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| CMS-TOP-16-010 ; CERN-EP-2017-069 | ||
| Measurements of $\mathrm{ t \bar{t} }$ cross sections in association with b jets and inclusive jets and their ratio using dilepton final states in pp collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 29 May 2017 | ||
| Phys. Lett. B 776 (2018) 355 | ||
| Abstract: The cross sections for the production of ${\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}$ and ${\mathrm{ t \bar{t} }\mathrm{jj}} $ events and their ratio $\sigma_{\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}/\sigma_{{\mathrm{ t \bar{t} }\mathrm{jj}} } $ are measured using data corresponding to an integrated luminosity of 2.3 fb$^{-1}$ collected in pp collisions at $\sqrt{s} = $ 13 TeV with the CMS detector at the LHC. Events with two leptons (e or $\mu$) and at least four reconstructed jets, including at least two identified as b quark jets, in the final state are selected. In the full phase space, the measured ratio is 0.022 $\pm$ 0.003 (stat) $\pm$ 0.006 (syst), the cross section $ \sigma_{\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}$ is 4.0 $\pm$ 0.6 (stat) $\pm$ 1.3 (syst) pb and $ \sigma_{{\mathrm{ t \bar{t} }\mathrm{jj}} }$ is 184 $\pm$ 6 (stat) $\pm$ 33 (syst) pb. The measurements are compared with the standard model expectations obtained from a POWHEG simulation at next-to-leading-order interfaced with PYTHIA. | ||
| Links: e-print arXiv:1705.10141 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Normalized 2D distributions of the b jet discriminators of the third ($x$-axis) and the fourth ($y$-axis) jets sorted in decreasing order of b tagging discriminator value, after the full event selection for $ { {\mathrm{ t } \mathrm{ \bar{t} } } {\mathrm{ b \bar{b} } } } $ (upper left), $ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm{ b } \mathrm {j}} $ (upper right), $ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm{c} \bar{\mathrm{c}} } $ (lower left), and $ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm {LF}} $ (lower right) processes. |
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Figure 1-a:
Normalized 2D distributions of the b jet discriminators of the third ($x$-axis) and the fourth ($y$-axis) jets sorted in decreasing order of b tagging discriminator value, after the full event selection for the $ { {\mathrm{ t } \mathrm{ \bar{t} } } {\mathrm{ b \bar{b} } } } $ process. |
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Figure 1-b:
Normalized 2D distributions of the b jet discriminators of the third ($x$-axis) and the fourth ($y$-axis) jets sorted in decreasing order of b tagging discriminator value, after the full event selection for the $ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm{ b } \mathrm {j}} $ process. |
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Figure 1-c:
Normalized 2D distributions of the b jet discriminators of the third ($x$-axis) and the fourth ($y$-axis) jets sorted in decreasing order of b tagging discriminator value, after the full event selection for the $ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm{c} \bar{\mathrm{c}} } $ process. |
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Figure 1-d:
Normalized 2D distributions of the b jet discriminators of the third ($x$-axis) and the fourth ($y$-axis) jets sorted in decreasing order of b tagging discriminator value, after the full event selection for the $ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm {LF}} $ process. |
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Figure 2:
Distributions of b jet discriminator for the third (left) and the fourth (right) jets in decreasing order of b tagging discriminator value, after the full event selection. The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
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Figure 2-a:
Distributions of b jet discriminator for the third jet in decreasing order of b tagging discriminator value, after the full event selection. The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
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Figure 2-b:
Distributions of b jet discriminator for the fourth jet in decreasing order of b tagging discriminator value, after the full event selection. The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
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Figure 3:
Distribution of b jet multiplicity after the four-jet requirement, but without the b tagging requirement, for the ${\mathrm{ e } ^\pm \mathrm{ e } ^\mp }$ (upper left), ${\mathrm{ e } ^\pm \mu ^\mp }$ (upper right), and ${\mu ^\pm \mu ^\mp }$ (lower left) final states and the sum of the three final states (lower right). The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
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Figure 3-a:
Distribution of b jet multiplicity after the four-jet requirement, but without the b tagging requirement, for the ${\mathrm{ e } ^\pm \mathrm{ e } ^\mp }$ final state. The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
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Figure 3-b:
Distribution of b jet multiplicity after the four-jet requirement, but without the b tagging requirement, for the ${\mathrm{ e } ^\pm \mu ^\mp }$ final state. The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
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Figure 3-c:
Distribution of b jet multiplicity after the four-jet requirement, but without the b tagging requirement, for the ${\mu ^\pm \mu ^\mp }$ final states. The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
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Figure 3-d:
Distribution of b jet multiplicity after the four-jet requirement, but without the b tagging requirement, for the sum of the ${\mathrm{ e } ^\pm \mathrm{ e } ^\mp }$, ${\mathrm{ e } ^\pm \mu ^\mp }$, and ${\mu ^\pm \mu ^\mp }$ final states. The points show the data and the stacked histograms are from simulated events, normalized by the results of the fit. The ratio of the number of data events to the expected number, as given by the stacked histograms, is shown in the lower panels. The hatched region indicates the modelling uncertainty in the MC simulation. |
| Tables | |
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Table 1:
Predicted number of events for each physics process and for each dilepton category, their total, and the observed number of events. Results are shown after the final event selection. The Z+jets normalization and uncertainty are calculated from data, while all other predictions and uncertainties come from simulated data samples. The $ {\mathrm{ t } \mathrm{ \bar{t} } } $ sample for event categorization is from the POWHEG (v2) event generator interfaced with PYTHIA (v8.205). |
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Table 2:
Summary of the systematic uncertainties in percentage (%) from various sources contributing to $ {\sigma _{ { {\mathrm{ t } \mathrm{ \bar{t} } } {\mathrm{ b \bar{b} } } } }} $, $ {\sigma _{ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm {jj}} }} $, and the ratio $ {\sigma _{ { {\mathrm{ t } \mathrm{ \bar{t} } } {\mathrm{ b \bar{b} } } } }} $/$ {\sigma _{ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm {jj}} }} $, for a jet ${p_{\mathrm {T}}}$ threshold of ${p_{\mathrm {T}}} > $ 20 GeV in the visible phase space. |
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Table 3:
The measured cross sections ${\sigma _{ { {\mathrm{ t } \mathrm{ \bar{t} } } {\mathrm{ b \bar{b} } } } }} $ and ${\sigma _{ { {\mathrm{ t } \mathrm{ \bar{t} } } \mathrm {jj}} }} $ and their ratio for the visible and the full phase space, corrected for acceptance and branching fractions. The uncertainties on the measurements show separately the statistical and systematic components, while those are combined for the POWHEG predictions. |
| Summary |
| Measurements of the cross sections ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}}} $ and ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{jj}} }} $ and their ratio ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}}} /{\sigma_{{\mathrm{ t \bar{t} }\mathrm{jj}} }} $ are presented using a data sample recorded in pp collisions at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 2.3 fb$^{-1}$. The cross section ratio has been measured in a visible phase space region using the dilepton decay mode of $\mathrm{ t \bar{t} }$ events and corrected to the particle level, corresponding to the detector acceptance. The measured cross section ratios in the visible and the full phase space are ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}}} /{\sigma_{{\mathrm{ t \bar{t} }\mathrm{jj}} }} = $ 0.024 $\pm$ 0.003 (stat) $\pm$ 0.007 (syst) and ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}}} /{\sigma_{{\mathrm{ t \bar{t} }\mathrm{jj}} }} = $ 0.022 $\pm$ 0.003 (stat) $\pm$ 0.006 (syst), respectively, where a minimum transverse momentum for the particle-level jets of 20 GeV is required. Theoretical ratios predicted from the POWHEG simulation (interfaced with PYTHIA) are 0.014 $\pm$ 0.001 for the visible and 0.012 $\pm$ 0.001 for the full phase space, which are lower than the measured values but consistent within two standard deviations. The individual cross sections ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }}}} = $ 4.0 $\pm$ 0.6 (stat) $\pm$ 1.3 (syst) pb and ${\sigma_{{\mathrm{ t \bar{t} }\mathrm{jj}} }} = $ 184 $\pm$ 6 (stat) $\pm$ 33 (syst) pb have also been measured. These results, in particular the ratio of the cross sections, provide important information for the ${\mathrm{ t \bar{t} }\mathrm{ H }} $ search, permitting the reduction of a dominant systematic uncertainty that derives from the uncertainty in the ${\mathrm{ t \bar{t} }\mathrm{ b \bar{b} }} $ background. They can also be used as a figure of merit for testing the validity of next-to-leading-order QCD calculations at $\sqrt{s} = $ 13 TeV. |
| Additional Figures | |
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Additional Figure 1:
The measured cross sections for ${\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ b \bar{b} } } $ (left), ${\mathrm{ t } {}\mathrm{ \bar{t} } } \mathrm {jj}$ (middle) and their ratio (right) for the full phase space in comparison with the theoretical calculations. Error bars for the Monte Carlo predictions correspond the scale uncertainty. |
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Additional Figure 2:
The measured cross sections for ${\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ b \bar{b} } } $ (left), ${\mathrm{ t } {}\mathrm{ \bar{t} } } \mathrm {jj}$ (middle) and their ratio (right) for the visible phase space in comparison with the theoretical calculations. Error bars for the Monte Carlo predictions correspond to the scale uncertainty. |
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Compact Muon Solenoid LHC, CERN |
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