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CMS-TOP-17-020 ; CERN-EP-2019-018
Search for new physics in top quark production in dilepton final states in proton-proton collisions at $\sqrt{s} =$ 13 TeV
Eur. Phys. J. C 79 (2019) 886
Abstract: A search for new physics in top quark production is performed in proton-proton collisions at 13 TeV. The data set corresponds to an integrated luminosity of 35.9 fb$^{-1}$ collected in 2016 with the CMS detector. Events with two opposite-sign isolated leptons (electrons or muons), and b quark jets in the final state are selected. The search is sensitive to new physics in top quark pair production and in single top quark production in association with a W boson. No significant deviation from the standard model expectation is observed. Results are interpreted in the framework of effective field theory and constraints on the relevant effective couplings are set, one at a time, using a dedicated multivariate analysis. This analysis differs from previous searches for new physics in the top quark sector by explicitly separating tW from $\mathrm{t\bar{t}}$ events and exploiting the specific sensitivity of the tW process to new physics.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Representative Feynman diagrams for the tW (left panel) and ${\mathrm{t} {}\mathrm{\bar{t}}}$ (right panel) production at leading order. The upper row presents the SM diagrams, the middle and lower rows present diagrams corresponding to the $O_{\phi \mathrm{q}}^{(3)}$, $O_{\mathrm{t} \mathrm{W}}$,$O_{\mathrm{t} \mathrm{G}}$, $O_{\mathrm{G}}$ and $O_{\mathrm {u/cG}}$ contributions.

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Figure 2:
The observed number of events and SM background predictions in the search regions of the analysis for the $\mathrm{e} \mathrm{e} $ (upper left), $\mu \mu $ (upper right), and $\mathrm{e} \mu $ (lower) channels. The hatched bands correspond to the quadratic sum of statistical and systematic uncertainties in the event yield for the SM background predictions. The ratios of data to the sum of the predicted yields are shown at the bottom of each plot. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation.

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Figure 2-a:
The observed number of events and SM background predictions in the search regions of the analysis for the $\mathrm{e} \mathrm{e} $ channel. The hatched bands correspond to the quadratic sum of statistical and systematic uncertainties in the event yield for the SM background predictions. The ratios of data to the sum of the predicted yields are shown at the bottom of the plot. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation.

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Figure 2-b:
The observed number of events and SM background predictions in the search regions of the analysis for the $\mu \mu $ $channel. The hatched bands correspond to the quadratic sum of statistical and systematic uncertainties in the event yield for the SM background predictions. The ratios of data to the sum of the predicted yields are shown at the bottom of the plot. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation.

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Figure 2-c:
The observed number of events and SM background predictions in the search regions of the analysis for the $\mathrm{e} \mu $ channel. The hatched bands correspond to the quadratic sum of statistical and systematic uncertainties in the event yield for the SM background predictions. The ratios of data to the sum of the predicted yields are shown at the bottom of the plot. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation.

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Figure 3:
The NN output distributions for data and simulation for the $\mathrm{e} \mathrm{e} $ (left) and $\mu \mu $ (right) channels in 1-jet, 1-tag (upper) and 2-jets, 1-tag (lower) categories. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the lower panel of each graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. In each plot, the expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 3-a:
The NN output distributions for data and simulation for the $\mathrm{e} \mathrm{e} $ channel in the 1-jet, 1-tag category. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 3-b:
The NN output distributions for data and simulation for the $\mu \mu $ channel in the 1-jet, 1-tag category. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 3-c:
The NN output distributions for data and simulation for the $\mathrm{e} \mathrm{e} $ channel in the 2-jets, 1-tag category. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 3-d:
The NN output distributions for data and simulation for the $\mu \mu $ channel in the 2-jets, 1-tag category. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 4:
The NN output distributions for (left) data and simulation for the $\mathrm{e} \mu $ channel in 1-jet, 1-tag (upper) and 2-jets, 1-tag (middle) and 1-jet, 0-tag (lower) categories; and for (right) data, simulation, and FCNC signals in the $n$-jets, 1-tag category used in the limit setting for the $\mathrm{e} \mathrm{e} $ (upper), $\mathrm{e} \mu $ (middle), and $\mu \mu $ (lower) channels. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the the lower panel of each graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. In each plot, the expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 4-a:
The NN output distributions for data and simulation for the $\mathrm{e} \mu $ channel in the 1-jet, 1-tag category. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 4-b:
The NN output distributions for data, simulation, and FCNC signals in the $n$-jets, 1-tag category used in the limit setting for the $\mathrm{e} \mathrm{e} $ channel. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 4-c:
The NN output distributions for data and simulation for the $\mathrm{e} \mu $ channel in the 2-jets, 1-tag category. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 4-d:
The NN output distributions for data, simulation, and FCNC signals in the $n$-jets, 1-tag category used in the limit setting for the $\mathrm{e} \mu $ channel. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 4-e:
The NN output distributions for data and simulation for the $\mathrm{e} \mu $ channel in the 1-jet, 0-tag category. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 4-f:
The NN output distributions for data, simulation, and FCNC signals in the $n$-jets, 1-tag category used in the limit setting for the $\mu \mu $ channel. The hatched bands correspond to the quadratic sum of the statistical and systematic uncertainties in the event yield for the sum of signal and background predictions. The ratios of data to the sum of the predicted yields are shown at the the lower panel of the graph. The narrow hatched bands represent the contribution from the statistical uncertainty in the MC simulation. The expected distributions assuming specific values for the effective couplings (given in the legend) are shown as the solid curves.

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Figure 5:
Observed (solid) and expected (dotted) log likelihoods for the effective couplings: $\mathrm{C}_{\mathrm{G}}$ (upper left), $\mathrm{C}_{\mathrm{t} \mathrm{G}}$ (upper right), $\mathrm{C}_{\mathrm{t} \mathrm{W}}$ (middle left), $\mathrm{C}_{\phi \mathrm{q}}$ (middle right), $\mathrm{C}_{\mathrm{u} \mathrm{G}}$ (lower left), and $\mathrm{C}_{\mathrm{c} \mathrm{G}}$ (lower right). The dashed curves represent fits to the observed data with the variations of normalization due to the theoretical uncertainties.

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Figure 5-a:
Observed (solid) and expected (dotted) log likelihoods for the $\mathrm{C}_{\mathrm{G}}$ effective coupling. The dashed curves represent fits to the observed data with the variations of normalization due to the theoretical uncertainties.

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Figure 5-b:
Observed (solid) and expected (dotted) log likelihoods for the $\mathrm{C}_{\mathrm{t} \mathrm{G}}$ $effective coupling. The dashed curves represent fits to the observed data with the variations of normalization due to the theoretical uncertainties.

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Figure 5-c:
Observed (solid) and expected (dotted) log likelihoods for the $\mathrm{C}_{\mathrm{t} \mathrm{W}}$ effective coupling. The dashed curves represent fits to the observed data with the variations of normalization due to the theoretical uncertainties.

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Figure 5-d:
Observed (solid) and expected (dotted) log likelihoods for the $\mathrm{C}_{\phi \mathrm{q}}$ effective coupling. The dashed curves represent fits to the observed data with the variations of normalization due to the theoretical uncertainties.

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Figure 5-e:
Observed (solid) and expected (dotted) log likelihoods for the $\mathrm{C}_{\mathrm{u} \mathrm{G}}$ effective coupling. The dashed curves represent fits to the observed data with the variations of normalization due to the theoretical uncertainties.

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Figure 5-f:
Observed (solid) and expected (dotted) log likelihoods for the $\mathrm{C}_{\mathrm{c} \mathrm{G}}$ effective coupling. The dashed curves represent fits to the observed data with the variations of normalization due to the theoretical uncertainties.

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Figure 6:
Observed best fits together with one and two standard deviation bounds on the top quark effective couplings. The dashed line shows the SM expectation and the vertical lines indicate the 95% CL bounds including the theoretical uncertainties.
Tables

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Table 1:
Contribution to the cross section due to the interference between the SM diagrams and diagrams with one EFT vertex ($\sigma _i^{(1)}$), and the pure new physics ($\sigma _i^{(2)}$) for ${\mathrm{t} {}\mathrm{\bar{t}}}$ and tW production [in {\text {\tmspace +\thinmuskip {.1667em}pb}} ] for the various effective couplings for $\Lambda = $ 1 TeV. The respective $K$ factors ($\sigma _i^{\mathrm {NLO}}/\sigma _i^{\mathrm {LO}}$) are also shown.

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Table 2:
Number of expected events from tW, ${\mathrm{t} {}\mathrm{\bar{t}}}$ and DY production, from the remaining backgrounds (other), total background contribution and observed events in data after all selections for the $\mathrm{e} \mathrm{e} $, $\mathrm{e} \mu $, and $\mu \mu $ channels and for different ($n$-jets,$m$-tags) categories. The uncertainties correspond to the statistical contribution only for the individual background predictions and to the quadratic sum of the statistical and systematic contributions for the total background predictions.

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Table 3:
Summary of the observables used to probe the effective couplings in various ($n$-jets,$m$-tags) categories in the $\mathrm{e} \mathrm{e} $, $\mathrm{e} \mu $, and $\mu \mu $ channels.

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Table 4:
Input variables for the NN used in the analysis in various bins of $n$-jets and $m$-tags. The symbols "$\times $" indicate the input variables used in the four NNs.

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Table 5:
Summary of the observed and expected allowed intervals on the effective couplings obtained in the $\mathrm{e} \mathrm{e} $, $\mathrm{e} \mu $, and $\mu \mu $ channels, and all channels combined. All sources of systematic uncertainty, described in Section 5, are taken into account with the exception of the uncertainties on the SM cross section predictions for the tt and tW processes.

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Table 6:
Estimation of the effect of the most important uncertainty sources on the observed allowed intervals of in the fit.
Summary
A search for new physics in top quark interactions is performed using $\mathrm{t\bar{t}}$ and tW events in dilepton final states. The analysis is based on data collected in pp collisions at 13 TeV by the CMS detector in 2016, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. No significant excess above the standard model background expectation is observed. For the first time, both $\mathrm{t\bar{t}}$ and tW production are used simultaneously in a model independent search for effective couplings. The six effective couplings, $\mathrm{C}_{\mathrm{G}}$, $\mathrm{C}_{\mathrm{t}\mathrm{G}}$, $\mathrm{C}_{\mathrm{t}\mathrm{W}}$, $\mathrm{C}_{\phi \mathrm{q}}^{(3)}$, $\mathrm{C}_{\mathrm{u}\mathrm{G}}$, and $\mathrm{C}_{\mathrm{c}\mathrm{G}}$ are constrained using a dedicated multivariate analysis. The constraints presented, obtained by considering one operator at a time, are a useful first step toward more global approaches.
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