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| CMS-B2G-16-015 ; CERN-EP-2017-049 | ||
| Search for $\mathrm{ t \bar{t} }$ resonances in highly-boosted lepton+jets and fully hadronic final states in proton-proton collisions at $ \sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 11 April 2017 | ||
| JHEP 07 (2017) 001 | ||
| Abstract: A search for the production of heavy resonances decaying into top quark-antiquark pairs is presented. The analysis is performed in the lepton+jets and fully hadronic channels using data collected in proton-proton collisions at $ \sqrt{s} = $ 13 TeV using the CMS detector at the LHC, corresponding to an integrated luminosity of 2.6 fb$^{-1}$. The selection is optimized for massive resonances, where the top quarks have large Lorentz boosts. No evidence for resonant $\mathrm{ t \bar{t} }$ production is found in the data, and upper limits on the production cross section of heavy resonances are set. The exclusion limits for resonances with masses above 2 TeV are significantly improved compared to those of previous analyses at $ \sqrt{s} = $ 8 TeV. | ||
| Links: e-print arXiv:1704.03366 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Distributions of generator-level $M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$ for the production of new particles with masses of 2 TeV (left) and 4 TeV (right), for the four signal hypotheses considered in this analysis. |
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Figure 1-a:
Distribution of generator-level $M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$ for the production of a new particle with mass 2 TeV, for the four signal hypotheses considered in this analysis. |
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Figure 1-b:
Distribution of generator-level $M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }$ for the production of a new particle with mass 4 TeV, for the four signal hypotheses considered in this analysis. |
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Figure 2:
Distributions of the N-subjettiness ratio, $\tau _{32}$, and the soft dropped mass, $M_{\mathrm {SD}}$, for AK8 jets in data and simulation, after the signal selection. For lepton+jets, with $ {p_{\mathrm {T}}} >$ 500 GeV (upper row). For the fully hadronic final state, with $ {p_{\mathrm {T}}} >$ 400 GeV and subjet b tag (lower row). The distribution of $\tau _{32}$ (left) is shown after the selection 110 $ < M_{\mathrm {SD}} < $ 210 GeV, and the distribution of $M_{\mathrm {SD}}$ (right) is shown after the selection $\tau _{32}< $ 0.69. The lepton+jets channel plots compare data to background simulation, where the latter is divided into contributions from jets matched at the generator level to top quarks and other jets in top pair or W+jets events. The fully hadronic channel plots compare data to ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and QCD multijet simulation. Contributions from a benchmark narrow Z' signal model are shown with the black dashed lines. In obtaining the final results, NTMJ production is estimated from data, and simulated QCD multijet events are not used. In all plots, the error bars include only statistical contributions. |
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Figure 2-a:
Distribution of the N-subjettiness ratio, $\tau _{32}$, for AK8 jets in data and simulation, after the signal selection for lepton+jets, with $ {p_{\mathrm {T}}} >$ 500 GeV. The distribution is shown after the selection 110 $ < M_{\mathrm {SD}} < $ 210 GeV. The plot compares data to background simulation, where the latter is divided into contributions from jets matched at the generator level to top quarks and other jets in top pair or W+jets events. The contribution from a benchmark narrow Z' signal model is shown with the black dashed line. In obtaining the final results, NTMJ production is estimated from data, and simulated QCD multijet events are not used. The error bars include only statistical contributions. |
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Figure 2-b:
Distribution of the soft dropped mass, $M_{\mathrm {SD}}$, for AK8 jets in data and simulation, after the signal selection for lepton+jets, with $ {p_{\mathrm {T}}} >$ 500 GeV. The distribution is shown after the selection $\tau _{32}< $ 0.69. The plot compares data to background simulation, where the latter is divided into contributions from jets matched at the generator level to top quarks and other jets in top pair or W+jets events. The contribution from a benchmark narrow Z' signal model is shown with the black dashed line. In obtaining the final results, NTMJ production is estimated from data, and simulated QCD multijet events are not used. The error bars include only statistical contributions. |
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Figure 2-c:
Distribution of the N-subjettiness ratio, $\tau _{32}$, for AK8 jets in data and simulation, after the signal selection for the fully hadronic final state, with $ {p_{\mathrm {T}}} >$ 400 GeV and subjet b tag. The distribution is shown after the selection 110 $ < M_{\mathrm {SD}} < $ 210 GeV. The plot compares data to ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and QCD multijet simulation. The contribution from a benchmark narrow Z' signal model is shown with the black dashed line. In obtaining the final results, NTMJ production is estimated from data, and simulated QCD multijet events are not used. The error bars include only statistical contributions. |
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Figure 2-d:
Distribution of the soft dropped mass, $M_{\mathrm {SD}}$, for AK8 jets in data and simulation, after the signal selection for the fully hadronic final state, with $ {p_{\mathrm {T}}} >$ 400 GeV and subjet b tag. The distribution is shown after the selection $\tau _{32}< $ 0.69. The plot compares data to ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ and QCD multijet simulation. The contribution from a benchmark narrow Z' signal model is shown with the black dashed line. In obtaining the final results, NTMJ production is estimated from data, and simulated QCD multijet events are not used. The error bars include only statistical contributions. |
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Figure 3:
The mistag rate for the t tagging algorithm in the fully hadronic channel, measured with data for the six event categories by an anti-tag and probe procedure. The round, square, and triangular points indicate the t mistag rate for events in the 0, 1, and 2 b tag categories, respectively. The left (right) plot contains events with $ {| \Delta y | } < $ 1.0 ($ > $ 1.0). The contamination from ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ production is removed by subtracting the distribution of ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ events in simulation, normalized to SM expectation. |
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Figure 3-a:
The mistag rate for the t tagging algorithm in the fully hadronic channel, measured with data for the six event categories by an anti-tag and probe procedure. The round, square, and triangular points indicate the t mistag rate for events in the 0, 1, and 2 b tag categories, respectively. The plot contains events with $ {| \Delta y | } < $ 1.0. The contamination from ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ production is removed by subtracting the distribution of ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ events in simulation, normalized to SM expectation. |
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Figure 3-b:
The mistag rate for the t tagging algorithm in the fully hadronic channel, measured with data for the six event categories by an anti-tag and probe procedure. The round, square, and triangular points indicate the t mistag rate for events in the 0, 1, and 2 b tag categories, respectively. The plot contains events with $ {| \Delta y | } > $ 1.0. The contamination from ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ production is removed by subtracting the distribution of ${\mathrm{ t } {}\mathrm{ \bar{t} } }$ events in simulation, normalized to SM expectation. |
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Figure 4:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the lepton+jets analysis ($\chi ^{2} < $ 30) after the maximum likelihood fit. Distributions are shown for the muon (left) and electron (right) channel. For each lepton flavor, events are split into three exclusive categories (from uppermost to lowest): (1 t tag), (0 t tag, 1 b tag), and (0 t tag, 0 b tag). The signal templates are normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel in each figure shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 4-a:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the lepton+jets analysis ($\chi ^{2} < $ 30) after the maximum likelihood fit. Distributions are shown for the muon channel, for events in the (1 t tag) category. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 4-b:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the lepton+jets analysis ($\chi ^{2} < $ 30) after the maximum likelihood fit. Distributions are shown for the electron channel, for events in the (1 t tag) category. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 4-c:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the lepton+jets analysis ($\chi ^{2} < $ 30) after the maximum likelihood fit. Distributions are shown for the muon channel, for events in the (0 t tag, 1 b tag) category. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 4-d:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the lepton+jets analysis ($\chi ^{2} < $ 30) after the maximum likelihood fit. Distributions are shown for the electron channel, for events in the (0 t tag, 1 b tag) category. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 4-e:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the lepton+jets analysis ($\chi ^{2} < $ 30) after the maximum likelihood fit. Distributions are shown for the muon channel, for events in the (0 t tag, 0 b tag) category. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 4-f:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the lepton+jets analysis ($\chi ^{2} < $ 30) after the maximum likelihood fit. Distributions are shown for the electron channel, for events in the (0 t tag, 0 b tag) category. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 5:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the fully hadronic analysis after the maximum likelihood fit. Distributions are shown for the regions with $ {| \Delta y | } < $ 1.0 (left) and $ {| \Delta y | } > $ 1.0 (right), for 0, 1, or 2 subjet b tags (from uppermost to lowest). The signal templates are normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel in each figure shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
png pdf |
Figure 5-a:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the fully hadronic analysis after the maximum likelihood fit. Distributions are shown for the regions with $ {| \Delta y | } < $ 1.0, for 0 subjet b tag. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
png pdf |
Figure 5-b:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the fully hadronic analysis after the maximum likelihood fit. Distributions are shown for the regions with $ {| \Delta y | } > $ 1.0, for 0 subjet b tag. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 5-c:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the fully hadronic analysis after the maximum likelihood fit. Distributions are shown for the regions with $ {| \Delta y | } < $ 1.0, for 1 subjet b tag. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
png pdf |
Figure 5-d:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the fully hadronic analysis after the maximum likelihood fit. Distributions are shown for the regions with $ {| \Delta y | } > $ 1.0, for 1 subjet b tag. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
png pdf |
Figure 5-e:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the fully hadronic analysis after the maximum likelihood fit. Distributions are shown for the regions with $ {| \Delta y | } < $ 1.0, for 2 subjet b tags. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
png pdf |
Figure 5-f:
Distributions in $ {M_{{\mathrm{ t } {}\mathrm{ \bar{t} } } }} $ for data and expected background, for events passing the signal selection of the fully hadronic analysis after the maximum likelihood fit. Distributions are shown for the regions with $ {| \Delta y | } > $ 1.0, for 2 subjet b tags. The signal template is normalized to a cross section of 1 pb. The uncertainties associated with the background expectation include the statistical and all post-fit systematic uncertainties. The lower panel shows the ratio of data to predicted SM background, with the statistical (light gray) and total (dark gray) uncertainties shown separately. |
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Figure 6:
Observed and expected upper limits at 95% CL on the product of the production cross section and branching fractions for the full combination of the analysis results, shown as function of the resonance mass. Limits are set using four extensions to the SM : (upper left) a Z' boson with $\Gamma /M$ of 1%, (upper right) a Z' boson with $\Gamma /M$ of 10%, (lower left) a Z' boson with $ \Gamma / M $ of 30% and (lower right) a KK excitation of a gluon in the RS model. The corresponding theoretical prediction as a function of the resonance mass is shown as a dot-dashed curve. |
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Figure 6-a:
Observed and expected upper limits at 95% CL on the product of the production cross section and branching fractions for the full combination of the analysis results, shown as function of the resonance mass. Limits are set for a Z' boson with $\Gamma /M$ of 1%. The corresponding theoretical prediction as a function of the resonance mass is shown as a dot-dashed curve. |
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Figure 6-b:
Observed and expected upper limits at 95% CL on the product of the production cross section and branching fractions for the full combination of the analysis results, shown as function of the resonance mass. Limits are set for a Z' boson with $\Gamma /M$ of 10%. The corresponding theoretical prediction as a function of the resonance mass is shown as a dot-dashed curve. |
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Figure 6-c:
Observed and expected upper limits at 95% CL on the product of the production cross section and branching fractions for the full combination of the analysis results, shown as function of the resonance mass. Limits are set for a Z' boson with $ \Gamma / M $ of 30%. The corresponding theoretical prediction as a function of the resonance mass is shown as a dot-dashed curve. |
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Figure 6-d:
Observed and expected upper limits at 95% CL on the product of the production cross section and branching fractions for the full combination of the analysis results, shown as function of the resonance mass. Limits are set for a KK excitation of a gluon in the RS model. The corresponding theoretical prediction as a function of the resonance mass is shown as a dot-dashed curve. |
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Figure 7:
Expected and observed limits presented as a function of width, for $M_{\mathrm{ Z }' } = $ 1, 2, 3, 4 TeV . The corresponding theoretical prediction as a function of width is shown as a dot-dashed curve in each case. |
| Tables | |
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Table 1:
The mistag rates in data and simulation, and their ratio (data/simulation SF), for AK8 jets in the $ \ell $+jets analysis. |
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Table 2:
Sources of uncertainty and the channels they affect. Uncorrelated uncertainties applied to a given channel are labeled with a $\odot $. Uncertainties that are correlated between channels are labeled with a $\oplus $. In this table, $\sigma $ denotes the uncertainty in the given prior value in the likelihood fit. |
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Table 3:
Numbers of events in the signal region for the lepton+jets analysis. The expected yields for SM backgrounds are obtained from the maximum likelihood fit to the data described in Section 6.4. The uncertainties reported in the total expected background include the statistical uncertainties in the simulation and all the posterior systematic uncertainties. For the W+jets background, LF (HF) indicates contributions from W bosons produced in association with light-flavor (heavy-flavor) jets. |
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Table 4:
Number of events in the signal region for the fully hadronic analysis. The expected yields for SM backgrounds are obtained from the maximum likelihood fit to data described in the text. The uncertainties reported for the total expected background include the statistical uncertainties on the simulation and all the posterior systematic uncertainties. |
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Table 5:
Comparison of mass exclusion results (in TeV) for the individual channels and for their combination. |
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Table 6:
Expected and observed cross section limits at 95% CL, for the 1% width Z' resonance hypothesis. |
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Table 7:
Expected and observed cross section limits at 95% CL, for the 10% width Z' resonance hypothesis. |
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Table 8:
Expected and observed cross section limits at 95% CL, for the 30% width Z' resonance hypothesis. |
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Table 9:
Expected and observed cross section limits at 95% CL, for the RS KK gluon hypothesis. |
| Summary |
| The first search for top quark pair resonances in the fully hadronic channel using $\sqrt{s} = $ 13 TeV data was performed using the CMS experiment at the CERN LHC. The search uses a new top-tagging algorithm, optimized for Run 2 analyses, using the jet mass from the modified mass-drop tagger and N-subjettiness jet substructure variables along with subjet b-tagging. The non-top multijet background was estimated using a mistag rate measured in a control region depleted of $\mathrm{ t \bar{t} }$ events. No excess above the standard model expectation is observed, and limits were set on the production cross sections of Z' bosons and RS gluons, for signal models with varying widths. For some signal models, previous limits are eclipsed, excluding Z' bosons with masses up to 3.3 (3.8) TeV, for Z' relative widths of 10% (30%) of their masses. |
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