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| CMS-TOP-18-002 ; CERN-EP-2020-011 | ||
| Measurement of the cross section for $\mathrm{t\bar{t}}$ production with additional jets and b jets in pp collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 14 March 2020 | ||
| JHEP 07 (2020) 125 | ||
| Abstract: Measurements of the cross section for the production of top quark pairs in association with a pair of jets from bottom quarks (${\sigma_{{\mathrm{t\bar{t}}\mathrm{b\bar{b}}} }} $) and in association with a pair of jets from quarks of any flavor or gluons (${\sigma_{{\mathrm{t\bar{t}} \mathrm{jj}} }} $) and their ratio are presented. The data were collected in proton-proton collisions at a center-of-mass energy of 13 TeV by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.9 fb$^{-1}$. The measurements are performed in a fiducial phase space, separately in the dilepton and lepton+jets channels, where lepton corresponds to either an electron or a muon. The results for the dilepton and lepton+jets channels, respectively, are ${\sigma_{{\mathrm{t\bar{t}} \mathrm{jj}} }} = $ 2.36 $\pm$ 0.02 (stat) $\pm$ 0.20 (syst) pb and 31.0 $\pm$ 0.2 (stat) $\pm$ 2.9 (syst) pb, and for the cross section ratio 0.017 $\pm$ 0.001 (stat) $\pm$ 0.001 (syst) and 0.020 $\pm$ 0.001 (stat) $\pm$ 0.001 (syst). The values of ${\sigma_{{\mathrm{t\bar{t}}\mathrm{b\bar{b}}} }} $ are determined from the product of the ${\sigma_{{\mathrm{t\bar{t}} \mathrm{jj}} }} $ and the cross section ratio, obtaining, respectively, 0.040 $\pm$ 0.002 (stat) $\pm$ 0.005 (syst) pb and 0.62 $\pm$ 0.03 (stat) $\pm$ 0.07 (syst) pb. These measurements are the most precise to date and are consistent, within the uncertainties, with the standard model expectations obtained at next-to-leading order in quantum chromodynamics. | ||
| Links: e-print arXiv:2003.06467 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The b tagging discriminant distribution from data (points) for the first (left) and second (right) additional jet for the dilepton (upper) and lepton+jets (lower) channels in decreasing order of the b tagging discriminant value after event selection, and the predicted distributions for the signal and background from simulation (shaded histograms). The contributions of single top quark, ${\mathrm{t} \mathrm{\bar{t}} {\mathrm{V}}}$, $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{H}} $, and the non-top quark processes are merged in the background (blue) entry. The $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ process is located in the bottom part of the stack due to its low contribution in comparison with the other entries. The lower panels display the ratio of the data to the expectations. The grey bands display the combination of the statistical and systematic uncertainties. |
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Figure 1-a:
The b tagging discriminant distribution from data (points) for the first additional jet for the dilepton channel in decreasing order of the b tagging discriminant value after event selection, and the predicted distributions for the signal and background from simulation (shaded histograms). The contributions of single top quark, ${\mathrm{t} \mathrm{\bar{t}} {\mathrm{V}}}$, $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{H}} $, and the non-top quark processes are merged in the background (blue) entry. The $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ process is located in the bottom part of the stack due to its low contribution in comparison with the other entries. The lower panel displays the ratio of the data to the expectations. The grey bands display the combination of the statistical and systematic uncertainties. |
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Figure 1-b:
The b tagging discriminant distribution from data (points) for the second additional jet for the dilepton channel in decreasing order of the b tagging discriminant value after event selection, and the predicted distributions for the signal and background from simulation (shaded histograms). The contributions of single top quark, ${\mathrm{t} \mathrm{\bar{t}} {\mathrm{V}}}$, $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{H}} $, and the non-top quark processes are merged in the background (blue) entry. The $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ process is located in the bottom part of the stack due to its low contribution in comparison with the other entries. The lower panel displays the ratio of the data to the expectations. The grey bands display the combination of the statistical and systematic uncertainties. |
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Figure 1-c:
The b tagging discriminant distribution from data (points) for the first additional jet for the lepton+jets channel in decreasing order of the b tagging discriminant value after event selection, and the predicted distributions for the signal and background from simulation (shaded histograms). The contributions of single top quark, ${\mathrm{t} \mathrm{\bar{t}} {\mathrm{V}}}$, $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{H}} $, and the non-top quark processes are merged in the background (blue) entry. The $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ process is located in the bottom part of the stack due to its low contribution in comparison with the other entries. The lower panel displays the ratio of the data to the expectations. The grey bands display the combination of the statistical and systematic uncertainties. |
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Figure 1-d:
The b tagging discriminant distribution from data (points) for the second additional jet for the lepton+jets channel in decreasing order of the b tagging discriminant value after event selection, and the predicted distributions for the signal and background from simulation (shaded histograms). The contributions of single top quark, ${\mathrm{t} \mathrm{\bar{t}} {\mathrm{V}}}$, $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{H}} $, and the non-top quark processes are merged in the background (blue) entry. The $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ process is located in the bottom part of the stack due to its low contribution in comparison with the other entries. The lower panel displays the ratio of the data to the expectations. The grey bands display the combination of the statistical and systematic uncertainties. |
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Figure 2:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the dilepton channel shown separately for different flavors of the additional jets: $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ (upper left), $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm {j}} $ (upper right), $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{c} \mathrm{\bar{c}}} $ (lower left) and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {LF}} $ (lower right). The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 2-a:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the dilepton channel shown for $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $. The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 2-b:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the dilepton channel shown for $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm {j}} $. The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 2-c:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the dilepton channel shown for $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{c} \mathrm{\bar{c}}} $. The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 2-d:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the dilepton channel shown for $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {LF}} $. The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 3:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the lepton+jets channel shown separately for different flavors of the additional jets: $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ (upper left), $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm {j}} $ (upper right), $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{c} \mathrm{\bar{c}}} $ (lower left) and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {LF}} $ (lower right). The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 3-a:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the lepton+jets channel shown for $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $. The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 3-b:
Two-dimensional dist$ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $. he number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 3-c:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the lepton+jets channel shown for $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{c} \mathrm{\bar{c}}} $. The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 3-d:
Two-dimensional distributions of the b tagging discriminant for the first and second additional jets in the lepton+jets channel shown for $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {LF}} $. The number of entries is normalized to unity. The histograms are obtained from the POWHEG MC simulation. |
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Figure 4:
Results of the simultaneous fit for ${R_{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} / {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} and {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}}$ (denoted by the cross) in the visible phase space, along with its 68 and 95% CL contours, are shown for the (left) dilepton and (right) lepton+jets channels. The solid circle shows the prediction by POWHEG + PYTHIA 8. The uncertainties in the MC prediction are a combination of statistical, $\mu _\mathrm {F}/\mu _\mathrm {R}$ scale, and PDF components; they are assumed to be uncorrelated between $ {R_{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} / {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $ and $ {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $. |
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Figure 4-a:
Results of the simultaneous fit for ${R_{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} / {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} and {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}}$ (denoted by the cross) in the visible phase space, along with its 68 and 95% CL contours, are shown for the dilepton channel. The solid circle shows the prediction by POWHEG + PYTHIA 8. The uncertainties in the MC prediction are a combination of statistical, $\mu _\mathrm {F}/\mu _\mathrm {R}$ scale, and PDF components; they are assumed to be uncorrelated between $ {R_{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} / {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $ and $ {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $. |
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Figure 4-b:
Results of the simultaneous fit for ${R_{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} / {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} and {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}}$ (denoted by the cross) in the visible phase space, along with its 68 and 95% CL contours, are shown for the lepton+jets channel. The solid circle shows the prediction by POWHEG + PYTHIA 8. The uncertainties in the MC prediction are a combination of statistical, $\mu _\mathrm {F}/\mu _\mathrm {R}$ scale, and PDF components; they are assumed to be uncorrelated between $ {R_{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} / {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $ and $ {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $. |
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Figure 5:
Measured values (vertical lines) of the $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}} $ cross sections and their ratio, along with their statistical and total uncertainties (dark and light bands) in the dilepton (upper) and lepton+jets (lower) channels in the FPS. Also shown are the theoretical predictions obtained from POWHEG and MG_aMC@NLO (5FS) interfaced with PYTHIA 8, and POWHEG interfaced with HERWIG++. The theoretical predictions for the $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}} $ and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ cross sections are normalized to $\sigma _{{\mathrm{t} \mathrm{\bar{t}}}}^{\text{NNLO}} = $ 832 pb. The previous measurement performed by the CMS Collaboration [18] is also shown with a rhombus marker in the lower plot. The uncertainties in the MC predictions are a combination of the statistical, $\mu _F/\mu _R$ scale, and PDF components. |
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Figure 5-a:
Measured values (vertical lines) of the $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}} $ cross sections and their ratio, along with their statistical and total uncertainties (dark and light bands) in the dilepton lepton+jets channel in the FPS. Also shown are the theoretical predictions obtained from POWHEG and MG_aMC@NLO (5FS) interfaced with PYTHIA 8, and POWHEG interfaced with HERWIG++. The theoretical predictions for the $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}} $ and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ cross sections are normalized to $\sigma _{{\mathrm{t} \mathrm{\bar{t}}}}^{\text{NNLO}} = $ 832 pb. The uncertainties in the MC predictions are a combination of the statistical, $\mu _F/\mu _R$ scale, and PDF components. |
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Figure 5-b:
Measured values (vertical lines) of the $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}} $ cross sections and their ratio, along with their statistical and total uncertainties (dark and light bands) in the dilepton lepton+jets channel in the FPS. Also shown are the theoretical predictions obtained from POWHEG and MG_aMC@NLO (5FS) interfaced with PYTHIA 8, and POWHEG interfaced with HERWIG++. The theoretical predictions for the $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}} $ and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ cross sections are normalized to $\sigma _{{\mathrm{t} \mathrm{\bar{t}}}}^{\text{NNLO}} = $ 832 pb. The previous measurement performed by the CMS Collaboration [18] is also shown with a rhombus marker. The uncertainties in the MC predictions are a combination of the statistical, $\mu _F/\mu _R$ scale, and PDF components. |
| Tables | |
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Table 1:
Summary of the requirements for a simulated event to be in the fiducial (VPS) and full (FPS) phase space regions for the $ {\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} $ and $ {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}} $ categories in the dilepton and lepton+jets channels. Details of the particle-level definitions are described in the text. The symbol $\ell $ denotes a lepton (e or $\mu$). |
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Table 2:
Expected and observed numbers of events in the dilepton and lepton+jets channels after applying the event selection. The results are given for the different ${\mathrm{t} \mathrm{\bar{t}}} $(+jets) categories, the individual sources of background (from MC simulation), normalized to an integrated luminosity of 35.9 fb$^{-1}$, and the observed number from data. The uncertainties quoted for each MC contribution include all the systematic uncertainties described in Section 7. |
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Table 3:
Summary of the individual contributions to the systematic uncertainty in the $ {R_{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}} / {\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $ and $ {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $ measurements for the VPS. The uncertainties are given as relative uncertainties. Some sources include a linear (lin.) or quadratic (quad.) dependency on the fluctuations in data and simulation. |
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Table 4:
The measured cross sections $ {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm{b} \mathrm{\bar{b}}}}} $ and $ {\sigma _{{\mathrm{t} \mathrm{\bar{t}} \mathrm {jj}}}} $, and their ratio, for the VPS and FPS, with the results in the latter corrected for the acceptance and branching fractions. In both the VPS and FPS definitions, the dilepton and lepton+jets channels require particle-level jets with $ {p_{\mathrm {T}}} > $ 30 GeV and 20 GeV, respectively. The predictions from several MC simulations are also shown. The uncertainties in the measurements are split into their statistical (first) and systematic (second) components, while the uncertainties in the MC predictions are a combination of the statistical, $\mu _\mathrm {F}/\mu _\mathrm {R}$ scale, and PDF components. |
| Summary |
|
Measurements of the ${\mathrm{t\bar{t}}\mathrm{b\bar{b}}} $ and ${\mathrm{t\bar{t}} \mathrm{jj}} $ cross sections and their ratio are performed independently in the dilepton and lepton+jets final states using a data sample of proton-proton collisions collected at $\sqrt{s} = $ 13 TeV by the CMS experiment at the LHC in 2016, and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Leptons and particle-level jets must be in the experimentally accessible kinematic region. The inclusive ${\mathrm{t\bar{t}} \mathrm{jj}} $ cross section and the ${\mathrm{t\bar{t}}\mathrm{b\bar{b}}} $ to ${\mathrm{t\bar{t}} \mathrm{jj}} $ cross section ratio in the fiducial phase space are measured by means of a binned maximum likelihood fit to the b tagging discriminant distribution of the additional jets, from which the inclusive ${\mathrm{t\bar{t}}\mathrm{b\bar{b}}} $ cross section measurement is inferred. The cross section ratio and the inclusive ${\mathrm{t\bar{t}} \mathrm{jj}} $ cross section in the fiducial phase space are extrapolated to the full phase space after correcting for the detector acceptance. The measured inclusive cross sections in the fiducial phase space for the dilepton and lepton+jets channels, respectively, are ${\sigma_{{\mathrm{t\bar{t}}\mathrm{b\bar{b}}} }} =$ 0.040 $\pm$ 0.002 (stat) $\pm$ 0.005 (syst) pb and 0.62 $\pm$ 0.03 (stat) $\pm$ 0.07 (syst) pb, performed by multiplying ${\sigma_{{\mathrm{t\bar{t}} \mathrm{jj}} }} $ with the ratio of ${\sigma_{{\mathrm{t\bar{t}}\mathrm{b\bar{b}}} }} $ to ${\sigma_{{\mathrm{t\bar{t}} \mathrm{jj}} }} $, where ${\sigma_{{\mathrm{t\bar{t}} \mathrm{jj}} }} = $ 2.36 $\pm$ 0.02 (stat) $\pm$ 0.20 (syst) pb and 31.0 $\pm$ 0.2 (stat) $\pm$ 2.9 (syst) pb and the ratios are 0.017 $\pm$ 0.001 (stat) $\pm$ 0.001 (syst) and 0.020 $\pm$ 0.001 (stat) $\pm$ 0.001 (syst). The treatment of the systematic uncertainties as nuisance parameters in the fit leads to an improvement in the precision compared to previous measurements. The inclusive ${\mathrm{t\bar{t}}\mathrm{b\bar{b}}} $ cross sections and the cross section ratios for both decay channels measured in the full phase space have values higher than, but consistent with, the predictions from several different Monte Carlo generators. A measured ${\mathrm{t\bar{t}}\mathrm{b\bar{b}}} $ cross section higher than Monte Carlo predictions is also reported in a recent measurement performed by the CMS Collaboration in the fully hadronic final state [19]. |
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