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| CMS-PAS-B2G-18-005 | ||
| Search for pair production of vector-like quarks in the fully hadronic channel | ||
| CMS Collaboration | ||
| March 2019 | ||
| Abstract: The results of two searches for vector-like T or B quark pair production using the fully hadronic final state are presented, using data from the CMS experiment at a center of mass energy of 13 TeV collected during the 2016 run, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. A multiclassification algorithm, the "boosted event shape tagger'', is deployed to label candidate jets as originating from top quarks, W, Z, or Higgs bosons. Candidate events are categorized according to the multiplicities of heavy objects observed in the final state, and the scalar sum of all observed jet momenta is used to discriminate signal events from the QCD multijet background. This novel strategy is complemented by a second, cut-based analysis that specifically targets the bW decay mode of the T quark, and allows for the reconstruction of the T quark candidates, providing additional sensitivity. Both analyses probe all possible branching fraction combinations of the T and B quarks and set limits on their masses, ranging from 740 to 1370 GeV. These results represent a significant improvement relative to existing searches in the fully hadronic final state. | ||
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Links:
CDS record (PDF) ;
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These preliminary results are superseded in this paper, PRD 100 (2019) 072001. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Classification rates for the six categories of the BEST algorithm, as measured in data events as a function of jet ${p_{\mathrm {T}}}$. Error bars shown indicate statistical errors on the rates to be propagated to the estimate of the QCD multijet background contribution. The value of the rate in the rightmost bin is also used for jets with ${p_{\mathrm {T}}}$ values above 3 TeV. |
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Figure 2:
The distributions of ${H_{\mathrm {T}}^{\mathrm {AK4}}}$ for each of the four signal region categories in the cut-based analysis. The top row shows channels with 2 W tags, and 2, 1 b tags respectively. The bottom row is for 1 W tag. The shaded error band represents the statistical uncertainty in the background. These distributions reflect the nuisance parameters evaluated after a likelihood fit to a background plus signal hypothesis, where the hypothesized signal is a T quark with a mass of 1200 GeV and 100% branching fraction to bW. The signal distributions show the expected yield of events assuming the cross section values in Table 1. The vertical axis label denotes that bin contents in these distributions have been scaled by their corresponding bin widths. |
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Figure 2-a:
The distribution of ${H_{\mathrm {T}}^{\mathrm {AK4}}}$ for the 2 W tags, 2 b tags signal region category in the cut-based analysis. The shaded error band represents the statistical uncertainty in the background. The distribution reflects the nuisance parameters evaluated after a likelihood fit to a background plus signal hypothesis, where the hypothesized signal is a T quark with a mass of 1200 GeV and 100% branching fraction to bW. The signal distribution shows the expected yield of events assuming the cross section values in Table 1. The vertical axis label denotes that bin contents in these distributions have been scaled by their corresponding bin widths. |
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Figure 2-b:
The distribution of ${H_{\mathrm {T}}^{\mathrm {AK4}}}$ for the 2 W tags, 1 b tags signal region category in the cut-based analysis. The shaded error band represents the statistical uncertainty in the background. The distribution reflects the nuisance parameters evaluated after a likelihood fit to a background plus signal hypothesis, where the hypothesized signal is a T quark with a mass of 1200 GeV and 100% branching fraction to bW. The signal distribution shows the expected yield of events assuming the cross section values in Table 1. The vertical axis label denotes that bin contents in these distributions have been scaled by their corresponding bin widths. |
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Figure 2-c:
The distribution of ${H_{\mathrm {T}}^{\mathrm {AK4}}}$ for the 1 W tag, 2 b tags signal region category in the cut-based analysis. The shaded error band represents the statistical uncertainty in the background. The distribution reflects the nuisance parameters evaluated after a likelihood fit to a background plus signal hypothesis, where the hypothesized signal is a T quark with a mass of 1200 GeV and 100% branching fraction to bW. The signal distribution shows the expected yield of events assuming the cross section values in Table 1. The vertical axis label denotes that bin contents in these distributions have been scaled by their corresponding bin widths. |
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Figure 2-d:
The distribution of ${H_{\mathrm {T}}^{\mathrm {AK4}}}$ for the 1 W tag, 1 b tags signal region category in the cut-based analysis. The shaded error band represents the statistical uncertainty in the background. The distribution reflects the nuisance parameters evaluated after a likelihood fit to a background plus signal hypothesis, where the hypothesized signal is a T quark with a mass of 1200 GeV and 100% branching fraction to bW. The signal distribution shows the expected yield of events assuming the cross section values in Table 1. The vertical axis label denotes that bin contents in these distributions have been scaled by their corresponding bin widths. |
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Figure 3:
A summary of the 126 signal region categories used in the NN analysis. This figure shows the expected yields in each category, while the signal discrimination is performed with the ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ distributions from each of the categories. The bottom panel shows the ratio of observed data to total background in each category, with Poisson error bars where applicable, along with the total background uncertainty shown for each category by the gray band. |
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Figure 4:
Distributions of ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ for all events entering the 126 signal regions of the NN analysis (top left), as well as for only categories containing at least one candidate of each of the particle types identified by the BEST algorithm. The plots shown here are not mutually exclusive, as the signal region may satisfy several of the criteria for the individual summary categories. The vertical axis label denotes that bin contents in these distributions have been scaled by their corresponding bin widths. |
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Figure 4-a:
Distribution of ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ for all events entering the 126 signal regions of the NN analysis. The plot shown here is not exclusive, as the signal region may satisfy several of the criteria for the individual summary categories. The vertical axis label denotes that bin contents in the distribution have been scaled by their corresponding bin widths. |
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Figure 4-b:
Distribution of ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ for only the category containing at least one W by the BEST algorithm. The plot shown here is not exclusive, as the signal region may satisfy several of the criteria for the individual summary categories. The vertical axis label denotes that bin contents in the distribution have been scaled by their corresponding bin widths. |
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Figure 4-c:
Distribution of ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ for only the category containing at least one Z identified by the BEST algorithm. The plot shown here is not exclusive, as the signal region may satisfy several of the criteria for the individual summary categories. The vertical axis label denotes that bin contents in the distribution have been scaled by their corresponding bin widths. |
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Figure 4-d:
Distribution of ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ for only the category containing at least one H identified by the BEST algorithm. The plot shown here is not exclusive, as the signal region may satisfy several of the criteria for the individual summary categories. The vertical axis label denotes that bin contents in the distribution have been scaled by their corresponding bin widths. |
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Figure 4-e:
Distribution of ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ for only the category containing at least one t jet identified by the BEST algorithm. The plot shown here is not exclusive, as the signal region may satisfy several of the criteria for the individual summary categories. The vertical axis label denotes that bin contents in the distribution have been scaled by their corresponding bin widths. |
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Figure 4-f:
Distribution of ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ for only the category containing at least one b jet identified by the BEST algorithm. The plot shown here is not exclusive, as the signal region may satisfy several of the criteria for the individual summary categories. The vertical axis label denotes that bin contents in the distribution have been scaled by their corresponding bin widths. |
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Figure 5:
Limits on the pair production cross section for T quarks (top) and B quarks (bottom) in the cut-based analysis, with decays solely to bW/tW (left), tZ/bZ (middle), and tH/bH (right). |
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Figure 5-a:
Limits on the pair production cross section for T quarks in the cut-based analysis, with decays solely to bW/tW. |
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Figure 5-b:
Limits on the pair production cross section for T quarks in the cut-based analysis, with decays solely to tZ/bZ. |
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Figure 5-c:
Limits on the pair production cross section for T quarks in the cut-based analysis, with decays solely to tH/bH. |
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Figure 5-d:
Limits on the pair production cross section for B quarks in the cut-based analysis, with decays solely to bW/tW. |
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Figure 5-e:
Limits on the pair production cross section for B quarks in the cut-based analysis, with decays solely to tZ/bZ. |
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Figure 5-f:
Limits on the pair production cross section for B quarks in the cut-based analysis, with decays solely to tH/bH. |
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Figure 6:
Limits on the pair production cross section for T quarks (top) and B quarks (bottom) in the NN analysis, with decays solely to bW/tW (left), tZ/bZ (middle), and tH/bH (right). |
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Figure 6-a:
Limits on the pair production cross section for T quarks in the NN analysis, with decays solely to bW/tW. |
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Figure 6-b:
Limits on the pair production cross section for T quarks in the NN analysis, with decays solely to tZ/bZ. |
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Figure 6-c:
Limits on the pair production cross section for T quarks in the NN analysis, with decays solely to tH/bH. |
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Figure 6-d:
Limits on the pair production cross section for B quarks in the NN analysis, with decays solely to bW/tW. |
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Figure 6-e:
Limits on the pair production cross section for B quarks in the NN analysis, with decays solely to tZ/bZ. |
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Figure 6-f:
Limits on the pair production cross section for B quarks in the NN analysis, with decays solely to tH/bH. |
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Figure 7:
Observed (left) and expected (right) mass exclusion limits for each combination of T quark branching fractions, in the cut-based analysis (top) and NN analysis (bottom). |
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Figure 7-a:
Observed mass exclusion limits for each combination of T quark branching fractions, in the cut-based analysis. |
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Figure 7-b:
Expected mass exclusion limits for each combination of T quark branching fractions, in the cut-based analysis. |
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Figure 7-c:
Observed mass exclusion limits for each combination of T quark branching fractions, in the NN analysis. |
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Figure 7-d:
Expected mass exclusion limits for each combination of T quark branching fractions, in the NN analysis. |
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Figure 8:
Observed (left) and expected (right) mass exclusion limits for each combination of B quark branching fractions, in the cut-based analysis (top) and NN analysis (bottom). |
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Figure 8-a:
Observed mass exclusion limits for each combination of B quark branching fractions, in the cut-based analysis. |
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Figure 8-b:
Expected mass exclusion limits for each combination of B quark branching fractions, in the cut-based analysis. |
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Figure 8-c:
Observed mass exclusion limits for each combination of B quark branching fractions, in the NN analysis. |
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Figure 8-d:
Expected mass exclusion limits for each combination of B quark branching fractions, in the NN analysis. |
| Tables | |
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Table 1:
Theoretical cross sections for TT and BB production. |
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Table 2:
Sources of systematic uncertainties that affect the ${H_{\mathrm {T}}^{\mathrm {AK4}}}$ or ${H_{\mathrm {T}}^{\mathrm {AK8}}}$ distribution in each analysis. Systematic sources with a prior uncertainty of "$ \pm 1 \sigma $'' affect the shape, all others affect the rate only. |
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Table 3:
Exclusion limits presented in terms of the T quark mass, for the different branching fraction scenarios considered, in each of the two analyses. |
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Table 4:
Exclusion limits presented in terms of the B quark mass, for the different branching fraction scenarios considered, in each of the two analyses. |
| Summary |
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Two independent searches for vector-like T and B quarks using the fully hadronic final state are presented. Both searches use data collected by the CMS experiment in 2016, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. A cut-based analysis, using jet substructure observables to identify hadronic decays of boosted W bosons, targets the bW decay mode of the T quark, and improves sensitivity relative to previous results most sensitive to the same decay mode. The cut-based analysis uses a data-driven QCD multijet estimation method based on shape and rate extrapolations from various control regions to the signal region. Improvements in W-tagging techniques, as well as additional signal regions requiring just a single W-tagged jet, enhance the performance of this analysis relative to previous strategies. The cut-based analysis extends the B quark mass exclusion to 1.03 TeV, relative to the previous exclusion of 705 GeV, obtained by a similar analysis targeting the bW decay mode using data collected at $\sqrt{s} = $ 8 TeV [50]. A new strategy is presented and compared with the traditional cut-based approach. The NN analysis uses a multiclassification technique, the BEST algorithm, to identify jets originating from heavy objects such as t or b quarks, and W, Z, or Higgs bosons. This allows the analysis to be sensitive to all decay modes of the T and B quarks. Using classification rates, the dominant QCD multijet background is estimated using observed data. The NN analysis provides sensitivity for the tH and tZ decay modes competitive with that obtained by other searches utilizing lepton+jets or multilepton topologies. For each analysis, results are presented in terms of cross section limits for the pair production of T and B quarks, along with exclusion limits in terms of the T and B quark masses, for the different combinations of branching fractions considered. The analyses presented here provide additional sensitivity for a possible combination result including all final states targeting VLQ production in CMS. The mass exclusion limits for the NN analysis range from 700-1370 GeV, providing comparable sensitivity to the searches utilizing leptons, which exclude VLQ masses in the range 910-1300 GeV. These results represent the most stringent limits on vector-like quarks in the fully hadronic channel to date. |
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