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CMS-TOP-20-003 ; CERN-EP-2020-234
First measurement of the cross section for top quark pair production with additional charm jets using dileptonic final states in pp collisions at $\sqrt{s} = $ 13 TeV
Phys. Lett. B. 820 (2021) 136565
Abstract: The first measurement of the inclusive cross section for top quark pairs ($\mathrm{t\bar{t}}$) produced in association with two additional charm jets is presented. The analysis uses the dileptonic final states of $\mathrm{t\bar{t}}$ events produced in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 41.5 fb$^{-1}$, recorded by the CMS experiment at the LHC. A new charm jet identification algorithm provides input to a neural network that is trained to distinguish among $\mathrm{t\bar{t}}$ events with two additional charm ($\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$), bottom ($\mathrm{t\bar{t}}\mathrm{b}\mathrm{\bar{b}}$), and light-flavour or gluon ($\mathrm{t\bar{t}}\text{LL}$) jets. By means of a template fitting procedure, the inclusive $\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$, $\mathrm{t\bar{t}}\mathrm{b}\mathrm{\bar{b}}$, and $\mathrm{t\bar{t}}\text{LL}$ cross sections are simultaneously measured, together with their ratios to the inclusive $\mathrm{t\bar{t}}$ + two jets cross section. This provides measurements of the $\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$ and $\mathrm{t\bar{t}}\mathrm{b}\mathrm{\bar{b}}$ cross sections of 8.0 $\pm$ 1.1 (stat) $\pm$ 1.3 (syst) pb and 4.09 $\pm$ 0.34 (stat) $\pm$ 0.55 (syst) pb, respectively, in the full phase space. The results are consistent with expectations from the standard model.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Example of a Feynman diagram at the lowest order in QCD, describing the dileptonic decay channel of a top quark pair with an additional $\mathrm{c} \mathrm{\bar{c}} $ pair produced via gluon splitting.

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Figure 2:
Comparison between data (points) and simulated predictions (histograms) for the distribution of the NN score for the best permutations of jet-parton assignments found in each event. Underflow is included in the first bin. The lower panel shows the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched band show the statistical uncertainty in the simulated predictions.

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Figure 3:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first additional jet, before (upper row) and after (lower row) applying the c tagging calibration. The lower panels show the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched band show the statistical uncertainty in the simulated predictions.

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Figure 3-a:
Comparison between data (points) and simulated predictions (histograms) for the CvsL c tagging discriminator distribution of the first additional jet, before applying the c tagging calibration. The lower panel shows the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched band show the statistical uncertainty in the simulated predictions.

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Figure 3-b:
Comparison between data (points) and simulated predictions (histograms) for the CvsB c tagging discriminator distribution of the first additional jet, before applying the c tagging calibration. The lower panel shows the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched band show the statistical uncertainty in the simulated predictions.

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Figure 3-c:
Comparison between data (points) and simulated predictions (histograms) for the CvsL c tagging discriminator distribution of the first additional jet, after applying the c tagging calibration. The lower panel shows the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched band show the statistical uncertainty in the simulated predictions.

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Figure 3-d:
Comparison between data (points) and simulated predictions (histograms) for the CvsB c tagging discriminator distribution of the first additional jet, after applying the c tagging calibration. The lower panel shows the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched band show the statistical uncertainty in the simulated predictions.

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Figure 4:
Normalized two-dimensional distributions of $\Delta _{\mathrm{b}}^{\mathrm{c}}$ vs. $\Delta _{\text {L}}^{\mathrm{c}}$ in simulated dileptonic ${\mathrm{t} {}\mathrm{\bar{t}}} $ events, for each of the event categories outlined in Section 4. The colour scale on the right shows the normalized event yields.

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Figure 5:
A one-dimensional representation of the two-dimensional $\Delta _{\text {L}}^{\mathrm{c}}$ vs. $\Delta _{\mathrm{b}}^{\mathrm{c}}$ distributions, in the simulations (histograms) and in data (points), after normalizing the simulated templates according to the fitted cross sections. The lower panel shows the ratio of the yields in data to those predicted in the simulations. The brown and grey uncertainty bands denote, respectively, the statistical and total uncertainties from the fit. The factors ($\mu $) by which the templates of the different processes (using the POWHEG ME generator) are scaled, are also displayed, together with their combined statistical and systematic uncertainties.

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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-fit value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level (CL) contours are shown, compared to the theoretical predictions using either the POWHEG (blue star) or MadGraph 5_aMC@NLO (red diamond) ME generators. Uncertainties in the theoretical predictions are displayed by the horizontal and vertical bars on the markers.

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Figure 6-a:
Result of the two-dimensional likelihood scan for $\sigma_{\mathrm{t\bar{t}b\bar{b}}}$ [fb] versus $\sigma_{\mathrm{t\bar{t}c\bar{c}}}$ [fb] in the fiducial phase space. The best-fit value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level (CL) contours are shown, compared to the theoretical predictions using either the POWHEG (blue star) or MadGraph 5_aMC@NLO (red diamond) ME generators. Uncertainties in the theoretical predictions are displayed by the horizontal and vertical bars on the markers.

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Figure 6-b:
Result of the two-dimensional likelihood scan for $\sigma_{\mathrm{t\bar{t}LL}}$ [fb] versus $\sigma_{\mathrm{t\bar{t}c\bar{c}}}$ [fb] in the fiducial phase space. The best-fit value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level (CL) contours are shown, compared to the theoretical predictions using either the POWHEG (blue star) or MadGraph 5_aMC@NLO (red diamond) ME generators. Uncertainties in the theoretical predictions are displayed by the horizontal and vertical bars on the markers.

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Figure 6-c:
Result of the two-dimensional likelihood scan for $\sigma_{\mathrm{t\bar{t}LL}}$ [fb] versus $\sigma_{\mathrm{t\bar{t}b\bar{b}}}$ [fb] in the fiducial phase space. The best-fit value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level (CL) contours are shown, compared to the theoretical predictions using either the POWHEG (blue star) or MadGraph 5_aMC@NLO (red diamond) ME generators. Uncertainties in the theoretical predictions are displayed by the horizontal and vertical bars on the markers.

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Figure 6-d:
Result of the two-dimensional likelihood scan for $R_{\mathrm{b}}$ [%] versus $R_{\mathrm{c}}$ [%] in the fiducial phase space. The best-fit value (black cross) with the corresponding 68% (full) and 95% (dashed) confidence level (CL) contours are shown, compared to the theoretical predictions using either the POWHEG (blue star) or MadGraph 5_aMC@NLO (red diamond) ME generators. Uncertainties in the theoretical predictions are displayed by the horizontal and vertical bars on the markers.

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Figure 7:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet before applying the c tagging calibration. The lower panels show the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched bands show the statistical uncertainty in the simulated predictions.

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Figure 7-a:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet before applying the c tagging calibration. The lower panels show the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched bands show the statistical uncertainty in the simulated predictions.

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Figure 7-b:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet before applying the c tagging calibration. The lower panels show the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched bands show the statistical uncertainty in the simulated predictions.

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Figure 7-c:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet before applying the c tagging calibration. The lower panels show the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched bands show the statistical uncertainty in the simulated predictions.

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Figure 7-d:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet before applying the c tagging calibration. The lower panels show the ratio of the yields in data to those predicted in simulations. The vertical bars represent the statistical uncertainties in data, while the hatched bands show the statistical uncertainty in the simulated predictions.

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Figure 8:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet, after normalizing the simulated templates according to the fitted cross sections. The lower panels show the ratio of the yields in data to those predicted in the simulations. The brown and grey uncertainty bands denote, respectively, the statistical and total uncertainties from the fit. The factors ($\mu $) by which the templates of the different processes (using the POWHEG ME generator) are scaled, are also displayed, together with their combined statistical and systematic uncertainties.

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Figure 8-a:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet, after normalizing the simulated templates according to the fitted cross sections. The lower panels show the ratio of the yields in data to those predicted in the simulations. The brown and grey uncertainty bands denote, respectively, the statistical and total uncertainties from the fit. The factors ($\mu $) by which the templates of the different processes (using the POWHEG ME generator) are scaled, are also displayed, together with their combined statistical and systematic uncertainties.

png pdf
Figure 8-b:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet, after normalizing the simulated templates according to the fitted cross sections. The lower panels show the ratio of the yields in data to those predicted in the simulations. The brown and grey uncertainty bands denote, respectively, the statistical and total uncertainties from the fit. The factors ($\mu $) by which the templates of the different processes (using the POWHEG ME generator) are scaled, are also displayed, together with their combined statistical and systematic uncertainties.

png pdf
Figure 8-c:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet, after normalizing the simulated templates according to the fitted cross sections. The lower panels show the ratio of the yields in data to those predicted in the simulations. The brown and grey uncertainty bands denote, respectively, the statistical and total uncertainties from the fit. The factors ($\mu $) by which the templates of the different processes (using the POWHEG ME generator) are scaled, are also displayed, together with their combined statistical and systematic uncertainties.

png pdf
Figure 8-d:
Comparison between data (points) and simulated predictions (histograms) for the CvsL (left column) and CvsB (right column) c tagging discriminator distributions of the first (upper row) and second (lower row) additional jet, after normalizing the simulated templates according to the fitted cross sections. The lower panels show the ratio of the yields in data to those predicted in the simulations. The brown and grey uncertainty bands denote, respectively, the statistical and total uncertainties from the fit. The factors ($\mu $) by which the templates of the different processes (using the POWHEG ME generator) are scaled, are also displayed, together with their combined statistical and systematic uncertainties.
Tables

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Table 1:
Selection efficiencies and acceptance factors for events in different signal categories. The values are obtained from simulated ${\mathrm{t} {}\mathrm{\bar{t}}} $ events.

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Table 2:
Sources of systematic uncertainties in the measured parameters and their individual impact in percent for the fiducial phase space. The upper (lower) rows of the table list uncertainties related to the experimental conditions (theoretical modelling). The last row gives the overall systematic uncertainty in each quantity, which results from the nuisance parameter variations in the fit and is not the quadrature sum of the individual components.

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Table 3:
Sources of theoretical uncertainties in the acceptance, used to extrapolate the results from the fiducial to the full phase space, for different signal categories, together with their individual impact in percent. The last row of the table quotes the total relative uncertainty in the acceptance, calculated by adding in quadrature the effects from individual sources.

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Table 4:
Measured parameter values in the fiducial (upper rows) and full (lower rows) phase spaces with their statistical and systematic uncertainties listed in that order. The last two columns display the expectations from the simulated ${\mathrm{t} {}\mathrm{\bar{t}}} $ samples using the POWHEG or MadGraph 5_aMC@NLO ME generators. The uncertainties quoted for these predictions include the contributions from the theoretical uncertainties listed in the lower rows of Table 2, as well as the uncertainty in the ${\mathrm{t} {}\mathrm{\bar{t}}} $ cross section.
Summary
The production of a top quark pair ($\mathrm{t\bar{t}}$) in association with additional bottom or charm jets at the LHC provides challenges both in the theoretical modelling and experimental measurement of this process. Whereas $\mathrm{t\bar{t}}$ production with two additional bottom jets ($\mathrm{t\bar{t}}\mathrm{b}\mathrm{\bar{b}}$) has been measured by the ATLAS and CMS Collaborations at different centre-of-mass energies [6,7,8,9,10,11,12,13], this analysis presents the first measurement of the cross section for $\mathrm{t\bar{t}}$ production with two additional charm jets ($\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$). The analysis is conducted using data from proton-proton collisions recorded 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 channel of the $\mathrm{t\bar{t}}$ decays and relies on the use of recently developed charm jet identification algorithms (c tagging). A template fitting method is used, based on the outputs of a neural network classifier trained to identify the signal categories defined by the flavour of the additional jets. This allows the simultaneous extraction of the cross section for the $\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$, $\mathrm{t\bar{t}}\mathrm{b}\mathrm{\bar{b}}$, and $\mathrm{t\bar{t}}$ with two additional light-flavour or gluon jets ($\mathrm{t\bar{t}}\text{LL}$) processes. A novel multidimensional calibration of the shape of the c tagging discriminator distributions is employed, such that this information can be reliably used in the neural network classifier.

The $\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$ cross section is measured for the first time to be 0.165 $\pm$ 0.023 (stat) $\pm$ 0.025 (syst) pb in the fiducial phase space (matching closely the sensitive region of the detector) and 8.0 $\pm$ 1.1 (stat) $\pm$ 1.3 (syst) pb in the full phase space. The ratio of the $\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$ to the inclusive $\mathrm{t\bar{t}}$ + two jets cross section is found to be (2.42 $\pm$ 0.32 (stat) $\pm$ 0.29 (syst))% in the fiducial phase space and (2.69 $\pm$ 0.36 (stat) $\pm$ 0.32 (syst))% in the full phase space. Agreement is observed at the level of one to two standard deviations between the measured values and theoretical predictions for the $\mathrm{t\bar{t}}\mathrm{c}\mathrm{\bar{c}}$, $\mathrm{t\bar{t}}\mathrm{b}\mathrm{\bar{b}}$, and $\mathrm{t\bar{t}}\text{LL}$ processes.
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