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| CMS-TOP-18-003 ; CERN-EP-2019-163 | ||
| Search for production of four top quarks in final states with same-sign or multiple leptons in proton-proton collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 18 August 2019 | ||
| Eur. Phys. J. C 80 (2020) 75 | ||
| Abstract: The standard model (SM) production of four top quarks (${\mathrm{t\bar{t}}\mathrm{t\bar{t}}} $) in proton-proton collision is studied by the CMS Collaboration. The data sample, collected during the 2016-2018 data taking of the LHC, corresponds to an integrated luminosity of 137 fb$^{-1}$ at a center-of-mass energy of 13 TeV. The events are required to contain two same-sign charged leptons (electrons or muons) or at least three leptons, and jets. The observed and expected significances for the ${\mathrm{t\bar{t}}\mathrm{t\bar{t}}} $ signal are respectively 2.6 and 2.7 standard deviations, and the ${\mathrm{t\bar{t}}\mathrm{t\bar{t}}} $ cross section is measured to be 12.6$^{+5.8}_{-5.2}$ fb. The results are used to constrain the Yukawa coupling of the top quark to the Higgs boson, $y_{\mathrm{t}}$, yielding a limit of $| {y_{\mathrm{t}}/y_{\mathrm{t}}^{\mathrm{SM}}} | < $ 1.7 at 95% confidence level, where $y_{\mathrm{t}}^{\mathrm{SM}}$ is the SM value of $y_{\mathrm{t}}$. They are also used to constrain the oblique parameter of the Higgs boson in an effective field theory framework, $\hat{H}$ < 0.12. Limits are set on the production of a heavy scalar or pseudoscalar boson in Type-II two-Higgs-doublet and simplified dark matter models, with exclusion limits reaching 350-470 GeV and 350-550 GeV for scalar and pseudoscalar bosons, respectively. Upper bounds are also set on couplings of the top quark to new light particles. | ||
| Links: e-print arXiv:1908.06463 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Typical Feynman diagrams for ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ production at leading order in the SM. |
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Figure 1-a:
Typical Feynman diagram for ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ production at leading order in the SM. |
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Figure 1-b:
Typical Feynman diagram for ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ production at leading order in the SM. |
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Figure 2:
Distributions of ${N_\text {jets}}$ (upper left), ${N_\mathrm{b}}$ (upper right), ${H_{\mathrm {T}}}$ (lower left), and ${{p_{\mathrm {T}}} ^\text {miss}}$ (lower right) in the summed SRs (1-14), before fitting to data, where the last bins include the overflows. The hatched areas represent the total uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panels show the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 2-a:
Distribution of ${N_\text {jets}}$ in the summed SRs (1-14), before fitting to data, where the last bin includes the overflows. The hatched areas represent the total uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 2-b:
Distribution of ${N_\mathrm{b}}$ in the summed SRs (1-14), before fitting to data, where the last bin includes the overflows. The hatched areas represent the total uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 2-c:
Distribution of ${H_{\mathrm {T}}}$ in the summed SRs (1-14), before fitting to data, where the last bin includes the overflows. The hatched areas represent the total uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 2-d:
Distribution of ${{p_{\mathrm {T}}} ^\text {miss}}$ in the summed SRs (1-14), before fitting to data, where the last bin includes the overflows. The hatched areas represent the total uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 3:
Distributions of ${N_\text {jets}}$ (left) and ${N_\mathrm{b}}$ (right) in the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{W}}$ (upper) and ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{Z}}$ (lower) CRs, before fitting to data. The hatched areas represent the uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panels show the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 3-a:
Distribution of ${N_\text {jets}}$ in the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{W}}$ CR, before fitting to data. The hatched areas represent the uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 3-b:
Distribution of ${N_\mathrm{b}}$ in the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{W}}$ CR, before fitting to data. The hatched areas represent the uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 3-c:
Distribution of ${N_\text {jets}}$ in the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{Z}}$ CR, before fitting to data. The hatched areas represent the uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 3-d:
Distribution of ${N_\mathrm{b}}$ in the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{Z}}$ CR, before fitting to data. The hatched areas represent the uncertainties in the SM signal and background predictions. The ${{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}$ signal assumes the SM cross section from Ref. [1]. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 4:
Observed yields in the control and signal regions for the cut-based (upper) and BDT (lower) analyses, compared to the post-fit predictions for signal and background processes. The hatched areas represent the total post-fit uncertainties in the signal and background predictions. The lower panels show the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 4-a:
Observed yields in the control and signal regions for the cut-based analysis, compared to the post-fit predictions for signal and background processes. The hatched areas represent the total post-fit uncertainties in the signal and background predictions. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 4-b:
Observed yields in the control and signal regions for the BDT analysis, compared to the post-fit predictions for signal and background processes. The hatched areas represent the total post-fit uncertainties in the signal and background predictions. The lower panel shows the ratios of the observed event yield to the total prediction of signal plus background. |
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Figure 5:
The observed ${\sigma ({\mathrm{p}} {\mathrm{p}} \to {{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}}})$ (solid line) and 95% CL upper limit (hatched line) are shown as a function of $ {| y_{\mathrm{t}}/y_{\mathrm{t}}^{\mathrm {SM}} |}$. The predicted value (dashed line) [2], calculated at LO and scaled to the calculation from Ref. [1], is also plotted. The shaded band around the measured value gives the total uncertainty, while the shaded band around the predicted curve shows the theoretical uncertainty associated with the renormalization and factorization scales. |
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Figure 6:
The 95% CL exclusion regions in the plane of the $\phi /\mathrm{Z'} $-top quark coupling versus $m_{\phi}$ or $m_{\mathrm{Z'}}$. The excluded regions are above the hatched lines. |
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Figure 7:
The observed (points) and expected (dashed line) 95% CL upper limits on the cross section times branching fraction to ${\mathrm{t} \mathrm{\bar{t}}}$ for the production of a new heavy scalar H (left) and pseudoscalar A (right), as a function of mass. The inner and outer bands around the expected limits indicate the regions containing 68 and 95%, respectively, of the distribution of limits under the background-only hypothesis. Theoretical values are shown for Type-II 2HDM in the alignment limit (solid line) and simplified dark matter (dot-dashed line) models. |
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Figure 7-a:
The observed (points) and expected (dashed line) 95% CL upper limits on the cross section times branching fraction to ${\mathrm{t} \mathrm{\bar{t}}}$ for the production of a new heavy scalar H, as a function of mass. The inner and outer bands around the expected limits indicate the regions containing 68 and 95%, respectively, of the distribution of limits under the background-only hypothesis. Theoretical values are shown for Type-II 2HDM in the alignment limit (solid line) and simplified dark matter (dot-dashed line) models. |
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Figure 7-b:
The observed (points) and expected (dashed line) 95% CL upper limits on the cross section times branching fraction to ${\mathrm{t} \mathrm{\bar{t}}}$ for the production of a new heavy pseudoscalar A, as a function of mass. The inner and outer bands around the expected limits indicate the regions containing 68 and 95%, respectively, of the distribution of limits under the background-only hypothesis. Theoretical values are shown for Type-II 2HDM in the alignment limit (solid line) and simplified dark matter (dot-dashed line) models. |
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Figure 8:
The observed (solid curve) and expected (long-dashed curve) 95% CL exclusion regions in the $\tan\beta $ versus mass plane for Type-II 2HDM models in the alignment limit for a new scalar H (upper left), pseudoscalar A (upper right), and both (lower) particles. The short-dashed curves around the expected limits indicate the region containing 68% of the distribution of limits expected under the background-only hypothesis. The excluded regions are below the curves. |
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Figure 8-a:
The observed (solid curve) and expected (long-dashed curve) 95% CL exclusion regions in the $\tan\beta $ versus mass plane for Type-II 2HDM models in the alignment limit for a new scalar H. The short-dashed curves around the expected limits indicate the region containing 68% of the distribution of limits expected under the background-only hypothesis. The excluded regions are below the curves. |
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Figure 8-b:
The observed (solid curve) and expected (long-dashed curve) 95% CL exclusion regions in the $\tan\beta $ versus mass plane for Type-II 2HDM models in the alignment limit for a new pseudoscalar A. The short-dashed curves around the expected limits indicate the region containing 68% of the distribution of limits expected under the background-only hypothesis. The excluded regions are below the curves. |
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Figure 8-c:
The observed (solid curve) and expected (long-dashed curve) 95% CL exclusion regions in the $\tan\beta $ versus mass plane for Type-II 2HDM models in the alignment limit for both a new scalar H and a new pseudoscalar A. The short-dashed curves around the expected limits indicate the region containing 68% of the distribution of limits expected under the background-only hypothesis. The excluded regions are below the curves. |
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Figure 9:
Exclusion regions at 95% CL in the plane of $m_\chi $ vs. $m_{\mathrm{H}}$ (left) or $m_{\mathrm{A} }$ (right). The outer lighter and inner darker solid curves show the expected and observed limits, respectively, assuming $g_\mathrm {SM} = g_\mathrm {DM} = $ 1. The excluded regions, shaded, are above the limit curves. The dashed lines show the limits assuming a weaker coupling between H/A and $\chi $, $g_\mathrm {DM} = $ 0.5. |
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Figure 9-a:
Exclusion regions at 95% CL in the plane of $m_\chi $ vs. $m_{\mathrm{H}}$. The outer lighter and inner darker solid curves show the expected and observed limits, respectively, assuming $g_\mathrm {SM} = g_\mathrm {DM} = $ 1. The excluded regions, shaded, are above the limit curves. The dashed lines show the limits assuming a weaker coupling between H/A and $\chi $, $g_\mathrm {DM} = $ 0.5. |
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Figure 9-b:
Exclusion regions at 95% CL in the plane of $m_\chi $ vs. $m_{\mathrm{A} }$. The outer lighter and inner darker solid curves show the expected and observed limits, respectively, assuming $g_\mathrm {SM} = g_\mathrm {DM} = $ 1. The excluded regions, shaded, are above the limit curves. The dashed lines show the limits assuming a weaker coupling between H/A and $\chi $, $g_\mathrm {DM} = $ 0.5. |
| Tables | |
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Table 1:
Definition of the 14 SRs and two CRs for the cut-based analysis. |
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Table 2:
Summary of the sources of uncertainty, their values, and their impact, defined as the relative change of the measurement of $\sigma ({\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}})$ induced by one-standard-deviation variations corresponding to each uncertainty source considered separately. The first group lists experimental and theoretical uncertainties in simulated signal and background processes. The second group lists normalization uncertainties in the estimated backgrounds. Uncertainties marked (not marked) with a $\dagger $ in the first column are treated as fully correlated (fully uncorrelated) across the three years of data taking. |
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Table 3:
The post-fit predicted background, $ {{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}} $ signal, and total yields with their total uncertainties and the observed number of events in the control and signal regions in data for the cut-based analysis. |
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Table 4:
The post-fit predicted background and $ {{\mathrm{t} \mathrm{\bar{t}}} {\mathrm{t} \mathrm{\bar{t}}}} $ signal, and total yields with their total uncertainties and the observed number of events in the control and signal regions in data for the BDT analysis. |
| Summary |
|
The standard model (SM) production of ${\mathrm{t\bar{t}}\mathrm{t\bar{t}}} $ has been studied in data from $\sqrt{s} = $ 13 TeV proton-proton collisions collected using the CMS detector during the LHC 2016-2018 data-taking period, corresponding to an integrated luminosity of 137 fb$^{-1}$. The final state with either two same-sign leptons or at least three leptons is analyzed using two strategies, the first relying on a cut-based categorization in lepton and jet multiplicity and jet flavor, the second taking advantage of a multivariate approach to distinguish the ${\mathrm{t\bar{t}}\mathrm{t\bar{t}}} $ signal from its many backgrounds. The more precise multivariate strategy yields an observed (expected) significance of 2.6 (2.7) standard deviations relative to the background-only hypothesis, and a measured value for the ${\mathrm{t\bar{t}}\mathrm{t\bar{t}}} $ cross section of 12.6$^{+5.8}_{-5.2}$ fb. The results based on the two strategies are in agreement with each other and with the SM prediction of 12.0$^{+2.2}_{-2.5}$ fb [1]. The results of the boosted decision tree (BDT) analysis are also used to constrain the top quark Yukawa coupling $y_{\mathrm{t}}$ relative to its SM value, based on the $| {y_{\mathrm{t}}} |$ dependence of ${\sigma({\mathrm{p}}{\mathrm{p}}\to{\mathrm{t\bar{t}}\mathrm{t\bar{t}}} })$ calculated at leading order in Ref. [2], resulting in the 95% confidence level (CL) limit of $| {y_{\mathrm{t}}/y_{\mathrm{t}}^{\mathrm{SM}}}| < $ 1.7. The Higgs boson oblique parameter in the effective field theory framework [11] is similarly constrained to $\hat{H} < 0.12$ at 95% CL. Upper limits ranging from 0.1 to 1.2 are also set on the coupling between the top quark and a new scalar ($\phi$) or vector (Z') particle with mass less than twice that of the top quark ($m_\mathrm{t}$) [9]. For new scalar (H) or pseudoscalar (A) particles with $m > 2m_\mathrm{t}$, and decaying to $\mathrm{t\bar{t}}$, their production in association with a single top quark or a top quark pair is probed. The resulting cross section upper limit, between 15 and 35 fb at 95% CL, is interpreted in the context of Type-II two-Higgs-doublet models [4,5,6,76] as a function of $\tan \beta$ and $m_{\mathrm{\mathrm{H}/\mathrm{A} }}$, and in the context of simplified dark matter models [7,8], as a function of $m_{\mathrm{\mathrm{H}/\mathrm{A} }}$ and the mass of the dark matter candidate. |
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Compact Muon Solenoid LHC, CERN |
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