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| CMS-PAS-TOP-20-003 | ||
| Measurement of the cross section of top quark pair production with additional charm jets using the dileptonic final state in pp collisions at $\sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| September 2020 | ||
| Abstract: For the first time, a measurement of the inclusive cross section of top quark pair ($\mathrm{t}\bar{\mathrm{t}}$) production with two additional charm jets is presented. The measurement is performed using the dileptonic decay channel of the top quark pairs produced in proton-proton collisions at a centre-of-mass energy of 13 TeV. The used dataset corresponds to an integrated luminosity of 41.5 fb$^{-1}$, collected with the CMS detector at the LHC. A neural network is trained to distinguish between top quark pair events with additional c jets ($\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}$), b jets ($\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}$), and light-flavour jets ($\mathrm{t}\bar{\mathrm{t}}\text{LF}$), based on observables related to dedicated charm jet identification algorithms, as well as kinematic properties of the event. By means of a template fitting procedure using simulated and observed neural network outputs, the inclusive $\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}$, $\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}$, and $\mathrm{t}\bar{\mathrm{t}}\text{LF}$ cross sections are simultaneously extracted, together with their ratios to the inclusive $\mathrm{t}\bar{\mathrm{t}}$ + two jets cross section. This results in a fully coherent treatment of different additional jet flavours in the production of $\mathrm{t}\bar{\mathrm{t}}$ + two jets, and provides the first measurement of the $\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}$ process with a measured cross section of 0.152 $\pm$ 0.022 (stat.) $\pm$ 0.019 (syst.) pb in the fiducial phase space and 7.43 $\pm$ 1.07 (stat.) $\pm$ 0.95 (syst.) pb in the full phase space. | ||
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Links:
CDS record (PDF) ;
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These preliminary results are superseded in this paper, Submitted to PLB. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Example of a Feynman diagram describing the dileptonic decay channel of a top quark pair with two additional c quarks produced via gluon splitting. |
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Figure 2:
Comparison between the data (points) and the simulated predictions (histograms) for the distribution of the neural network score for the best permutation found in each event. Underflow is included in the first bin. |
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Figure 3:
Comparison between the data (points) and the simulated predictions (histograms) for the CvsL (left) and CvsB (right) c-tagging discriminator distributions of the first additional jet, before (top) and after (bottom) the c-tagging calibration is applied. |
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Figure 3-a:
Comparison between the data (points) and the simulated predictions (histograms) for the CvsL c-tagging discriminator distributions of the first additional jet, before the c-tagging calibration is applied. |
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Figure 3-b:
Comparison between the data (points) and the simulated predictions (histograms) for the CvsB c-tagging discriminator distributions of the first additional jet, before the c-tagging calibration is applied. |
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Figure 3-c:
Comparison between the data (points) and the simulated predictions (histograms) for the CvsL c-tagging discriminator distributions of the first additional jet, after the c-tagging calibration is applied. |
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Figure 3-d:
Comparison between the data (points) and the simulated predictions (histograms) for the CvsB c-tagging discriminator distributions of the first additional jet, after the c-tagging calibration is applied. |
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Figure 4:
Normalised two-dimensional distributions of $\Delta _{\text {L}}^{\mathrm{c}}$ on the x-axis and $\Delta _{\mathrm{b}}^{\mathrm{c}}$ on the y-axis in simulated dileptonic top quark pair events after the event selection outlined in Sec. 5. |
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Figure 5:
A one-dimensional representation of the two-dimensional $\Delta _{\text {L}}^{\mathrm{c}}$ and $\Delta _{\mathrm{b}}^{\mathrm{c}}$ distributions, in the simulation and in data after scaling the simulated templates according to the fitted cross sections. The bottom panel shows the ratio of the yields in data to those in the simulation. The brown (grey) uncertainty band denotes the statistical uncertainty from the fit (the statistical and systematic uncertainties combined). The signal strengths $\mu _{{\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}}}$, $\mu _{{\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}}}$ and $\mu _{{\mathrm{t}\bar{\mathrm{t}}\text {LF}}}$ are also shown with their uncertainties (not including theoretical uncertainties on the NNLO $\mathrm{t}\bar{\mathrm{t}}$ cross section). |
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Figure 6:
Results of the two-dimensional likelihood scans for several combinations of the parameters of interest in the fiducial phase space. The best fitted value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level contours are shown, compared to the theory predictions using either the {POWHEG} (blue star) or MG5_aMC@NLO (red diamond) matrix element generators. Uncertainties on the theoretical predictions are displayed by the horizontal and vertical error bars on the markers. |
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Figure 6-a:
Results of the two-dimensional likelihood scans for $\sigma_{\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}}$ vs $\sigma_{\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}}$ in the fiducial phase space. The best fitted value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level contours are shown, compared to the theory predictions using either the {POWHEG} (blue star) or MG5_aMC@NLO (red diamond) matrix element generators. Uncertainties on the theoretical predictions are displayed by the horizontal and vertical error bars on the markers. |
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Figure 6-b:
Results of the two-dimensional likelihood scans for $\sigma_{\mathrm{t}\bar{\mathrm{t}}\text{LF}}$ vs $\sigma_{\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}}$ in the fiducial phase space. The best fitted value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level contours are shown, compared to the theory predictions using either the {POWHEG} (blue star) or MG5_aMC@NLO (red diamond) matrix element generators. Uncertainties on the theoretical predictions are displayed by the horizontal and vertical error bars on the markers. |
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Figure 6-c:
Results of the two-dimensional likelihood scans for $\sigma_{\mathrm{t}\bar{\mathrm{t}}\text{LF}}$ vs $\sigma_{\mathrm{t}\bar{\mathrm{b}}\mathrm{c}\bar{\mathrm{b}}}$ in the fiducial phase space. The best fitted value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level contours are shown, compared to the theory predictions using either the {POWHEG} (blue star) or MG5_aMC@NLO (red diamond) matrix element generators. Uncertainties on the theoretical predictions are displayed by the horizontal and vertical error bars on the markers. |
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Figure 6-d:
Results of the two-dimensional likelihood scans for $\text{R}_{\mathrm{b}}$ vs $\text{R}_{\mathrm{c}}$ in the fiducial phase space. The best fitted value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level contours are shown, compared to the theory predictions using either the {POWHEG} (blue star) or MG5_aMC@NLO (red diamond) matrix element generators. Uncertainties on the theoretical predictions are displayed by the horizontal and vertical error bars on the markers. |
| Tables | |
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Table 1:
Selection efficiencies and acceptance factors for events in different signal categories. These values were derived from simulated top quark pair events. |
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Table 2:
Summary of the individual impacts of the uncertainties on the different parameters of interest in the fiducial phase space. The upper (lower) rows of the table list uncertainties related to the experimental conditions (theoretical modelling). |
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Table 3:
Results on the parameters of interest in the fiducial and full phase space with uncertainties. The last two columns display the theoretical predictions from the simulated top quark pair samples using either POWHEG or MG5_aMC@NLO as a matrix element generator. The uncertainty quoted for these predictions includes uncertainties from variations of the QCD scales ($\mu _{\text {R}}$ and $\mu _{\text {F}}$) in the ME, $\alpha _{|text{S}}$ uncertainties in the PS and in the proton PDF, uncertainties related to the underlying event and the matching between the ME and the PS (hdamp), as well as the uncertainty on the NNLO $\mathrm{t}\bar{\mathrm{t}}$ cross section. |
| Summary |
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The production of a top quark pair in association with additional bottom or charm jets at the LHC presents challenges both in the theoretical modelling as well as in the experimental measurement of this process. Whereas the $\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}$ process has been measured by the CMS and ATLAS collaborations at different centre-of-mass energies, this analysis presents the first measurement of the $\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}$ cross section. The analysis is conducted using a data sample of proton-proton collision events collected by the CMS experiment at a centre-of-mass energy of $13 TeV$, corresponding to an integrated luminosity of 41.5 fb$^{-1}$. The measurement is performed in the dileptonic decay channel of the top quark pairs and relies on the use of recently developed charm-jet identification algorithms. A template fitting method is used, based on the outputs of a neural network classifier that is trained to identify the different signal categories defined by the flavour of the additional jets. This allows the simultaneous extraction of the $\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}$, $\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}$ and $\mathrm{t}\bar{\mathrm{t}}\text{LF}$ cross sections, as well as the ratios R$_{\mathrm{c}} = \sigma_{\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}}/\sigma_{\mathrm{t}\bar{\mathrm{t}}\mathrm{jj}}$ and R$_{\mathrm{b}} = \sigma_{\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}}/\sigma_{\mathrm{t}\bar{\mathrm{t}}\mathrm{jj}}$. A novel calibration of the full shape of the c-tagging discriminator distributions is employed, such that this information can be reliably used in the neural network classifier. The $\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}$ cross section is measured to be 0.152 $\pm$ 0.022 (stat.) $\pm$ 0.019 (syst.) pb in the fiducial phase space and 7.43 $\pm$ 1.07 (stat.) $\pm$ 0.95 (syst.) pb in the full phase space. The ratio $\text{R}_{\mathrm{c}}$ is found to be 2.37 $\pm$ 0.32 (stat.) $\pm$ 0.25 (syst.)% in the fiducial phase space and 2.64 $\pm$ 0.36 (stat.) $\pm$ 0.28 (syst.)% in the full phase space. An overall agreement is observed between the measured values and the theoretical predictions at the level of one to two standard deviations for the $\mathrm{t}\bar{\mathrm{t}}\mathrm{c}\bar{\mathrm{c}}$, $\mathrm{t}\bar{\mathrm{t}}\mathrm{b}\bar{\mathrm{b}}$ and $\mathrm{t}\bar{\mathrm{t}}\text{LF}$ processes. The largest disagreement is observed for the ratio $\text{R}_{\mathrm{b}}$, at the level of 2.5 standard deviations, which nevertheless is found to be consistent with observations from previous analyses [4,5,6,7,8,9,10] targeting specifically this final state. |
| Additional Figures | |
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Additional Figure 1:
Normalised distribution of the DeepCSV CvsB discriminator for the first additional jet for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 2:
Normalised distribution of the DeepCSV CvsL discriminator for the first additional jet for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 3:
Normalised distribution of the DeepCSV CvsB discriminator for the second additional jet for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 4:
Normalised distribution of the DeepCSV CvsL discriminator for the second additional jet for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 5:
Normalised distribution of the angular separation in $\Delta R$ between the two additional jets for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 6:
Normalised distribution of the neural network score for the best permutation found in each event for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 7:
Normalised distribution of the $\Delta _{\text {b}}^{\text {c}}$ discriminator for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 8:
Normalised distribution of the $\Delta _{\text {L}}^{\text {c}}$ discriminator for different signal categories in simulated dileptonic top quark pair events. |
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Additional Figure 9:
Comparison between the data (points) and the simulated predictions (histograms) for the distribution of the $\Delta _{\text {b}}^{\text {c}}$ discriminator before the fit. |
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Additional Figure 10:
Comparison between the data (points) and the simulated predictions (histograms) for the distribution of the $\Delta _{\text {L}}^{\text {c}}$ discriminator before the fit. |
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Additional Figure 11:
One-dimensional likelihood scan for the $\mathrm{t\bar{t}}\mathrm{b\bar{b}}$ cross section in the fiducial phase space, taking into account only the statistical uncertainty in red, and both systematic and statistical uncertainties in black. Intersections with $-\Delta \text {log}(\text {L}) = $ 0.5 and $-\Delta \text {log}(\text {L}) = $ 2 show respectively the 68% and 95% confidence level intervals on the parameter of interest. The theory prediction from the POWHEG + PYTHIA-8 simulation is also shown on the figure with the corresponding 68% confidence level interval superimposed. The latter includes uncertainties from variations of the QCD scales ($\mu _{\text {R}}$ and $\mu _{\text {F}}$) in the ME, $\alpha _s$ uncertainties in the PS and in the proton pdf, uncertainties related to the underlying event and the matching between the ME and the PS (hdamp), as well as the uncertainty on the NNLO $\mathrm{t\bar{t}}$ cross section. |
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Additional Figure 12:
One-dimensional likelihood scan for the $\mathrm{t\bar{t}}\text {c}\overline {\text {c}}$ cross section in the fiducial phase space, taking into account only the statistical uncertainty in red, and both systematic and statistical uncertainties in black. Intersections with $-\Delta \text {log}(\text {L}) = $ 0.5 and $-\Delta \text {log}(\text {L}) = $ 2 show respectively the 68% and 95% confidence level intervals on the parameter of interest. The theory prediction from the POWHEG + PYTHIA-8 simulation is also shown on the figure with the corresponding 68% confidence level interval superimposed. The latter includes uncertainties from variations of the QCD scales ($\mu _{\text {R}}$ and $\mu _{\text {F}}$) in the ME, $\alpha _s$ uncertainties in the PS and in the proton pdf, uncertainties related to the underlying event and the matching between the ME and the PS (hdamp), as well as the uncertainty on the NNLO $\mathrm{t\bar{t}}$ cross section. |
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Additional Figure 13:
One-dimensional likelihood scan for the $\mathrm{t\bar{t}}\text {LF}$ cross section in the fiducial phase space, taking into account only the statistical uncertainty in red, and both systematic and statistical uncertainties in black. Intersections with $-\Delta \text {log}(\text {L}) = $ 0.5 and $-\Delta \text {log}(\text {L}) = $ 2 show respectively the 68% and 95% confidence level intervals on the parameter of interest. The theory prediction from the POWHEG + PYTHIA-8 simulation is also shown on the figure with the corresponding 68% confidence level interval superimposed. The latter includes uncertainties from variations of the QCD scales ($\mu _{\text {R}}$ and $\mu _{\text {F}}$) in the ME, $\alpha _s$ uncertainties in the PS and in the proton pdf, uncertainties related to the underlying event and the matching between the ME and the PS (hdamp), as well as the uncertainty on the NNLO $\mathrm{t\bar{t}}$ cross section. |
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Additional Figure 14:
One-dimensional likelihood scan for the ratio $\text {R}_{\text {b}}$ in the fiducial phase space, taking into account only the statistical uncertainty in red, and both systematic and statistical uncertainties in black. Intersections with $-\Delta \text {log}(\text {L}) = $ 0.5 and $-\Delta \text {log}(\text {L}) = $ 2 show respectively the 68% and 95% confidence level intervals on the parameter of interest. The theory prediction from the POWHEG + PYTHIA-8 simulation is also shown on the figure with the corresponding 68% confidence level interval superimposed. The latter includes uncertainties from variations of the QCD scales ($\mu _{\text {R}}$ and $\mu _{\text {F}}$) in the ME, $\alpha _s$ uncertainties in the PS and in the proton pdf, uncertainties related to the underlying event and the matching between the ME and the PS (hdamp), as well as the uncertainty on the NNLO $\mathrm{t\bar{t}}$ cross section. |
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Additional Figure 15:
One-dimensional likelihood scan for the ratio $\text {R}_{\text {c}}$ in the fiducial phase space, taking into account only the statistical uncertainty in red, and both systematic and statistical uncertainties in black. Intersections with $-\Delta \text {log}(\text {L}) = $ 0.5 and $-\Delta \text {log}(\text {L}) = $ 2 show respectively the 68% and 95% confidence level intervals on the parameter of interest. The theory prediction from the POWHEG +PYTHIA-8 simulation is also shown on the figure with the corresponding 68% confidence level interval superimposed. The latter includes uncertainties from variations of the QCD scales ($\mu _{\text {R}}$ and $\mu _{\text {F}}$) in the ME, $\alpha _s$ uncertainties in the PS and in the proton pdf, uncertainties related to the underlying event and the matching between the ME and the PS (hdamp), as well as the uncertainty on the NNLO $\mathrm{t\bar{t}}$ cross section. |
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Additional Figure 16:
A one-dimensional representation of the two-dimensional $\Delta _{\text {L}}^{\text {c}}$ and $\Delta _{\text {b}}^{\text {c}}$ distributions, in the simulation and in data before performing the fit. The bottom panel shows the ratio of the yields in data to those in the simulation. The brown (grey) uncertainty band denotes the statistical uncertainty from the simulation (the statistical and systematic uncertainties combined). |
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Additional Figure 17:
Summary of the measured cross sections and ratios for the fiducial phase space, compared to predictions from the POWHEG and MG5_aMC@NLO matrix element generators, interfaced with PYTHIA-8 for the parton shower. |
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