet, significant missing transverse momentum, and a low transverse momentum electron or muon. Two analysis techniques are used, targeting different decay modes of the $ {\tilde{\mathrm{t}}_{1}} $: a sequential selection and a multivariate technique. No evidence for the production of top squarks is found, and mass limits at 95% confidence level are set that reach up to 560 GeV, depending on the $m( {\tilde{\mathrm{t}}_{1}} ) - m(\tilde{\chi}^0_1)$ mass difference and the decay mode. | ||
| Links: e-print arXiv:1805.05784 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| 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 can be found here |
| Figures | |
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Figure 1:
Top squark pair production at the LHC with four-body (left) or chargino-mediated (right) decays. |
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Figure 1-a:
Top squark pair production at the LHC with four-body (left) or chargino-mediated (right) decays. |
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Figure 1-b:
Top squark pair production at the LHC with four-body (left) or chargino-mediated (right) decays. |
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Figure 2:
Distributions of lepton $ {p_{\mathrm {T}}} $ (left) and $ {M_{\mathrm {T}}} $ (right) at the preselection level in data and simulation. The lower panels show the ratio of data to the sum of the SM backgrounds where the dark shaded bands indicate the statistical uncertainties of the simulation. The distributions of two signal points of the four-body decay are also represented, while not being added to the background: ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}})) = $ (500,490) and (500,420) GeV. The background distributions are obtained directly from simulation, and are normalized to an integrated luminosity of 35.9 fb$^{-1}$. The last bin in each plot includes events beyond 200 GeV. |
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Figure 2-a:
Distribution of lepton $ {p_{\mathrm {T}}} $ at the preselection level in data and simulation. The lower panel shows the ratio of data to the sum of the SM backgrounds where the dark shaded bands indicate the statistical uncertainties of the simulation. The distributions of two signal points of the four-body decay are also represented, while not being added to the background: ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}})) = $ (500,490) and (500,420) GeV. The background distributions are obtained directly from simulation, and are normalized to an integrated luminosity of 35.9 fb$^{-1}$. The last bin in each plot includes events beyond 200 GeV. |
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Figure 2-b:
Distribution of $ {M_{\mathrm {T}}} $ at the preselection level in data and simulation. The lower panel shows the ratio of data to the sum of the SM backgrounds where the dark shaded bands indicate the statistical uncertainties of the simulation. The distributions of two signal points of the four-body decay are also represented, while not being added to the background: ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}})) = $ (500,490) and (500,420) GeV. The background distributions are obtained directly from simulation, and are normalized to an integrated luminosity of 35.9 fb$^{-1}$. The last bin in each plot includes events beyond 200 GeV. |
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Figure 3:
Simulated distributions of ${{p_{\mathrm {T}}} (\ell)}$ (left) and $ {M_{\mathrm {T}}} $ (right) at the preselection level for signal samples with different ${\Delta {m}}$, and W+jets and $ {{\mathrm {t}\overline {\mathrm {t}}}} $ background events. The area of each signal distribution, and the total background contribution, are normalized to unit area. |
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Figure 3-a:
Simulated distribution of ${{p_{\mathrm {T}}} (\ell)}$ at the preselection level for signal samples with different ${\Delta {m}}$, and W+jets and $ {{\mathrm {t}\overline {\mathrm {t}}}} $ background events. The area of the signal distribution and the total background contribution are normalized to unit area. |
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Figure 3-b:
Simulated distribution of $ {M_{\mathrm {T}}} $ at the preselection level for signal samples with different ${\Delta {m}}$, and W+jets and $ {{\mathrm {t}\overline {\mathrm {t}}}} $ background events. The area of the signal distribution and the total background contribution are normalized to unit area. |
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Figure 4:
Distributions of the BDT output at the preselection level in data and simulation in 10 GeV steps of $ {\Delta {m}} $ from 10 (top-left) to 40 GeV (bottom-right). For each case, a representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 4-a:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 10 GeV. A representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 4-b:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 20 GeV. A representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 4-c:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 30 GeV. A representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 4-d:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 40 GeV. A representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 5:
Distributions of the BDT output at the preselection level in data and simulation in 10 GeV steps of $ {\Delta {m}} $ from 50 (top-left) to 80 GeV (bottom-right). For each case, a representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 5-a:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 50 GeV. For each case, a representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 5-b:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 60 GeV. For each case, a representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 5-c:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 70 GeV. For each case, a representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 5-d:
Distribution of the BDT output at the preselection level in data and simulation for $ {\Delta {m}} = $ 80 GeV. For each case, a representative ${({m}({\tilde{\mathrm {t}}} _{1}}), {m}({\tilde{\chi}^{0}_{1}}))$ signal point is also shown, but is not added to the SM background. The shaded area on the Data/MC ratio represents the statistical uncertainty of the simulated background. |
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Figure 6:
The CC approach: summary of observed and expected background yields in all SRs as defined in Table yyyyy. The vertical bars and the shaded areas represent the statistical uncertainty of the data and the total uncertainty in the prediction, respectively. The lower panel shows the ratio of data to prediction. |
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Figure 7:
Exclusion limit at 95% CL for the four-body decay of the top squark as a function of $m({\tilde{\mathrm {t}}} _{1})$ and $ {\Delta {m}} $ for the CC (upper) and MVA (lower) approaches. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines represent the observed (expected) limits, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical or experimental uncertainties. |
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Figure 7-a:
Exclusion limit at 95% CL for the four-body decay of the top squark as a function of $m({\tilde{\mathrm {t}}} _{1})$ and $ {\Delta {m}} $ for the CC approach. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines represent the observed (expected) limits, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical or experimental uncertainties. |
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Figure 7-b:
Exclusion limit at 95% CL for the four-body decay of the top squark as a function of $m({\tilde{\mathrm {t}}} _{1})$ and $ {\Delta {m}} $ for the MVA approach. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines represent the observed (expected) limits, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical or experimental uncertainties. |
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Figure 8:
Exclusion limit at 95% CL for the chargino-mediated decay of the top squark as a function of $m({\tilde{\mathrm {t}}} _{1})$ and $ {\Delta {m}} $ for the CC search. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines represent the observed (expected) limits, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical (experimental) uncertainties. |
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Figure 9:
Combined limits at 95% CL between the CC single-lepton (1$ \ell $) and all-hadronic (0$ \ell $) [19] searches for the four-body decay (upper) and the chargino-mediated decay (lower) of the top squark in the $m({\tilde{\mathrm {t}}} _{1})$-$\Delta m({\tilde{\mathrm {t}}} _{1}, {\tilde{\chi}^{0}_{1}})$ plane. The correlations between the two searches have been taken into account. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines show the observed (expected) mass limits, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical (experimental) uncertainties. The dot-dashed blue and dotted green lines show the individual expected mass limits for the 1$ \ell $ and 0$ \ell $ searches, respectively. |
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Figure 9-a:
Combined limits at 95% CL between the CC single-lepton (1$ \ell $) and all-hadronic (0$ \ell $) [19] searches for the four-body decay of the top squark in the $m({\tilde{\mathrm {t}}} _{1})$-$\Delta m({\tilde{\mathrm {t}}} _{1}, {\tilde{\chi}^{0}_{1}})$ plane. The correlation with the other search has been taken into account. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines show the observed (expected) mass limits, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical (experimental) uncertainties. The dot-dashed blue and dotted green lines show the individual expected mass limits for the 1$ \ell $ and 0$ \ell $ searches, respectively. |
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Figure 9-b:
Combined limits at 95% CL between the CC single-lepton (1$ \ell $) and all-hadronic (0$ \ell $) [19] searches the chargino-mediated decay of the top squark in the $m({\tilde{\mathrm {t}}} _{1})$-$\Delta m({\tilde{\mathrm {t}}} _{1}, {\tilde{\chi}^{0}_{1}})$ plane. The correlation with the other search has been taken into account. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines show the observed (expected) mass limits, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical (experimental) uncertainties. The dot-dashed blue and dotted green lines show the individual expected mass limits for the 1$ \ell $ and 0$ \ell $ searches, respectively. |
| Tables | |
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Table 1:
The CC search: definition of SRs. The subregions of SRs are denoted by tags in parentheses, as described in the text: VL, L, M, and H refer to the four bins in lepton $ {p_{\mathrm {T}}} $, and X and Y to the $ {C_{\mathrm {T}}} $ ranges specified in the table. The corresponding control regions (CR) use the same selection with the exception of the lepton $ {p_{\mathrm {T}}} $ as shown in the table. For jets, the attributes "soft'' and "hard'' refer to the $ {p_{\mathrm {T}}} $ ranges 30-60 GeV and $ > $60 GeV, respectively. |
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Table 2:
The CC search: observed yields and simulated background contributions to CRs normalized to an integrated luminosity of 35.9 fb$^{-1}$. The nonprompt contributions are estimated from data. The last column shows the scale factors used for the normalization of the W+jets and $ {{\mathrm {t}\overline {\mathrm {t}}}} $ samples. Only statistical uncertainties are reported. |
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Table 3:
The CC search: typical ranges for relative systematic uncertainties (in%) on the total background prediction and signal prediction in the main SRs. The "--'' means that a certain source of uncertainty is not applicable. |
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Table 4:
The MVA search: relative systematic uncertainties (in%) on the total background and signal prediction. The "--'' means that a certain source of uncertainty is not applicable. In the case of the background, the uncertainties are on the total background. Systematic uncertainties on the data-driven prediction of the W+jets, $ {{\mathrm {t}\overline {\mathrm {t}}}} $, and nonprompt lepton backgrounds are reported. |
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Table 5:
The MVA search: prediction of the W+jets, $ {{\mathrm {t}\overline {\mathrm {t}}}} $, nonprompt lepton, and other backgrounds in the eight SRs defined by the threshold on the BDT output reported in the second column. The prediction of the first three processes is based on data, while that of $N^{SR}(\text {Rare})$, i.e. rare backgrounds, is based on simulation. The uncertainties are the quadrature sum of the statistical uncertainties, the systematic uncertainties of Table xxxxx, and for the backgrounds predicted from simulation, the cross section uncertainties. The number of total expected background ($N^{SR}\text {(B)}$) and observed data ($N^{SR}\text {(D)}$) events in each SR are also reported. |
| Summary |
|
A search for direct top squark pair production is performed in a compressed scenario where the mass difference $ {\Delta {m}} $ between the lightest top squark and the lightest supersymmetric particle (LSP), taken to be the lightest neutralino $ \tilde{\chi}^0_1 $, does not exceed the W boson mass. Two decay modes of the top squark are targeted: the four-body prompt decay to $\mathrm{b} \mathrm{f} \overline{\mathrm{f}}^{\,\prime} \tilde{\chi}^0_1$, and the chargino-mediated decay to $\mathrm{b} \tilde{\chi}^{+}_{1}$ with a subsequent decay $\tilde{\chi}^{+}_{1} \to \mathrm{f} \overline{\mathrm{f}}^{\,\prime} \tilde{\chi}^0_1$. Results are based on proton-proton collision data at $\sqrt{s} = $ 13 TeV, recorded with the CMS detector in 2016 and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Selected events are required to have a single lepton (electron or muon), and significant missing transverse momentum ($ p_{\mathrm{T}}^{\text{miss}} $). Because of the small mass difference between the top squark and the LSP, the decay products of the top squark are expected to have low $ {p_{\mathrm{T}}} $. Events where the presence of a jet from initial-state radiation leads to a boost of the top squark pair and sizeable $ p_{\mathrm{T}}^{\text{miss}} $ are selected. Two search strategies are pursued. In the sequential selection approach (CC), signal regions are defined based on discriminating variables, particularly the transverse mass of the lepton-$ p_{\mathrm{T}}^{\text{miss}} $ system and the lepton momentum. In another approach, a multivariate analysis (MVA) is employed that uses both kinematic and topological variables and is specifically trained for different $ {\Delta {m}} $ regions of the four-body decay mode. In both approaches, the dominant contributions to the signal regions from standard model processes (W+jets, $\mathrm{t\bar{t}}$, and events with misidentified leptons) are estimated from control regions in data. Data are found to be compatible with the predicted standard model backgrounds. The results are used to set limits at 95% confidence level on the production cross section as a function of the $ {\tilde{\mathrm{t}}_{1}} $ and $ \tilde{\chi}^0_1 $ masses, within the context of simplified models. Assuming 100% branching fraction in the decay channel under consideration and the top squark pair production cross section computed at NLO+NLL precision [40,41,42,43,44,45,46], these limits are converted into mass limits. Both search strategies are applied to the four-body decay mode. For this decay mode, the MVA search excludes top squark masses up to 420 and 560 GeV at ${\Delta {m}} = $ 10 and 80 GeV, respectively. There is less sensitivity at lower $ {\Delta {m}} $ due to the smaller available phase space, where the very soft kinematics of the decay products lead to a lower acceptance. The limits obtained in the CC approach are comparable with the MVA approach for ${\Delta {m}} = $ 30 GeV. The CC approach also covers the chargino-mediated decays, where the chargino mass is taken as the average of the top squark and the neutralino masses, probing $ {\tilde{\mathrm{t}}_{1}} $ masses up to 540 GeV for ${\Delta {m}} \approx $ 40 GeV. The results of the CC search have been combined with a search for top squark pair production in the fully hadronic channel [19]. The combined mass limits reach up to 590 and 670 GeV for four-body and chargino-mediated decays, respectively. The reach of the $ {\Delta {m}} $ dependent MVA search in the four-body decay mode is noteworthy, as the exclusion limit goes beyond that of the combined result at high ${\Delta {m}}$. The results summarized in this paper represent the most stringent limits to date on the top squark pair production cross section for mass differences between the top squark and the lightest neutralino below the W boson mass, and for decays proceeding through the four-body or the chargino-mediated modes. |
| Additional Figures | |
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Additional Figure 1:
The CC approach. Summary of observed and expected background yields in all SRs as defined in Table 1 of the paper. The ranges for $M_{\mathrm {T}}$ are given in units of GeV. The vertical bars and the shaded areas represent the statistical uncertainty of the data and the total uncertainty in the prediction, respectively. The lower panel shows the ratio of data to prediction. |
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Additional Figure 2:
The CC approach. Covariance matrix of the background estimates between signal regions. An electronic version of this figure is available. |
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Additional Figure 3:
The CC approach. Correlation matrix of the background estimates between signal regions. An electronic version of this figure is available. |
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Additional Figure 4:
Observed significance for the four-body decay of the top squark in the CC approach. |
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Additional Figure 5:
Observed significance for the chargino-mediated decay of the top squark in the CC approach. |
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Additional Figure 6:
The CC approach. Limits at 95% CL for the four-body decay of the top squark in the $ \tilde{\mathrm {t}} _{1}$-$ {\tilde{\chi}^{0}_{1}} $ mass plane. |
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Additional Figure 7:
The CC approach. Limits at 95% CL for the chargino-mediated decay of the top squark in the $ {\tilde{\mathrm {t}}} _{1}$-$ {\tilde{\chi}^{0}_{1}} $ mass plane. |
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Additional Figure 8:
The MVA approach. Total efficiency of the selection (preselection and threshold applied on the BDT output) as function of $m_{{\tilde{\mathrm {t}}} _{1}}$ and $\Delta m$ for the four-body decay scenario (see Figure 1). |
| Additional Tables | |
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Additional Table 1:
Summary of expected background and observed data yields in the CC signal regions. The uncertainties on the background prediction include the statistical and systematic sources. |
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Additional Table 2:
The CC approach. Cut flow table for selection of signal regions with two signal points from the four-body decay. The values are normalized to an integrated luminosity of 35.9 fb$^{-1}$. |
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Additional Table 3:
The CC approach. Cut flow table for selection of signal regions with two signal points from the chargino-mediated scenario. The values are normalized to an integrated luminosity of 35.9 fb$^{-1}$. |
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The efficiency maps of the simplified models in the CC signal region are provided in the following ROOT files: - Four-body decay of the top squark (T2tt): CCAccpEffMap_T2tt - Chargino-mediated decay of the top squark (T2bW): CCAccpEffMap_T2bW |
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