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CMS-TOP-18-011 ; CERN-EP-2019-183
Measurement of the $\mathrm{t\bar{t}}\mathrm{b\bar{b}}$ production cross section in the all-jet final state in pp collisions at $\sqrt{s} = $ 13 TeV
Phys. Lett. B 803 (2020) 135285
Abstract: A measurement of the production cross section of top quark pairs in association with two b jets ($\mathrm{t\bar{t}}\mathrm{b\bar{b}}$) is presented using data collected in proton-proton collisions at $\sqrt{s} = $ 13 TeV by the CMS detector at the LHC corresponding to an integrated luminosity of 35.9 fb$^{-1}$ . The cross section is measured in the all-jet decay channel of the top quark pair by selecting events containing at least eight jets, of which at least two are identified as originating from the hadronization of b quarks. A combination of multivariate analysis techniques is used to reduce the large background from multijet events not containing a top quark pair, and to help discriminate between jets originating from top quark decays and other additional jets. The cross section is determined for the total phase space to be 5.5 $\pm$ 0.3 (stat) $^{+1.6}_{-1.3}$ (syst) pb and also measured for two fiducial $\mathrm{t\bar{t}}\mathrm{b\bar{b}}$ definitions. The measured cross sections are found to be larger than theoretical predictions by a factor of 1.5-2.4, corresponding to 1-2 standard deviations.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Distributions in the QGLR (left) and the CWoLa BDT discriminants (right). Both are after preselection, requiring $P(\chi ^2) > 10^{-6}$ and at least eight selected jets. All the contributions are based on simulation. The multijet contribution is scaled to match the total yields in data, after the other processes including the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$ signal have been normalized to their corresponding theoretical cross sections. This choice takes into account only the effect of the shape variation from the multijet background. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. The lower panels show the ratio between the observed data and the predictions. The dashed lines indicate the boundaries between the signal and control regions defined in Section 6. Hatched bands indicate the statistical uncertainty in the predictions without considering the systematic sources, dominated by the uncertainties in the simulated multijet background.

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Figure 1-a:
Distribution in the QGLR BDT discriminant, after preselection, requiring $P(\chi ^2) > 10^{-6}$ and at least eight selected jets. All the contributions are based on simulation. The multijet contribution is scaled to match the total yields in data, after the other processes including the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$ signal have been normalized to their corresponding theoretical cross sections. This choice takes into account only the effect of the shape variation from the multijet background. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. The lower panel shows the ratio between the observed data and the predictions. The dashed lines indicate the boundaries between the signal and control regions defined in Section 6. Hatched bands indicate the statistical uncertainty in the predictions without considering the systematic sources, dominated by the uncertainties in the simulated multijet background.

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Figure 1-b:
Distribution in the CWoLa BDT discriminant, after preselection, requiring $P(\chi ^2) > 10^{-6}$ and at least eight selected jets. All the contributions are based on simulation. The multijet contribution is scaled to match the total yields in data, after the other processes including the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$ signal have been normalized to their corresponding theoretical cross sections. This choice takes into account only the effect of the shape variation from the multijet background. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. The lower panel shows the ratio between the observed data and the predictions. The dashed lines indicate the boundaries between the signal and control regions defined in Section 6. Hatched bands indicate the statistical uncertainty in the predictions without considering the systematic sources, dominated by the uncertainties in the simulated multijet background.

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Figure 2:
Distribution in the 2DCSV in the SR (upper left), CR1 (upper right), CR2 (lower right), and CR3 (lower left) regions. For clarity, the two-dimensional distribution with largest and next-to-largest b tagging discriminant scores for the additional jets have been unrolled to one dimension, and the resulting bins ordered according to increasing values of the ratio between expected signal and background yields in each bin of the SR. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. Hatched bands correspond to uncertainties. The bottom panels show the pull distribution. The pull is defined as the bin by bin difference between data and predicted yields after the fit, divided by the uncertainties accounted for correlations between data and predictions after the fit.

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Figure 2-a:
Distribution in the 2DCSV in the SR region. For clarity, the two-dimensional distribution with largest and next-to-largest b tagging discriminant scores for the additional jets have been unrolled to one dimension, and the resulting bins ordered according to increasing values of the ratio between expected signal and background yields in each bin of the SR. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. Hatched bands correspond to uncertainties. The bottom panel shows the pull distribution. The pull is defined as the bin by bin difference between data and predicted yields after the fit, divided by the uncertainties accounted for correlations between data and predictions after the fit.

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Figure 2-b:
Distribution in the 2DCSV in the CR1 region. For clarity, the two-dimensional distribution with largest and next-to-largest b tagging discriminant scores for the additional jets have been unrolled to one dimension, and the resulting bins ordered according to increasing values of the ratio between expected signal and background yields in each bin of the SR. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. Hatched bands correspond to uncertainties. The bottom panel shows the pull distribution. The pull is defined as the bin by bin difference between data and predicted yields after the fit, divided by the uncertainties accounted for correlations between data and predictions after the fit.

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Figure 2-c:
Distribution in the 2DCSV in the CR2 region. For clarity, the two-dimensional distribution with largest and next-to-largest b tagging discriminant scores for the additional jets have been unrolled to one dimension, and the resulting bins ordered according to increasing values of the ratio between expected signal and background yields in each bin of the SR. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. Hatched bands correspond to uncertainties. The bottom panel shows the pull distribution. The pull is defined as the bin by bin difference between data and predicted yields after the fit, divided by the uncertainties accounted for correlations between data and predictions after the fit.

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Figure 2-d:
Distribution in the 2DCSV in the CR3 region. For clarity, the two-dimensional distribution with largest and next-to-largest b tagging discriminant scores for the additional jets have been unrolled to one dimension, and the resulting bins ordered according to increasing values of the ratio between expected signal and background yields in each bin of the SR. The small backgrounds include ${\mathrm{t} \mathrm{\bar{t}}} {\mathrm{V}} $, ${\mathrm{t} \mathrm{\bar{t}}} \mathrm{H} $, single top quark, V+jets, and diboson production. Hatched bands correspond to uncertainties. The bottom panel shows the pull distribution. The pull is defined as the bin by bin difference between data and predicted yields after the fit, divided by the uncertainties accounted for correlations between data and predictions after the fit.

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Figure 3:
Comparison of the measured ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$ production cross sections (vertical lines) with predictions from several Monte Carlo generators (squares), for three definitions of our ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$ regions of phase space: fiducial PI (left), fiducial PB (middle), total (right). The dark (light) shaded bands show the statistical (total) uncertainties in the measured value. Uncertainty intervals in the theoretical cross sections include the statistical uncertainty as well as the uncertainties in the PDFs and the $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ scales.
Tables

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Table 1:
The considered sources of systematic uncertainties and their respective contributions to the total systematic uncertainty in the measured ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$ cross section in the FPS for the two defined ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$. The upper (lower) portion of the table lists uncertainties related to the experimental conditions (theoretical modelling). The numbers are obtained by taking the difference in quadrature of the profile likelihood width when fixing nuisance parameters corresponding to a given source of uncertainty and leaving the others free to vary.

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Table 2:
Measured and predicted cross sections for the different definitions of the ${{\mathrm{t} \mathrm{\bar{t}}} \mathrm{b} \mathrm{\bar{b}}}$ phase space considered in this analysis. For measurements, the first uncertainty is statistical, while the second one is from the systematic sources. The uncertainties in the predicted cross sections include the statistical uncertainty, the PDF uncertainties, and the $\mu _\mathrm {R}$ and $\mu _\mathrm {F}$ dependences on changes in scale. The uncertainties in scale for parton showers are not included, and amount to about 15% for POWHEG+PYTHIA. Unless specified otherwise, {pythia} is used for the modelling the parton shower, hadronization, and the underlying event.
Summary
The first measurement of the $\mathrm{t\bar{t}}\mathrm{b\bar{b}}$ cross section in the all-jet final state was presented, using 35.9 fb$^{-1}$ of data collected in pp collisions at $\sqrt{s} = $ 13 TeV. The cross section is first measured in a fiducial region of particle-level phase space by defining two categories of $\mathrm{t\bar{t}}\mathrm{b\bar{b}}$ events, and subsequently this result is corrected to the total phase space. One of the defined fiducial regions corresponds to ignoring parton-level information, while the other uses parton-level information to identify the particle-level jets that do not originate from the decay of top quarks. For both definitions, the cross section is measured to be 1.6 $\pm$ 0.1 (stat)$^{+0.5}_{-0.4}$ (syst) pb. The cross section in the total phase space is obtained by correcting this measurement for the experimental acceptance on the jets originating from the top quarks, which yields 5.5 $\pm$ 0.3 (stat)$^{+1.6}_{-1.3}$ (syst) pb. This measurement provides valuable input to studies of the $\mathrm{t\bar{t}}\mathrm{H}$ process, where the Higgs boson decays into a pair of b quarks, and for which the normalization and modelling of the $\mathrm{t\bar{t}}\mathrm{b\bar{b}}$ process represent a leading source of systematic uncertainty. Furthermore, these results represent a stringent test of perturbative quantum chromodynamics at the LHC. Predictions from several generators are compared with measurements and found to be smaller than the measured values by a factor of 1.5-2.4, corresponding to 1-2 standard deviations. This is consistent with previous results for the $\mathrm{t\bar{t}}\mathrm{b\bar{b}}$ cross section and calls for further experimental and theoretical studies of the associated production of top quark pairs and b jets.
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